|
SYNTHESIS AND X-RAY STRUCTURE OF
IRON STABILIZED STRAINED CYCLIC ALLENES.
VALENCE ISOMERIZATION BETWEEN LINEAR PERPENDICULAR
AND BENT PLANAR ALLENE
BY
SU-MIN OON
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1987
To my mother and
in memory of my father
ACKNOWLEDGEMENTS
I would like to thank Dr. W.M. Jones for providing an
atmosphere conducive to my personal and intellectual growth
during my stay. He has kept me entertained with a fun and
exciting problem and his guidance and encouragement are
crucial toward the successful completion of this study. He
has attracted many fine individuals who are a pleasure to
work with. I would also like to thank Dr. Palenik and
Dr. Koziol for the X-ray crystal structure of my iron
stabilized cyclic allene. The witty, sarcastic and some-
times acrimonious remarks of Mr. Nicholas Conti made me
laugh at even the worst of my scientific blunders. The most
special thanks go to Miss Margaret Easley; a soprano, a
pianist, an artist and a dear friend. I have spent many
happy moments with her engaged in intriguing conversations
spanning an extremely wide range of topics. She introduced
me to opera and classical music and therefore made an
everlasting impact on my life. Her friendship is greatly
appreciated and she will always be remembered with fondness.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS . . . . . . . . .
LIST OF TABLES . . . . . . . . . .
LIST OF FIGURES . . . . . . . . . .
ABSTRACT . . . . . . . . . . ..
CHAPTER
Page
ix
x
xi
* xi~
I INTRODUCTION . . . . . . . . . 1
II DICARBONYL(h5-CYCLOPENTADIENYL)IRON(II)
COMPLETED 1,2-CYCLOHEPTADIENE . . . ... 16
III CARBONYL(h5-CYCLOPENTADIENYL)TRIPHENYLPHOSPHINE-
IRON(II) COMPLETED 1,2-CYCLOHEPTADIENE .... .61
IV IRON(II) COMPLETED 1,2-CYCLOHEXADIENE . . .. .98
V EXPERIMENTAL . . . . . . . ... .108
Improved synthesis ?f dicarbonyl(h -cyclo
pentadienyl)(h -(7-methoxy)cyclohepten-1-
yl)iron(II) . . . . . . .. 109
Thermal decomposition ot dicarbonyl(h -1,2-
cycloheptadiene)(h -cyclopentadienyl)
iron(II) trifluoromethanesulfonate .... .109
NMR scale synthesis of dicarbonyl(h -cyclo
pentadienyl)iron(II) trifluoromethane-
sulfonate . . . . . . . . 110
Attempted thermal decomposition of dicarbonyl
(h -1,2-cycloheptadiene)(h -cyclopentadienyl)
iron(II) tetrafluoroborate . . . . .. 110
13
Variable temperature C NMR studies of the
dicarbonyl(h -1,2-cycloheptadiene)(h -
cyclopentadienyl)iron(II) tetrafluoro-
borate . . . . . . . . . 110
Page
2
The reaction between dicarbonyl(h -1,2-cyclo-
heptadiene)(h -cyclopentadienyl)iron(II)
tetrafluoroborate and triphenylmethane . 111
Attempted enantio eric enrichment of race ic
dicarbonyl(h -1,2-cycloheptadiene)(h -
cyclopentadienyl)iron(II) tetrafluoroborate
with (S)-(-)-2-methylbutanol . . ... Ill
Resolution of 7-methoxy-l-cycloheptenecarboxylic
acid . . . . . .... . . . 112
Synthesis of optically active dicarbonyl(h -
cyclopentadienyl)(h -(7-methoxy)cyclo-
hepten-l-yl)iron(II) (N) . . . ... .112
Synthesis of optically active 1-bromo-
7-methoxycycloheptene . . . ... 113
Synthesis of optically aTtive dicarbonyl(h -
cyclopentadienyl)(h -(7-methoxy)cyclohepen-
1-yl)iron(II) (P) . . . . . . 114
Synthesis of dicarbonyl(h -cyclopentadienyl)
(h -(7-methoxy)cycloheptenl1-yl)
iron(II) from5dicarbonyl(h -1,2-cyclo-
heptadiene)(h -cyclopentadienyl)iron(II)
trifluoromethanesulfonate . . . ... 114
Synthesis of dicarbonyl(l -carbonyl-(7-methoxy)
cyclohepten-1-yl) (h -cyclopentadienyl)
iron(II) via carbonylation of dicarbonyl
(h -cyclopentadienyl)(h -(7-methoxy)
cyclohepten-1-yl)iron(II) . . . ... .114
Synthes s of carbonyl(h5-cyclopentadienyl)
(h -(7-methoxy)cyclohepten-l-yl)triphenyl-
phosphine iron(II) . . . . . ... 115
Synthes s of carbonyl(h2-1,2-cycloheptadiene)
(h -cyclopentadienyl)triphenylphosphine-
iron(II) trifluoromethanesulfonate .... .116
Synthesis of l-bromo-7-methoxycycloheptene-7d1 117
Synthesis of carbonyl(h -1,2-cycloheptadiene-
ld ) (h -cyclopentadienyl)triphenylphos-
phineiron(II) trifluoromethanesulfonate 117
Page
The exchange of hexafluorophosphate anion for
tr fluoromethanesulfonate anion in carbonyl
(h -1,2-cycloheptadiene)(h -cyclopenta-
dienyl)triphenylphosphineiron(II) . .. .118
Synthesis of carbonyl(h 5carbonyl-(7-methoxy)
cyclohepten-l-yl)(h -cyclopentadienyl)
triphenylphosphineiron(II) . . . ... .118
Separation of diastereomers of carbonyl(h -
cagbonyl-(7-methoxy)cyclohepten-l-yl)
(h -cydopentadienyl)triphenylphosphine-
iron(II) . . . . . .... . . 119
Attempted decarbonylation of carbonyl(h -
carbonyl-(7-methoxy)cyclohepten-1-yl)
(h -cyclopentadienyl)triphenylphosphine-
iron(II) . . . . . .... . . 120
Separation of diastereomer of carbonyl(h -cyclo-
pentadienyl)(h -(7-methoxy)cyclohepten-1-
yl)triphenylphosphineiron(II) . . ... 121
Synthes s of carbonyl(h2-1,2-cycloheptadiene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) trifluoromethanesulfonate from the
enriched carbonyl(h -cyclopentadienyl)
(h -(7-methoxy)-cycloheptne-1-yl)triphenyl-
phosphineiron(II) (80:20) . . ... 122
Reaction between carbonyl(h5-cyclopentadienyl)
(h -(7-methoxy)cyclohepten-l-yl)triphenyl-
phosphineiron(II) with a half equivalent
of trimethylsilyl trifluoromethane-
sulfonate . . . . . . . . 122
Synthes s of carbonyl(h -1,2-cycloheptadiene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) trifluoromethapesulfonate from
th enriched carbonyl(h -cyclopentadienyl)
(h -(7-methoxy)cyclohepten-1-yl)triphenyl-
phosphineiron(II) (15:85) . . . ... .123
Synthes s of dicarbonyl(h -cyclopentadienyl)
(h -(7-ethoxy)cycloheptadien-l-yl)
iron(II) . . . . . . . ... 123
Synthesis of carbonyl(h -cyclopentadienyl)
(h -(7-ethoxy)cycloheptadien-1-yl)
triphenylphosphineiron(II) . . . ... .123
Page
Separat on of the diastereom r of carbonyl
(h -cyclopentadienyl)(h -(7-ethoxy)cyclo-
heptadien-1-yl)triphenylphosphineiron(II). 124
2
Synthes s of carbonyl(h -1,2-cycloheptadiene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) tgifluoromethanesulfonate from
carbonyl(h -cyclopentadienyl)(h -(7-
ethoxy)cyclohepten-1-yl)triphenylphosphine
iron(II) . . . . . . . . . 125
Epimerization of carbonyl(h2-1,2-cycloheptene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) hexafluorophosphate in the
presence of triphenylphosphite ...... 125
2
Epimerization5of carbonyl(h -1,2-cyclohepta-
diene)(h -cyclopentadienyl)triphenylphos-
phineiron(II) trifluoromethanesulfonate
in the presence of triphenylphosphine . 126
Synthes s of dicarbonyl(h -cyclopentadienyl)
(h -(6-methoxy)cyclohexen-l-yl)iron(II) 126
2
Attempted synthesis of dicarbonyl(h -1,2-
cyclohexadiene)(h -cyclopentadienyl)iron
(II) trifluoromethanesulfonate ...... 127
2
Attempted synthesis of dicarbonyl(h -1,2-
cyclohexadiene)(h -cyclopentadienyl)
iron(II) tetrafluoroborate . . . ... 127
Low temperature NMR scale synthesis of 5
dicarbonyl(h -1,2-cyclohexadiene)(h -
cyclopentadienyl)iron(II) trifluoromethane-
sulfonate . . . . . . . . 128
Synthesis of carbonyl(h -cyclopentadienyl)
(h -(6-methoxy)cyclohexen-1-yl)triphenyl-
phosphineiron(II) . . . . . .. 129
Attempted synthesis of carbonyl(h -1,2-
cyclohexadiene)(h -cyclopentadienyl)
triphenylphosphineiron(II) trifluoro-
methanesulfonate . . . . . ... .129
Reaction of carbonyl(hl-(6-methoxy)cyclohexen-l-
yl)(h -cyclopentadienyl)triphenylphosphine-
iron(II) with trimethylsilyl trifluoro-
methanesulfonate followed by additions of
ethanol . . . . . .... . . 130
Page
APPENDIX . . . . . . . . . . . 132
REFERENCES . . . . . . . . ... . .133
BIOGRAPHICAL SKETCH . . . . . . . . .. .137
viii
LIST OF TABLES
Table Page
1. Geometries of cyclic allenes calculated
by MNDO . . . . . . . . . . 5
2. Methoxy abstraction from hl-(7-methoxy)
cyclohepten-l-yl Fpp (34) to give
h -1-2-cycloheptadiene Fpp (35a) . . . . 82
LIST OF FIGURES
Figure Page
1. Thermal decomposition of (4a) in CD2Cl2 .. 19
2. Fp trifluoromethanesulfonate and 1,3-
cycloheptadiene from the thermal decomposition
of (4a) in CD2C12 .............. 20
3. High temperature 1C NMR of (4b) in CD3NO2 . 31
4. A CDC1 solution of 10 mole % Eu(hfc) and (32)
from ( ) showing a 25:75 diastereomeric
composition . . . . . . . . . 56
5. A CDC1 solution of 10 mole % Eu(hfc) and (32)
from methoxy abstraction and addition of (N)
showing a 50:50 diastereomeric composition 57
6. A CDC1 solution of 10 mole % Eu(hfc) and (32)
from (P) showing a 60:40 diastereomeric
composition . . . . . . . . . 58
7. A CDC1 solution of 10 mole % Eu(hfc) and (32)
from mathoxy abstraction and addition of (P)
showing a 50:50 diastereomeric composition 59
8. 1H NMR of (36) in CDC1 showing a 20:80
diastereomeric composition . . . . .. 69
9. 1H NMR of (34) in CDC1 showing a 85:15
diastereomeric composition . . . ... 72
10. 1H NMR of (34) in CDC1 showing a 10:90
diastereomeric composition . . . ... 73
11. IH NMR of (35a) in CDC1 .......... 78
12. H S.S.T. of (35a) in CDC13 . . . . . 91
13. X-ray structure of h2-1,2-cycloheptadiene Fpp
cation . . . . . . . . . . 97
14. Reaction between h -(6-methoxy)cyclohexen-l-yl
Fp and trimethylsilyl trifluromethanesulfonate
followed by low temperature H NMR . . . 104
Abstract of Dissertation Presented to the Graduate
School of the University of Florida in Partial
Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
SYNTHESIS AND X-RAY STRUCTURE OF
IRON STABILIZED STRAINED CYCLIC ALLENES.
VALENCE ISOMERIZATION BETWEEN LINEAR PERPENDICULAR
AND BENT PLANAR ALLENE.
By
Su-Min Oon
May 1987
Chairman: William M. Jones
Major Department: Chemistry
The synthesis and isolation of strained organic mole-
cules has been an area of active research for over
forty years. Strained cyclic allenes are of no exception
and have been studied both theoretically and experimentally.
Calculations have shown that strained cyclic allenes down to
1,2-cyclopentadiene remained twisted and chiral but with a
2-5 kcal/mole barrier for racemization. Experimentally,
1,2-cyclohexadiene was found to be chiral but with a very
low barrier for racemization.
Extended Huckel molecular orbital (EHMO) calculations
have shown that the racemization barrier of the twisted
1,2-cycloheptadiene via its bent planar allyl cation is
lowered considerably upon complexation with a transition
metal.
We have synthesized and studied the racemization
process of dicarbonyl(h5-cyclopentadienyl)iron(II) [Fp] and
carbonyl(h5-cyclopentadienyl)triphenylphosphineiron(II)
[Fpp] completed 1,2-cycloheptadiene.
Although we have failed to observe the racemization of
Fp completed 1,2-cycloheptadiene directly, we have experi-
mental evidence which indicates that the bent planar allyl
cation was formed either via methoxy abstraction from the
hl-(7-methoxy)cyclohepten-l-yl Fp leading to the formation
of h2-1,2-cycloheptadiene Fp or from the isomerization of
h2-1,2-cycloheptadiene Fp itself. We have also determined
that the allene to allyl cation isomerization must be at
least 14.7 kcal/mole.
The Fpp completed 1,2-cycloheptadiene was synthesized
as enantiomeric pairs of diastereomers. Its fluxional
barrier for 1,2-Fpp shift is between 15 kcal/mole and
18 kcal/mole. An enriched mixture of one diastereomer
slowly equilibrates at room temperature to its thermodynamic
mixture. The iron center here is stereochemically rigid and
epimerization through dissociation and reassociation of the
triphenylphosphine ligand does not occur. We infer from our
observation that the thermal equilibration occurs via the
allyl cation. Methoxy abstraction from the h -(7-methoxy)
cycloheptadien-1-yl Fpp proceeds via the allyl cation as in
the case with h -(7-methoxy)cycloheptadien-1-yl Fp.
We also report the first X-ray crystal structure of the
h -1,2-cycloheptadiene Fpp cation.
Finally, we were able to synthesize and isolate the
h2-1,2-cyclohexadiene Fpp complex as a thermally unstable
solid. This solid reacted with ethanol, in a manner typical
of most metal olefin complexes, to give hl-(6-ethoxy)cyclo-
hexen-l-yl Fpp.
xiii
CHAPTER I
INTRODUCTION
Allenes are a class of organic compounds which contain
two cumulated double bonds arranged in an orthogonal
geometry. Acyclic allene has a linear structure and the
planes defined by R1R2C1 and R3R4C3 are mutually perpendicu-
lar. R3
R2
R4
One of the most fascinating problems both experimental-
ists and theoreticians have been concerned with regarding
the allene structure is the energy gap separating the ground
state linear perpendicular allene (A) from its excited bent
planar valence isomer (B). 9
A B
Equally interesting is the extent to which this energy gap
can be lessened and perhaps be inverted such that the bent
planar allene becomes the ground state.
One can approach this problem by either increasing the
ground state energy of the linear perpendicular allene or
decreasing the excited state energy of the bent planar
allene or both.
Incorporating a linear perpendicular allenic unit into
a small ring will deform this allenic unit in two ways in
order to facilitate ring closure and reduce ring strain.
The allene will bend at the C2 carbon about an axis perpen-
dicular to the R R2C plane as defined by 8. The allene will
also twist about the R1R2C1 and the R3R4C3 planes as defined
by 1.
This combined effect raises the ground state energy of the
allenic unit and is reflected by its kinetic instability.
1,2-Cyclononadiene1 is a distillable liquid while 1,2-cyclo-
octadiene2 dimerizes within hours at room temperature.
1,2-Cycloheptadiene1 and 1,2-cyclohexadiene3 have only a
fleeting existence and can only be trapped chemically or, in
the second case, by trapping in an argon matrix.
Although cyclic allenes lack an asymmetric carbon, they
are chiral molecules with a C2 point group. Their R and S
configuration, using 1,2-cycloheptadiene as an example, are
shown below.4
H .H H H
R S
In order for one enantiomer to convert to the other,
the allenic pi-bonds of that enantiomer must be rotated out
of orthogonality, passing through a planar state to the
other enantiomer. The smaller the ring size of the cyclic
allene the greater the allene is bent and twisted towards
its planar isomer. When ring constraints force the pi-bonds
of an optically active cyclic allene (1) to rotate out of
orthogonality and become planar (2), optical activity is
lost.
H*H, H H H H
CH2)n (CH2)n CH2)n
1 2
Racemization of an optically active cyclic allene is suffi-
cient proof that a planar intermediate is accessible. The
energy barrier for racemization can be measured via the loss
of optical activity and represents the minimum energy
separating the two forms of the allene, (A) and (B).
Numerous calculations have been performed over the last
20 years on 1,2-propadiene and 1,2-cyclohexadiene, the
archetype cyclic allene, in order to estimate this barrier
for racemization and to address the electronic nature of the
bent planar allene. The ground state geometry for
1,2-cyclononadiene, 1,2-cyclooctadiene and 1,2-cyclohepta-
diene are accepted generally as slightly bent and twisted
towards planarity. The results from the latest MNDO calcu-
lations5 are given in Table 1.
Allenic geometries for this series of three cyclic
allenes clearly show the effect of ring strain on the
allenic unit. An X-ray crystal structure is available for a
phenylurethane derivative of 1,2-cyclononadiene6 and agrees
closely with the calculated results.
Historically, there have been considerable doubts as to
the ground state structure of 1,2-cyclohexadiene. Moore and
Moser7 predicted the singlet diradical as the ground state.
INDO calculations by Dillon and Underwood suggested that
the triplet diradical might be lower in energy. Recent SCF
calculations of Johnson favor a chiral allenic ground state
structure for 1,2-cyclohexadiene.5 The calculations further
predict a chiral ground state structure for 1,2-cyclopenta-
diene but with a 2-5 kcal/mole barrier for racemization.
Mechanistic studies of 1,2-cyclohexadiene by Bottini et
al. suggested that 1,2-cyclohexadiene has an allenic
structure but rapidly racemizes to a singlet diradical form.
He also showed some chemistry arising from the zwitterionic
form of 1,2-cyclohexadiene.
Table 1. Geometries of cyclic allenes calculated by
MNDO.
C1-C2-C3 out of plane H benda
1,2-cyclononadiene 170.40 33.70
1,2-cyclooctadiene 161.5 31.00
1,2-cycloheptadiene 153.40 27.60
defined as the angle made by the C-H bond with the C -C2-C3
plane containing the C2 axis, e.g., 450 for 1,2 propadiene.
Optically active 1,2-cyclohexadiene, synthesized via
asymmetric dehydrobromination of resolved 1-bromocyclo-
hexene-6d1 with potassium tert-butoxide, was trapped in situ
with diphenylisobenzofuran.0 The adduct (3) is optically
active when the reaction is done at low temperatures but
optically inactive above +800C suggesting that racemization
of the cyclic allene is competitive with cycloaddition and
the inversion barrier is low.
0
Br
H K t-OBu > 0
at low temperatures 3
H D
The adduct from reaction of 1,2-cycloheptadiene and
diphenylisobenzofuran remained optically active under
similar reaction conditions. The allene subunit in this
larger ring has maintained its orthogonality.
We have now extended our studies from the free strained
cyclic allenes to the transition metal completed strained
cyclic allenes. We are interested in the effect the transi-
tion metal has on the allenic unit, in particular with
regard to the isomerization of the allene unit from its
linear perpendicular form to its bent planar isomer.
EHMO calculations predict that the energy gap
separating the linear perpendicular allene from its bent
planar isomer is lessened considerably when the allene is
completed onto the (Cp)Fe(CO)2 [Fp] cationic fragment. Here
lies the impetus to study Fp completed allenes.
1,2-Propadiene lies about 80 kcal/mole below that of
its bent planar isomer according to EHMO. The relative
energy between 1,2-propadiene and its bent planar isomer is
only 25 kcal/mole when both are bound to the Fp fragment.
Fp
80 kcal/mole 25 kal/mole
Fp
Similarly, Fp bound 1,2-cycloheptadiene (4) is cal-
culated to be only 17 kcal/mole below its bent planar form
(5). Furthermore, EHMO predicts the Fp bound bent planar
1,2-cyclohexadiene (7) is actually 14 kcal/mole more stable
than the allene form (6).
Fp
Fp+
5 6
17 kcal/mole 14 kcal/mole
Fp+ Fp
4 7
Although energies derived from EHMO should not be
accepted as absolute values, one does see a trend favoring a
bent planar allenic ground state as one proceeds towards Fp
completed allene with the allene incorporated into a smaller
ring. This decrease in relative energy between the linear
perpendicular allene and its bent planar isomer is largely
due to the greater stabilizing interaction between the
Fp-LUMO and the bent planar allenic HOMO.
We decided to choose the Fp completed 1,2-cyclohepta-
diene as the starting point of our work.
Transition metal complexes of acyclic allenes are well
known and have been reviewed extensively.12 However cyclic
counterparts are limited to Fp complexes of 1,2-cyclonona-
diene13 (8) and 1,2-cycloheptadienel4 (4), (PPh3)2Pt(O)
complexesl5 of 1,2-cyclononadiene (9), 1,2-cyclooctadiene
(10) and 1,2-cycloheptadiene (11) and (Am)(Cl)2Pt(II)
complex of 1,2-cyclononadienel6 (12).
Fp+ Pt(O)
sFop+'
8 4 9
Pt(lH)
Pt(0) P(0)
10 11 12
The bonding between the metal fragment and the allene
can be described using the Dewar-Chatt-Duncanson model.17
The bond is formed by the interaction of the HOMO of one of
the allene double bonds with an empty acceptor on the metal
(13). The filled metal d-orbital in turn back bonds with
the LUMO of the same double bond (14).
1c
H H
HOMO
13
H H
C
LUMO H H
LUMO
40'
HOMO
HOMO
This back bonding causes substituents on the completed
double bond to bend away from the metal, the degree of which
is directly related to the extent of this back bonding. The
metal to olefin pi-bond is often looked upon as a metallo-
cyclopropane.18 The metal is not positioned symmetrically
about the C1 and C2 carbons of the allene. The metal is
closer to the C2 carbon because of its greater s character
compared to the C1 carbon (15).
/"
H
H C2
I| M
C1
15
Allene complexes of metals, like olefin complexes of
metals, rotate about the metal to olefin pi-bond. In
addition to this rotational motion, allene complexes of most
metals also exhibit fluxional behavior whereby the completed
metal fragment moves from one double bond of the allene to
the adjacent double bond. Such fluxional behavior of
acyclic allenes completed to Fp was studied extensively by
Rosenblum et al.13 and was shown to be nondissociative (i.e.
intramolecular).
Fp+
\=.^'
)Fp+VA
Unlike the Fp completed allenes, not all platinum
completed allenes are fluxional. The Pt(0) complex of
19
1,3-diphenylpropadiene (16) is not fluxional.1 Ha and Hb
remain distinct in the IH NMR even at high temperatures.
Ha
c/
I I
C
-H-H Pt
C
Ph Hb
PPh3
PPh3
The Pt(II) complex of
with the metal moving
double bonds.20
double bonds.
tetramethylallene however is fluxional
back and forth between the two allenic
1,2-Fp shifts do not involve a bent planar intermedi-
ate. Optical rotation of an optically active 1,2-cyclo-
nonadiene completed onto Fp remained unchanged3 up to
1.2 Fp shift
+800C. A racemate would result if a bent planar intermedi-
ate were involved. In contrast, the optical activity of the
Pt(II) complex of 1,2-cyclononadiene (12) diminishes with
time. The 1,2-cyclononadiene is believed to racemize via a
h3-allyl Pt intermediate (18) and not the bent planar
allene16 (17).
Pt(Am)(CI)2 CI
SPt(Am)(CI)2 Pt(Am)(CI)
17 12 18
1,2-Cycloheptadiene completed to Fp has a measured
fluxional barrier of 13.9 kcal/mole and is the lowest
14
barrier observed for Fp allene complexes. Fluxional
barriers for h2-methylallene, h2-1,1-dimethylallene and
h2-tetramethylallene Fp cationic complexes are
23.1 kcal/mole, 18.0 kcal/mole and 16.3 kcal/mole, respec-
tively.13 h2-1,2-Cyclononadiene Fp has a fluxional barrier
of 16.9 kcal/mole. Although the decrease in fluxional
barrier with increasing methyl substitution is probably
steric in origin, one could not help but to wonder if a bent
planar allene intermediate were involved. The bent planar
allene is an allyl cation and should be favored by electron
donating methyl substituents. Moreover, if one assumes that
the steric environment immediate to the Fp center is the
same for 1,3-dimethylpropadiene, 1,2-cyclononadiene and
1,2-cycloheptadiene, bending and twisting the linear perpen-
dicular allenic unit toward its bent planar form by tying it
into a ring lowers the fluxional barrier. Therefore, it was
not known at the time it was reported whether the fluxional
barrier of 13.9 kcal/mole for h2-1,2-cycloheptadiene Fp (4)
represented a simple 1,2-Fp shift or that an allyl cation
intermediate (5) was involved.
Fp
Fp+ Fp+
allene to allyl 1,2-Fp shift
cation 4
5 4
X-ray crystal data is available for many allene com-
plexes of Rh, Pt and Pd.21 In all cases the completed
double bond is longer than the uncomplexed double bond. The
metal is unsymmetrically positioned about the bound double
bond and is further away from the terminal carbon than the
central carbon. The C1-C2-C3 bond angle falls within the
range of 1580 to 1420. The completed allenes are not
planar. Allene complexes of transition metal dimers (19)
where each of the allene double bonds is bound to a
different metal in the dimer are also known. The C1-C2-C3
angles of such complexes are, on the average, much
smaller.22
LnM MLn
19
The only X-ray structure of an iron allene complex is the
h2-tetramethylallene Fp.23 The completed allene is oriented
parallel to the cyclopentadienyl ring as expected. The bond
length of the uncomplexed double bond is 1.335A and the
completed double bond is 1.367A. The completed double bond
is longer as a result of back bonding from the Fp to the pi*
of the allene double bond. The iron is asymmetrically
placed about the double bond and is closer to the central
0 0
carbon (2.063A) than the outer carbon (2.237A). The
C1-C2-C3 bond angle is 145.7 but the allene is still
orthogonal.
There have been no X-ray data reported on any metal
completed cyclic allenes to date although such compounds are
well known. It would be interesting to compare the X-ray
structure of h2-1,2-cycloheptadiene Fp with h2-tetramethyl-
allene Fp. Of particular importance is how the ring affects
the allene C1-C2-C3 bond angle and how far the allene is
away from orthogonality.
The objective of this study was to determine whether a
1,2-Fp shift between the double bond of the cyclic allene
15
involves a bent planar intermediate and, if not, to
determine if the latter is accessible. It was also to
provide the first X-ray structure of a metal completed
cyclic allene.
CHAPTER II
DICARBONYL(h5-CYCLOPENTADIENYL)IRON(II) COMPLETED
1,2-CYCLOHEPTADIENNE
The h1-(7-methoxy)cyclohepten-l-yl Fp was synthesized
in moderate yields by reacting the l-lithio-7-methoxycyclo-
heptene with either Fp chloride or Fp bromide. Treating a
methylene chloride solution of the hl-(7-methoxy)cyclo-
hepten-l-yl Fp with trimethylsilyl trifluoromethanesulfonate
followed by precipitation with diethyl ether yielded the
desired h2-1,2-cycloheptadiene Fp cation complex (4a) as an
air and thermally sensitive yellow solid.
Br Fp
OCH3 OCH3 Fp
1) n-BuLi TMSOTF OTF
2)FpClorFpBr
4a
Attempts to synthesize the h -(7-methoxy)cyclohepten-l-yl Fp
by this route failed with Fp iodide. It is critical that
the l-bromo-7-methoxycycloheptene be in a slight excess
relative to n-butyllithium when generating the l-lithio-7-
methoxycycloheptene in order to ensure a maximum yield of
the hl-(7-methoxy)cyclohepten-l-yl Fp. Synthesis
of h1-(7-methoxy)cyclohepten-l-yl Fp by this method was far
superior to a longer and lower yield route used pre-
viously.14
The yellow colored methylene chloride solution of the
h2-1,2-cycloheptadiene Fp trifluoromethanesulfonate (4a)
turned red within 30 mins at room temperature. In contrast,
the h2-1,2-cyclononadiene Fp tetrafluoroborate (8) is not
only air stable as a solid but a methylene chloride solution
of it is also stable for over 14 hrs at +830C. Although the
extreme thermal instability of the h2-1,2-cycloheptadiene Fp
(4a) in methylene chloride is an inconvenience, it may
signal a different pathway for the fluxional process; a
pathway that is different from that of the h2-1,2-cyclo-
nonadiene Fp (8) and perhaps involves the bent planar allene
(allyl cation) intermediate from which decomposition may
occur. We will now refer to (4) as the allene form and (5)
as the allyl cationic form.
Fp
Fp+
4 5
1H NMR was used to follow the decomposition of the
h2-1,2-cycloheptadiene Fp (4a) in a methylene chloride
solution at +400C. Decomposition was complete within 3 hrs
and Fp trifluoromethanesulfonate and 1,3-cycloheptadiene
were the only major decomposition products observed. The
former was identified by comparing its 1H NMR with that of
Fp trifluoromethanesulfonate synthesized independently from
Fp chloride and silver trifluoromethanesulfonate and the
latter with an authentic sample of 1,3-cycloheptadiene. The
decomposition mixture was red in color. The result of this
experiment is in agreement with that by Manganiello14
Neither cycloheptene nor the 1,2-cycloheptadiene dimer were
observed in the H NMR.
According to the 1H NMR, the decomposition of the
h -1,2-cycloheptadiene Fp (4a) to Fp trifluoromethane-
sulfonate and 1,3-cycloheptadiene proceeded via a short
lived, thermally unstable intermediate. This intermediate
had a singlet at 5.58 ppm, Fig. 1. Its concentration
increased rapidly up to a certain point and then decreased
to the baseline with a concurrent increase in the concen-
tration of both Fp trifluoromethanesulfonate and 1,3-cyclo-
heptadiene, Fig. 2. Based upon this observation, the
thermally labile intermediate was suspected to be the
h2-1,3-cycloheptadiene Fp cation complex. The singlet at
5.58 ppm is certainly within the range for cyclopentadienyl
hydrogen resonances of cationic Fp-olefin complexes. The
h2-1,3-cycloheptadiene Fp could not be synthesized via a
common thermal exchange reaction between 1,3-cycloheptadiene
and the h2-propene Fp cation,24 which lends further evidence
for its thermal liability.
C4-
r.
0
4-)
.rA
ull
0
-4
E-
4-)
0
4
4-4
4-J
0
>1
4-4
1-
QI)
E
04J N
0
r-4 -HJ
-r~
Wa
P4J
=
-dO.
~rd
(N
H
A reasonable mechanism for the decomposition of the
h2-1,2-cycloheptadiene Fp (4a) is as follows.
FpFp Fp
S+ HOTF )
4a 20
Fp Fp+
FpOTF +0
5
We believe that an initial deprotonation of an appro-
priately situated hydrogen occurs from either the allene (4)
or allyl cation intermediate (5) to form the h -1,3-cyclo-
heptadien-2-yl Fp and trifluomethanesulfonate acid. In the
presence of trifluomethanesulfonic acid, this hl-1,3-cyclo-
heptadien-2-yl Fp is then converted via the conjugated
carbene (20) to the h2-1,3-cycloheptadiene Fp. Under the
experimental conditions it dissociates to the Fp cation and
1,3-cycloheptadiene.
The decomposition reaction would have stopped after the
first step, in the absence of trifluomethanesulfonic acid,
giving hl-1,3-cycloheptadien-2-yl Fp as the only product.
In order to test the validity of this mechanism, the h -1,2-
cycloheptadiene Fp (4a) was subjected to the same decomposi-
tion conditions but in a slurry of lithium or sodium
carbonate to neutralize the trifluoromethanesulfonic acid.
The decomposition mixture did not turn red but remained
brown in color. The 1H NMR cyclopentadienyl resonance of
this brown substance falls within the range typical for most
hl-alkyl Fp. Presumably the hl-1,3-cyclopentadien-2-yl Fp
was formed, but we were unable to purify it sufficiently for
a positive identification. 1,3-Cycloheptadiene and Fp
trifluoromethanesulfonate were no longer the decomposition
products.
Counter ions have been known to affect the way an
organometallic species behaves in solution, a recent example
being the intramolecular rearrangement of the h2-3-bromo-
propene Fp complex.25
Fp+ Fp+
D H p H F D H
H rH Y
Br Br
The 1,3 shift of a bromine atom exo to the Fp-olefin bond
was greatly accelerated when the counter ion was changed
from trifluoromethanesulfonate to hexafluorophosphate. Ion
pair association in the salt may have been responsible for
such behavior. Caseyet al.26 has also reported that a
solution of the tetrafluoroborate salt of the (Cp)Fe(CO)
(PPh3) isopropylidene complex is more stable than its
trifluoromethanesulfonate salt.
Fe+ C H3
CC
Ph3P I
CH3
Since we believe that the basicity of the trifluoro-
methanesulfonate anion is responsible for the decomposition
of the h2-1,2-cycloheptadiene Fp (4a), we thought it would
be desirable to replace it with a non-basic anion. The
tetraphenylborate anion was successfully exchanged for the
trifluoromethanesulfonate anion in a cold (-350C) methanol
solution. We were disappointed that a methylene chloride
solution of the tetraphenylborate salt of h2-1,2-cyclo-
heptadiene Fp (4) was also thermally unstable and decomposed
within 30 mins at +500C to several unidentifiable pro-
ducts.27
The h2-1,2-cyclononadiene Fp tetrafluoroborate (8) is
the only known stable cyclic allene of iron. If by chance
the thermal stability of (8) were due to the tetra-
fluoroborate anion, this would suggest that the tetrafluoro-
borate anion may be the counterion of choice for the
h -1,2-cycloheptadiene Fp (4).
The h2-1,2-cycloheptadiene Fp tetrafluoroborate (4b)
was synthesized in good yields by reacting the h -(7-
methoxy)cyclohepten-1-yl Fp with trimethyloxonium tetra-
fluoroborate.
Fp*
OCH3
+ (CH3)30BF4 BF4
4b
The yellow solid h2-1,2-cycloheptadiene Fp (4b) turns red
when exposed to air, but a room temperature methylene
chloride solution of it is stable for over 3 days. The same
solution is also stable for over 2 hrs at +400C. Thus it
would appear that the thermal decomposition of the
h2-1,2-cycloheptadiene Fp (4a) in methylene chloride is due
to the basicity of the trifluoromethanesulfonate anion. The
tetrafluoroborate and the hexafluorophosphate salt of
h2-1,2-cycloheptadiene Fp (4) may also be synthesized by
reacting the h -(7-methoxy)cycloheptadien-l-yl Fp with
triphenylcarbenium tetrafluoroborate and hexafluorophos-
phate, respectively. This procedure has a drawback in that
the product is sometimes contaminated with the unreacted
triphenylcarbenium cation.
Fp
OCH3 Fp+
+ Ph3CBF4/PF6 BF4/PF6
4b/c
We now have on hand the stable h -1, 2-cycloheptadiene
Fp (4b) and (4c). Both h2-1,2-cycloheptadiene Fp (4b) and
(4c) react with alcohols to give the same ether adducts as
are given by the h2-1,2-cycloheptadiene -Fp (4a). It was
interesting to note that their fluxional barriers were
unaffected by the counterions.
The h2-1,2-cycloheptadiene Fp (4) has a fluxional
barrier of 13.9 kcal/mole and is the lowest barrier yet
measured when compared with other h2-Fp completed acyclic
and cyclic allenes. Does this low fluxional barrier signal
a new mechanism for the fluxional process, perhaps via the
bent planar allene (allyl cation) intermediate (5),
Scheme I?
Scheme I
Fp
Fp+ Fp Fp+
0-6S
4 5
Or perhaps the fluxional process merely involves a simple
intramolecular 1,2-Fp shift from one allenic double bond to
the other, Scheme II, passing through the allene intermedi-
ate (21)? The low fluxional barrier merely reflects in some
manner the effect of ring strain in the smaller cyclic
allene. Such a simple intramolecular 1,2-Fp shift was
demonstrated conclusively for the h2-1,2-cyclononadiene Fp
(8).
Scheme II
Fp+
Fp + Fp +
4 21
The allene (4) to allyl cation (5) valence isomeriza-
tion for Fp complexes is unprecedented. The racemization of
optically active 1,2-cyclononadiene completed onto Pt(II)
(12) was initially proposed to proceed via an allyl spe-
cies16 (17).
Pt(Am)(CI)2
Pt(Am)(CI)2 Pt(Am)(CI)2
12 17
This allyl species (17), with two electrons in its
pi-system, would be formally equivalent to an allyl cation.
It is now accepted that a reversible h2-pi-allene (12)
to h3-pi-allyl (18) isomerization, whereby a chloride is
transferred from the Pt(II) to the pi-allyl, is responsible
for the racemization.
Pt(Am)(CI) Pt(Am)(CI) Pt(Am)(CI)2
n- -S
12 18
Such a h2-pi-allene to h3-pi-allyl isomerization in conjunc-
tion with a ligand transfer from the metal to the pi-allyl,
although rare for h2-allene Pt(II) complexes, is quite
common among its cogener, the h -allene Pd complexes.2 In
the case of the h2-1,2-cycloheptadiene Fp (4), no such
ligand is available on the Fp for an analogous transfer to
the 1,2-cycloheptadiene ligand. This precludes such a
mechanism for an allene to allyl isomerization.
Numerous barriers to the fluxional behavior for h2-
allene complexes of Pt(II) and Fe(II) have been measured.
Fluxional barriers for h2-tetramethylallene complexes of
Pt(II) are generally low and lie between 7 to 10 kcal/
mole.20,29 The h2-1,1-dimethylallene complex of Pt(II) is
not fluxional. The Pt(II) remained bonded to the C2-C3
olefinic bond, the less hindered bond. The Fp in the
h2-1,1-dimethylallene Fp also prefers the distal position.13
This preference for the distal position is presumed to be
steric in origin. Fluxional behavior has not been reported
for the h2-allene, h2-tetramethylallene and h2-1,3-diphenyl-
allene Pt(0) complexes.19 30 They are presumed to be
2 16
dynamically rigid. The h -1,2-cyclononadiene Pt(0)1 is
also not fluxional at room temperature.
For comparison with the h2-1,2-cycloheptadiene Fp (4),
the fluxional barriers of h2-1-methylallene, h2-1,3-
dimethylallene, h2-tetramethylallene and h2-1,2-cyclo-
nonadiene (8) Fp complexes3 are 23.1 kcal/mole, 18.0 kcal/
mole, 16.3 kcal/mole and 16.9 kcal/mole, respectively.
It has been well established that the fluxional process
for these complexes occurs by a concerted 1,2-Fp shift
between the two allenic double bonds and not via an allyl
cation. The fluxional barriers decrease gradually with
increasing methyl substitution on the allene and are
attributed to a steric factor rather than the stabilization
of an allyl cationic intermediate by the methyl substit-
uents. The steric bulk of the methyl substituents increases
the liability of the Fp-olefin pi-bond making it easier for
the Fp to move between the two adjacent double bonds.
The h2-1,2-cyclononadiene Fp (8) has a fluxional
barrier of 16.9 kcal/mole. In this case it was conclusively
demonstrated that the fluxional process did not proceed via
an allyl cationic species (22). When a solution of the
optically active h2-1,2-cyclononadiene Fp (8) was heated to
the point of rapid fluxionality, reisolation gave the allene
complex without any loss in optical activity.3 The
optically active h2-1,2-cyclononadiene Fp (8) would have
racemized if the achiral allyl cation (22) were involved in
the fluxional process either as an intermediate or as a
transition state.
FpFp Fp
H. Fp H Fp H HFp H
8 22
At first thought one would not expect the h2-1,2-cyclo-
heptadiene Fp (4) fluxional barrier to be much different
from that of the h2-1,2-cyclononadiene Fp (8) if both
proceeded by the same mechanism. In fact, we would expect
the fluxional barrier for the h2-1,2-cycloheptadiene Fp (4)
to be higher should the Fp-olefin pi-bond strength parallel
that of the Pt(0)-olefin pi-bond. The coordinating ability
of cyclic allenes onto Pt(PPh3)2 was reported to increase
with decreasing ring size.15 This is due to the greater
release of ring strain upon coordination. The low fluxional
barrier for the h2-1,2-cycloheptadiene Fp (4) coupled with
the strained ring therefore causes one to wonder if the
fluxionality occurs via a mechanism involving an allyl
cationic intermediate (5).
To test for this possibility, a temperature dependent
13C NMR study of h2-1,2-cycloheptadiene Fp (4b) was under-
taken. 1,2-Cycloheptadiene is chiral and when completed to
Fp causes the carbonyl ligands on the Fp to become
diastereotopic and be distinguishable. The two carbonyl
[CO] resonances appear at 207.2 ppm and 210.2 ppm. A simple
intramolecular 1,2-Fp shift would render the COa and COb
unchanged (23). In contrast an allene to allyl cation
isomerization would cause the carbonyls to become equivalent
and coalesce to a single resonance (24).
Cp Cp
Fe COb \ e
Fe Ca Fe COb
H 0
23 24
As the temperature of a nitromethane-d3 solution of
h2-1,2-cycloheptadiene Fp (4b) was raised, the two carbonyl
resonances remained unchanged. The allenic carbons C1 and
C3 however became equivalent at +30C. The allenic carbon
C2 remained unchanged, Fig. 3. Taken together, it means
that the rapid fluxional motion of the h2-1,2-cyclohepta-
diene Fp (4) at +30C does not involve an allyl cationic
intermediate because under this condition, C1 and C3 become
equivalent whereas the two carbonyls remain distinct. If an
allyl cationic intermediate (24) were involved in the
fluxional process at +300C, this would correspond to a free
energy of activation of 14.7 kcal/mole. We can safely say
that the fluxional barrier of 13.9 kcal/mole as determined
by 1H NMR corresponds to a simple intramolecular 1,2-Fp
shift, Scheme II, and that the allene to allyl cation
isomerization, Scheme I, requires an energy of greater than
14.7 kcal/mole.
It is reasonable to expect the allene to allyl cation
isomerization to be the higher energy process of the two.
This isomerization requires bonding and structural changes
and a transfer of a positive charge from the iron to the
organic ligand, whereas a 1,2-Fp shift merely involves a
repositioning of the Fp from one double bond to the other.
It is inconvenient to study any dynamic process of
organometallic compounds at high temperatures by 1H NMR due
to their thermal instability. For example, when seeking NMR
evidence for allyl cation formation, a nitromethane-d3
solution of the h2-1,2-cycloheptadiene Fp (4b) was heated to
+600C and was found to decompose rapidly within the time
needed to acquire a 1C spectrum.
A possible way to study the fluxionality of a thermally
sensitive organometallic complex by NMR without raising the
probe temperature is to use the spin saturation transfer
technique, S.S.T.31
The spin saturation technique involves finding a
condition whereby saturating one of the spins of an exchang-
ing two-spin system results in a partial saturation of the
other spin. From this information and the T1 of both spins,
the energy barrier for the exchange can be calculated. The
advantage of this method was exemplified in the case for
h2-1,2-cycloheptadiene Fp (4a). The conditions required for
obtaining a fluxional barrier from spin saturation transfer
described above were met at -20C. It was necessary to heat
(4a) to +290C before its fluxional barrier could be obtained
by the coalescence method.
For the case of the h2-1,2-cycloheptadiene Fp (4), if a
simple 1,2-Fp shift were the only process occurring, irra-
diation of one of the carbonyl resonances would not affect
the intensity of the other because they would remain dis-
tinct at all times. On the other hand, if an allyl cation
were accessible, the carbonyl ligands would become equiva-
lent and if the relaxation of the carbonyl carbon were
slower than the fluxional process, irradiation of one of the
carbonyl resonances would cause the other carbonyl resonance
to diminish in intensity. From this information, it would
be possible to calculate the barrier for the fluxional
process while maintaining a reasonable probe temperature.
Unfortunately, S.S.T. is impractical here for the two car-
bonyl resonances are too close to each other. We could not
selectively irradiate one carbonyl without irradiating the
other.
Another possible way to detect the allyl cationic
intermediate would be to trap it as it is formed. 2-Sub-
stituted allyl cations (25) are known to react with
1,3-dienes.
+ 2- Y -- I K0 Y
25 Y=cO,OR,OM,NR2,R,etc.
Extensive reviews have been written about the cycloaddition
of such allyl cations (25), the most recent one by
Hoffmann.32 These reactions have been exploited in organic
synthesis of bi- and tricyclic compounds. Several possible
products can result from the cycloaddition of allyl cations
(25) to 1,3 dienes depending upon whether the reaction is
concerted or stepwise and upon the nature of Y. The reader
32
is best referred to the review article by Hoffmann32 for all
the possible products from concerted and stepwise cycloaddi-
tions.
The pi-donating strength of a Fp in the homocycloocta-
trienylidene Fp was found to be similar to that of a methoxy
33
group. This would make the 2-Fp substituted allyl cation
a good candidate to be trapped by 1,3-dienes. There is
however no literature precedent that such a reaction will
occur. In fact, allyl cations with Fp substitution at C2
are not even known to exist at this point.
The cycloaddition products expected from cyclopenta-
diene and furan with the h1-allyl Fp cation (5) by either
the concerted or stepwise addition are shown in Scheme III
and IV, respectively.
Scheme III
Concerted addition
I Z +
Z=O,CH2
S- Fp
5
Scheme IV
Stepwise addition
: Fp -
JjH+
a z Fp+
,, 0 H+
jZ +
Z=O,CH2
We do not expect to isolate the carbene adduct (26) from the
concerted cycloaddition of the h1-allyl Fp cation (5) with
the 1,3-diene. A rapid 1,2-alkyl shift would probably occur
to give the corresponding h2-olefin Fp complexes, (27) and
(28).
Fp Fp+
Z a Fpz
27
b
b + Fp Fp+
26
28
Such a 1,2-alkyl shift to a Fp-carbene carbon is well
precedented in the literature.34
A methylene chloride solution of cyclopentadiene or
furan and the h2-1,2-cycloheptadiene Fp (4b) was stirred
for a day. In no case was there any reaction and (4b) was
recovered unchanged. However this does not mean that the
allyl cation (5) is not accessible. The allyl cation (5)
may have formed but did not react or the concentration of
the allyl cation (5) may have been so small that the rate of
reaction was negligible. In fact, the allyl cation (5) may
possibly be a transition state and not an intermediate in
the isomerization pathway.
A methylene chloride solution of tetramethylethylene
was also treated with the h2-1,2-cycloheptadiene Fp (4b) at
room temperature hoping to obtain the h2-olefin Fp complexes
(29). Again (4) was recovered unchanged.
Fp
Fp Fp
5 29
14
In a much earlier study, Manganiello4 discovered that
the thermal decomposition of a methylene chloride solution
of the trifluoromethanesulfonate salt of h2-1,2-cyclo-
heptadiene Fp (4) followed by treatment of the solution with
sodium iodide/acetone yielded 1,3-cycloheptadiene(40%) and a
trace amount of cycloheptene. The mechanism in Scheme V was
proposed to account for the decomposition reaction based on
the products observed.
Scheme V
Fp
y+H-
5
Fp
6H
1 H
H+
Nal
0+ FplI
INal
\ + Fpl
In this scheme, proton loss from the h -allyl Fp cation
(5) is responsible for the formation of the 1,3-cyclohepta-
diene, whereas a hydride abstraction gives the cycloheptene.
It was argued that if a hydride source were available to the
allyl cation (5) then the amount of the cycloheptene formed
relative to 1,3-cycloheptadiene would increase. Indeed,
when triphenylmethane (a hydride source) was added to a
methylene chloride solution of the h2-1,2-cycloheptadiene Fp
(4a), cycloheptene was isolated in amounts up to 3/5 of that
of 1,3-cycloheptadiene after workup.
The above experiment was repeated using the tetra-
fluoroborate salt of the h2-1,2-cycloheptadiene Fp (4).
Neither cycloheptene nor 1,3-cycloheptadiene were observed
but the h2-1,2-cycloheptadiene Fp (4b) was reisolated
unchanged. There was no reaction between (4b) and tri-
phenylmethane. This cast doubts on the previously proposed
mechanism involving the intermediacy of the allyl cation
(5).
In principle, clear evidence for the accessibility of
the allyl cation (5) may be obtained by using an optically
active cyclic allene as the ligand. As can be seen from
Scheme I, if the allyl cation (5) is accessible, whether as
an intermediate or a transition state, an optically active
complex must racemize. One approach to the synthesis of an
optically active h2-1,2-cycloheptadiene Fp (4) is to dis-
place isobutylene from h2-isobutylene Fp with an optically
active 1,2-cycloheptadiene in much the same way as was used
in the preparation of optically active h2-1,2-cyclononadiene
Fp (8). Unfortunately, unlike 1,2-cyclononadiene, 1,2-
cycloheptadiene dimerizes too rapidly to displace the
isobutylene.
Alternative methods were therefore sought. In the
first approach, racemic h2-1,2-cycloheptadiene Fp (11) could
be treated with less than an equivalent of an optically
active alcohol in order to selectively remove one the allene
enantiomers. This procedure would only be effective if the
racemization of (4) via the allyl cation (5) did not occur
or occurred at a very slow rate under the conditions for
enantiomeric enrichment.
2-Methylbutanol was tried first. When the racemic
alcohol was allowed to react with a suspension of h -1,2-
cycloheptadiene Fp (4) in diethyl ether, it gave the ether
adduct (30).
Fp4 Fp
+"OH
+ OH 0
30
Interestingly, although the ether adduct (30) has two chiral
centers as marked, and therefore should exist as a pair of
diastereomers, only one set of signals was observed in its
1H NMR (we have insufficient material for a C NMR
spectrum). It is quite unfortunate that the diastereomers
have coincidental chemical shifts. One would expect that
the reaction between an optically active alcohol with a
racemic h2-1,2-cycloheptadiene Fp (4) should give unequal
amounts of the diastereomeric ether adduct (30) provided the
reaction is incomplete. If the chemical shifts of the
diastereomeric ether adduct (30) were non-coincidental, NMR
measurement of the diastereomeric composition of the ether
adduct (30) would enable us to draw a conclusion as to the
enantiomeric purity of the h2-1,2-cycloheptadiene Fp (4)
left behind.
Half an equivalent of the (S)-(-)-2-methylbutanol was
allowed to react with the racemic h2-1,2-cycloheptadiene Fp
(4b). The unreacted (4b) recovered, which amounted to a
half of the starting material, showed no detectable optical
rotation.
The chiral carbon on (S)-(-)-2-Methylbutanol is one
carbon away from the alcohol group and may be too far
removed for it to effectively induce asymmetry. We decided
to try (-)-menthol instead. Under the same conditions,
(-)-menthol did not give the ether adduct but a mixture of
unidentifiable products and menthol.
Fp
Fp HO Men
31
Less than half of the starting h2-1,2-cycloheptadiene Fp
(4b) was recovered. A solution of the recovered h2-1,2-
cycloheptadiene Fp (4b) showed no detectable optical
rotation. A reaction between h2-1,2-cycloheptadiene Fp (4b)
and potassium (-)-menthoxide in THF at 0C resulted in total
decomposition of the allene (4b).
Zero optical rotation of the recovered h2-1,2-cyclo-
heptadiene Fp (4b) from the above reactions is consistent
with an allene to allyl cation isomerization, Scheme I.
However, absence of rotation can also be due to other
reasons. First, enantiomeric enrichment by (S)-(-)-2-
methylbutanol and (-)-menthol might not be successful. Even
if the enantiomeric enrichment were successful, the
recovered h2-1,2-cycloheptadiene Fp (4b) might have such low
inherent optical rotation that it could not be measured with
certainty. Unfortunately, it is not possible to increase
the concentration of the solution of h2-1,2-cycloheptadiene
Fp (4b) in order to get a measurable optical rotation
because the solution which is highly colored absorbs so much
light that rotation measurements become quite impossible.
In the second approach, we attempted to synthesize the
optically active h2-1,2-cycloheptadiene Fp (4) directly via
methoxy abstraction with trimethylsilyl trifluoromethane-
sulfonate starting from an optically active hl-(7-methoxy)-
cyclohepten-1-yl Fp.
Let us first consider the conformation of the R and S
enantiomers of the h -(7-methoxy)cyclohepten-1-yl Fp as
obtained from CPK space filling models.
H Fp
OCH3
R
H
Fp
OCH3
S
Se Sg
Of the two conformers available to the h -((R)-7-methoxy)-
cycloheptadien-1-yl Fp (R) we would expect the gauche
conformer (Rg) to be the preferred conformer. Here, the
larger methoxy groups is away from the bulky Fp. Similarly,
we expect the (Sg) to be the preferred conformer for the
h -((S)-7-methoxy)cyclohepten-1-yl Fp (S).
We shall limit our discussion on methoxy abstraction to
one enantiomer, h -((R)-7-methoxy)cyclohepten-l-yl Fp (R).
Cationic h2-olefin Fp complexes are typically made by a
B-hydride abstraction from the h -alkyl Fp.35 Such
B-hydride abstractions are conformationally dependant and
widely accepted to proceed via an antiperiplanar transition
state with the Fp assisting from the anti face in concert
with the hydride loss.
M
N/PE
/1
H
Methoxy abstraction, like hydride abstraction, is also
believed to proceed with anti Fp assistance leading to the
h2-olefin Fp complex. A concerted methoxy abstraction with
anti Fp assistance will give an optically active h2-olefin
Fp complex from an optically active hl-alkyl Fp. When the
methoxy group is part of a ring as in the case with h -
(7-methoxy)cyclohepten-l-yl Fp, it is prevented by the ring
from adopting a conformation where it is anti to the Fp. An
anti alignment of the Fp with the methoxy group would
necessarily force the methoxy group inside the cycloheptene
ring, a conformation which is impossibly strained.
Since a concerted anti methoxy abstraction is unlikely,
a concerted syn methoxy abstraction with the Fp moving over
to the same face as the leaving methoxy group will also give
an optically active h2-olefin Fp complex from an optically
active h -alkyl Fp. Thus by a concerted mechanism with syn
1
Fp assistance the h -((R)-7-methoxy)cyclohepten-1-yl Fp
(R) should give only the h2-(S)-1,2-cycloheptadiene Fp (45),
Scheme VI.
Scheme VI
H p HFp
H FpH Fp H
/CH3 Fp -
`Q
/-- OTF
Rg 4S
Although syn Fp assisted 8-hydride and B-methoxy
abstractions have not been substantiated, there are examples
where an h2-olefin Fp complex is formed by an apparent syn
Fp assistance.36
*
The discussion is still valid should for any reason methoxy
abstraction involve anti Fp assistance; thus R would give
4R.
Fp
H Fp+
H Ph3C+ F
H
H Fp Fp+
Ph3C+
SH
H
There are no hydrogens anti to the Fp in h -cyclobutyl and
hl-cyclopentyl Fp, and yet they undergo hydride abstraction
to give their respective h2-cycloalkene Fp complexes. It is
not known whether hydride abstraction occurs via a distorted
transition state, a e-carbonium cation or by some other
mechanism.
If Fp migration is not concerted with methoxy abstrac-
tion, an allyl cation intermediate (5) would be formed and
should give the racemic h2-1,2-cycloheptadiene Fp (4),
Scheme VII.
Scheme VII
-Fpb a
In this case, the h -((R)-7-methoxy)cyclohepten-l-yl Fp (R)
would give both the (R)- and the (S)-h2-1,2-cycloheptadiene
Fp, (4R) and (4S).
In short, it is possible to synthesize an optically
active h2-1,2-cycloheptadiene Fp (4) from syn Fp assisted
methoxy abstraction of optically active h -(7-methoxy)cyclo-
hepten-l-yl Fp. An optically inactive h2-1,2-cyclohepta-
diene Fp (4) from this reaction would necessarily mean that
methoxy abstraction proceeded via an allyl cation or that
the barrier for isomerization between the allene (4) and the
allyl cation (5) is very low.
Optically active h -(7-methoxy)cyclohepten-l-yl Fp was
prepared according to Scheme VIII.
Scheme VIII
,OCH3
+ Qui
0
II
COOQui
OCH3
separate
The racemic 7-methoxycycloheptenecarboxylic acid was
resolved as its quinine salt via two recrystallizations from
absolute ethanol. The acid was released from the quinine
salt readily in aqueous acid and was found to have a nega-
tive optical rotation. The acid was subsequently converted
to the acid chloride which was then reacted with potassium
Fp to yield the hl-carbonyl-(7-methoxy)cyclohepten-l-yl Fp
(32). The 1H NMR of the cyclopentadienyl hydrogen
.OCH3
0CH3
00
II II
CICCCI
lii
resonances are well separated in a 10 mole % chloroform-d1
solution of Eu(hfc)3 and have an integrated ratio of 25 to
75 which indicates a 50% excess of one enantiomer. Decar-
bonylation under photolytic conditions yielded h -(7-
methoxy)cyclohepten-l-yl Fp (N). We presumed that the
h (7-methoxy)cyclohepten-l-yl Fp (N) is optically active
with the same optical purity as the hl-carbonyl-(7-
methoxy)cyclohepten-l-yl Fp (32). Eu(hfc)3 does not
separate the cyclopentadienyl and methoxy resonances of
h -(7-methoxy)cyclohepten-l-yl Fp (N) and a direct measure-
ment is not possible. It is also very difficult to obtain
optical rotations on these Fp complexes because their
solutions are highly colored.
A mixture weighted with the other enantiomer was
prepared according to Scheme IX.
Scheme IX
Br Br Br
0 H OCH3
+ LAH/Qui CNaH3
(+) 20% e.e.
33 CL Fp
Fp OCH3
OCH3
1) n-BuLi CP2Fe+
2) FpC CO 20% e.e.
P 32
The 2-bromo-2-cycloheptenone was reduced to its alcohol (33)
with a preformed LAH/quinine mixture. The alcohol (33) has
a positive optical rotation and was obtained with a 20%
enantiomeric excess as determined by Eu(hfc)3. Treating
this mixture of alcohols (+33) with sodium hydride followed
by methyl iodide gave the l-bromo-7-methoxycycloheptene.
The h -(7-methoxy)cyclohepten-l-yl Fp was obtained by the
well established route. As mentioned above, the h -(7-
methoxy)cycloheptadien-l-yl Fp does not form a complex with
Eu(hfc)3 and must be converted to its h -carbonyl-
(7-methoxy)cyclohepten-l-yl Fp (32) in order to determine
its enantiomeric purity. The h -carbonyl-(7-methoxy)cyclo-
hepten-l-yl Fp (32) was prepared by stirring a methylene
chloride solution of hl-(7-methoxy)cyclohepten-l-yl Fp in
10 mole % of ferrocenium tetrafluoroborate under 55 psi of
CO gas. The reaction was complete within an hour giving a
quantitative yield of the desired Fp-acyl complex (32). The
h -carbonyl-(7-methoxy)cyclohepten-l-yl Fp (32), in a
10 mole % chloroform-d1 solution of Eu(hfc)3 has a cyclo-
pentadienyl hydrogen integrated ratio of 60 to 40 giving us
a 20% excess of the other enantiomer. We shall call this
mixture (P). It was comforting to know that this 20%
enantiomeric excess was carried forward from the (+)-alcohol
(33). We infer from this information that the
h1-(7-methoxy)cyclohepten-1-yl Fp (P) made was optically
active (20% e.e.) and with an optical rotation opposite to
that prepared by Scheme VIII.
The carbonylation of the h -(7-methoxy)cyclohepten-1-yl
Fp to h1-carbonyl-(7-methoxy)cyclohepten-1-yl Fp (32) is of
particular importance here. As far as we know, this is the
first case of an unassisted CO insertion into a h -vinyl Fp
bond to give a h -acyl Fp complex.
All CO insertions into hl-alkyl Fp or h -vinyl Fp
complexes are assisted by phosphines under thermal condi-
tions to give the corresponding hl-acyl (Cp)Fe(CO)(PPh3)
complexes.37
Fe R P3 > Fe -
0/C OC R
CO R'3P
R allyl, vinyl
CO insertions catalyzed by oxidants, eg. Ce(IV), Ag(I),
Cp2Fe or Lewis acids have only been reported for h -alkyl
(Cp)Fe(CO)(L) where L = PPh3 and P(OPh)3 and h1-vinyl
(Cp)Fe(CO)(P(OPh)3) complexes;38 the former complexes
carbonylate faster than the latter. To date, CO insertions
into h -(alkoxy)methylene Fp to give the corresponding acyls
39
have not been successful.3
0
Fp C2 OR C CH2II
Fp CH2 OR CX)> Fp C- CH20R
We have also attempted without success to carbonylate the
h -(7-methoxy)cyclohepten-l-yl Fp with boron trifluoride
etherate under 1 atm. of CO.
The optically active h -(7-methoxy)cyclohepten-l-yl Fp
(N) and (P) were treated with trimethylsilyl trifluoro-
methanesulfonate to yield the h2-1,2-cycloheptadiene Fp
(4a). A dilute solution of the h2-1,2-cycloheptadiene Fp
(4a) has a negligible optical rotation. When the concentra-
tion of (4a) was increased, the solution became strongly
colored and made optical measurement impossible.
Since rotational measurements were experimentally
impossible, we decided that an alternative solution would be
to convert the h2-1,2-cycloheptadiene Fp (4a) to its methyl
ether adduct and carbonylate the ether adduct to the h-
carbonyl-(7-methoxy)cyclohepten-l-yl Fp (32).
FP Fp
Fp+ OCH3 OCH3
CH3OH CO
4 32
The enantiomeric composition of the h carbonyl-(7-
methoxy)cyclohepten-l-yl Fp (32) can then be determined by
H NMR with Eu(hfc)3 and this will give us a clue as to the
enantiomeric composition of the h2-1,2-cycloheptadiene Fp
(4).
~
To simplify the discussion, let us assume that the
fluxional process proceeds only via the 1,2-Fp shift,
Scheme II, and that the addition of methanol is stereo-
specific.
Anti attack of a nucleophile at a h2-olefin Fp bond is
40
well documented in the literature.40 Anti attack of a
methanol onto the h2-(R)-1,2-cycloheptadiene Fp (4R) regard-
less of which double bond the Fp is bonded to should give
the same h -((R)-7-methoxy)cyclohepten-l-yl Fp (R),
Scheme X.
Scheme X
H .H
Fp' H Fp
O H
Fp CH3 / 1,.CH3
4R -<
H R
The above statement is true because only two of the four
sides of the allene double bonds are available for complex-
ation with the Fp. If the Fp were bonded to the other
two sides of the allene double bond, an impossible geometry
which puts the Fp inside the ring (see 4R below), the allene
(4R) would add methanol in an anti fashion to give the (S)
methyl ether adduct.
Fp
Fp H+ CH30O.i
CH3OH
4R S
It should be noted that the anti face of the h -(R)-
1,2-cycloheptadiene Fp (4R), where the methanol comes in, is
only partially shrouded by the allene ring. A syn attack,
however, would be hindered by the bulky Fp group.
At first glance, eliminating the possibility of a syn
attack by methanol would seem to violate the principle of
microscopic reversibility;41 especially when we suggest syn
assistance as one of the ways methoxy abstraction can occur.
Furthermore, by the principle, anti attack of methanol would
necessarily require the 7-membered ring to adopt an
impossible conformation of putting the methanol inside the
ring. We must keep in mind that neither the methoxy
abstraction nor the methanol addition reactions are
reversible and that the methoxy group is leaving the
h -(7-methoxy)cyclohepten-1-yl Fp as a methyltrimethylsilyl
ether and not as a methanol. Thus, methoxy abstraction and
addition is not bound by microscopic reversibility. Regard-
less of the pathway methoxy abstraction follows (syn or
anti), we should be able to arrive at the enantiomeric
composition of the h2-1,2cycloheptadiene Fp (4) from the
enantiomeric composition of the hl-carbonyl-(7-methoxy)-
cyclohepten-1-yl Fp (32) provided methanol addition is
stereospecific.
The h2-1,2-cycloheptadiene Fp (4a), from the reaction
of the optically active hl-(7-methoxy)cyclohepten-1-yl Fp
(N) with trimethylsilyl trifluoromethanesulfonate, was
reacted with methanol and then carbonylated to give the
hl-carbonyl-(7-methoxy)cyclohepten-1-yl Fp (32). The
integrated cyclopentadienyl hydrogen intensities of h-
carbonyl-(7-methoxy)cyclohepten-1-yl Fp (32) in a chloro-
form-d1 solution containing Eu(hfc)3 were equal, Fig. 4 and
5. At some point complete racemization had occurred. The
optically active h -(7-methoxy)cyclohepten-1-yl Fp (P) was
put through the same series of reactions with the same
results, Fig. 6 and 7. Again complete racemization had
occurred.
Based upon the experimental results it appeared that an
allyl cation (5) had been formed at some point in the
sequence of reactions. One possibility is the concerted
formation of the optically active h2-1,2-cycloheptadiene Fp
(4) which then racemizes via the allyl cation (5). An
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alternative is allyl cation formation as methoxy is
abstracted followed by collapse to the h2-1,2-cyclo-
heptadiene Fp (4). In either case we would have to invoke
an allyl cation intermediate (5).
CHAPTER III
CARBONYL(h5-CYCLOPENTADIENYL)TRIPHENYLPHOSPHINEIRON(II)
COMPLETED 1,2-CYCLOHEPTADIENE
Methoxy abstraction from optically active hl-(7-
methoxy)cyclohepten-l-yl Fp led to the racemic h2-1,2-cyclo-
heptadiene Fp (4). Two mechanistic pathways were con-
sidered; one involving a concerted stereospecific methoxy
abstraction to the optically active h2-1,2-cycloheptadiene
Fp (4) followed by racemization via the achiral allyl cation
intermediate (5) and the other involving the achiral allyl
cation intermediate (5) from methoxy abstraction and prior
to complexation by the Fp.
In order to complete the stereochemical picture and to
shed light on the methoxy abstraction and the racemization
process, we shall turn our attention to the h and
h2- carbonyl(h -cyclopentadienyl)triphenylphosphineiron(II)
[Fpp] complexes of 7-methoxycycloheptadien-l-yl (34) and
1,2-cycloheptadiene (35), respectively.
L 0 3OCH3
Fe Fe
o0// 0o
03P 03P
34 35
There are quite a few advantages to moving from the Fp
system to the Fpp system. The most obvious is that the
triphenylphosphine ligand generally imparts additional
stability to these type of complexes against exposure to air
and therefore makes them easier to manipulate.
More important to us is that the triphenylphosphine
ligand introduces another chiral center, at the iron, in
both the hl-(7-methoxy)cyclohepten-l-yl Fpp (34) and the
h2-1,2-cycloheptadiene Fpp (35) complexes. The consequence
of this is that both complexes exist as enantiomeric pairs
of diastereomers. Diastereomers, unlike enantiomers, can
often be separated by physical methods and their diastere-
omeric compositions determined directly by NMR spectroscopic
measurements.
Phosphinylation therefore provides us a means to
separate hl-(7-methoxy)cyclohepten-l-yl Fpp (34) into its
diastereomers. We felt that examination of the diastere-
omeric composition of h2-1,2-cycloheptadiene Fpp (35) formed
from different diastereomers of hl-(7-methoxy)cycloheptene-
l-yl Fpp (34) should enable us to elucidate the pathway by
which methoxy abstraction occurs. Also, if h2-1,2-cyclo-
heptadiene Fpp (35) can be separated into its diastereomers,
we can follow directly by NMR the allene to allyl cation
isomerization because such isomerization necessarily con-
verts one diastereomer to a mixture of both diastereomers.
Lastly, since we were unable to grow crystals of the
h2-1,2-cycloheptadiene Fp (4) suitable for X-ray studies, we
would like to try to grow crystals of its Fpp analogue.
Before we proceed any further, the iron center of the
Fpp can be designated as either R or S according to the
Cahn-Ingold-Prelog (CIP) system adapted to organotransition
metal complexes.42 The configurations of the isomers of
hl-(7-methoxy)cycloheptadien-l-yl Fpp (34) and h2-1,2-cyclo-
heptadiene Fpp (35) according to the modified CIP system are
shown below.
H OCH3 Fe H CH3
0C.0 03P j
03P CO
R R S R
and enantiomer RS
and enantiorner SS
FeN Fe+
0H H HN-
OC O3P'*
03P OC
S R R R
and enantiomer RS and enantiomer SS
note that the iron center with the R configuration in
h -(7-methoxy)cyclohepten-l-yl Fpp (34) and h -1,2-cyclo-
heptadiene Fpp (35) have opposite absolute configuration.
h2-(7-Methoxy)cyclohepten-1-yl Fpp (34) was synthesized
by photolyzing a mixture of the hl-(7-methoxy)cyclohepten-
1-yl Fp with excess triphenylphosphine in an equal mixture
of n-pentane and benzene.
/ OCH3 Z OCH3
Fe Fe
/// \
OC CO + P03 hv 03
34
Photolysis was conducted in a quartz photochemical reactor
at room temperature under a stream of N2 to remove the CO
gas evolved. The reaction proceeded rapidly and was com-
plete within 20 mins without forming the bistriphenyl-
phosphine adduct. hl-(7-Methoxy)cycloheptadien-1-yl Fpp
(34) was purified via column chromatography and was obtained
as an air stable red paste. Residual triphenylphosphine may
be removed by filtration as its insoluble methyltriphenyl-
phosphonium iodide salt after warming a n-pentane solution
of the crude h1-(7-methoxy)cyclohepten-1-yl Fpp (34) with
methyl iodide.
The H NMR for h -(7-methoxy)cyclohepten-1-yl Fpp (34)
showed two sets of resonances, one for each diastereomer.
The hydrogen resonances for the methoxy hydrogens and the
ring hydrogens on carbons 2 and 7 are well separated. The
cyclopentadienyl hydrogen resonances are barely separated
from each other at 100 MHz and each is coupled to the
phosphorus.
Let us now consider first the methoxy abstraction of
hl-(7-methoxy)cyclohepten-l-yl Fpp (34) within the context
of a concerted stereospecific Fpp assisted pathway.
Following the same lines of analysis as the Fp ana-
logue, let us assume that the ring conformation necessary to
place the methoxy group anti to the Fpp cannot be achieved
and that any concerted stereospecific pathway for methoxy
abstraction must necessarily proceed with syn Fpp
assistance. Therefore, concerted stereospecific methoxy
abstraction of a single diastereomer of hl-(7-methoxy)
cyclohepten-l-yl Fpp (34) should lead to a single
diastereomer of h2-1,2-cycloheptadiene Fpp (35), Scheme XI.
Scheme XI.
OC_ OC'
03P3 Fe H 03P FeH
H\ H H
O III'OCH3
RR SS
This argument still holds should for any reason methoxy
abstraction proceed via anti Fpp assistance.
The other alternative is for methoxy abstraction to
proceed via a free allyl cation. The Fpp then collapses
onto the free allyl cation to give the h2-1,2-cyclo-
heptadiene Fpp (35). In this case, methoxy abstraction from
one diastereomer of h -(7-methoxy)cyclohepten-l-yl Fpp (34)
should lead to a mixture of diastereomers of h2-1,2-cyclo-
heptadiene Fpp (35), Scheme XII.
Scheme XII.
03P Fe
03P I -Fe
OC
03P Fe
H I H
SOCH3
It is clear that in order to establish unequivocally
the mechanism of methoxy abstraction, we need to separate
h -(7-methoxy)cyclohepten-1-yl Fpp (34) into its
two diastereomers.
hl-(7-Methoxy)cyclohepten-l-yl Fpp is isolated as a red
oil and its diastereomeric composition typically falls
around 60:40. The diastereomeric composition was deter-
mined by 1H NMR integration of the methoxy hydrogen reso-
nances because the cyclopentadienyl hydrogen resonances were
not well resolved. We first attempted the separation by
recrystalization at -350C from a variety of solvents; the
Diastereomeric composition will always be presented as such
a : b where (a) corresponds to the composition of the low
field cyclopentadienyl hydrogen resonance and (b) corre-
sponds to the high field resonance.
solvents tried include pentane, hexane, hexane/benzene
mixture and acetone/water mixture. In each case the
diastereomeric composition of the crystals of hl-(7-
methoxy)cyclohepten-1-yl Fpp (34) remained unchanged. The
diastereomers were also not separated by TLC.
An alternative approach to this problem was to synthe-
size the h -carbonyl-(7-methoxy)cyclohepten-l-yl Fpp (36),
which also exists as enantiomeric pairs of diastereomers,
separate this complex into its diastereomers and then
convert them back to the corresponding hl-(7-methoxy)cyclo-
hepten-l-yl Fpp (34).
The hl-(carbonyl-(7-methoxy)cyclohepten-1-yl Fpp (36)
was readily synthesized by warming h -(7-methoxy)cyclo-
hepten-l-yl Fp (34) with triphenylphosphine in acetonitrile.
6OCH3 0 OCH3
Fe Fe
OC 0 + P3 //
03P
36
The acyl complex (36) is an air stable orange solid and is
formed as a mixture of equal amounts of the two diastere-
omers. After numerous recrystalization attempts, we dis-
covered that a 20:80 ratio of diastereomers of (36) could be
obtained in one recrystalization from a 1:2 v/v mixture of
ethyl acetate and n-pentane at +100C with excellent recov-
ery, Fig. 8. The mother liquor was concentrated to give the
other diastereomer in a 65:35 ratio. A second recrystali-
zation led to further enrichment giving a 10:90 ratio of
diastereomers of (36).
Attempts to decarbonylate the hl-carbonyl-(7-
methoxycyclohepten-l-yl Fpp by either of the standard
photolytic or chemical methods were unsuccessful. Surpris-
ingly, CO was not evolved when a benzene solution of the
acyl complex (36) was photolyzed with a low pressure Hg lamp
even though other phosphinylated iron acyl alkyl complexes
have been reported not only to decarbonylate under photo-
lytic conditions but also to retain high
stereospecificity at the iron.43 The acyl complex (36) was
isolated with its diastereomeric composition unchanged. No
reaction was observed when the acyl complex (36) was allowed
to react with trimethylamine-N-oxide, chlorobis(tri-
phenylphosphine)rhodium dimer or iodosobenzene. In all
cases the acyl complex (36) was reisolated with its
diastereomeric composition unchanged. One should note the
tenacity of this acyl complex (36) against epimerization at
the iron center. The acyl complex (36) gave predominantly
decomposition products when refluxed in dioxane although a
small amount of hl-(7-methoxy)cyclohepten-1-yl Fp was formed
presumably via phosphine dissociation followed by migration
of the organic ligand onto the iron.
We finally succeeded in inducing decarbonylation in the
acyl complex (36) by a combined photolysis and ultrasoni-
cation process. Unfortunately, the h -(7-methoxy)cyclo-
hepten-l-yl Fpp (34) isolated had epimerized to a 55:45
diastereomeric mixture.
It was later discovered that eluting h1-(7-methoxy)
cyclohepten-l-yl Fpp (34) from a 8" x 1" alumina column
(neutral, grade II) with a 1:1 v/v n-pentane/benzene as the
eluant afforded an enriched diastereomeric mixture of the
complex (34). The diastereomeric mixture of complexes (34)
appeared on the column as a broad red band and was collected
in two halves. Surprisingly, the first half and the second
half of this red band both gave mixtures of hl-(7-methoxy)-
cyclohepten-1-yl Fpp (34) enriched in the same diastereomer
(75:25 and 85:15, respectively), Fig. 9. The alumina
apparently did not separate the mixture of diastereomers but
selectively decomposed one of them. Selective decomposition
of h -(7-methoxy)cyclohepten-1-yl Fpp (34) with trimethyl-
silyl trifluoromethanesulfonate also afforded an enriched
diastereomeric mixture of the complex but this time in a
10:90 ratio, Fig. 10. We shall postpone the discussion of
the latter selective decomposition to an appropriate time.
We have also attempted to synthesize other phos-
phinylated h1-(7-methoxy)cyclohepten-1-yl iron complexes but
without much success. The respective phosphinylated com-
plexes were not isolated when h -(7-methoxy)cyclohepten-1-yl
Fp was photolyzed in the presence of either trimethyl-
phosphine or triphenylphosphite. When a cold (-780C)
solution of l-lithio-7-methoxycycloheptene was treated with
carbonyl(h5-cyclopentadienyl)tri-n-butylphosphineiron(II)
73
0
.H
4J
.rl
U3
0
0
(
U
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ro
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-4
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[Fppbu] iodide, the major product isolated was unreacted
Fppbu iodide and a minor product perhaps resulting from the
attack of the vinyl lithium reagent on the cyclopentadienyl
ring.
Triphenylphosphite substituted hl-(7-methoxy)cyclo-
hepten-l-yl iron complex was synthesized via a thermal
exchange for triphenylphosphine in h -(7-methoxy)cyclo-
hepten-l-yl Fpp with an excess triphenylphosphite in
refluxing THF.37a
Z OCH3 0 OCH3
Fe j /<
Fe
OC 03P I + P(O0)3 THF> o/ /
(00)3P
The equilibrium of this reaction lies strongly to the right
despite the stronger nucleophilicity of the triphenylphos-
phine and is attributed to a steric effect. The triphenyl-
phosphine ligand being very bulky is unable to displace the
triphenylphosphite ligand once the latter is coordinated at
the iron. We could only isolate this triphenylphosphite
iron complex as a crude mixture containing free triphenyl-
phosphine and triphenylphosphite.
At this point, we have not succeeded in isolating
h -(7-methoxy)cyclohepten-l-yl Fpp (34) as a single
diastereomer but we have developed the methodology which
enables us to separate hl-(7-methoxy)cyclohepten-l-yl Fpp
(34) from a 60:40 diastereomeric mixture to a mixture with a
diastereomeric ratio of 85:15 and 10:90.
Before we look at the methoxy abstraction process, it
is imperative to examine the stereochemical integrity of
hl-(7-methoxy)cyclohepten-1-yl Fpp (34). We discovered that
h -(7-methoxy)cyclohepten-1-yl Fpp (34) with diastereomeric
compositions of 85:15 or 60:40 maintained their respective
compositions indefinitely when frozen in the refrigerator.
A chloroform-dl solution of hl-(7-methoxy)cyclohepten-l-yl
Fpp (34) with a 85:15 diastereomeric composition however
epimerizes very slowly as measured by 1H NMR but only at
elevated temperatures. The diastereomeric composition of a
60:40 mixture of hl-(7-methoxy)cyclohepten-l-yl Fpp (34)
remained unchanged by 1H NMR under the same conditions.
Therefore, we feel it is safe to assume that h -(7-
methoxy)cyclohepten-l-yl Fpp (34) does not epimerize under
the conditions for methoxy abstraction, i.e., short duration
and low temperature.
We tried a number of methods to effect methoxy abstrac-
tion. Trimethyloxonium tetrafluoroborate was used first
instead of the trimethylsilyl trifluoromethanesulfonate
because we anticipated that the trifluoromethanesulfonate
anion might affect the h2-1,2-cycloheptadiene Fpp (35) in an
adverse manner similar to what was observed with the
h2-1,2-cycloheptadiene Fp (4). Treating h -(7-
methoxy)cyclohepten-l-yl Fpp (34) with trimethyloxonium
tetrafluoroborate in methylene chloride at -100C for 7 hrs
gave an impure green substance after the reaction mixture
was quenched with diethyl ether.
Discouraged by the ineffectiveness of trimethyloxonium
tetrafluoroborate as a methoxy abstracting reagent, we went
back to trimethysilyl trifluoromethanesulfonate. The
reaction did not fair any better and a black intractable
paste was obtained. The same reaction in pentane at 0C
gave a greenish yellow precipitate which dissolved when
washed with diethyl ether. If the reaction in pentane was
quenched with diethyl ether, again a black pasty residue was
formed. We discovered, quite by accident, that if this
black paste is dissolved in ethyl acetate, the trifluoro-
methanesulfonate salt of h2-1,2-cycloheptadiene Fpp (35a)
precipitates from the solution as a bright orange air stable
solid. In fact, prior quenching with diethyl ether is
unnecessary. Pentane was removed from the reaction mixture
in vacuo to leave a dark greenish brown paste from which
h -1,2-cycloheptadiene Fpp (35a) was precipitated with ethyl
acetate in the open air. The reaction was also run in an
equal mixture of n-pentane and benzene at 0C with similar
results; the solvent mixture was removed in vacuo to leave a
residue from which h2-1,2-cycloheptadiene Fpp (35) was
precipitated with ethyl acetate.
The 1H NMR of the h2-1,2-cycloheptadiene Fpp (35a) is
consistent with a pair of diastereomers, Fig. 11. The
two cyclopentadienyl hydrogen resonances, each coupled to
the phosphorus, and three of the four allenic hydrogen reso-
nances are well separated. One allenic hydrogen resonance
is obscured by the ring methylene resonances and is
confirmed by 2H NMR of the appropriately deuterated complex.
The assigned structure was confirmed by X-ray videe infra).
The trifluoromethanesulfonate anion was readily
exchanged for the hexafluorophosphate anion by adding water
to a methanol solution of the trifluoromethansulfonate salt
of the h2-1,2-cycloheptadiene Fpp (35) containing an excess
of ammonium hexafluorophosphate. This gave a yellow pre-
cipitate which was filtered to give the pure hexafluoro-
phosphate salt (35b) in quantitative yield. The procedure
was performed in open air which demonstrated the stability
of h -1,2-cycloheptadiene Fpp (35). The h -cycloheptadiene
Fp (4a,b) would have decomposed in the presence of air,
methanol or moisture.
Unfortunately, treating carbonyl(h5-cyclopentadienyl)
(hl-(7-methoxy)cyclohepten-l-yl)triphenylphosphiteiron(II)
with trimethylsilyl trifluoromethanesulfonate failed to give
the h2-1,2-cycloheptadiene complex. we believe that
trimethysilyl trifluoromethanesulfonate is incompatible with
the triphenylphosphite ligand and results in complete
decomposition of the iron complex.
We have thus developed the methodology to synthesize
and purify both the trifluoromethanesulfonate and hexa-
fluorophosphate salt of the h2-1,2-cycloheptadiene Fpp (35).
Both (35a) and (35b) are air stable and moderately stable in
solution.
We are finally ready to look into the process of
methoxy abstraction, paying particular attention to the
h2-1,2-cycloheptadiene Fpp (35) diastereomeric composition
obtained starting with a majority of one of the
diastereomers of h -(7-methoxy)cyclohepten-l-yl Fpp (34).
Methoxy abstraction from a 60:40 mixture of h -(7-
methoxy)cycloheptadien-l-yl Fpp (34) in a 1:1 v/v
n-pentane/benzene solvent mixture at 0C typically gives a
65:35 to 60:40 diastereomeric mixtures of h2-1,2-cyclo-
heptadiene Fpp (35) after precipitation from ethyl acetate.
The diastereomeric composition was determined by 1H NMR
based on the integrated cyclopentadienyl hydrogen
resonances. Repeated precipitation from ethyl acetate does
not change this diastereomeric composition.
The diastereomeric composition of h2-1,2-cyclo-
heptadiene Fpp (35) was found to be dependent upon the
solvent in which methoxy abstraction was performed. When
the reaction was repeated under identical conditions but in
n-pentane, the crude product (35a) was found to have a
diastereomeric composition of around 65:35. The 1H NMR of
the crude product (35a) is broad and poorly resolved and one
should consider the diastereomeric ratio of 65:35 a very
rough value. A single precipitation from ethyl acetate
gave (35b) in a 80:20 diasteromeric composition. Just in
case the discrepancy in the crude and pure diastereomeric
compositions of h2-1,2-cycloheptadiene Fpp (35a) was in some
way caused by the precipitation process, the filtrate was
collected and was examined for h2-1,2-cycloheptadiene Fpp
(35a). We were unable to detect by 1H NMR any trace of
h -1,2-cycloheptadiene Fpp (35a). This 80:20 composition
also remained invariant after repeated precipitations.
Regardless of the solvent used, the h2-1,2-cycloheptadiene
Fpp was obtained at 45% to 55% yields.
1
Methoxy abstraction of a 80:20 mixture of h -(7-
methoxy)cyclohepten-1-yl Fpp (34) in the n-pentane/benzene
solvent mixture at 0C gives a 60:40 diastereomeric mixture
of the h2-1,2-cycloheptadiene Fpp (35a) after precipitation.
The yields are slightly higher; over 55%.
The most interesting observation regarding methoxy
abstraction is from a 60:40 mixture of h -(7-methoxy)cyclo-
hepten-1-yl Fpp (34) with half an equivalent of trimethyl-
silyl trifluoromethanesulfonate at 0C in n-pentane/benzene
solvent mixture. We were able to isolate a 60:40
diastereomeric mixture of h2-1,2-cycloheptadiene Fpp (35a)
in over 90% yield (based on trimethylsilyl trifluoromethane-
sulfonate used). Recovered from the reaction mixture in
The 85:15 mixture of (34) has epimerized to a 80:20
mixture.
almost a quantitative amount was hl-(7-methoxy)cyclo-
heptene-l-yl Fpp (34) with a 10:90 diastereomeric ratio!
Note that the major diastereomer now is the one with a
cyclopentadienyl hydrogen resonance at a higher field.
Methoxy abstraction of the 10:90 mixture of h -(7-
methoxy)cyclohepten-l-yl Fpp with trimethylsilyl trifluoro-
methanesulfonate at 0C in the n-pentane/benzene solvent
gave h2-1,2-cycloheptadiene Fpp (35a) in less than a 10%
yield but with a 80:20 diastereomeric ratio!
The results of the methoxy abstractions were summarized
in Table 2.
Ethoxy abstraction from h -(7-ethoxy)cyclohepten-1-yl
Fpp by trimethylsilyl trifluoromethanesulfonate under the
conditions developed for methoxy abstraction also gave
h2-1,2-cycloheptadiene Fpp (35a). The results of ethoxy
abstraction from enriched diastereomeric mixtures of h -(7-
ethoxy)cycloheptene-1-yl Fpp parallel that of its methoxy
analogue in terms of diastereomeric compositions and percent
yields of h2-1,2-cycloheptadiene Fpp (35a).
We have a few reasons to believe that the diastere-
omeric composition of h2-1,2-cycloheptadiene Fpp (35a) from
methoxy abstraction of various diastereomeric mixtures of
h -(7-methoxy)cyclohepten-1-yl Fpp (34) represents the
initial diastereomeric ratio immediately upon methoxy
abstraction and does not reflect the different solubilities
of the h2-1,2-cycloheptadiene Fpp (35a) diastereomers in
ethyl acetate.
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First of all, the ethyl acetate filtrate from the
initial precipitation of h2-1,2-cycloheptadiene Fpp (35a)
was collected and examined for any residual h2-1,2-cyclo-
heptadiene Fpp (35a). We did not detect any trace of
h2-1,2-cycloheptadiene Fpp (35a) by 1H NMR from this
filtrate. Also, repeated precipitations of h2-1,2-cyclo-
heptadiene Fpp (35a) from ethyl acetate did not affect its
diastereomeric compositions.
Lastly, we also tried a different procedure to precipi-
tate h2-1,2-cycloheptadiene Fpp (35) from the reaction
mixture. Instead of precipitating h2-1,2-cycloheptadiene
Fpp (35a) from the crude reaction mixture with ethyl
acetate, we dissolved the crude reaction mixture in methanol
saturated with ammonium hexafluorophosphate and precipitated
the h2-1,2-cycloheptadiene Fpp as its hexafluorophosphate
salt (35b) with water. We found by 1H NMR measurements that
the diastereomeric compositions of h2-1,2-cycloheptadiene
Fpp (35b) obtained by this method are similar to those
obtained by the previous method.
We conclude that methoxy abstraction of h -(7-methoxy)
cyclohepten-l-yl Fpp (34) by trimethylsilyl trifluoro-
methanesulfonate occurs from only one diastereomer. The
other diastereomer merely decomposes in the presence of
trimethylsilyl trifluoromethanesulfonate. Therefore, when a
60:40 diastereomeric ratio of h -(7-methoxy)cyclohepten-1-yl
Fpp (34) was treated with half an equivalent of trimethyl-
silyl trifluoromethanesulfonate, only one diastereomer
reacted to give h2-1,2-cycloheptadiene Fpp (35a) in a 60:40
diastereomeric ratio in almost quantitative yields while the
other diastereomer was recovered from the reaction mixture.
This other diastereomer gave negligible amounts of h2-1,2-
cycloheptadiene Fpp (35a) when treated with trimethylsilyl
trifluoromethanesulfonate.
From the above observation, it becomes clear why
methoxy abstraction in a pentane/benzene solvent from either
a 80:20 or a 60:40 mixture of hl-(7-methoxy)cyclohepten-1-yl
Fpp (34) results in the same 60:40 mixture of h2-1,2-cyclo-
heptadiene Fpp (35a). In each case only one diastereomer,
the major diastereomer, reacted with the trimethylsilyl
trifluoromethanesulfonate.
The choice of solvent for methoxy abstraction affects
the diastereomeric ratio of h2-1,2-cycloheptadiene Fpp
(35a). In a non-polar pentane solvent, we are probably
seeing methoxy loss through Fpp assistance (probably syn Fpp
assistance) leading to an enriched diastereomeric mixture of
h -1,2-cycloheptadiene Fpp (35a). Methoxy loss in a more
polar pentane/benzene solvent probably proceeds via the
allyl cation to give an almost equal diastereomeric mixture
of the h2-1,2-cycloheptadiene Fpp (35a).
At the present moment we are unable to guess as to the
significance of the 80:20 diastereomeric ratio of h2-1,2-
cycloheptadiene Fpp (35a) obtained from the methoxy
abstraction of a 10:90 diastereomeric mixture of h -(7-
methoxy)cyclohepten-1-yl Fpp (34).
Our conjecture as to why only one h1-(7-methoxy)-
cyclohepten-1-yl Fpp (34) diastereomer undergoes methoxy
abstraction to give the desired complex (35a) is that only
this diastereomer has its methoxy group accessible to
trimethylsilyl trifluoromethanesulfonate. The methoxy group
of the other diastereomer is occluded from trimethylsilyl
trifluoromethanesulfonate.
Based upon extended Huckel calculations of Seeman and
Davies44 and X-ray crystal structures45 of similar types of
complexes, we arrived at what we believe to be the most
stable conformers of each of the two hl-(7-methoxy)cyclo-
hepten-l-yl Fpp (34) diastereomers shown below.
OCH3 H
Fe --*1-- Fe
OC H C OCH3
02P 02?--<>P
SR SS
The phosphorus to phenyl bond here is eclipsed with the iron
to (7-methoxy)cyclohepten-1-yl bond forcing the planes of
both rings to lie parallel to each other. This conformation
places the methoxy group of one diastereomer (SR) above the
plane of the cycloheptene ring and places the methoxy group
of the other diastereomer (SS) below the plane of the
cycloheptene ring. The methoxy group of the diastereomer
(SR) is therefore exposed to trimethylsilyl trifluoro-
methanesulfonate whereas the methoxy group of the other
diastereomer (SS) is shielded from abstraction by the phenyl
ring below. MMX calculations for the two diastereomers of
hl-(7-methoxy)cyclohepten-l-yl Fpp (34) confirmed the
conformation of the structures drawn above.
Let us now consider the consequences of an Fpp cation
alternating between the adjacent double bonds of the 1,2-
cycloheptadiene in terms of its IH NMR spectrum and
diastereomeric ratio based upon the two schemes described
below.
In one case, the Fpp fragment moves between the
two allenic double bonds as shown in Scheme XIII. The allyl
cation intermediate is not involved in the 1,2-Fpp shift and
the stereochemical integrity of each diastereomer is
retained; ie. the SR diastereomer (and its RS enantiomer)
does not interconvert with its RR (or SS) diastereomer.
Scheme XIII.
OC 03P-- Fe -CO
03P-e Fe+ Fe+ .09 4CO
H1 \ H3 H 3
H H H 1 H
SR SR SR
A rapid 1,2-Fpp shift in the NMR time scale will render the
two allenic hydrogens of each diastereomer equal. This
effect should be similar to that observed in the case of
h2-1,2-cycloheptadiene Fp (4) when the allenic hydrogen
resonances coalesce. In this case both diastereomers would
still be observed and their diastereomeric composition after
coalescence would remain unchanged. In other words, the
two sets of allenic hydrogen resonances will coalesce and at
the same time the two cyclopentadienyl hydrogen resonances
will remain distinct and separated.
In the other case involving an allyl cation inter-
mediate as shown in Scheme XIV, both cyclopentadienyl
hydrogen resonances and the two pairs of the allenic hydro-
gen resonances would coalesce.
|
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PAGE 1
SYNTHESIS AND X-RAY STRUCTURE OF IRON STABILIZED STRAINED CYCLIC ALLENES . VALENCE ISOMERIZATION BETWEEN LINEAR PERPENDICULAR AND BENT PLANAR ALLENE BY SU-MIN OON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1987
PAGE 2
To my mother and in memory of my father
PAGE 3
ACKNOWLEDGEMENTS I would like to thank Dr. W.M. Jones for providing an atmosphere conducive to my personal and intellectual growth during my stay. He has kept me entertained with a fun and exciting problem and his guidance and encouragement are crucial toward the successful completion of this study. He has attracted many fine individuals who are a pleasure to work with. I would also like to thank Dr. Palenik and Dr. Koziol for the X-ray crystal structure of my iron stabilized cyclic allene. The witty, sarcastic and sometimes acrimonious remarks of Mr. Nicholas Conti made me laugh at even the worst of my scientific blunders. The most special thanks go to Miss Margaret Easley; a soprano, a pianist, an artist and a dear friend. I have spent many happy moments with her engaged in intriguing conversations spanning an extremely wide range of topics. She introduced me to opera and classical music and therefore made an everlasting impact on my life. Her friendship is greatly appreciated and she will always be remembered with fondness.
PAGE 4
TABLE OF CONTENTS Page ACKNOWLEDGEMENTS iii LIST OF TABLES ix LIST OF FIGURES x ABSTRACT xi CHAPTER I INTRODUCTION 1 II DICARBONYL (h 5 -CYCLOPENTADIENYL) IRON (II) COMPLEXED 1,2-CYCLOHEPTADIENE 16 III CARBONYL (h 5 -CYCLOPENTADIENYL) TRIPHENYLPHOSPHINEIRON(II) COMPLEXED 1,2-CYCLOHEPTADIENE 61 IV IRON (II) COMPLEXED 1 , 2-CYCLOHEXADIENE 9 8 V EXPERIMENTAL 108 5 Improved synthesis of dicarbonyl(h -cyclo pentadienyl) (h (7-methoxy) cyclohepten-1yl)iron(II) 109 2 Thermal decomposition of dicarbonyl(h -1,2cycloheptadiene) (h -cyclopentadienyl) iron (II) trif luoromethanesulfonate 109 5 NMR scale synthesis of dicarbonyKh -cyclo pentadienyl) iron (II) trif luoromethanesulfonate 110 Attempted thermal decomposition of dicarbonyl (h -1 ,2-cycloheptadiene) (h -cyclopentadienyl) iron (II) tetrafluoroborate 110 13 Variable temperature C NMR studies of th_e dicarbonyKh -1 ,2-cycloheptadiene) (h cyclopentadienyl) iron (II) tetrafluoroborate 110 IV
PAGE 5
Page 2 The reaction between dicarbonyl (h -1,2-cycloheptadiene) (h -cyclopentadienyl) iron (II) tetrafluoroborate and triphenylmethane . . . Ill Attempted enantiomeric enrichment of racemic dicarbonyl (h -1 ,2-cycloheptadiene) (h cyclopentadienyl) iron (II) tetrafluoroborate with (S)-(-)-2-methylbutanol Ill Resolution of 7-methoxy-l-cycloheptenecarboxylic acid 112 5 Synthesis of optically active dicarbonyl (h cyclopentadienyl) (h (7-methoxy) cyclohepten-l-yl)iron(II) (N) 112 Synthesis of optically active 1-bromo7-methoxycycloheptene 113 Synthesis of optically active dicarbonyl (h cyclopentadienyl) (h (7-methoxy) cyclohepenl-yl)iron(II) (P) 114 Synthesis of dicarbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten^l-yl) iron (II) from 5 dicarbonyl(h -1,2-cycloheptadiene) (h -cyclopentadienyl) iron (II) trif luoromethanesulfonate 114 Synthesis of dicarbonyl (h -carbonyl(7-methoxy) cyclohepten-1-yl) (h -cyclopentadienyl) iron (II) via carbonylation of dicarbonyl (h -cyclopentadienyl) (h -(7-methoxy) cyclohepten-1-yl) iron (II) 114 Synthesis of carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphine iron (II) 115 2 Synthesis of carbonyl (h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron(II) trif luoromethanesulfonate 116 Synthesis of l-bromo-7-methoxycycloheptene-7d 1 . 117 2 Synthesis of carbonyl (h -1 , 2-cycloheptadieneld 1 ) (h -cyclopentadienyl) triphenylphosphineiron (II) trif luoromethanesulfonate . . 117
PAGE 6
Page The exchange of hexaf luorophosphate anion for trifluoromethanesulfonate anion in carbonyl (h -1,2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) 118 Synthesis of carbonyl (h ^carbonyl(7-methoxy) cyclohepten-1-yl) (h -cyclopentadienyl) triphenylphosphineiron (II) 118 Separation of diastereomers of carbonyl (h carbonyl( 7-methoxy) cyclohepten-1-yl) (h -cydopentadienyl) triphenylphosphineiron(II) 119 Attempted decarbonylation of carbonyl (h carbonyl( 7-methoxy) cyclohepten-1-yl) (h -cyclopentadienyl) triphenylphosphineiron(II) 120 Separation of diastereomer of carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1yl) triphenylphosphineiron (II) 121 2 Synthesis of carbonyl (h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphine iron (II) trifluoromg thanesulfonate from the ei (] phosphineiron(II) (80:20) 122 Reaction between carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) with a half equivalent of trimethylsilyl trifluoromethanesulfonate 122 2 Synthesis of carbonyl (h -1,2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphine iron (II) trifluoromethanesulfonate from the enriched carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) (15:85) 123 5 Synthesis of dicarbonyl (h -cyclopentadienyl) (h (7-ethoxy) cycloheptadien-1-yl) iron(II) 123 5 Synthesis of carbonyl (h -cyclopentadienyl) (h ( 7-ethoxy) cycloheptadien-1-yl) triphenylphosphineiron (II) 123 mriched carbonyl (h -cyclopentadienyl) (h (7-methoxy) -cycloheptne-1-yl) triphenyl-
PAGE 7
Page Separation of the diastereomer of carbonyl (h -cyclopentadienyl) (h (7-ethoxy) cycloheptadien-1-yl) triphenylphosphineiron (II) . . 124 2 Synthesis of carbonyl (h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphine iron (II) trifluoromethanesulfonate from carbonyl (h -cyclopentadienyl) (h -(7ethoxy) cyclohepten-1-yl) triphenylphosphine iron(II) 125 2 Epimerization of carbonyl (h -1 , 2-cycloheptene) (h -cyclopentadienyl) triphenylphosphine iron (II) hexafluorophosphate in the presence of triphenylphosphite 125 2 Epimerization-of carbonyl (h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) trifluoromethanesulfonate in the presence of triphenylphosphine . . . 126 Synthesis of dicarbonyKh -cyclopentadienyl) (h(6-methoxy) cyclohexen-1-yl) iron (II) . . 126 2 Attempted synthesis of ,-dicarbonyl (h -1,2cyclohexadiene) (h -cyclopentadienyl) iron (II) trifluoromethanesulfonate 127 2 Attempted synthesis of ,-dicarbonyl (h -1,2cyclohexadiene) (h -cyclopentadienyl) iron (II) tetraf luoroborate 127 Low temperature NMR scale synthesis of 5 dicarbonyl(h -1 , 2-cyclohexadiene) (h cyclopentadienyl) iron (II) trifluoromethanesulfonate 128 Synthesis of carbonyl (h -cyclopentadienyl) (h (6-methoxy) cyclohexen-1-yl) triphenylphosphineiron (II) 129 2 Attempted synthesis of carbonyl (h -1,2cyclohexadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) trifluoromethanesulfonate 129 Reaction of carbonyl (h -( 6-methoxy ) cyclohexen-1yl) (h -cyclopentadienyl) triphenylphosphineiron (II) with trimethylsilyl trifluoromethanesulfonate followed by additions of ethanol 130 VII
PAGE 8
Page APPENDIX 13 2 REFERENCES 133 BIOGRAPHICAL SKETCH 137
PAGE 9
LIST OF TABLES Table Page 1. Geometries of cyclic allenes calculated by MNDO 5 2. Methoxy abstraction from h -(7-methoxy) cyclohepten-1-yl Fpp (34) to give h -1-2-cycloheptadiene Fpp (35a) 82
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LIST OF FIGURES Figure Page 1. Thermal decomposition of (4a) in CD 2 C1 2 .... 19 2. Fp trif luoromethanesulfonate and 1,3cycloheptadiene from the thermal decomposition of (4a) in CD 2 C1 2 20 3. High temperature 13 C NMR of (4b) in CD 3 N0 2 . . 31 4. A CDCl^ solution of 10 mole % Eu(hfc) and (32) from (N) showing a 25:75 diastereomeric composition 56 5. A CDC1 3 solution of 10 mole % Eu(hfc) and (32) from methoxy abstraction and addition of (N) showing a 50:50 diastereomeric composition . . 57 6. A CDC1solution of 10 mole % Eu(hfc) and (32) from (P) showing a 60:40 diastereomeric composition 58 7. A CDC1solution of 10 mole % Eu(hfc) and (32) from methoxy abstraction and addition of (P) showing a 50:50 diastereomeric composition . . 59 8. 1 H NMR of (36) in CDC1., showing a 20:80 diastereomeric composition 69 9. 1 H NMR of (34) in CDC1 3 showing a 85:15 diastereomeric composition 72 10. 1 H NMR of (34) in CDC1 3 showing a 10:90 diastereomeric composition 73 11. 1 H NMR of (35a) in CDC1 3 78 12. X H S.S.T. of (35a) in CDC1 3 91 2 13. X-ray structure of h -1 , 2-cycloheptadiene Fpp cation 97 14. Reaction between h (6-methoxy) cyclohexen-1-yl Fp and trimethylsilyl trif luoromethanesulfonate followed by low temperature H NMR 104
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHESIS AND X-RAY STRUCTURE OF IRON STABILIZED STRAINED CYCLIC ALLENES . VALENCE ISOMERIZATION BETWEEN LINEAR PERPENDICULAR AND BENT PLANAR ALLENE. By Su-Min Oon May 1987 Chairman: William M. Jones Major Department: Chemistry The synthesis and isolation of strained organic molecules has been an area of active research for over forty years. Strained cyclic allenes are of no exception and have been studied both theoretically and experimentally. Calculations have shown that strained cyclic allenes down to 1,2-cyclopentadiene remained twisted and chiral but with a 2-5 kcal/mole barrier for racemization. Experimentally, 1 ,2-cyclohexadiene was found to be chiral but with a very low barrier for racemization. Extended Huckel molecular orbital (EHMO) calculations have shown that the racemization barrier of the twisted 1,2-cycloheptadiene via its bent planar allyl cation is
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lowered considerably upon complexation with a transition metal. We have synthesized and studied the racemization 5 process of dicarbonyl (h -cyclopentadienyl) iron (II) [Fp] and carbonyl (h -cyclopentadienyl) triphenylphosphineiron (II) [Fpp] complexed 1 , 2-cycloheptadiene. Although we have failed to observe the racemization of Fp complexed 1 , 2-cycloheptadiene directly, we have experimental evidence which indicates that the bent planar allyl cation was formed either via methoxy abstraction from the h (7-methoxy) cyclohepten-1-yl Fp leading to the formation 2 of h -1, 2-cycloheptadiene Fp or from the isomerization of o h-1, 2-cycloheptadiene Fp itself. We have also determined that the allene to allyl cation isomerization must be at least 14.7 kcal/mole. The Fpp complexed 1 , 2-cycloheptadiene was synthesized as enantiomeric pairs of diastereomers . Its fluxional barrier for 1,2-Fpp shift is between 15 kcal/mole and 18 kcal/mole. An enriched mixture of one diastereomer slowly equilibrates at room temperature to its thermodynamic mixture. The iron center here is stereochemically rigid and epimerization through dissociation and reassociation of the triphenylphosphine ligand does not occur. We infer from our observation that the thermal equilibration occurs via the allyl cation. Methoxy abstraction from the h -(7-methoxy) cycloheptadien-1-yl Fpp proceeds via the allyl cation as in the case with h (7-methoxy) cycloheptadien-1-yl Fp. xn
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We also report the first X-ray crystal structure of the 2 h -1 ,2-cycloheptadiene Fpp cation. Finally, we were able to synthesize and isolate the h -1 ,2-cyclohexadiene Fpp complex as a thermally unstable solid. This solid reacted with ethanol, in a manner typical of most metal olefin complexes, to give h (6-ethoxy) cyclohexen-1-yl Fpp. xm
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CHAPTER I INTRODUCTION Allenes are a class of organic compounds which contain two cumulated double bonds arranged in an orthogonal geometry. Acyclic allene has a linear structure and the planes defined by R-iR? ! and R 3 R 4 C 3 are mutuall Y perpendicular. ^ R3 c 2 ^~ p3' R 4 One of the most fascinating problems both experimentalists and theoreticians have been concerned with regarding the allene structure is the energy gap separating the ground state linear perpendicular allene (A) from its excited bent planar valence isomer (B) . 11 — . — / -» Equally interesting is the extent to which this energy gap can be lessened and perhaps be inverted such that the bent planar allene becomes the ground state.
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One can approach this problem by either increasing the ground state energy of the linear perpendicular allene or decreasing the excited state energy of the bent planar allene or both. Incorporating a linear perpendicular allenic unit into a small ring will deform this allenic unit in two ways in order to facilitate ring closure and reduce ring strain. The allene will bend at the C 2 carbon about an axis perpendicular to the R 1 R„C plane as defined by 6. The allene will also twist about the R.R-C, and the R 3 R.C 3 planes as defined by +. This combined effect raises the ground state energy of the allenic unit and is reflected by its kinetic instability. 1,2-Cyclononadiene is a distillable liquid while 1,2-cyclo2 octadiene dimerizes within hours at room temperature. 1 ,2-Cycloheptadiene and 1 , 2-cyclohexadiene have only a fleeting existence and can only be trapped chemically or, in the second case, by trapping in an argon matrix. Although cyclic allenes lack an asymmetric carbon, they are chiral molecules with a C„ point group. Their R and S configuration, using 1 , 2-cycloheptadiene as an example, are shown below.
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H., .. ^H R In order for one enantiomer to convert to the other, the allenic pi-bonds of that enantiomer must be rotated out of orthogonality, passing through a planar state to the other enantiomer. The smaller the ring size of the cyclic allene the greater the allene is bent and twisted towards its planar isomer. When ring constraints force the pi-bonds of an optically active cyclic allene (1) to rotate out of orthogonality and become planar (2) , optical activity is lost. Racemization of an optically active cyclic allene is sufficient proof that a planar intermediate is accessible. The energy barrier for racemization can be measured via the loss of optical activity and represents the minimum energy separating the two forms of the allene, (A) and (B) . Numerous calculations have been performed over the last 20 years on 1 , 2-propadiene and 1 , 2-cyclohexadiene, the
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archetype cyclic allene, in order to estimate this barrier for racemization and to address the electronic nature of the bent planar allene. The ground state geometry for 1 ,2-cyclononadiene, 1 , 2-cyclooctadiene and 1 , 2-cycloheptadiene are accepted generally as slightly bent and twisted towards planarity. The results from the latest MNDO calculations are given in Table 1. Allenic geometries for this series of three cyclic allenes clearly show the effect of ring strain on the allenic unit. An X-ray crystal structure is available for a phenylurethane derivative of 1 , 2-cyclononadiene and agrees closely with the calculated results. Historically, there have been considerable doubts as to the ground state structure of 1 , 2-cyclohexadiene. Moore and Moser predicted the singlet diradical as the ground state. 8 INDO calculations by Dillon and Underwood suggested that the triplet diradical might be lower in energy. Recent SCF calculations of Johnson favor a chiral allenic ground state structure for 1 , 2-cyclohexadiene. The calculations further predict a chiral ground state structure for 1 , 2-cyclopentadiene but with a 2-5 kcal/mole barrier for racemization. Mechanistic studies of 1 , 2-cyclohexadiene by Bottini et al. suggested that 1 , 2-cyclohexadiene has an allenic structure but rapidly racemizes to a singlet diradical form. He also showed some chemistry arising from the zwitterionic form of 1 , 2-cyclohexadiene.
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Table 1. Geometries of cyclic allenes calculated by MNDO. c -c -c 1 2 3 out of plane H bend' 1 , 2-cyclononadiene 1 , 2-cyclooctadiene 1 , 2-cycloheptadiene 170.4° 161.5° 153.4° 33.7° 31.0° 27.6° l defined as the angle made by the C-H bond with the c i~ C 2~ C 3 plane containing the C_ axis, e.g., 45° for 1,2 propadiene.
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Optically active 1 , 2-cyclohexadiene, synthesized via asymmetric dehydrobromination of resolved 1-bromocyclohexene-6d 1 with potassium tert-butoxide , was trapped in situ with diphenylisobenzofuran. The adduct (3) is optically active when the reaction is done at low temperatures but optically inactive above +80 °C suggesting that racemization of the cyclic allene is competitive with cycloaddition and the inversion barrier is low. at low temperatures « 3" . The adduct from reaction of 1 ,2-cycloheptadiene and diphenylisobenzofuran remained optically active under similar reaction conditions. The allene subunit in this larger ring has maintained its orthogonality. We have now extended our studies from the free strained cyclic allenes to the transition metal complexed strained cyclic allenes. We are interested in the effect the transition metal has on the allenic unit, in particular with
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regard to the isomerization of the allene unit from its linear perpendicular form to its bent planar isomer. EHMO calculations predict that the energy gap separating the linear perpendicular allene from its bent planar isomer is lessened considerably when the allene is complexed onto the (Cp)Fe(CO)„ [Fp] cationic fragment. Here lies the impetus to study Fp complexed allenes. 1 ,2-Propadiene lies about 80 kcal/mole below that of its bent planar isomer according to EHMO. The relative energy between 1 , 2-propadiene and its bent planar isomer is only 2 5 kcal/mole when both are bound to the Fp fragment. Fp + t 80 kcal/mole FP + 25 kcal/mole H= Similarly, Fp bound 1 ,2-cycloheptadiene (4) is calculated to be only 17 kcal/mole below its bent planar form (5) . Furthermore, EHMO predicts the Fp bound bent planar 1 , 2-cyclohexadiene (7) is actually 14 kcal/mole more stable than the allene form (6).
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^< F P + A 1 4 kcal/mole Although energies derived from EHMO should not be accepted as absolute values, one does see a trend favoring a bent planar allenic ground state as one proceeds towards Fp complexed allene with the allene incorporated into a smaller ring. This decrease in relative energy between the linear perpendicular allene and its bent planar isomer is largely due to the greater stabilizing interaction between the Fp-LUMO and the bent planar allenic HOMO. We decided to choose the Fp complexed 1 , 2-cycloheptadiene as the starting point of our work. Transition metal complexes of acyclic allenes are well 12 known and have been reviewed extensively. However cyclic counterparts are limited to Fp complexes of 1 , 2-cyclononadiene 13 (8) and 1 , 2-cycloheptadiene 14 (4), (PPh 3 ) 2 Pt(0) 1 5 complexes of 1 , 2-cyclononadiene (9), 1 , 2-cyclooctadiene
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(10) and 1,2-cycloheptadiene (11) and (Am) (CI) 2 Pt (II) complex of 1 , 2-cyclononadiene (12). Pt(0) Pt(n) The bonding between the metal fragment and the allene 17 can be described using the Dewar-Chatt-Duncanson model. The bond is formed by the interaction of the HOMO of one of the allene double bonds with an empty acceptor on the metal (13) . The filled metal d-orbital in turn back bonds with the LUMO of the same double bond (14) . H £ o o M H H LUMO HOMO 13 o n a o" H H LUMO 14 O HOMO
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10 This back bonding causes substituents on the complexed double bond to bend away from the metal, the degree of which is directly related to the extent of this back bonding. The metal to olefin pi-bond is often looked upon as a metallo1 8 cyclopropane. The metal is not positioned symmetrically about the C. and C_ carbons of the allene. The metal is closer to the Ccarbon because of its greater s character compared to the C. carbon (15) . H / M Allene complexes of metals, like olefin complexes of metals, rotate about the metal to olefin pi-bond. In addition to this rotational motion, allene complexes of most metals also exhibit fluxional behavior whereby the complexed metal fragment moves from one double bond of the allene to the adjacent double bond. Such fluxional behavior of acyclic allenes complexed to Fp was studied extensively by 13 Rosenblum et al. and was shown to be nondissociative (i.e. intramolecular) .
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11 Fp + 1 1.2 -Fp shift s \ -j — . 'II'"' ' ^ Unlike the Fp complexed allenes, not all platinum complexed allenes are fluxional. The Pt(0) complex of 19 1,3-diphenylpropadiene (16) is not fluxional. H and H, remain distinct in the H NMR even at high temperatures. Ph C H-n C £ PPh 3 PPh 3 Ph H b 16 The Pt(II) complex of tetramethylallene however is fluxional with the metal moving back and forth between the two allenic double bonds. 1,2-Fp shifts do not involve a bent planar intermediate. Optical rotation of an optically active 1,2-cyclo13 nonadiene complexed onto Fp remained unchanged up to
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12 +80 °C. A racemate would result if a bent planar intermediate were involved. In contrast, the optical activity of the Pt(II) complex of 1 ,2-cyclononadiene (12) diminishes with time. The 1 , 2-cyclononadiene is believed to racemize via a h -allyl Pt intermediate (18) and not the bent planar Pt(Am)(CI)2 Cl allene 16 (17) tJib. Pt(Am)(CI) 2 Pt(Am)(CI) 1,2-Cycloheptadiene complexed to Fp has a measured fluxional barrier of 13.9 kcal/mole and is the lowest 14 barrier observed for Fp allene complexes. Fluxional 2 2 barriers for h -methylallene , h -1 , 1-dimethylallene and 2 h -tetramethylallene Fp cationic complexes are 23.1 kcal/mole, 18.0 kcal/mole and 16.3 kcal/mole, respectively. h -1, 2-Cyclononadiene Fp has a fluxional barrier of 16.9 kcal/mole. Although the decrease in fluxional barrier with increasing methyl substitution is probably steric in origin, one could not help but to wonder if a bent planar allene intermediate were involved. The bent planar allene is an allyl cation and should be favored by electron donating methyl substituents. Moreover, if one assumes that the steric environment immediate to the Fp center is the same for 1 ,3-dimethylpropadiene, 1 , 2-cyclononadiene and
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13 1,2-cycloheptadiene, bending and twisting the linear perpendicular allenic unit toward its bent planar form by tying it into a ring lowers the fluxional barrier. Therefore, it was not known at the time it was reported whether the fluxional 2 barrier of 13.9 kcal/mole for h -1,2-cycloheptadiene Fp (4) represented a simple 1,2-Fp shift or that an allyl cation intermediate (5) was involved. -^ allene to allyl cation X-ray crystal data is available for many allene com21 plexes of Rh, Pt and Pd. In all cases the complexed double bond is longer than the uncomplexed double bond. The metal is unsymmetrically positioned about the bound double bond and is further away from the terminal carbon than the central carbon. The C,-C 2 ~C^ bond angle falls within the range of 158° to 142°. The complexed allenes are not planar. Allene complexes of transition metal dimers (19) where each of the allene double bonds is bound to a different metal in the dimer are also known. The C i~ c 2 ~ C 3 angles of such complexes are, on the average, much 22 smaller.
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14 LnM ML r 19 The only X-ray structure of an iron allene complex is the 2 23 h -tetramethylallene Fp. The complexed allene is oriented parallel to the cyclopentadienyl ring as expected. The bond o length of the uncomplexed double bond is 1.33 5A and the o complexed double bond is 1.367A. The complexed double bond is longer as a result of back bonding from the Fp to the pi* of the allene double bond. The iron is asymmetrically placed about the double bond and is closer to the central o o carbon (2.063A) than the outer carbon (2.237A). The C 1 -C 2 -C 3 bond angle is 145.7° but the allene is still orthogonal. There have been no X-ray data reported on any metal complexed cyclic allenes to date although such compounds are well known. It would be interesting to compare the X-ray 2 2 structure of h -1 , 2-cycloheptadiene Fp with h -tetramethylallene Fp. Of particular importance is how the ring affects the allene C.-C 2 -C.. bond angle and how far the allene is away from orthogonality. The objective of this study was to determine whether a 1,2-Fp shift between the double bond of the cyclic allene
PAGE 28
15 involves a bent planar intermediate and, if not, to determine if the latter is accessible. It was also to provide the first X-ray structure of a metal complexed cyclic allene.
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CHAPTER II DICARBONYL (h 5 -CYCLOPENTADIENYL) IRON (II) COMPLEXED 1 , 2-CYCLOHEPTADIENNE The h(7-methoxy) cyclohepten-1-yl Fp was synthesized in moderate yields by reacting the 1-lithio7-methoxycycloheptene with either Fp chloride or Fp bromide. Treating a methylene chloride solution of the h (7-methoxy) eye lohepten-1-yl Fp with trimethylsilyl trifluoromethanesulfonate followed by precipitation with diethyl ether yielded the 2 desired h -1 , 2-cycloheptadiene Fp cation complex (4a) as an air and thermally sensitive yellow solid. Attempts to synthesize the h (7-methoxy) cyclohepten-1-yl Fp by this route failed with Fp iodide. It is critical that the l-bromo-7-methoxycycloheptene be in a slight excess relative to n-butyllithium when generating the l-lithio-7methoxycycloheptene in order to ensure a maximum yield of the h (7-methoxy) cyclohepten-1-yl Fp. Synthesis 16
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17 of h (7-methoxy) cyclohepten-1-yl Fp by this method was far superior to a longer and lower yield route used previously. The yellow colored methylene chloride solution of the h -1 ,2-cycloheptadiene Fp trif luoromethanesulfonate (4a) turned red within 30 mins at room temperature. In contrast, the h -1 ,2-cyclononadiene Fp tetraf luoroborate (8) is not only air stable as a solid but a methylene chloride solution of it is also stable for over 14 hrs at +83°C. Although the 2 extreme thermal instability of the h -1 , 2-cycloheptadiene Fp (4a) in methylene chloride is an inconvenience, it may signal a different pathway for the fluxional process; a 2 pathway that is different from that of the h -1,2-cyclononadiene Fp (8) and perhaps involves the bent planar allene (allyl cation) intermediate from which decomposition may occur. We will now refer to (4) as the allene form and (5) as the allyl cationic form. Fp H NMR was used to follow the decomposition of the h 2 -l, 2-cycloheptadiene Fp (4a) in a methylene chloride solution at +40 °C. Decomposition was complete within 3 hrs and Fp trif luoromethanesulfonate and 1 , 3-cycloheptadiene
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were the only major decomposition products observed. The former was identified by comparing its H NMR with that of Fp trifluoromethanesulfonate synthesized independently from Fp chloride and silver trifluoromethanesulfonate and the latter with an authentic sample of 1 , 3-cycloheptadiene. The decomposition mixture was red in color. The result of this 14 experiment is in agreement with that by Manganiello Neither cycloheptene nor the 1 , 2-cycloheptadiene dimer were observed in the H NMR. According to the H NMR, the decomposition of the 2 h-1, 2-cycloheptadiene Fp (4a) to Fp trifluoromethanesulfonate and 1 , 3-cycloheptadiene proceeded via a short lived, thermally unstable intermediate. This intermediate had a singlet at 5.58 ppm, Fig. 1. Its concentration increased rapidly up to a certain point and then decreased to the baseline with a concurrent increase in the concentration of both Fp trifluoromethanesulfonate and 1, 3-cycloheptadiene, Fig. 2. Based upon this observation, the thermally labile intermediate was suspected to be the h-1, 3-cycloheptadiene Fp cation complex. The singlet at 5.58 ppm is certainly within the range for cyclopentadienyl hydrogen resonances of cationic Fp-olefin complexes. The h-1, 3-cycloheptadiene Fp could not be synthesized via a common thermal exchange reaction between 1 , 3-cycloheptadiene 2 24 and the h -propene Fp cation, which lends further evidence for its thermal lability.
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19 0") o a. E O U
PAGE 33
20 .H
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21 A reasonable mechanism for the decomposition of the 2 h -1,2-cycloheptadiene Fp (4a) is as follows. We believe that an initial deprotonation of an appropriately situated hydrogen occurs from either the allene (4) or allyl cation intermediate (5) to form the h -1,3-cycloheptadien-2-yl Fp and trif luomethanesulfonate acid. In the presence of trifluomethanesulfonic acid, this h -1,3-cycloheptadien-2-yl Fp is then converted via the conjugated 2 carbene (20) to the h -1 ,3-cycloheptadiene Fp. Under the experimental conditions it dissociates to the Fp cation and 1 , 3-cycloheptadiene . The decomposition reaction would have stopped after the first step, in the absence of trifluomethanesulfonic acid, giving h -1 , 3-cycloheptadien-2-yl Fp as the only product. 2 In order to test the validity of this mechanism, the h -1,2-
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22 cycloheptadiene Fp (4a) was subjected to the same decomposition conditions but in a slurry of lithium or sodium carbonate to neutralize the trif luoromethanesulfonic acid. The decomposition mixture did not turn red but remained brown in color. The H NMR cyclopentadienyl resonance of this brown substance falls within the range typical for most h -alkyl Fp. Presumably the h -1 , 3-cyclopentadien-2-yl Fp was formed, but we were unable to purify it sufficiently for a positive identification. 1 , 3-Cycloheptadiene and Fp trifluoromethanesulfonate were no longer the decomposition products. Counter ions have been known to affect the way an organometallic species behaves in solution, a recent example 2 being the intramolecular rearrangement of the h -3-bromoi 25 propene Fp complex. Fp + Fp + D H i H D. . H H Br Br The 1,3 shift of a bromine atom exo to the Fp-olefin bond was greatly accelerated when the counter ion was changed from trifluoromethanesulfonate to hexaf luorophosphate . Ion pair association in the salt may have been responsible for such behavior. Caseyet al. has also reported that a solution of the tetraf luoroborate salt of the (Cp)Fe(CO) (PPh-.) isopropylidene complex is more stable than its trifluoromethanesulfonate salt.
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£ // Fe ^ c CH3 C0 / / i Ph 3 P I CH 3 Since we believe that the basicity of the trifluoromethanesulfonate anion is responsible for the decomposition 2 of the h -1,2-cycloheptadiene Fp (4a), we thought it would be desirable to replace it with a non-basic anion. The tetraphenylborate anion was successfully exchanged for the trifluoromethanesulfonate anion in a cold (-3 5°C) methanol solution. We were disappointed that a methylene chloride 2 solution of the tetraphenylborate salt of h -1,2-cycloheptadiene Fp (4) was also thermally unstable and decomposed within 30 mins at +50°C to several unidentifiable pro27 ducts. 2 The h -1 ,2-cyclononadiene Fp tetraf luoroborate (8) is the only known stable cyclic allene of iron. If by chance the thermal stability of (8) were due to the tetrafluoroborate anion, this would suggest that the tetrafluoroborate anion may be the counterion of choice for the 2 h -1,2-cycloheptadiene Fp (4). 2 The h -1,2-cycloheptadiene Fp tetraf luoroborate (4b) was synthesized in good yields by reacting the h -(7methoxy) cyclohepten-1-yl Fp with trimethyloxonium tetraf luoroborate.
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24 Fp + + (CH 3 ) 3 OBF 4 _> / \ BF 4 4b 2 The yellow solid h -1 , 2-cycloheptadiene Fp (4b) turns red when exposed to air, but a room temperature methylene chloride solution of it is stable for over 3 days. The same solution is also stable for over 2 hrs at +40°C. Thus it would appear that the thermal decomposition of the h-1, 2-cycloheptadiene Fp (4a) in methylene chloride is due to the basicity of the trif luoromethanesulfonate anion. The tetrafluoroborate and the hexaf luorophosphate salt of h-1, 2-cycloheptadiene Fp (4) may also be synthesized by reacting the h (7-methoxy) cycloheptadien-1-yl Fp with triphenylcarbenium tetrafluoroborate and hexaf luorophosphate, respectively. This procedure has a drawback in that the product is sometimes contaminated with the unreacted triphenylcarbenium cation. FP Fp + + Ph 3 CBF 4 /PF 6 — > C ) BF4/PF 6 " 4b/c
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25 2 We now have on hand the stable h •»!', 2-cycloheptadiene 2 Fp (4b) and (4c). Both h-1, 2-cycloheptadiene Fp (4b) and (4c) react with alcohols to give the same ether adducts as 2 are given by the h -1, 2-cycloheptadiene Fp (4a). It was interesting to note that their fluxional barriers were unaffected by the counterions. The h-1, 2-cycloheptadiene Fp (4) has a fluxional barrier of 13.9 kcal/mole and is the lowest barrier yet 2 measured when compared with other h -Fp complexed acyclic and cyclic allenes. Does this low fluxional barrier signal a new mechanism for the fluxional process, perhaps via the bent planar allene (allyl cation) intermediate (5) , Scheme I? Scheme I Fp A Fp + o 4 Or perhaps the fluxional process merely involves a simple intramolecular 1,2-Fp shift from one allenic double bond to the other, Scheme II, passing through the allene intermediate (21)? The low fluxional barrier merely reflects in some manner the effect of ring strain in the smaller cyclic allene. Such a simple intramolecular 1,2-Fp shift was 2 demonstrated conclusively for the h -1 , 2-cyclononadiene Fp (8).
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26 Scheme II The allene (4) to allyl cation (5) valence isomerization for Fp complexes is unprecedented. The racemization of optically active 1 , 2-cyclononadiene complexed onto Pt(II) (12) was initially proposed to proceed via an allyl species (17) . Pt(Am)(CI) 2 Pt(Am)(CI) 2 Pt(Am)(CI) 2 This allyl species (17) , with two electrons in its pi-system, would be formally equivalent to an allyl cation. 2 It is now accepted that a reversible h -pi-allene (12) •3 to h -pi-allyl (18) isomerization, whereby a chloride is transferred from the Pt(II) to the pi-allyl, is responsible for the racemization.
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27 CI Pt(Am)(CI) I R(Am)(CI) Pt(Am)(CI) 2 2 3 .... Such a h -pi-allene to h -pi-allyl isomerization in conjunction with a ligand transfer from the metal to the pi-allyl, 2 although rare for h -allene Pt(II) complexes, is quite 2 2 8 common among its cogener, the h -allene Pd complexes. In 2 the case of the h -1 ,2-cycloheptadiene Fp (4), no such ligand is available on the Fp for an analogous transfer to the 1,2-cycloheptadiene ligand. This precludes such a mechanism for an allene to allyl isomerization. 2 Numerous barriers to the fluxional behavior for h allene complexes of Pt(II) and Fe(II) have been measured. 2 Fluxional barriers for h -tetramethylallene complexes of Pt(II) are generally low and lie between 7 to 10 kcal/ mole. 20 ' 29 The h 2 -l , 1-dimethylallene complex of Pt(II) is not fluxional. The Pt(II) remained bonded to the C2-C3 olefinic bond, the less hindered bond. The Fp in the 2 . . 13 h-1, 1-dimethylallene Fp also prefers the distal position. This preference for the distal position is presumed to be steric in origin. Fluxional behavior has not been reported oo 2 for the h -allene, h -tetramethylallene and h -1 , 3-diphenyl19 30 allene Pt(0) complexes. ' They are presumed to be
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28 9 16 dynamically rigid. The h -1 , 2-cyclononadiene Pt(0) is also not fluxional at room temperature. 2 For comparison with the h -1 , 2-cycloheptadiene Fp (4), 2 2 the fluxional barriers of h -1-methylallene , h -1,32 2 dimethylallene, h -tetramethylallene and h -1, 2-cyclononadiene (8) Fp complexes are 23.1 kcal/mole, 18.0 kcal/ mole, 16.3 kcal/mole and 16.9 kcal/mole, respectively. It has been well established that the fluxional process for these complexes occurs by a concerted 1,2-Fp shift between the two allenic double bonds and not via an allyl cation. The fluxional barriers decrease gradually with increasing methyl substitution on the allene and are attributed to a steric factor rather than the stabilization of an allyl cationic intermediate by the methyl substituents. The steric bulk of the methyl substituents increases the lability of the Fp-olefin pi-bond making it easier for the Fp to move between the two adjacent double bonds. 2 The h -1 , 2-cyclononadiene Fp (8) has a fluxional barrier of 16.9 kcal/mole. In this case it was conclusively demonstrated that the fluxional process did not proceed via an allyl cationic species (22) . When a solution of the 2 optically active h -1 , 2-cyclononadiene Fp (8) was heated to the point of rapid f luxionality , reisolation gave the allene 13 complex without any loss in optical activity. The 2 optically active h -1 , 2-cyclononadiene Fp (8) would have racemized if the achiral allyl cation (22) were involved in
PAGE 42
29 the fluxional process either as an intermediate or as a transition state. 22 At first thought one would not expect the h -1,2-cycloheptadiene Fp (4) fluxional barrier to be much different 2 from that of the h -1 , 2-cyclononadiene Fp (8) if both proceeded by the same mechanism. In fact, we would expect 2 the fluxional barrier for the h -1 , 2-cycloheptadiene Fp (4) to be higher should the Fp-olefin pi-bond strength parallel that of the Pt(0) -olefin pi-bond. The coordinating ability of cyclic allenes onto Pt(PPh 3 ) 2 was reported to increase with decreasing ring size. This is due to the greater release of ring strain upon coordination. The low fluxional 2 barrier for the h -1 , 2-cycloheptadiene Fp (4) coupled with the strained ring therefore causes one to wonder if the fluxionality occurs via a mechanism involving an allyl cationic intermediate (5). To test for this possibility, a temperature dependent C NMR study of h-1, 2-cycloheptadiene Fp (4b) was undertaken. 1 ,2-Cycloheptadiene is chiral and when complexed to
PAGE 43
30 Fp causes the carbonyl ligands on the Fp to become diastereotopic and be distinguishable. The two carbonyl [CO] resonances appear at 207.2 ppm and 210.2 ppm. A simple intramolecular 1,2-Fp shift would render the CO and CO^ unchanged (23) . In contrast an allene to allyl cation isomerization would cause the carbonyls to become equivalent and coalesce to a single resonance (24) . 23 As the temperature of a nitromethane-dsolution of 2 h -1,2-cycloheptadiene Fp (4b) was raised, the two carbonyl resonances remained unchanged. The allenic carbons C. and C. however became equivalent at +30 °C. The allenic carbon C„ remained unchanged, Fig. 3. Taken together, it means 2 that the rapid fluxional motion of the h -1,2-cycloheptadiene Fp (4) at +30°C does not involve an allyl cationic intermediate because under this condition, C. and Cbecome equivalent whereas the two carbonyls remain distinct. If an allyl cationic intermediate (24) were involved in the fluxional process at +30 °C, this would correspond to a free energy of activation of 14.7 kcal/mole. We can safely say
PAGE 44
31 id u p U (U e
PAGE 45
32 that the fluxional barrier of 13.9 kcal/mole as determined by H NMR corresponds to a simple intramolecular 1,2-Fp shift, Scheme II, and that the allene to allyl cation isomerization, Scheme I, requires an energy of greater than 14.7 kcal/mole. It is reasonable to expect the allene to allyl cation isomerization to be the higher energy process of the two. This isomerization requires bonding and structural changes and a transfer of a positive charge from the iron to the organic ligand, whereas a 1,2-Fp shift merely involves a repositioning of the Fp from one double bond to the other. It is inconvenient to study any dynamic process of organometallic compounds at high temperatures by H NMR due to their thermal instability. For example, when seeking NMR evidence for allyl cation formation, a nitromethane-d^ 2 solution of the h -1 , 2-cycloheptadiene Fp (4b) was heated to +60 °C and was found to decompose rapidly within the time 13 needed to acquire a C spectrum. A possible way to study the fluxionality of a thermally sensitive organometallic complex by NMR without raising the probe temperature is to use the spin saturation transfer 31 technique, S.S.T. The spin saturation technique involves finding a condition whereby saturating one of the spins of an exchanging two-spin system results in a partial saturation of the other spin. From this information and the T.. of both spins, the energy barrier for the exchange can be calculated. The
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33 advantage of this method was exemplified in the case for h -1,2-cycloheptadiene Fp (4a). The conditions required for obtaining a fluxional barrier from spin saturation transfer described above were met at -20°C. It was necessary to heat (4a) to +29°C before its fluxional barrier could be obtained by the coalescence method. 2 For the case of the h -1,2-cycloheptadiene Fp (4), if a simple 1,2-Fp shift were the only process occurring, irradiation of one of the carbonyl resonances would not affect the intensity of the other because they would remain distinct at all times. On the other hand, if an allyl cation were accessible, the carbonyl ligands would become equivalent and if the relaxation of the carbonyl carbon were slower than the fluxional process, irradiation of one of the carbonyl resonances would cause the other carbonyl resonance to diminish in intensity. From this information, it would be possible to calculate the barrier for the fluxional process while maintaining a reasonable probe temperature. Unfortunately, S.S.T. is impractical here for the two carbonyl resonances are too close to each other. We could not selectively irradiate one carbonyl without irradiating the other. Another possible way to detect the allyl cationic intermediate would be to trap it as it is formed. 2-Substituted allyl cations (25) are known to react with 1,3-dienes.
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34 25 1 Y=0",0R,0M,NR2,R,etc. Extensive reviews have been written about the cycloaddition of such allyl cations (25) , the most recent one by 3 2 Hoffmann. These reactions have been exploited in organic synthesis of biand tricyclic compounds. Several possible products can result from the cycloaddition of allyl cations (25) to 1,3 dienes depending upon whether the reaction is concerted or stepwise and upon the nature of Y. The reader 32 is best referred to the review article by Hoffmann for all the possible products from concerted and stepwise cycloadditions. The pi-donating strength of a Fp in the homocyclooctatrienylidene Fp was found to be similar to that of a methoxy 3 3 group. This would make the 2-Fp substituted allyl cation a good candidate to be trapped by 1,3-dienes. There is however no literature precedent that such a reaction will occur. In fact, allyl cations with Fp substitution at C 2 are not even known to exist at this point. The cycloaddition products expected from cyclopentadiene and furan with the h -allyl Fp cation (5) by either the concerted or stepwise addition are shown in Scheme III and IV, respectively.
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Scheme III Concerted addition 35 Z + Fp > Z=0,CH 2 5 Fp + 26 Scheme IV Stepwise addition
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36 We do not expect to isolate the carbene adduct (26) from the concerted cycloaddition of the h -allyl Fp cation (5) with the 1,3-diene. A rapid 1,2-alkyl shift would probably occur 2 to give the corresponding h -olefin Fp complexes, (27) and (28) . Fp Such a 1,2-alkyl shift to a Fp-carbene carbon is well 34 precedented in the literature. A methylene chloride solution of cyclopentadiene or 2 furan and the h -1 ,2-cycloheptadiene Fp (4b) was stirred for a day. In no case was there any reaction and (4b) was recovered unchanged. However this does not mean that the allyl cation (5) is not accessible. The allyl cation (5) may have formed but did not react or the concentration of the allyl cation (5) may have been so small that the rate of reaction was negligible. In fact, the allyl cation (5) may possibly be a transition state and not an intermediate in the isomerization pathway.
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37 A methylene chloride solution of tetramethylethylene 2 was also treated with the h -1 , 2-cycloheptadiene Fp (4b) at 2 room temperature hoping to obtain the h -olefin Fp complexes (29) . Again (4) was recovered unchanged. 14 In a much earlier study, Manganiello discovered that the thermal decomposition of a methylene chloride solution 2 of the trifluoromethanesulfonate salt of h -1, 2-cycloheptadiene Fp (4) followed by treatment of the solution with sodium iodide/acetone yielded 1 ,3-cycloheptadiene (40%) and a trace amount of cycloheptene. The mechanism in Scheme V was proposed to account for the decomposition reaction based on the products observed.
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Scheme V 38 Fp F P + \k NaT \|/ Fp Fp + H + NJ/ Nal \y + Fpl Fpl
PAGE 52
39 In this scheme, proton loss from the h -allyl Fp cation (5) is responsible for the formation of the 1 , 3-cycloheptadiene, whereas a hydride abstraction gives the cycloheptene. It was argued that if a hydride source were available to the allyl cation (5) then the amount of the cycloheptene formed relative to 1 ,3-cycloheptadiene would increase. Indeed, when triphenylmethane (a hydride source) was added to a 2 methylene chloride solution of the h -1 , 2-cycloheptadiene Fp (4a) , cycloheptene was isolated in amounts up to 3/5 of that of 1 ,3-cycloheptadiene after workup. The above experiment was repeated using the tetra2 fluoroborate salt of the h -1 ,2-cycloheptadiene Fp (4). Neither cycloheptene nor 1 ,3-cycloheptadiene were observed 2 but the h -1 , 2-cycloheptadiene Fp (4b) was reisolated unchanged. There was no reaction between (4b) and triphenylmethane. This cast doubts on the previously proposed mechanism involving the intermediacy of the allyl cation (5). In principle, clear evidence for the accessibility of the allyl cation (5) may be obtained by using an optically active cyclic allene as the ligand. As can be seen from Scheme I, if the allyl cation (5) is accessible, whether as an intermediate or a transition state, an optically active complex must racemize. One approach to the synthesis of an 2 optically active h -1 , 2-cycloheptadiene Fp (4) is to dis2 place isobutylene from h -isobutylene Fp with an optically active 1 , 2-cycloheptadiene in much the same way as was used
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40 in the preparation of optically active h -1 , 2-cyclononadiene Fp (8). Unfortunately, unlike 1 , 2-cyclononadiene , 1,2cycloheptadiene dimerizes too rapidly to displace the isobutylene. Alternative methods were therefore sought. In the 2 first approach, racemic h -1 , 2-cycloheptadiene Fp (11) could be treated with less than an equivalent of an optically active alcohol in order to selectively remove one the allene enantiomers. This procedure would only be effective if the racemization of (4) via the allyl cation (5) did not occur or occurred at a very slow rate under the conditions for enantiomeric enrichment. 2-Methylbutanol was tried first. When the racemic 2 alcohol was allowed to react with a suspension of h -1,2cycloheptadiene Fp (4) in diethyl ether, it gave the ether adduct (30) . 30 Interestingly, although the ether adduct (30) has two chiral centers as marked, and therefore should exist as a pair of diastereomers , only one set of signals was observed in its H NMR (we have insufficient material for a C NMR spectrum) . It is quite unfortunate that the diastereomers
PAGE 54
41 have coincidental chemical shifts. One would expect that the reaction between an optically active alcohol with a 2 racemic h -1 , 2-cycloheptadiene Fp (4) should give unequal amounts of the diastereomeric ether adduct (30) provided the reaction is incomplete. If the chemical shifts of the diastereomeric ether adduct (30) were non-coincidental, NMR measurement of the diastereomeric composition of the ether adduct (30) would enable us to draw a conclusion as to the 2 enantiomeric purity of the h -1 , 2-cycloheptadiene Fp (4) left behind. Half an equivalent of the (S) (-) -2-methylbutanol was 2 allowed to react with the racemic h -1 ,2-cycloheptadiene Fp (4b) . The unreacted (4b) recovered, which amounted to a half of the starting material, showed no detectable optical rotation. The chiral carbon on (S) (-) -2-Methylbutanol is one carbon away from the alcohol group and may be too far removed for it to effectively induce asymmetry. We decided to try (-) -menthol instead. Under the same conditions, (-) -menthol did not give the ether adduct but a mixture of unidentifiable products and menthol. Fp + HO Men Men
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42 2 Less than half of the starting h -1 , 2-cycloheptadiene Fp 2 (4b) was recovered. A solution of the recovered h -1,2cycloheptadiene Fp (4b) showed no detectable optical 2 rotation. A reaction between h -1 , 2-cycloheptadiene Fp (4b) and potassium (-) -menthoxide in THF at 0°C resulted in total decomposition of the allene (4b). 2 Zero optical rotation of the recovered h -1, 2-cycloheptadiene Fp (4b) from the above reactions is consistent with an allene to allyl cation isomerization, Scheme I. However, absence of rotation can also be due to other reasons. First, enantiomeric enrichment by (S)-(-)-2methylbutanol and (-) -menthol might not be successful. Even if the enantiomeric enrichment were successful, the 2 recovered h -1 , 2-cycloheptadiene Fp (4b) might have such low inherent optical rotation that it could not be measured with certainty. Unfortunately, it is not possible to increase 2 the concentration of the solution of h -1 , 2-cycloheptadiene Fp (4b) in order to get a measurable optical rotation because the solution which is highly colored absorbs so much light that rotation measurements become quite impossible. In the second approach, we attempted to synthesize the 2 optically active h -1 , 2-cycloheptadiene Fp (4) directly via methoxy abstraction with trimethylsilyl trif luoromethanesulfonate starting from an optically active h (7-methoxy) cyclohepten-1-yl Fp.
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43 Let us first consider the conformation of the R and S enantiomers of the h ( 7-methoxy ) cyclohepten-1-yl Fp as obtained from CPK space filling models. H Fp OCH 3 Rg or F P OCH3 Re CH3O H '\\ ^ Fp OCH3 Se Fp H sg Of the two conformers available to the h -( (R) -7-methoxy) cycloheptadien-1-yl Fp (R) we would expect the gauche conformer (Rg) to be the preferred conformer. Here, the
PAGE 57
44 larger methoxy groups is away from the bulky Fp. Similarly, we expect the (Sg) to be the preferred conformer for the 2 h ( (S) -7-methoxy) cyclohepten-1-yl Fp (S) . We shall limit our discussion on methoxy abstraction to one enantiomer, h -( (R) -7-methoxy) cyclohepten-1-yl Fp (R) . 2 Cationic h -olefin Fp complexes are typically made by a 1 3 5 6-hydride abstraction from the h -alkyl Fp. Such 3-hydride abstractions are conformationally dependant and widely accepted to proceed via an antiperiplanar transition state with the Fp assisting from the anti face in concert with the hydride loss. Methoxy abstraction, like hydride abstraction, is also believed to proceed with anti Fp assistance leading to the 2 h -olefin Fp complex. A concerted methoxy abstraction with 2 anti Fp assistance will give an optically active h -olefin Fp complex from an optically active h -alkyl Fp. When the 1 methoxy group is part of a ring as in the case with h (7-methoxy) cyclohepten-1-yl Fp, it is prevented by the ring from adopting a conformation where it is anti to the Fp. An anti alignment of the Fp with the methoxy group would necessarily force the methoxy group inside the cycloheptene ring, a conformation which is impossibly strained.
PAGE 58
45 Since a concerted anti methoxy abstraction is unlikely, a concerted syn methoxy abstraction with the Fp moving over to the same face as the leaving methoxy group will also give 2 an optically active h -olefin Fp complex from an optically active h -alkyl Fp. Thus by a concerted mechanism with syn * 1 Fp assistance , the h ( (R) -7-methoxy) cyclohepten-1-yl Fp 2 (R) should give only the h (S) -1 , 2-cycloheptadiene Fp (4S), Scheme VI. Although syn Fp assisted 0-hydride and 6-methoxy abstractions have not been substantiated, there are examples 2 where an h -olefin Fp complex is formed by an apparent syn 36 Fp assistance. The discussion is still valid should for any reason methoxy abstraction involve anti Fp assistance; thus R would give 4R.
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46 Fp + FP + There are no hydrogens anti to the Fp in h -cyclobutyl and h -cyclopentyl Fp, and yet they undergo hydride abstraction 2 to give their respective h -cycloalkene Fp complexes. It is not known whether hydride abstraction occurs via a distorted transition state, a B-carbonium cation or by some other mechanism. If Fp migration is not concerted with methoxy abstraction, an allyl cation intermediate (5) would be formed and 2 should give the racemic h -1 , 2-cycloheptadiene Fp (4), Scheme VII.
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47 Scheme VII H F .P + .H H Fp OCH 3 Rg In this case, the h ( (R) -7-methoxy) cyclohepten-1-yl Fp (R) 2 would give both the (R) and the (S)-h -1 , 2-cycloheptadiene Fp, (4R) and (4S) . In short, it is possible to synthesize an optically 2 active h -1 , 2-cycloheptadiene Fp (4) from syn Fp assisted methoxy abstraction of optically active h (7-methoxy) cyclo2 hepten-1-yl Fp. An optically inactive h -1, 2-cycloheptadiene Fp (4) from this reaction would necessarily mean that
PAGE 61
48 methoxy abstraction proceeded via an allyl cation or that the barrier for isomerization between the allene (4) and the allyl cation (5) is very low. Optically active h ( 7-methoxy ) cyclohepten-1-yl Fp was prepared according to Scheme VIII. Scheme VIII COOH COOQui COOH OCH 3 JL ^ 0CH 3 J^ / 0CH 3 + Qui -> separate 00 IMI CICCCI 50% e.e 32 OCHThe racemic 7-methoxycycloheptenecarboxylic acid was resolved as its quinine salt via two recrystallizations from absolute ethanol. The acid was released from the quinine salt readily in aqueous acid and was found to have a negative optical rotation. The acid was subsequently converted to the acid chloride which was then reacted with potassium Fp to yield the h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32) . The H NMR of the cyclopentadienyl hydrogen
PAGE 62
49 resonances are well separated in a 10 mole % chloroform-cL solution of Eu(hfc)-. and have an integrated ratio of 25 to 75 which indicates a 50% excess of one enantiomer. Decarbonylation under photolytic conditions yielded h -(7methoxy) cyclohepten-1-yl Fp (N) . We presumed that the h (7-methoxy) cyclohepten-1-yl Fp (N) is optically active with the same optical purity as the h -carbonyl(7methoxy) cyclohepten-1-yl Fp (32). Eu(hfc) 3 does not separate the cyclopentadienyl and methoxy resonances of h (7-methoxy) cyclohepten-1-yl Fp (N) and a direct measurement is not possible. It is also very difficult to obtain optical rotations on these Fp complexes because their solutions are highly colored. A mixture weighted with the other enantiomer was prepared according to Scheme IX. Scheme IX OCH 3 (+) 20% e.e. 33 1)n-BuLi 2) FpCI "> OCH3 20% e.e. 32
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50 The 2-bromo-2-cycloheptenone was reduced to its alcohol (33) with a preformed LAH/quinine mixture. The alcohol (33) has a positive optical rotation and was obtained with a 20% enantiomeric excess as determined by Eu(hfc).,. Treating this mixture of alcohols (+33) with sodium hydride followed by methyl iodide gave the l-bromo-7-methoxycycloheptene. The h (7-methoxy) cyclohepten-1-yl Fp was obtained by the well established route. As mentioned above, the h -(7methoxy) cycloheptadien-1-yl Fp does not form a complex with Eu(hfc) 3 and must be converted to its h -carbonyl( 7-methoxy) cyclohepten-1-yl Fp (32) in order to determine its enantiomeric purity. The h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32) was prepared by stirring a methylene chloride solution of h (7-methoxy) cyclohepten-1-yl Fp in 10 mole % of ferrocenium tetrafluoroborate under 55 psi of CO gas. The reaction was complete within an hour giving a quantitative yield of the desired Fp-acyl complex (32) . The h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32), in a 10 mole % chloroform-d.. solution of Eu(hfc)_ has a cyclopentadienyl hydrogen integrated ratio of 60 to 40 giving us a 20% excess of the other enantiomer. We shall call this mixture (P) . It was comforting to know that this 20% enantiomeric excess was carried forward from the (+) -alcohol (33) . We infer from this information that the h (7-methoxy) cyclohepten-1-yl Fp (P) made was optically active (20% e.e.) and with an optical rotation opposite to that prepared by Scheme VIII.
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51 The carbonylation of the h (7-methoxy) cyclohepten-1-yl Fp to h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32) is of particular importance here. As far as we know, this is the first case of an unassisted CO insertion into a h -vinyl Fp bond to give a h -acyl Fp complex. All CO insertions into h -alkyl Fp or h -vinyl Fp complexes are assisted by phosphines under thermal conditions to give the corresponding h -acyl (Cp)Fe(CO) (PPh.J 37 complexes . FeR P*3 v ^ > 9 _/ \ R 0Cr CO 0C r. 3 p R = allyl, vinyl CO insertions catalyzed by oxidants, eg. Ce(IV), Ag(I), Cp 2 Fe , or Lewis acids have only been reported for h -alkyl (Cp)Fe(CO) (L) where L = PPhand P (OPh) and h 1 -vinyl 3 8 (Cp)Fe (CO) (P (OPh) 3 ) complexes; the former complexes carbonylate faster than the latter. To date, CO insertions into h (alkoxy) methylene Fp to give the corresponding acyls 39 have not been successful. O rrv II Fp — CH 2 — OR V<> Fp C — CH 2 OR
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52 We have also attempted without success to carbonylate the h (7-methoxy) cyclohepten-1-yl Fp with boron trifluoride etherate under 1 atm. of CO. The optically active h (7-methoxy) cyclohepten-1-yl Fp (N) and (P) were treated with trimethylsilyl trifluoro2 methanesulfonate to yield the h -1 , 2-cycloheptadiene Fp 2 (4a). A dilute solution of the h -1 ,2-cycloheptadiene Fp (4a) has a negligible optical rotation. When the concentration of (4a) was increased, the solution became strongly colored and made optical measurement impossible. Since rotational measurements were experimentally impossible, we decided that an alternative solution would be 2 to convert the h -1 , 2-cycloheptadiene Fp (4a) to its methyl ether adduct and carbonylate the ether adduct to the h carbonyl(7-methoxy) cyclohepten-1-yl Fp (32). OCH 3 The enantiomeric composition of the h carbonyl(7methoxy) cyclohepten-1-yl Fp (32) can then be determined by H NMR with Eu(hfc)-. and this will give us a clue as to the 2 enantiomeric composition of the h -1 , 2-cycloheptadiene Fp (4).
PAGE 66
53 To simplify the discussion, let us assume that the fluxional process proceeds only via the 1,2-Fp shift, Scheme II, and that the addition of methanol is stereospecific. 2 Anti attack of a nucleophile at a h -olefin Fp bond is 40 well documented in the literature. Anti attack of a 2 methanol onto the h (R) -1 , 2-cycloheptadiene Fp (4R) regardless of which double bond the Fp is bonded to should give the same h ( (R) -7-methoxy) cyclohepten-1-yl Fp (R) , Scheme X. Scheme X h. ?! jh
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54 The above statement is true because only two of the four sides of the allene double bonds are available for complexation with the Fp. If the Fp were bonded to the other two sides of the allene double bond, an impossible geometry which puts the Fp inside the ring (see 4R below) , the allene (4R) would add methanol in an anti fashion to give the (S) methyl ether adduct. CH3OH ^/FP + It should be noted that the anti face of the h -(R)1 ,2-cycloheptadiene Fp (4R) , where the methanol comes in, is only partially shrouded by the allene ring. A syn attack, however, would be hindered by the bulky Fp group. At first glance, eliminating the possibility of a syn attack by methanol would seem to violate the principle of 41 microscopic reversibility; especially when we suggest syn assistance as one of the ways methoxy abstraction can occur. Furthermore, by the principle, anti attack of methanol would necessarily require the 7-membered ring to adopt an impossible conformation of putting the methanol inside the ring. We must keep in mind that neither the methoxy abstraction nor the methanol addition reactions are reversible and that the methoxy group is leaving the
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55 h (7-methoxy) cyclohepten-1-yl Fp as a methyltrimethylsilyl ether and not as a methanol. Thus, raethoxy abstraction and addition is not bound by microscopic reversibility. Regardless of the pathway methoxy abstraction follows (syn or anti) , we should be able to arrive at the enantiomeric 2 composition of the h -1 , 2cycloheptadiene Fp (4) from the enantiomeric composition of the h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32) provided methanol addition is stereospecif ic. 2 The h -1 ,2-cycloheptadiene Fp (4a), from the reaction of the optically active h (7-methoxy) cyclohepten-1-yl Fp (N) with trimethylsilyl trif luoromethanesulfonate, was reacted with methanol and then carbonylated to give the h -carbonyl(7-methoxy) cyclohepten-1-yl Fp (32). The integrated cyclopentadienyl hydrogen intensities of h carbonyl(7-methoxy) cyclohepten-1-yl Fp (32) in a chloroform-d. solution containing Eu(hfc)., were equal, Fig. 4 and 5. At some point complete racemization had occurred. The optically active h (7-methoxy) cyclohepten-1-yl Fp (P) was put through the same series of reactions with the same results, Fig. 6 and 7. Again complete racemization had occurred. Based upon the experimental results it appeared that an allyl cation (5) had been formed at some point in the sequence of reactions. One possibility is the concerted 2 formation of the optically active h -1 ,2-cycloheptadiene Fp (4) which then racemizes via the allyl cation (5) . An
PAGE 69
56 c •H o A tfl T ^2, .H
PAGE 70
57 V \ X. a
PAGE 71
< C •H o A en T3
PAGE 72
59 T3
PAGE 73
60 alternative is allyl cation formation as methoxy is 2 abstracted followed by collapse to the h -1,2-cycloheptadiene Fp (4) . In either case we would have to invoke an allyl cation intermediate (5).
PAGE 74
CHAPTER III CARBONYL (h 5 -CYCLOPENTADIENYL) TRIPHENYLPHOSPHINEIRON ( II ) COMPLEXED 1,2-CYCLOHEPTADIENE Methoxy abstraction from optically active h -(72 methoxy) cyclohepten-1-yl Fp led to the racemic h -1,2-cycloheptadiene Fp (4) . Two mechanistic pathways were considered; one involving a concerted stereospecif ic methoxy 2 abstraction to the optically active h -1 , 2-cycloheptadiene Fp (4) followed by racemization via the achiral allyl cation intermediate (5) and the other involving the achiral allyl cation intermediate (5) from methoxy abstraction and prior to complexation by the Fp. In order to complete the stereochemical picture and to shed light on the methoxy abstraction and the racemization process, we shall turn our attention to the h and 2 5 h carbonyl(h -cyclopentadienyl) triphenylphosphineiron (II) [Fpp] complexes of 7-methoxycycloheptadien-l-yl (34) and 1 , 2-cycloheptadiene (35), respectively. 61
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62 There are quite a few advantages to moving from the Fp system to the Fpp system. The most obvious is that the triphenylphosphine ligand generally imparts additional stability to these type of complexes against exposure to air and therefore makes them easier to manipulate. More important to us is that the triphenylphosphine ligand introduces another chiral center, at the iron, in both the h (7-methoxy) cyclohepten-1-yl Fpp (34) and the 2 h -1 ,2-cycloheptadiene Fpp (35) complexes. The consequence of this is that both complexes exist as enantiomeric pairs of diastereomers. Diastereomers , unlike enantiomers, can often be separated by physical methods and their diastereomeric compositions determined directly by NMR spectroscopic measurements. Phosphinylation therefore provides us a means to separate h (7-methoxy) cyclohepten-1-yl Fpp (34) into its diastereomers. We felt that examination of the diastere2 omeric composition of h -1 , 2-cycloheptadiene Fpp (35) formed from different diastereomers of h (7-methoxy) cycloheptene1-yl Fpp (34) should enable us to elucidate the pathway by
PAGE 76
63 which methoxy abstraction occurs. Also, if h -1,2-cycloheptadiene Fpp (35) can be separated into its diastereomers, we can follow directly by NMR the allene to allyl cation isomerization because such isomerization necessarily converts one diastereomer to a mixture of both diastereomers. Lastly, since we were unable to grow crystals of the 2 h -1,2-cycloheptadiene Fp (4) suitable for X-ray studies, we would like to try to grow crystals of its Fpp analogue. Before we proceed any further, the iron center of the Fpp can be designated as either R or S according to the Cahn-Ingold-Prelog (CIP) system adapted to organotransition 42 metal complexes. The configurations of the isomers of i 2 h (7-methoxy)cycloheptadien-l-yl Fpp (34) and h -1,2-cycloheptadiene Fpp (35) according to the modified CIP system are shown below. OCH 3 3 POCH< and enantiomer SS and enantiomer RS
PAGE 77
64 oc 3 p \ S R and enantiomer RS R R and enantiomer SS note that the iron center with the R configuration in h -(7-methoxy)cyclohepten-l-yl Fpp (34) and h -1,2-cycloheptadiene Fpp (35) have opposite absolute configuration. h (7-Methoxy) cyclohepten-1-yl Fpp (34) was synthesized by photolyzing a mixture of the h (7-methoxy) cyclohepten1-yl Fp with excess triphenylphosphine in an equal mixture of n-pentane and benzene. + P03 hv Photolysis was conducted in a quartz photochemical reactor at room temperature under a stream of N~ to remove the CO gas evolved. The reaction proceeded rapidly and was complete within 2 mins without forming the bistriphenylphosphine adduct. h (7-Methoxy) cycloheptadien-1-yl Fpp (34) was purified via column chromatography and was obtained as an air stable red paste. Residual triphenylphosphine may
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65 be removed by filtration as its insoluble methyltriphenylphosphonium iodide salt after warming a n-pentane solution of the crude h (7-methoxy) cyclohepten-1-yl Fpp (34) with methyl iodide. The H NMR for h (7-methoxy) cyclohepten-1-yl Fpp (34) showed two sets of resonances, one for each diastereomer. The hydrogen resonances for the methoxy hydrogens and the ring hydrogens on carbons 2 and 7 are well separated. The cyclopentadienyl hydrogen resonances are barely separated from each other at 100 MHz and each is coupled to the phosphorus. Let us now consider first the methoxy abstraction of h (7-methoxy) cyclohepten-1-yl Fpp (34) within the context of a concerted stereospecific Fpp assisted pathway. Following the same lines of analysis as the Fp analogue, let us assume that the ring conformation necessary to place the methoxy group anti to the Fpp cannot be achieved and that any concerted stereospecific pathway for methoxy abstraction must necessarily proceed with syn Fpp assistance. Therefore, concerted stereospecific methoxy abstraction of a single diastereomer of h -(7-methoxy) cyclohepten-1-yl Fpp (34) should lead to a single diastereomer of h -1 , 2-cycloheptadiene Fpp (35), Scheme XI.
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Scheme XI . 66 OC 3 Pl Fe I H\^> ^/^^Nl OCHo O RR This argument still holds should for any reason methoxy abstraction proceed via anti Fpp assistance. The other alternative is for methoxy abstraction to proceed via a free allyl cation. The Fpp then collapses 2 onto the free allyl cation to give the h -1,2-cycloheptadiene Fpp (3 5) . In this case, methoxy abstraction from one diastereomer of h (7-methoxy) cyclohepten-1-yl Fpp (34) 2 should lead to a mixture of diastereomers of h -1,2-cycloheptadiene Fpp (35), Scheme XII.
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67 Scheme XII It is clear that in order to establish unequivocally the mechanism of methoxy abstraction, we need to separate h (7-methoxy) cyclohepten-1-yl FPP (34) into its two diastereomers. h (7-Methoxy)cyclohepten-l-yl Fpp is isolated as a red oil and its diastereomeric composition typically falls around 60:40. The diastereomeric composition was determined by H NMR intergration of the methoxy hydrogen resonances because the cyclopentadienyl hydrogen resonances were not well resolved. We first attempted the separation by recrystalization at -35°C from a variety of solvents; the Diastereomeric composition will always be presented as such a : b where (a) corresponds to the composition of the low field cyclopentadienyl hydrogen resonance and (b) corresponds to the high field resonance.
PAGE 81
68 solvents tried include pentane , hexane, hexane/benzene mixture and acetone/water mixture. In each case the diastereomeric composition of the crystals of h -(7methoxy) cyclohepten-1-yl Fpp (34) remained unchanged. The diastereomers were also not separated by TLC. An alternative approach to this problem was to synthesize the h -carbonyl(7-methoxy) cyclohepten-1-yl Fpp (36), which also exists as enantiomeric pairs of diastereomers, separate this complex into its diastereomers and then convert them back to the corresponding h (7-methoxy) cyclohepten-1-yl Fpp (34) . The h (carbonyl(7-methoxy) cyclohepten-1-yl Fpp (36) was readily synthesized by warming h (7-methoxy) cyclohepten-1-yl Fp (34) with triphenylphosphine in acetonitrile. OCH 3 ^/\ OCH3 Fe' re // x// I + p0 3 ~ > * 3 P 36 The acyl complex (36) is an air stable orange solid and is formed as a mixture of equal amounts of the two diastereomers. After numerous recrystalization attempts, we discovered that a 20:80 ratio of diastereomers of (36) could be obtained in one recrystalization from a 1:2 v/v mixture of ethyl acetate and n-pentane at +10°C with excellent recovery, Fig. 8. The mother liquor was concentrated to give the
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69 ZD c o •H -P H en o & o u u H I o
PAGE 83
70 other diastereomer in a 65:35 ratio. A second recrystalization led to further enrichment giving a 10:90 ratio of diastereomers of (36) . Attempts to decarbonylate the h -carbonyl(7methoxycyclohepten-1-yl Fpp by either of the standard photolytic or chemical methods were unsuccessful. Surprisingly, CO was not evolved when a benzene solution of the acyl complex (36) was photolyzed with a low pressure Hg lamp even though other phosphinylated iron acyl alkyl complexes have been reported not only to decarbonylate under photolytic conditions but also to retain high 43 stereospecificity at the iron. The acyl complex (36) was isolated with its diastereomeric composition unchanged. No reaction was observed when the acyl complex (36) was allowed to react with trimethylamine-N-oxide, chlorobis (triphenylphosphine) rhodium dimer or iodosobenzene. In all cases the acyl complex (36) was reisolated with its diastereomeric composition unchanged. One should note the tenacity of this acyl complex (36) against epimerization at the iron center. The acyl complex (36) gave predominantly decomposition products when refluxed in dioxane although a small amount of h (7-methoxy) cyclohepten-1-yl Fp was formed presumably via phosphine dissociation followed by migration of the organic ligand onto the iron. We finally succeeded in inducing decarbonylation in the acyl complex (36) by a combined photolysis and ultrasonication process. Unfortunately, the h (7-methoxy) cyclo-
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71 hepten-1-yl Fpp (34) isolated had epimerized to a 55:45 diastereomeric mixture. It was later discovered that eluting h (7-methoxy) cyclohepten-1-yl Fpp (34) from a 8" x 1" alumina column (neutral, grade II) with a 1:1 v/v n-pentane/benzene as the eluant afforded an enriched diastereomeric mixture of the complex (3 4) . The diastereomeric mixture of complexes (34) appeared on the column as a broad red band and was collected in two halves. Surprisingly, the first half and the second half of this red band both gave mixtures of h (7-methoxy) cyclohepten-1-yl Fpp (34) enriched in the same diastereomer (75:25 and 85:15, respectively), Fig. 9. The alumina apparently did not separate the mixture of diastereomers but selectively decomposed one of them. Selective decomposition of h (7-methoxy) cyclohepten-1-yl Fpp (34) with trimethylsilyl trifluoromethanesulfonate also afforded an enriched diastereomeric mixture of the complex but this time in a 10:90 ratio, Fig. 10. We shall postpone the discussion of the latter selective decomposition to an appropriate time. We have also attempted to synthesize other phosphinylated h (7-methoxy) cyclohepten-1-yl iron complexes but without much success. The respective phosphinylated complexes were not isolated when h (7-methoxy) cyclohepten-1-yl Fp was photolyzed in the presence of either trimethylphosphine or triphenylphosphite . When a cold (-78 °C) solution of l-lithio-7-methoxycycloheptene was treated with carbonyl (h -cyclopentadienyl) tri-n-butylphosphineiron (II)
PAGE 85
72 c o H p •H U) O 04 e o u C H 3s O •H
PAGE 86
73 en C •H o Cn •H Pn
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74 [Fpp, ] iodide, the major product isolated was unreacted Fpp, iodide and a minor product perhaps resulting from the attack of the vinyl lithium reagent on the cyclopentadienyl ring. Triphenylphosphite substituted h (7-methoxy) cyclohepten-1-yl iron complex was synthesized via a thermal exchange for triphenylphosphine in h (7-methoxy) cyclohepten-1-yl Fpp with an excess triphenylphosphite in refluxing THF. 37a Fe 0C 3 P (I \ P(O0) 3 -Ig-> „/'/ (0O) 3 P The equilibrium of this reaction lies strongly to the right despite the stronger nucleophilicity of the triphenylphosphine and is attributed to a steric effect. The triphenylphosphine ligand being very bulky is unable to displace the triphenylphosphite ligand once the latter is coordinated at the iron. We could only isolate this triphenylphosphite iron complex as a crude mixture containing free triphenylphosphine and triphenylphosphite. At this point, we have not succeeded in isolating h (7-methoxy) cyclohepten-1-yl Fpp (34) as a single
PAGE 88
75 diastereomer but we have developed the methodology which enables us to separate h (7-methoxy) cyclohepten-1-yl Fpp (34) from a 60:40 diastereomeric mixture to a mixture with a diastereomeric ratio of 85:15 and 10:90. Before we look at the methoxy abstraction process, it is imperative to examine the stereochemical integrity of h (7-methoxy) cyclohepten-1-yl Fpp (34). We discovered that h (7-methoxy) cyclohepten-1-yl Fpp (34) with diastereomeric compositions of 85:15 or 60:40 maintained their respective compositions indefinitely when frozen in the refrigerator. A chloroform-d 1 solution of h (7-methoxy) cyclohepten-1-yl Fpp (34) with a 85:15 diastereomeric composition however epimerizes very slowly as measured by H NMR but only at elevated temperatures. The diastereomeric composition of a 60:40 mixture of h (7-methoxy) cyclohepten-1-yl Fpp (34) remained unchanged by H NMR under the same conditions. Therefore, we feel it is safe to assume that h -(7methoxy) cyclohepten-1-yl Fpp (34) does not epimerize under the conditions for methoxy abstraction, i.e., short duration and low temperature. We tried a number of methods to effect methoxy abstraction. Trimethyloxonium tetraf luoroborate was used first instead of the trimethylsilyl trif luoromethanesulfonate because we anticipated that the trif luoromethanesulfonate 2 anion might affect the h -1 , 2-cycloheptadiene Fpp (35) in an adverse manner similar to what was observed with the
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76 2 1 h -1,2-cycloheptadiene Fp (4). Treating h -(7methoxy) cyclohepten-1-yl Fpp (34) with trimethyloxonium tetraf luoroborate in methylene chloride at -10°C for 7 hrs gave an impure green substance after the reaction mixture was quenched with diethyl ether. Discouraged by the ineffectiveness of trimethyloxonium tetraf luoroborate as a methoxy abstracting reagent, we went back to trimethysilyl trif luoromethanesulfonate. The reaction did not fair any better and a black intractable paste was obtained. The same reaction in pentane at 0°C gave a greenish yellow precipitate which dissolved when washed with diethyl ether. If the reaction in pentane was quenched with diethyl ether, again a black pasty residue was formed. We discovered, quite by accident, that if this black paste is dissolved in ethyl acetate, the trifluoro2 methanesulfonate salt of h -1,2-cycloheptadiene Fpp (35a) precipitates from the solution as a bright orange air stable solid. In fact, prior quenching with diethyl ether is unnecessary. Pentane was removed from the reaction mixture in vacuo to leave a dark greenish brown paste from which 2 h -1,2-cycloheptadiene Fpp (35a) was precipitated with ethyl acetate in the open air. The reaction was also run in an equal mixture of n-pentane and benzene at 0°C with similar results; the solvent mixture was removed in vacuo to leave a 2 residue from which h -1,2-cycloheptadiene Fpp (35) was precipitated with ethyl acetate.
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77 1 2 The H NMR of the h -1 , 2-cycloheptadiene Fpp (35a) is consistent with a pair of diastereomers , Fig. 11. The two cyclopentadienyl hydrogen resonances, each coupled to the phosphorus, and three of the four allenic hydrogen resonances are well separated. One allenic hydrogen resonance is obscured by the ring methylene resonances and is 2 confirmed by H NMR of the appropriately deuterated complex. The assigned structure was confirmed by X-ray (vide infra) . The trif luoromethanesulfonate anion was readily exchanged for the hexaf luorophosphate anion by adding water to a methanol solution of the trif luoromethansulfonate salt 2 of the h -1 , 2-cycloheptadiene Fpp (3 5) containing an excess of ammonium hexaf luorophosphate. This gave a yellow precipitate which was filtered to give the pure hexafluorophosphate salt (35b) in quantitative yield. The procedure was performed in open air which demonstrated the stability 2 2 of h -1 , 2-cycloheptadiene Fpp (35). The h -cycloheptadiene Fp (4a,b) would have decomposed in the presence of air, methanol or moisture. 5 Unfortunately, treating carbonyl(h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphiteiron (II) with trimethylsilyl trif luoromethanesulfonate failed to give 2 the h -1 , 2-cycloheptadiene complex. We believe that trimethysilyl trif luoromethanesulfonate is incompatible with the triphenylphosphite ligand and results in complete decomposition of the iron complex.
PAGE 91
78 •H
PAGE 92
79 We have thus developed the methodology to synthesize and purify both the trif luoromethanesulfonate and hexa2 fluorophosphate salt of the h -1 , 2-cycloheptadiene Fpp (35). Both (35a) and (35b) are air stable and moderately stable in solution. We are finally ready to look into the process of methoxy abstraction, paying particular attention to the 2 h -1 , 2-cycloheptadiene Fpp (35) diastereomeric composition obtained starting with a majority of one of the diastereomers of h (7-methoxy) cyclohepten-1-yl Fpp (34). Methoxy abstraction from a 60:40 mixture of h -(7methoxy) cycloheptadien-1-yl Fpp (34) in a 1:1 v/v n-pentane/benzene solvent mixture at 0°C typically gives a 2 65:35 to 60:40 diastereomeric mixtures of h -1, 2-cycloheptadiene Fpp (35) after precipitation from ethyl acetate. The diastereomeric composition was determined by H NMR based on the integrated cyclopentadienyl hydrogen resonances. Repeated precipitation from ethyl acetate does not change this diastereomeric composition. 2 The diastereomeric composition of h -1, 2-cycloheptadiene Fpp (35) was found to be dependent upon the solvent in which methoxy abstraction was performed. When the reaction was repeated under identical conditions but in n-pentane , the crude product (35a) was found to have a diastereomeric composition of around 65:35. The H NMR of the crude product (35a) is broad and poorly resolved and one should consider the diastereomeric ratio of 65:35 a very
PAGE 93
80 rough value. A single precipitation from ethyl acetate gave (35b) in a 80:20 diasteromeric composition. Just in case the discrepancy in the crude and pure diastereomeric 2 compositions of h -1 , 2-cycloheptadiene Fpp (35a) was in some way caused by the precipitation process, the filtrate was 2 collected and was examined for h -1 ,2-cycloheptadiene Fpp (35a) . We were unable to detect by H NMR any trace of 2 h -1 ,2-cycloheptadiene Fpp (35a). This 80:20 composition also remained invariant after repeated precipitations. 2 Regardless of the solvent used, the h -1 , 2-cycloheptadiene Fpp was obtained at 45% to 55% yields. * 1 Methoxy abstraction of a 80:20 mixture of h -(7methoxy) cyclohepten-1-yl Fpp (34) in the n-pentane/benzene solvent mixture at 0°C gives a 60:40 diastereomeric mixture 2 ... of the h -1,2-cycloheptadiene Fpp (35a) after precipitation. The yields are slightly higher; over 55%. The most interesting observation regarding methoxy abstraction is from a 60:40 mixture of h (7-methoxy) cyclohepten-1-yl Fpp (34) with half an equivalent of trimethylsilyl trifluoromethanesulfonate at 0°C in n-pentane/benzene solvent mixture. We were able to isolate a 60:40 2 diastereomeric mixture of h -1,2-cycloheptadiene Fpp (3 5a) in over 90% yield (based on trimethylsilyl trifluoromethanesulfonate used) . Recovered from the reaction mixture in The 85:15 mixture of (34) has epimerized to a 80:20 mixture.
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almost a quantitative amount was h (7-methoxy) cycloheptene-1-yl Fpp (34) with a 10:90 diastereomeric ratio! Note that the major diastereomer now is the one with a cyclopentadienyl hydrogen resonance at a higher field. Methoxy abstraction of the 10:90 mixture of h -(7methoxy) cyclohepten-1-yl Fpp with trimethylsilyl trifluoromethanesulfonate at 0°C in the n-pentane/benzene solvent 2 gave h -1 , 2-cycloheptadiene Fpp (35a) in less than a 10% yield but with a 80:20 diastereomeric ratio! The results of the methoxy abstractions were summarized in Table 2. Ethoxy abstraction from h (7-ethoxy) cyclohepten-1-yl Fpp by trimethylsilyl trif luoromethanesulfonate under the conditions developed for methoxy abstraction also gave 2 h -1 , 2-cycloheptadiene Fpp (35a). The results of ethoxy abstraction from enriched diastereomeric mixtures of h -(7ethoxy) cycloheptene-1-yl Fpp parallel that of its methoxy analogue in terms of diastereomeric compositions and percent 2 yields of h -1 ,2-cycloheptadiene Fpp (35a). We have a few reasons to believe that the diastere2 omeric composition of h -1 ,2-cycloheptadiene Fpp (35a) from methoxy abstraction of various diastereomeric mixtures of h (7-methoxy) cyclohepten-1-yl Fpp (34) represents the initial diastereomeric ratio immediately upon methoxy abstraction and does not reflect the different solubilities 2 of the h -1 , 2-cycloheptadiene Fpp (3 5a) diastereomers in ethyl acetate.
PAGE 95
82 > H -p ex En c 0) •P a 0) o u >1 u >! o -p 0) e I e
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83 First of all, the ethyl acetate filtrate from the 2 initial precipitation of h -1 , 2-cycloheptadiene Fpp (35a) 2 was collected and examined for any residual h -1, 2-cycloheptadiene Fpp (3 5a) . We did not detect any trace of 2 1 h -1 , 2-cycloheptadiene Fpp (3 5a) by H NMR from this 2 filtrate. Also, repeated precipitations of h -1, 2-cycloheptadiene Fpp (35a) from ethyl acetate did not affect its diastereomeric compositions. Lastly, we also tried a different procedure to precipi2 tate h -1 , 2-cycloheptadiene Fpp (35) from the reaction 2 mixture. Instead of precipitating h -1 , 2-cycloheptadiene Fpp (3 5a) from the crude reaction mixture with ethyl acetate, we dissolved the crude reaction mixture in methanol saturated with ammonium hexaf luorophosphate and precipitated 2 the h -1 , 2-cycloheptadiene Fpp as its hexaf luorophosphate salt (35b) with water. We found by H NMR measurements that 2 the diastereomeric compositions of h -1 , 2-cycloheptadiene Fpp (35b) obtained by this method are similar to those obtained by the previous method. We conclude that methoxy abstraction of h -(7-methoxy) cyclohepten-1-yl Fpp (34) by trimethylsilyl trifluoromethanesulfonate occurs from only one diastereomer. The other diastereomer merely decomposes in the presence of trimethylsilyl trif luoromethanesulfonate. Therefore, when a 60:40 diastereomeric ratio of h -( 7-methoxy) cyclohepten-1-yl Fpp (34) was treated with half an equivalent of trimethylsilyl trif luoromethanesulfonate, only one diastereomer
PAGE 97
84 2 reacted to give h -1 , 2-cycloheptadiene Fpp (35a) in a 60:40 diastereomeric ratio in almost quantitative yields while the other diastereomer was recovered from the reaction mixture. 2 This other diastereomer gave negligible amounts of h -1,2cycloheptadiene Fpp (3 5a) when treated with trimethylsilyl trif luoromethanesulf onate . From the above observation, it becomes clear why methoxy abstraction in a pentane/benzene solvent from either a 80:20 or a 60:40 mixture of h (7-methoxy) cyclohepten-1-yl 2 Fpp (34) results in the same 60:40 mixture of h -1, 2-cycloheptadiene Fpp (3 5a) . In each case only one diastereomer, the major diastereomer, reacted with the trimethylsilyl trif luoromethanesulf onate. The choice of solvent for methoxy abstraction affects 2 the diastereomeric ratio of h -1 , 2-cycloheptadiene Fpp (35a) . In a non-polar pentane solvent, we are probably seeing methoxy loss through Fpp assistance (probably syn Fpp assistance) leading to an enriched diastereomeric mixture of 2 h -1, 2-cycloheptadiene Fpp (35a). Methoxy loss in a more polar pentane/benzene solvent probably proceeds via the allyl cation to give an almost equal diastereomeric mixture 2 of the h -1 , 2-cycloheptadiene Fpp (35a). At the present moment we are unable to guess as to the 2 significance of the 80:20 diastereomeric ratio of h -1,2cycloheptadiene Fpp (35a) obtained from the methoxy abstraction of a 10:90 diastereomeric mixture of h -(7methoxy) cyclohepten-1-yl Fpp (34).
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85 Our conjecture as to why only one h (7-methoxy) cyclohepten-1-yl Fpp (34) diastereomer undergoes methoxy abstraction to give the desired complex (35a) is that only this diastereomer has its methoxy group accessible to trimethylsilyl trif luoromethanesulfonate. The methoxy group of the other diastereomer is occluded from trimethylsilyl trif luoromethanesulfonate. Based upon extended Huckel calculations of Seeman and 44 45 Davies and X-ray crystal structures of similar types of complexes, we arrived at what we believe to be the most stable conformers of each of the two h (7-methoxy) cyclohepten-1-yl Fpp (34) diastereomers shown below. 2 P <^> 02 p — SR SS The phosphorus to phenyl bond here is eclipsed with the iron to ( 7-methoxy ) cyclohepten-1-yl bond forcing the planes of both rings to lie parallel to each other. This conformation places the methoxy group of one diastereomer (SR) above the
PAGE 99
86 plane of the cycloheptene ring and places the methoxy group of the other diastereomer (SS) below the plane of the cycloheptene ring. The methoxy group of the diastereomer (SR) is therefore exposed to trimethylsilyl trifluoromethanesulfonate whereas the methoxy group of the other diastereomer (SS) is shielded from abstraction by the phenyl 46 ring below. MMX calculations for the two diastereomers of h (7-methoxy) cyclohepten-1-yl Fpp (34) confirmed the conformation of the structures drawn above. Let us now consider the consequences of an Fpp cation alternating between the adjacent double bonds of the 1,2cycloheptadiene in terms of its H NMR spectrum and diastereomeric ratio based upon the two schemes described below. In one case, the Fpp fragment moves between the two allenic double bonds as shown in Scheme XIII. The allyl cation intermediate is not involved in the 1,2-Fpp shift and the stereochemical integrity of each diastereomer is retained; ie. the SR diastereomer (and its RS enantiomer) does not interconvert with its RR (or SS) diastereomer.
PAGE 100
87 Scheme XIII. A rapid 1,2-Fpp shift in the NMR time scale will render the two allenic hydrogens of each diastereomer equal. This effect should be similar to that observed in the case of 2 h -1,2-cycloheptadiene Fp (4) when the allenic hydrogen resonances coalesce. In this case both diastereomers would still be observed and their diastereomeric composition after coalescence would remain unchanged. In other words, the two sets of allenic hydrogen resonances will coalesce and at the same time the two cyclopentadienyl hydrogen resonances will remain distinct and separated. In the other case involving an allyl cation intermediate as shown in Scheme XIV, both cyclopentadienyl hydrogen resonances and the two pairs of the allenic hydrogen resonances would coalesce.
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Scheme XIV. The allyl cation intermediate (S-) no longer exists as enantiomeric pairs of diastereomers but just as a pair of enantiomers. The consequence of this is that starting out with the SR diastereomer we end up with both SR and SS diastereomers . The H NMR of a chloroform-dsolution of the tri2 f luoromethanesulfonate salt of the h -1 ,2-cycloheptene Fpp (35) with a diastereomeric composition of 60:40 remained unchanged up to +60°C. We saw neither coalescence of the allenic hydrogen resonances or the cyclopentadienyl hydrogen resonances. We also did not see any change in the diastereomeric composition of the complex up to 3 hrs at that temperature. Any attempt to raise the temperature beyond this point resulted in decomposition of the complex. This negative result does not necessarily mean that the
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89 1 , 2-cycloheptadiene Fpp (3 5a) is not fluxional because the fluxional process may not be within the appropriate time scale to be observed. This process may be observed at a higher temperature which is unattainable due to thermal instability of the complex. -The fact that the diastereomeric composition remains unchanged does not necessarily mean that the allyl cation is not accessible because we may have started with a thermodynamic mixture of 2 h -1, 2-cycloheptadiene Fpp (35a). If coalescence of the allenic hydrogens were to occur at +60 °C, this would put the free energy of activation for a 1,2-Fpp shift at about 15 kcal/mole. The free energy of activation for an allene to allyl cation isomerization is therefore higher than 15 kcal/mole. Since we were unable to raise the solution temperature over +60 °C, a way that might overcome this limitation is to use spin saturation transfer. The spin saturation transfer 31 technique allows one to observe by NMR a dynamic process at a lower probe temperature than is required for coalesence. 2 Take the SR diastereomer of the h -1 , 2-cycloheptadiene Fpp (3 5) as an example and consider the case where the Fpp cation moves rapidly between the two allenic double bonds via a 1,2-Fpp shift, Scheme XIII. We see that this process causes the allenic H_ to exchange with the allenic H. . Therefore, if H.. were saturated by an irradiation frequency, this saturation could be transferred to H 1 . The net effect
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90 is a diminished H. signal. Of course, in order to observe saturation transfer, the relaxation time, T. , of the allenic hydrogens must be long compared to the fluxional process. If this condition is not met, then the saturation on H-. would have decayed before the 1,2-Fpp shift could occur and saturation would not be transferred to H.. . Experimentally, it was found that irradiation of the allenic hydrogen H, (6.40 ppm) of the minor diastereomer at +60 °C led to complete saturation transfer to the allenic hydrogen H 1 (2.92 ppm). Similarly, irradiating the allenic hydrogen Hresulted in complete saturation transfer to 47 allenic hydrogen H-. Blind irradiation of the allenic hydrogen H(1.96 ppm) of the major diastereomer resulted in a diminished signal for the allenic hydrogen H_ (6.15 ppm) of this diastereomer. It is clear that the Fpp is indeed fluxional about the adjacent double bonds of the 1,2-cycloheptadiene at +60 °C even though this fluxional process is not observed by the coalescence method at the same temperature. Approximately half saturation transfer is observed at +40°C, Fig. 12. It was not possible to derive an accurate free energy of activation for the fluxional motion from the S.S.T. experiments because only the allenic hydrogens belonging to the minor diastereomer could be used. The major diastereomer has one of its allenic hydrogen buried beneath the ring methylene hydrogen resonances.
PAGE 104
91 Eh CO • • c CO >H u x a
PAGE 105
92 Furthermore, the allenic hydrogen resonances are quite broad which make accurate T.. and area measurements very inaccurate. However, if we take +40°C as the condition for a half saturation transfer and assume conservatively a T.. between 0.1 s and 0.5 s, we can arrive at a very rough value of no more than 17-18 kcal/mole for the fluxional barrier. It is important to note that irradiating the allenic hydrogen of the minor diastereomer did not affect the allenic hydrogen's intensities of the major diastereomer and vice versa. A pathway by which saturation transfer can occur between the allenic hydrogens of the two diastereomers is through the allyl cation. We can see from Scheme XIV that the allyl cationic species is the common link between the two diastereomers. If the allenic hydrogen H., of the SR diastereomer is irradiated, saturation transfer would not be confined only to the SR diastereomer but should also leak into the SS diastereomer. Thus this result shows that within the time scale for the S.S.T. experiment, the allene to allyl cation isomerization is slower than the 1,2-Fpp shift. 2 Although the 60:40 mixture of the h -1,2-cycloheptadiene Fpp (35b) diastereomers did not isomerize in 3 hrs at +60°C during coalescense studies, a chloroform-d.. 2 solution of h -1 , 2-cycloheptadiene Fpp (35b) with a diastereomeric composition of 80:20 did change slowly at room temperature. The equilibration was followed by H NMR and ultimately stopped at a 65:35 diastereomeric mixture
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93 within one day. A second solution containing a 65:35 2 mixture of h -1,2-cycloheptadiene Fpp (35b) remained unchanged during this time period. This observation is consistent with an allene to allyl cation isomerization which interconverts diastereomers and the diastereomeric ratio of 65:35 represents a thermodynamical mixture of 2 h -1,2-cycloheptadiene Fpp (35). It should be noted that the above arguments assume that the triphenylphosphine ligand is not labile under the equilibration conditions. When a phosphine ligand leaves the Fpp fragment, it can recoordinate with the iron resulting in a Fpp fragment with the same or opposite stereochemistry. 3 P^Fe + 0CFe + OC^-Fe + SR -R RR Such epimerization of the iron center is indistinguishable from the allene to allyl cation isomerization in that if we 2 start with one diastereomer of the h -1,2-cycloheptadiene Fpp (SR) we end up with a mixture of the two diastereomers (SR and RR) . We do not think that this process is occurring because h -olefin Fpp cationic complexes generally lose the
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94 olefin ligand (and decompose) before the phosphine ligand. Nevertheless, we did not see any exchange of triphenylphosphite for triphenylphosphine when a chloroform solution of the h -1,2-cycloheptene Fpp (35) and triphenylphosphite * was allowed to stand at room temperature for 95 hrs. Moreover, the rate of equilibration of a 80:20 diastereomeric composition of 1 , 2-cycloheptadiene Fpp (35a) is unaffected in the presence of 0.15 M triphenylphosphine. One would expect the rate of equilibration to be supressed should epimerization occur about the iron center by dissociation and reassociation of triphenylphosphine. 2 2 Both h -1, 2-cycloheptadiene Fp (4) and h -1, 2-cycloheptadiene Fpp (3 5) exhibit fluxional behavior via a 2 1,2-iron shift. Whereas the fluxional barrier of h -1,2cycloheptadiene Fp (4) is 13.9 kcal/mole, the fluxional 2 barrier for h -1 , 2-cycloheptadiene Fpp (35) falls between 15 kcal/mole to no more than 18 kcal/mole. The higher fluxional barrier for h -1 , 2-cycloheptadiene Fpp (35) is probably the result of a stronger Fpp to olefin back bonding. This stronger back bonding is reflected in a longer o C(2)-C(l) bond (1.385A) and a larger difference in the o complexed and uncomplexed allene double bond (0.047A) (vide infra) when compared with the Fp complexed tetramethyl23 ° ° propadiene (1.267A and 0.032A, respectively). The steric bulk of the more nucleophilic triphenylphosphine prevents it from displacing the coordinated triphenylphosphite.
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95 2 The allene to allyl cation isomerization for h -1,2cycloheptadiene Fpp (35) is very slow and should have a barrier of over 18 kcal/mole, the upper limit for a 1,2-Fpp shift. Allene to allyl cation isomerization involves the transfer of a positive charge from the iron to the allyl moiety. This isomerization is necessarily slower for the relatively electron rich Fpp because the Fpp is better able to stabilize the positive charge in its allene form compared to the relatively electron poor Fp. For this reason, we infer that the barrier for allene to allyl cation 2 isomerization of h -1 , 2-cycloheptadiene Fp (4) is lower than 2 h -1 , 2-cycloheptadiene Fpp (35). Another area of interest to us besides isomerization of allene to allyl cation is the effect of ring strain on the structure and bonding of cyclic allenes. Theoretical calculations of Johnson in particular predicted substantial deformation of both C^-C.-C-. bond angles and dihedral angles of strained cyclic allenes, see Table 1. There are no theoretical studies yet on transition metal complexed cyclic allenes. Although transition metal complexed cyclic allenes are 1 C 1 (L well known ' , no X-ray studies have been reported. 4 8 X-ray studies of acyclic allenes however revealed a bent structure with the C.-C -C., bond angles between 134° and 159° but with little distortion of the dihedral angles. 2 We have succeeded in growing crystals of h -1, 2-cycloheptadiene Fpp hexaf luorophosphate (3 5b) suitable for X-ray
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96 studies. The air stable orange colored crystals were grown from a methylene chloride/n-heptane solvent mixture at -35°C in a nitrogen atmosphere. This is the first cyclic allene transition metal complex characterized by X-ray and its structure is shown in Fig. 13. Interactions between iron and the C(2)-C(l) double bond o lengthens it (1.385 (6)A) whereas the uncomplexed C(l)-C(7) o bond length is typical of a double bond (1.303 (7) A). The 3 2 hybridization at C(2) and C(l) is closer to sp and sp , respectively. The consequence of rehybridization is that the C(2)-C(l)-C(7) angle contracts to 138.1(3)° from a calculated value of 153.4° for the free 1 ,2-cycloheptadiene. Considerable ring strain is perhaps relieved. This angle is one of the smallest for a monomeric transition metal allene 2 complex. Only the h -phenylallene complex of (diphos) 2 ReCl has a smaller angle of 138.1(3)°. a The iron is also displaced toward the C(l) carbon; the Fe-C(l) and Fe-C(2) o o bond lengths are 2.007 (4)A and 2.168 (3)A, respectively. The dihedral angle defined by the planes C(3)-C(2)C(l)-H(21) and C (6) -C (7) -C (1) -H (71) is 70.7° versus 90° for an unstrained acyclic allene and 55.2° calculated for 1 ,2-cycloheptadiene. The dihedral angle is increased upon complexation but the ring still prevents it from returning to its normal value of 90°. The bonding between 1 , 2-cycloheptadiene and Fpp is typical of all metal to olefin bonds.
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97 C(10) Fig. 13. X-ray structure of h -1 , 2-cycloheptadiene Fpp cation. , An illustration of the [FeCp (CO) (PPh~) (C ? H Q ) ] ion. Selected bond lengths: Fe-C(lf, 2.007(4); Fe-C(2), 2.168(3); C(l)-C(2), 1.385(6); C(l)-C(7); 1.303(7); Fe-Cc, 1.777(5); Cc-Oc , 1.137(6); Fe-P(l), 2.250(1), P(1)-C(1A), 1.828(4); P(l)C(1B), 1.82544); P(1)-C(1C), 1.831(3); Fe-C(Cp) Avg 2.097 (5) A. Selected bond angles: P(l) -Fe-Cc, 89.5(1); P(l)-Fe-C(l) , 82.9(1); Cc-Fe-C(l), 111.3(2); Fe-C(l)-C(2) , 77.0(2); Fe-C ( 1) -C (7) , 141.9(3); C(2)-C(l)-C(7) , 138.1(3); Fe-Cc-Oc, 177.8(3); Fe-P(l)-C(1A) , 110.8(1); Fe-P ( 1) -C ( IB) , 117.3(1); Fe-P(l)-C(lC) , 115.1(1).
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CHAPTER IV h -IRON COMPLEX OF 1 , 2-CYCLOHEXADIENE One can further strain the linear and perpendicular allene by incorporating it into a smaller six-member ed ring. MNDO calculated geometries of 1 , 2-cyclohexadiene show that it is even more distorted compared with the larger cyclic allenes. The allene C,-C 2 ~C 3 bond angle is 138.5° versus 153.4° calculated for 1 , 2-cycloheptadiene and the dihedral angle of 22.9° is very close to planarity. Despite the large bending and torsional strain, the allene form is still the ground state but the allyl cationic form lies only 15 kcal/mole higher. On the contrary, EHMO predicts that the h -allyl Fp cationic form (38) is actually 14 kcal/mole 2 lower in energy than the h -allene Fp form (3 7) . r^ Fp + 98
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99 Earlier attempts to generate h -1 , 2-cyclohexadiene Fp (37) and observe this allene to allyl cation isomerization were not successful. Treating a solution of the h -(6methoxy)cyclohexen-l-yl Fp with trimethylsilyl trifluoromethanesulfonate followed by ethanol gave h -cyclohexen-1-yl Fp instead of the expected ethyl ether adduct. Fp OCH 3 + TMSOTF C 2 H 5 OH -> X C 2 H 5 OH Fp OC 2 H 5 Fp Adding an acetone solution of sodium iodide to the crude reaction mixture gave a 3:1 mixture of cyclohexene and cyclohexadiene, respectively. A mechanism similar to that 2 for the decomposition of the h -1 , 2-cycloheptadiene Fp (4a), Scheme V, was proposed to account for the decomposition products observed. The h (6-methoxy) cyclohexen-1-yl Fp was synthesized in good yields according to the method developed for its 7-membered ring analogue.
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100 Br Li Fp OCH 3 J^ OCH3 X 9 CH 3 h-BuLi -> FpCI -> The reaction between a methylene chloride solution of h (6-methoxy) cyclohexen-1-yl Fp and trimethysilyl trifluoromethanesulfonate at -78 °C gave a flocculant yellow precipitate upon quenching with diethyl ether (-78 °C) . The precipitate was extremely sensitive to temperature and turned to a dark red paste when we attempted to isolate it via filtration at room temperature. The red paste could not be characterized but was presumed to contain Fp trifluoromethanesulfonate along with other decomposition products. An acetone solution of sodium iodide added to this red paste gave Fp iodide as the only identifiable product. 2 This suggested that h -1 , 2-cyclohexadiene Fp (37) may be extremely prone to decomposition. We therefore attempted to trap it at low temperatures. The yellow flocculant precipitate from the reaction of h (6-methoxy) cyclohexen1-yl Fp with trimethysilyl trif luoromethanesulfonate can be isolated and kept from turning red momentarily via a cold filtration (-78°C) under N„ in a jacketed filter funnel. However, when ethanol with a slurry of sodium carbonate was
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101 added to this precipitate again the ethyl ether adduct could not be detected from the reaction mixture. The same reaction in pentane also afforded a yellow precipitate as soon as trimethylsilyl trif luoromethylsulfonate was added. Again the ethyl ether adduct could not be detected from the reaction mixture when ethanol/sodium carbonate was added to this precipitate. Trimethylsilyl bromide is known to be a methoxy 49 abstracting reagent. The advantage of this reagent is that the bromide anion resulting from the reaction between trimethylsilyl bromide and h (6-methoxy) cyclohexen-1-yl Fp may act as an internal trap to give the h (6-bromo) cyclohexen-1-yl Fp. However, the reaction gave Fp bromide as the only isolatable product. 1 ,3-Cyclohexadiene was not detected in the reaction mixture by H NMR. Fp OCH 3 + — Si — Br r-^X F P + Previous experimental evidence showed that trifluoromethanesulfonate anion was responsible for the thermal
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102 2 decomposition of h -1 , 2-cycloheptadiene Fp (4a). It would be reasonable to assume that the trif luoromethanesulfonate 2 anion may also decompose the h -1 , 2-cyclohexadiene Fp (37); perhaps even more rapidly if indeed it is formed. The procedure for room temperature methoxy abstraction with trimethyloxonium tetrafluoroborate developed for the sevenmembered ring analogue was therefore tried here. Unfortunately, unlike the seven-membered ring, the reaction at room temperature yielded only a black intractable solid. There was no reaction between the h (6-methoxy) cyclohexen-1-yl Fp and trimethyloxonium tetrafluoroborate at low temperatures (-78°C) and the starting material was isolated unchanged. Trimethyloxonixim tetrafluoroborate is not very soluble in methylene chloride used in the above experiment and it was suspected that the long reaction times necessary for methoxy abstraction may be detrimental to either the 1 2 h -allyl Fp cation (38) or the h -1 , 2-cyclohexadiene Fp (37) . Triethyloxonium tetrafluoroborate was substituted as the methoxy abstracting reagent. The advantage of this reagent is that it is soluble in methylene chloride, the reaction solvent. We therefore expect reaction times to be shortened considerably. Methoxy abstraction with triethyloxonium tetrafluoroborate at -78 °C in methylene chloride did not give a yellow solid upon quenching with diethyl ether but turned red when warmed to room temperature. The same reaction in either n-pentane or diethyl ether gave an intractable red substance which coats the reaction vessel.
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103 Triphenylmethanecarbenium tetraf luoroborate was also reacted with h (6-methoxy) cyclohexen-1-yl Fp according to the procedure developed for its seven-membered ring analogue but this reaction mixture also decomposed to a red substance . 2 Having failed to synthesize or trap the h -1,2-cyclohexadiene Fp (37) , or its h 1 -allyl Fp cation (38) , we decided, as a last recourse, to attempt to generate and 2 1 observe h -1 , 2-cyclohexadiene Fp (37) by low temperature H NMR. Trimethylsilyl trif luoromethanesulfonate and h -(6methoxy) cyclohexen-1-yl Fp were introduced into a low temperature NMR tube reactor as separate plugs frozen in methylene chloride. The plugs were thawed in cold pentane (-80°C) , mixed and then refroze in liq. N_ and the NMR tube was then sealed under vacuum. The contents of the tube were thawed inside a precooled NMR probe and the reaction was followed by H NMR. There was no evidence for either the h -1,2-cyclohexadiene Fp (37) or h -allylic Fp cation (38) at -85°C. Instead, only a signal of 5.21 ppm taken to be the Fp trifluoromethanesulfonate grew steadily while the cyclopentadienyl hydrogen resonances for h (6-methoxy) cyclohexen-1-yl Fp decrease with time, Fig. 14. The contents of the NMR tube also turned red again indicating the presence of Fp trifluoromethanesulfonate. There was no sign of cyclohexene and cyclohexadiene or the Fp cation complexes of either. Methyltrimethylsilyl ether was the only other product identified by H NMR.
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104 >,
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105 We have seen the dramatic effect of a triphenylphosphine ligand on the physical and chemical properties of the 2 h -1,2-cycloheptadiene Fpp (35). We therefore hoped that the triphenylphosphine ligand would impart additional 2 stability to the h -1 , 2-cyclohexadiene Fp such that it could be synthesized and isolated or observed by low temperature NMR. h (6-Methoxy) cyclohexen-1-yl Fpp was synthesized in reasonable yields according to the procedure developed for its seven-membered ring analogue. Fe OC CO OCH+ P0 3 hv Fe OC 3 P OCH 3 When h (6-methoxy) cyclohexen-1-yl Fpp was treated with trimethysilyl trif luoromethanesulfonate in a n-pentane/ benzene solvent mixture at 0°C, a brownish yellow precipitate fell out of solution. This pasty precipitate could not be isolated via filtration and turned dark rust colored after the reaction solvent was removed in vacuo. The paste dissolved in ethyl acetate but unlike its seven-membered ring analogue, did not give an orange colored precipitate.
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106 Treating h (6-methoxy) cyclohexen-1-yl Fpp with triphenylmethanecarbenium fluoroborate led to a black decomposed mixture. The same reaction in pentane , however, gave a greenish yellow precipitate which can be isolated by filtration under N„ at room temperature. The solid decomposes instantly in solution but can be kept in solid form for several days at -35°C. We were unable to characterize this solid fully due to its instability but we believed it to be the 2 h -1, 2-cyclohexadiene Fpp. If this solid were the 2 h -cyclohexadiene Fpp, we hope to trap it as its ethyl ether adduct. Fp Fpp + I OCH 2 CH 3 ^^ + CH 3 CH 2 OH
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107 Replacing a carbonyl ligand on the iron with triphenylphos2 phine stabilizes the h -1 , 2-cyclohexadiene Fpp to a point where we can isolate it as a solid.
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CHAPTER V EXPERIMENTAL Hexane, diethyl ether and tetrahydrofuran were distilled from benzophenone ketyl. Methylene chloride was distilled from P 4 0, n under N and degassed. Benzene was distilled from SilicaPent under N 2> Acetonitrile, p-dioxane, furan and cyclopentadiene were distilled under N_. The initial fractions were discarded. n-Pentane and n-heptane were degassed by bubbling a stream of N_ into them. Alumina was Brockman 80-200 mesh (neutral, activity I) and was reduced to activity II by adding 3% by 1 13 weight of water and degassed before use. H NMR and C NMR were taken on a JEOL FX-100 (100 MHz) . IR were taken on a Perkin Elmer 283B. X-ray crystal structure was obtained from a Nicolet R3m dif fractometer. Elemental analyses were performed by Atlantic Microlabs, Atlanta. Melting points were uncorrected and obtained using a Thomas Hoover apparatus. The following compounds were prepared as described in the literature: 7 , 7-dibromobicyclo [4 . 1 . ' ] heptane, 15 14 6,6-dibromobicyclo[3. 1.0 ' ] hexane, l-bromo-7-methoxycycloheptene, l-bromo-6-methoxychclohexene, l-bromo-7ethoxycycloheptene, 7-methoxycycloheptene-l-carboxylic acid, 7-methoxycycloheptene-l-carboxylic acid chloride, 108
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109 (h 5 -C 5 H 5 ) (CO) 2 FeCl, 51 (h 5 -C 5 H 5 ) (CO) 2 Fe (h 2 -CH=C=CH(CH 2 ) 3 ~ CH 2 ) + OTF~/BF 4 .~ 50 All organometallic reactions, unless otherwise specified, were conducted under N» using Schlenk techniques or in a dry box by Vacuum Atmospheres Corporation. Improved synthesis of dicarbonyl(h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) iron (II) n-Butyllithium (1.6 M in hexane) (6 ml; 9.6 mmole) was added dropwise to a cold (-78°C) solution of 7-methoxy-lbromocycloheptene (2.24 g; 11 mmole) in 20 ml of THF. The reaction mixture was stirred for 2 hrs at -78°C after which a solution of Fp chloride (2.1 g; 10 mmole) in 5 ml of THF was added. The reaction mixture was allowed to stir for 1 hr at -78 °C and then warmed to room temperature. The reaction mixture was then adsorbed onto alumina and eluted down a 3" x 1" alumina column (neutral; grade II) with hexane. The only yellow band was collected and yielded 2.0 8 g (63%) of the product as a brown oil. The NMR and IR are consistent with those of the known compound. IR (neat) 2010s, 1920s cm -1 ; 1 E NMR (60 MHz, CDC1 3 > 0.80-2.50 (m,8H), 3.30 (s,3H), 3.8 (m,lH), 4.75 (s,5H), 5.80 (t,lH). 2 Thermal decomposition of dicarbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) iron (II) trif luoromethanesulfonate A methylene chloride-d„ solution of the title compound (10 mg) was vacuum sealed in an NMR tube and heated to +40 °C
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110 in the NMR. The progress of the thermal decomposition was monitored at regular intervals. Refer to Fig. la and lb and discussion in Chapter II. NMR scale synthesis of dicarbonyl (h -cyclopentadienyl) iron (II) trifluoromethanesulfonate To a dilute solution of Fp chloride in methylene chloride-d~ (cyclopentadienyl hydrogen resonance at 5.0 6 ppm) was added a small amount of silver trifluomethanesulfonate. The orange colored solution turned red with the appearance of a new cyclopentadienyl hydrogen resonance at 5.20 ppm in the H NMR. The result is in 52 agreement with that reported by Mattson and Graham (5.25 ppm, CDC1 3 ) 2 Attempted Thermal decomposition of dicarbonyl (h -1,2cycloheptadiene) (h -cyclopentadienyl) iron (II) tetrafluoroborate A methylene chloride solution of the title compound was heated to +40 °C in a vacuum sealed NMR tube for 16 hrs. The H NMR remained unchanged. 13 2 Variable temperature C NMR studies of the dicarbonyl (h 1 ,2-cycloheptadiene) (h -cyclopentadienyl) iron (II) tetraf luoroborate A nitromethane-dsolution of the title compound was 13 sealed under vacuum in an NMR tube. The C NMR was obtained at 15°C intervals. The carbonyl signals remained distinct up to +45°C whereas the C. and C^ signals vanished.
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Ill The compound decomposed at +60 °C. Refer to Fig. 2 and discussion in Chapter II. 2 The reaction between dicarbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) iron (II) tetraf luoroborate and triphenylme thane A methylene chloride-dsolution of the title compound (6.8 mg; 0.019 mmole) and triphenylme thane (5.4 mg; 0.022 mmole) was vacuum sealed in an NMR tube. The solution was heated to +40°C for 3 hrs with no detectable change in its H NMR. The solution remained yellow in color and clear. There was no reaction between the two compounds. Attempted enantiomeric enrichment of racemic dicarbonyl 2 5 (h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) iron (II) tetraf luoroborate with (S) (-) -2-methylbutanol To a suspension of the title compound (0.10 g; 0.36 mmole) in diethyl ether was added (S) -(-) -2-methylbutanol (0.02 ml; 0.18 mmole). The reaction was allowed to stir overnight at room temperature. The remaining solid (0.07 g) was isolated via filtration and washed with diethyl 2 ether. The solid, h -1 , 2-cycloheptadiene Fp, has a negligible optical rotation, [<*]J 4 £ =-0.7° (c 0.001 g/ml CH 2 C1 2 ). The solution was too dark for rotation measurements at higher concentrations. The ether adduct was isolated from the filtrate and eluted down a short alumina column (neutral, grade II) with hexane. IR (CDCl 3 > 2015s, 1955s cm" 1 ; 1 H NMR (100 MHz, CDC1 3 ) 0.80-1.00 (m,6H), 1.08-2.32
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112 (m,HH), 3.16 (dd,2H), 3.94 (d,lH), 4.80 (s,5H), 5.80 (t,lH) . Resolution of 7-methoxy-l-cycloheptenecarboxylic acid The title compound was dissolved in abs . ethanol at the concentration of 1 g of the acid to 3 ml of the ethanol. One equivalent of the (-) -quinine was added and the mixture was heated to dissolution. The solution was filtered and allowed to stand at room temperature. Translucent crystals were formed. The solution was allowed to stand until the first sign of a white crystal appeared. The translucent crystals were collected via filtration, washed with abs. ethanol and air dried to give a white solid, m.p. 160°C162°C. The acid was regenerated by adding a few milliliters of dilute HCl(aq) to a diethyl ether solution the crystals. The solution was heated briefly over a steam cone. The diethyl ether layer was collected, dried and the diethyl ether removed in vacuo to give the acid as a yellowish clear 546 oil, [ct]^^ = -8.41°. Synthesis of optically active dicarbonyl(h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) iron (II) (N) The title compound was made in two steps in a 3 5% yield from the optically active 7-methoxycycloheptene-l-carboxylic acid chloride and potassium Fp followed by photolytic decarbonylation according to a previously established route
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113 50 for the racemic complex. The NMR and IR are in agreement with the known racemic compound synthesized here by treating l-lithio-7-methoxycycloheptene with Fp chloride. Optical rotation measurments were not obtained. The enantiomeric excess of 50% was determined by NMR measurements of the cyclopentadienyl hydrogen resonances of a 10 mole % Eu(hfc) 3 chloroform-d.. solution of the acyl complex (before photolytic decarbonylation) . Synthesis of optically active l-bromo-7-methoxycycloheptene The title compound was synthesized in two steps. Stereospecific reduction of 2-bromo-2-cycloheptenone with LAH/quinine according to the method of Jones and Balci gave the optically active alcohol (20% e.e.) in a 82% yield. The enantiomeric purity was determined by NMR measurements of the C7 hydrogen in a chloroform-d, solution of Eu(hfc) and not by optical rotation. The optically active alcohol was treated with sodium hydride followed by methyl iodide 14 according to the method of Manganiello to give the optically active l-bromo-7-methoxycycloheptene in 77% yield. Optical rotation was not determined. H NMR is consistent with the known racemic compound . H NMR (60 MHz, CDC1 3 ) 1.20, 1.75 (m,8H), 3.40 (s,3H), 4.05 (m,lH), 6.30 (t,lH).
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114 Synthesis of the optically active dicarbonyl (h -cyclopentadienyl) (h ( 7-methoxy) cyclohepten-1-yl) iron (II) (P) The title compound was made from the optically active l-bromo-7-methoxycycloheptene according to the method developed here. The NMR and IR are in agreement with the known racemic compound synthesized here by treating l-lithio-7-methoxycycloheptene with Fp chloride. The enantiomeric excess of 20% was determined by NMR measurement of the cyclopentadienyl hydrogen resonances of a 10 mole % Eu(hfc) chloroform-d.. solution of the acylated compound. Synthesis of dicarbonyl (h -cyclopentadienyl) h -(7-methoxy) 2 cyclopentadien-1-yl) iron (II) from dicarbonyl (h -1,25 cycloheptadiene) (h -cyclopentadienyl) iron (II) trifluoromethanesulfonate The title compound was made according to the method by 14 Manganiello in 30% yields substituting methanol for ethanol. The NMR and IR are in agreement with the known compound synthesized here by treating l-lithio-7-methoxycycloheptene with Fp chloride. Synthesis of dicarbonyl (h -carbonyl(7-methoxy) cycloheptene5 1-yl) (h -cyclopentadienyl iron (II) via carbonylation of 5 1 dicarbonyl (h -cyclopentadienyl) (h (7-methoxy) cycloheptenel-yl)iron(II) A methylene chlorde solution of the h -(7-methoxy) cyclohepten-1-yl Fp (0.08 g; 0.26 mmole) and ferrocenium tetrafluoroborate (0.01 g; 0.037 mmole) in a thick wall glass reactor was cooled to -78°C and charged with 55 psi of
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115 CO gas. The reaction mixture was stirred for 1 hr after which the pressure was released. The reaction mixture was adsorbed onto alumina and eluted down a 1" x 1" alumina column (neutral, grade II) first with hexane then with methylene chloride. A quantitative yield of the product was obtained. The NMR and IR 50 are consistent with the known compound. IR (neat) 2010s, 1960s cm" 1 ; 1 H NMR (60 MHz, CDC1 3 > 1.00-2.90 (m,8H), 3.20 (s,3H), 4.20 (m,lH), 4.90 (s,5H), 6.45 (m,lH). 5 1 Synthesis of carbonyl(h -cyclopentadienyl) (h -(7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) The h (7-methoxy) cycloheptene Fp (1.10 g; 3.6 mmole) dissolved in a 1:1 v/v mixture of n-pentane and benzene was introduced into a quartz photolysis well equipped with a low pressure Hg lamp (Hanovia, 4 50 W) . The solution was photolyzed while being flushed with a stream of N„ and triphenylphosphine dissolved in 15 ml of the same solvent mixture was added at a rate of 1 ml/min. The photolysis was terminated 5 mins after all of the triphenylphosphine was added. The solvent was removed and the red viscous residue was chromatographed down a 3" x 1" alumina column (neutral, grade II) first with hexane then with benzene giving 0.94 g (48%) yield of a red oil. The compound was obtained in a 60:40 diastereomeric composition determined by NMR measurements of the methoxy hydrogen resonances. An analytical sample was recrystalized from a 4:1 v/v of acetone/water to
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116 give a dark red solid. Mp. 138°-140°d; IR (CDC1 3 ) 1920s cm" 1 ; 1 E NMR (100 MHz, CDC1 3 ) a) diastereomer A 0.802.30(bm,8H), 3.19(s,3H), 3.90(dd,lH), 4.46(d, 3 J pH = 0.98Hz, 5H) , 5.32(dt,lH), 7 . 33 ( s, 15H) ; b) diastereomer B 0.80-230(bm,8H) , 3.08(s,3H), 4 . 43 (d, 3 J pR =0 . 98Hz , 5H) , 5.76(dt,lH), 7.33(s,15H); 13 C NMR (25 MHz, CDC1 3 ) 25.8, 26.7, 27.9, 31.7(C3-C6), 55.8(C8), 84.4(Cp), 92.0(C7), 127.8(o-C, 2 J pc =8.54Hz) , 133 . 5 (m-C, 3 J pc =9 . 76Hz) , 136.8 (i-C^Jp^ 39.06Hz), 141 . 7 (C2 , 3 J pc =6 . 1Hz) , 151.KC1, 1 J c =23.19Hz) , 211.6(CO); Anal, calcd. for ^32^30^^^ C,71.65; H,6.20; Found: 0,71.62; H,6.22. TLC (benzene) rf: 0.35. 2 5 Synthesis of carbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) trif luoromethanesulfonate To the h (7-methoxy) cyclohepten-1-yl Fpp (0.15 g; 0.28 mmole) dissolved in 15 ml of an equal volume mixture of n-pentane and benzene at 0°C was added an excess of TMSOTF (0.10 ml; 0.4 mmole). The reaction mixture was stirred for 10 mins at 0°C and the solvent removed to yield a dark greenish paste. Ethyl acetate was added to the paste and the desired product precipitated as an orange solid (0.1 g; 56%) with a diastereomic composition of 60:40. The same reaction in n-pentane gave the product in a 80:20 ratio after precipitation from ethyl acetate. The diastereomeric compositions were determined by NMR measurements of the cyclopentadienyl hydrogen resonances. This orange solid is
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117 air stable. An analytical sample was recrystallized from ethyl acetate. Mp. 170°-173 °d; IR (KBr) 1996s cm -1 ; 1 H NMR (100 MHz, CDC1 3 ) a) diastereomer A 1.73, 2.23(b,8H), 2.92(b,lH), 5.12(d, 3 J pH =1.22Hz,5H) , 6.40(b,lH), 7.007.60(b,15H); b) diastereomer B 1.73, 2.23(b,8H), 1.96(b,lH), 5.23 (d, 3 J pH =l. 22Hz, 5H) , 6.15(b,lH), 7 . 00-7 . 60 (b, 15H) ; 13 C NMR (25 MHz, CDC1 3 ) 17 . 5 (CI , 2 J pc =170 . 5Hz) , 27.63, 28.75, 31.29, 32.11 (C3-C6) , 90.4(Cp), 126 . 71 (C3 , 3 J pc = 17.25Hz), 129. 0-132. 8(PPh 3 ) , 154.27(C2, 2 J pc =20.75Hz) , 217.94(CO); Anal, calcd. for C 32 HO.F-P^SjFe^ C,58.72; H,4.62; Found: C,58.74; H,4.72. Synthesis of l-bromo-7-methoxycycloheptene-7d 1 The title compound was made in four steps by the method adapted from Jones and Balci with two changes. The 2-bromo-l-cycloheptenone was reduced with LAD instead of LAH/quinine mixture and the resulting alcohol was reacted with methyl iodide instead of methanesulfonyl chloride to give the compound at a 25% overall yield. With the exception of the deuterium, the NMR is consistent with the known protiated compound. 1 H NMR (60 MHz, CDC1 3 ) 1.20, 1.75 (m,8H), 3.40 (s,3H), 6.30 (t,lH). 2 5 Synthesis of carbonyl(h -1 , 2-cycloheptadiene-ld.. ) (h -cyclopentadienyl) triphenylphosphineiron (II) trif luoromethanesulfonate The title compound was made in three steps starting from l-bromo-7-methoxycycloheptene-7d, and Fp chloride
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118 according to the method developed here. The NMR and IP. are in agreement with the known protiated compound synthesized here. The exchange of hexaf luorophosphate anion for trifluo2 methanesulfonate anion in carbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) tripehnylphosphineiron (II) 2 A methanol solution containing the h -1 , 2-cycloheptadiene Fpp trif luoromethanesulfonate and an excess ammonium hexaf luorophosphate was filtered into water. A yellow precipitate fell out of solution. The compound was isolated via filtration, washed with water and air dried. The exchange was quantitative and the NMR is identical to the trifluoromethanesulfonate salt of the compound synthesized here. The IR shows hexaf luorophosphate absorption (850b cm" ) in place of the trifluoromethanesulfonate absorption (1270, 1150 cm" ). Synthesis of carbonyl(h -carbonyl(7-methoxy) cyclohepten5 1-yl) (h -cyclopentadienyl) triphenylphosphineiron (II) A mixture of h (7-methoxy) cyclohepten-1-yl) Fp (1.11 g; 3.6 mmole) and triphenylphosphine (1.29 g; 4.8 mmole) in 30 ml of acetonitrile was heated to +50°C for 20 hrs. Acetonitrile was removed and the red oil was eluted down a 3" x 1" alumina column (neutral, grade II) first with a 1:1 v/v mixture of n-pentane/benzene followed by a 1:1 v/v mixture of benzene/ethyl acetate. A red band was collected
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119 giving 1.0 g (37%) of the desired product as an orange solid. The compound was obtained in a 50:50 diastereomeric composition determined by NMR measurements of the methoxy hydrogen resonances. Mp. 120°-122°d; IR (KBr) 1914s, 1558s cm" ; 1 H NMR (100 MHz, CDC1 3 ) a) diastereomer A 1.00-2.70(b,8H) , 3.11(S,3H), 3.44(d,lH), 4.45(d, 3 J pH =1.16Hz,5H) , 6.87(dt,lH), 7 . 24-7 . 64 (b, 15H) ; b) diastereomer B 1 . 00-2. 70 (b, 8H) , 2.89(s,3H), 4.02(d,lH), 4.41 (d, 3 J pH =1.22Hz,5H) , 6.54(dt,lH), 7 . 24-7 . 64 (b, 15H) ; 13 C NMR (25 MHz, CDC1 3 ) 24.95, 27.05, 27.53, 29 . 97 (C3-C6) , 55.55(C8), 75.49(C7), 85.09(Cp), 127 . 78-137 . 57 (PPh 3 ) , 146.88(C2), 158.67, 158.48(C1), 210.97, 211.11(CO), 272.71 (>CO); Anal, calcd. for C 33 H 33 3 P 1 Fe 1 : C, 70.22; H,5.89; Found: C,70.21; H,5.94. Separation of diastereomers,-of carbonyl (h -carbonyl( 7methoxy) cyclohepten-1-yl) (h -cyclopentadienyl) triphenylphosphineiron (II) The title compound dissolved in a 1:2 v/v mixture of ethyl acetate/n-pentane was cooled to +10°C. The solid residue was enriched up to 80% of the diastereomer B. The process may be repeated once more with up to 90% enrichment of that diastereomer. The filtrate was only moderately enriched (40%) with the diastereomer A. The diastereomeric compositions were determined by NMR measurements of the methoxy hydrogen resonances.
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120 Attempted decarbonylation of carbonyl (h -carbonyl(7methoxy) cyclohepten-1-yl) (h -cyclopentadienyl) triphenylphosphineiron (II) 1) Photolysis A solution of the title compound was photolyzed at room temperature in benzene with a low pressure Hg lamp (Hanovia; 450 W) . The progress of the reaction was followed by IR. There was no reaction after 2 1/2 hrs. 2) Photolysis in an ultrasonic bath The above experiment was repeated in an ultrasonic bath. A rapid evolution a gas (CO) was observed. The decarbonylated compound was isolated but as a racemate. The unreacted compound recovered has its diastereomeric composition intact. 3) Reaction with trimethylamine-N-oxide A 5 ml benzene solution of the title compound (0.05 g; 0.08 mmole) and trimethylamine-N-oxide (0.04 g; 0.53 mmole) was stirred at room temperature for 2 4 hrs. There was no reaction and the compound was recovered with its diastereomeric composition intact. 4) Reaction with chlorobis (triphenylphosphine) rhodium dimer A 4 ml acetonitrile solution of the title compound (0.02 g; 0.035 mmole) and the rhodium dimer (0.02 g; 0.018 mmole) was stirred at room temperature for 3 6 hrs.
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121 There was no reaction and the compound was recovered with its diastereomeric composition intact. 5) Reaction with iodosobenzene A 10 ml methylene chloride solution of the title compound (0.12 g; 0.21 mmole) and iodosobenzene (0.07 g; 0.32 mmole) was stirred at room temperature for 24 hrs. There was no reaction and the compound was recovered unchanged. 6) Thermal decarbonylation The title compound was heated to +100°C in a p-dioxane for 4 hrs. Both the starting compound and the dephosphinylated compound was recovered from the reaction. Separation of diastereomer of carbonyl (h -cyclopentadienyl) h (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) The title compound was eluted down a 8" x 1" alumina column (neutral; grade II) with a 1:1 v/v n-pentane/benzene solvent mixture. The broad red band was collected in two halves. Both fractions were enriched in the same diastereomer, diastereomer A, the second fraction (70% enrichment) greater than the first (50% enrichment) . Diastereomeric compositions were determined by H NMR measurements of the methoxy hydrogen resonances.
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122 2 5 Synthesis of carbonyl (h"-l , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) trif luoromethanesulfonate from the enriched carbonyl (h -cyclopentadienyl) (h 1 (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) (80:20) The title compound was synthesized from a 80:20 diastereomeric mixture of h (7-methoxy) cyclohepten-1-yl Fpp in a 1:1 v/v pentane/benzene solvent mixture according to procedures developed here. The compound with a diastereomeric ratio of 60:40 was isolated in 40% yields. The diastereomeric composition was determined by NMR measurements of the cyclopentadienyl hydrogen resonances. The H NMR and IR are in agreement with the known compound synthesized here. Reaction between carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) with a half equivalent of trimethylsilyl trif luoromethanesulfonate To the title compound (0.63 g; 1.17 mmole) with a 55:45 diastereomeric ratio in a 1:1 v/v n-pentane/benzene solvent mixture was added TMSOTF (0.11 ml; 0.6 mmole). The solvent 2 was removed leaving behind a residue from which h -1,2cycloheptadiene Fpp was precipitated from ethyl acetate and isolated via filtration (0.33 g; 85% yield based on TMSOTF) in a 64:36 diastereomeric ratio. The starting material was reisolated quantitatively (based on TMSOTF) from the filtrate in a 10:90 diastereomeric ratio.
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123 2 5 Synthesis of carbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II ) trif luoromethanesulfonate from the enriched carbonyl (h -cyclopentadienyl) (h (7-methoxy) cyclohepten-1-yl) triphenylphosphineiron (II) (15:85) The title compound was synthesized from a 15:85 diastereomeric mixture of h (7-methoxy) cyclohepten-1-yl Fpp in a 1:1 v/v pentane/benzene solvent mixture according to procedures developed here. The compound with a diastereomeric ratio of 80:20 was isolated in negligible yields. The H NMR and IR are in agreement with the known compound synthesized here. 5 1 Synthesis of dicarbonyl(h -cyclopentadienyl) (h -(7-ethoxy) cycloheptadien-1-yl) iron (II) The title compound was made according to the procedure developed here substituting l-bromo-7-ethoxychcloheptene for l-bromo-7-methoxychcloheptene with a 48% yield. The NMR and 14 IR are consistent with the known compound. IR (neat) 2005s, 1950s cm" 1 , X H NMR (100 MHz, CDCl 3 ) 1.20 (t,3H), 1.08-2.40 (m,8H), 3.43 (m,2H), 3.97 (d,lH), 4.80 (s,5H), 5.81 (t,lH). Synthesis of carbonyl(h -cyclopentadienyl) (h -(7-ethoxy) cycloheptadien-1-yl) triphenylphosphineiron (II) The title compound with a diastereomeric of 50:50 was synthesized according to the same procedure developed here for its 7-methoxy analogue in a 76% yield. Diastereomeric composition was determined by NMR measurements of the
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124 cyclopentadienyl hydrogen resonances. IR (CHC1.J 1910s cm -1 ; 1 H NMR (100 MHz, CDC1,) a) diastereomer A 1.11 (t,3H), 0.92-2.32 (b,8H) , 3 . 10 (m, 2H) , 3.79(d,lH), 4.35(d, 3 J pH =0.95Hz,5H) , 5.63(t.lH), 7 . 12-7 . 43 (m, 15H) ; b) diastereomer B 1.11(6,3H), . 92-2 . 32 (b, 8H) , 3.50(m,2H), 3.91(d,lH), 4.40(d, 3 J DH =0.97Hz) , 5.25(t,lH), 7.12-7.43 (m,15H) ; 13 C NMR (25 MHz, CDC1 3 ) 15.89(-CH 3 ), 25.93, 17.63, 20.17, 31 . 58 (C3-C6) , 63 . 21 (-OCHj-) , 84.36(Cp), 91.28(C7), 127. 54-137. 82(PPh 3 ) , 141.76(C2), 151,15(C1), 223.49, 222.08(CO); Anal. Calcd. for c 3 3 H 35 2 P i Pe i : C, 72.00; H,6.41; Found: C,71.90; H,6.48. 5 Separation of the diastereomer of carbonyl(h -cyclopentadienyl) (h (7-ethoxy)cycloheptadien-l-yl) triphenylphosphineiron (II) The title compound was e luted down a 8" x 1" alumina column (neutral, grade II) with a 1:1 v/v n-pentane/benzene mixture. The broad red band was collected in two parts, the first 1/3 and the second 2/3. Both fractions were enriched in the same diastereomer, diastereomer B, the second fraction (60% enrichment) greater than the first fraction (20% enrichment) . Diastereomeric compositions were determined by H NMR measurements of cyclopentadienyl hydrogen resonances.
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125 2 5 Synthesis of carbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) trif luoromethane5 1 sulfonate from carbonyl(h -cyclopentadienyl) (h -(7-ethoxy) cyclohepten-1-yl) triphenylphosphineiron (II) 1) Equal mixture of each diastereomer The title compound was synthesized according to the procedures developed for its 7-methoxy analogue with a 31% yield using a 1:1 v/v pentane/benzene solvent mixture. The product was isolated as an equal mixture of two diastereomer s. The NMR, IR and m.p. are in agreement with the product synthesized here from its 7-methoxy analogue. 2) A 4:1 mixture of diastereomer The title compound was synthesized according to the above procedures. The product was isolated as an equal mixture of two diastereomers. 2 5 Epimerization of carbonyl(h -1 , 2-cycloheptadiene) (h -cyclopentadienyl) triphenylphosphineiron (II) hexaf luorophosphate in the presence of triphenylphosphite A chloroform-d. solution of the title compound (0.01 g; 0.018 mmole) and triphenylphosphite (0.03 g; 0.097 mmole) were followed by H NMR for 4 5 hrs. The NMR remained unchanged and there was no indication (e.g. new cyclopentadienyl hydrogen resonance) that the free triphenylphosphite had replaced the bound triphenylphosphine.
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126 2 5 Epimerization of carbonyl (h -1 , 2-cycloheptadiene) (h cyclopentadienyl) triphenylphosphineiron (II) trifluoromethanesulfonate in the presence of triphenylphosphine Two chloroform-cL solutions of the title compound with a 80:20 diastereomeric composition were made up; the first (a) containing 10 mg of the compound in 1/2 ml of chloroform-d.. and the second (b) containing 10 mg of the compound and 2 mg of triphenylphosphone (0.15 M) in 1/2 ml of chloroform-d. . The solutions were heated at +30°C and the change in diastereomeric compositions with time were determined by NMR measurements of the cyclopentadienyl hydrogen resonances (accurate to ±2) . a b t= min. 83:17 81:19 10 min. 73:27 72:28 20 min. 70:30 70:30 30 min. 67:33 65:35 oo 63:37 65:35 There was no difference in the rate of equilibration between (a) and (b) . The equilibration process is consistant with allene to allyl cation racemization and not epimerization about the iron center via dissociation and reassociation of triphenylphosphine. Synthesis of dicarbonyl (h -cyclopentadienyl) (h -(6-methoxy) cyclohexen-1-yl) iron (II) The title compound was synthesized according to the method developed here for its cycloheptene analogue with a
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127 60% yield. The NMR and IR are consistent with the known compound. 14 IR (neat) 2005s, 1950s cm" 1 ; 1 H NMR (60 MHz, CDC1 3 ) 1.15-2.35 (b,6H), 3.40 (s,3H), 3.50 (bs,lH), 4.87 (s,5H), 5.75 (t,lH). 2 5 Attempted synthesis of dicarbonyl(h -1 , 2-cyclohexadiene) (h cyclopentadienyl) iron (II) trif luoromethanesulfonate To a cold (-78°C) solution of title compound (0.034 g; 0.12 mmole) in 10 ml of methylene chloride was added TMSOTF. The reaction was allowed to stir for 30 mins at -78°C and was quenched with cold (-78°C) diethyl ether. A reddish yellow flocculent precipitate appeared. The precipitate promptly decomposed to a black paste while being filtered at room temperature in an inert atmosphere box. Attempted synthesis of dicarbonyl(h -1 , 2-cyclohexadiene) (h -cyclopentadienyl) iron (II) tetraf luroborate 1) With trimethyloxonium tetraf luoroborate To a suspension of trimethyloxonium tetraf luoroborate (0.34 g; 2.3 mmole) in 7 ml of cold (-78°) methylene chloride was added a 2 ml methylene chloride solution of the h (6-methoxy) cyclohexen-1-yl Fp (0.45 g; 1.6 mmole). There was no reaction after 4 hrs and the starting compound was recovered unchanged. 2) With triethyloxonium tetraf luoroborate To a 5 ml methylene chlorde solution cyclohexen-1-yl Fp (0.26 g; 0.9 mmole) at -78°C was added To a 5 ml methylene chlorde solution of h -(6-methoxy)
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128 triethyloxonium tetraf luoroborate (1 M solution in CH 2 C1 2 ; 0.9 ml; 0.9 mmole) . Cold (-78°C) diethyl ether was added 1 hr later to quench the reaction. The reaction mixture remained unchanged but decomposed to a red oil after the solvent was removed and warmed to room temperature. 2 Low temperature NMR scale synthesis of dicarbonyl(h -1,25 cyclohexadiene) (h -cyclopentadienyl) iron (II) trifluorome thane sulfonate The open end of an NMR tube was attached to a 19/22 male joint with a side mounted 3-way stopcock. A narrow stir rod was inserted into the NMR tube and the whole assembly was capped with a closed end 19/22 female joint. This NMR tube reactor was charged with a plug of a TMSOTF/methylene chloride-d 2 mixture followed by methylene chloride-d„ and a h (6-methoxy) cyclohexen-1-yl Fp/methylene chloride-dmixture. Each of the solvent mixture or solvent was frozen in liq. N» before the next one was added. The frozen plugs were then thawed in cold (-78°C) n-pentane. The female joint was removed under a rapid flushing stream of N_ and the reaction mixture were stirred. The stir rod was withdrawn, the female joint replaced and the reaction mixture was frozen again in liq. N~ . The NMR tube was sealed under vacuum. The reaction mixture was thawed in the NMR probe with the probe temperature preset at -9 5 °C. The progress of the reaction was followed at low temperatures. Refer to Fig. 11 and discussion in Chapter IV.
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129 Synthesis of carbonyl (h -cyclopentadienyl) (h (6-methoxy) cyclohexen-1-yl) triphenylphosphineiron (II) The title compound was synthesized in a 35:65 diastereomeric ratio according to the method developed here for its cycloheptene analogue with a 15% yield. Diastereomeric composition was determined by NMR measurements of the methoxy hydrogen resonances. IR (CHC1-) 1915s cm ; H NMR (100 MHz, CDC1-) 1.0-2.2 (b,6H) , 3.18, 3.35(s,3H), 3.23(m,lH), 4.42, 4.30 (d, 3 J_=1.22, 5H) , 5.16, 5.40(m,lH), 7.32(s,15H); 13 C NMR (25 MHz, CDC1 3 ) 17.93, 22,97, 31,26(C3-C5) , 55.61(07), 84.11(Cp), 86.65, 86.75(C6), 140.06, 140.21(C2), 144,50, 145.47(C1), 127 . 58-136 . 06 (PPh 3 ) , 222.13, 223.42(CO); Anal, calcd. for C^H^OjP^e^ C, 71.27; H,5.98; Found: C, 72.69; H, 6.30. 2 5 Attempted synthesis of carbonyl (h -1 , 2-cyclohexadiene) (h cyclopentadieny) triphenylphosphineiron (II) trif luoromethanesulfonate To h (6-methoxy) cyclohexen-1-yl Fpp (0.55 g; 1.0 mmole) dissolved in an 1:1 v/v mixture of n-pentane and benzene at -10°C was added TMSOTF (0.2 ml; 1.0 mmole). The reaction mixture was allowed to stir for 1 hr after which the solvent mixture was removed in vacuo. Ethyl acetate was added to the dark residue but the desired product failed to precipitate from the solution. The same reaction in pentane yielded a greenish yellow solid isolated via filtration under N„. The solid is stable and can be kept at -35°C for several days. A solution of
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130 the solid decomposes instantly to a dark green solution. M.p.>200°C (the solid does not melt); IR (KBr) 1962s cm" Reaction of carbonyl(h (6-methoxy) cyclohexen-1-yl) (h cyclopentadienyl) triphenylphosphineiron (II) with trimethylsilyl trifluoromethanesulfonate followed by additiona of ethanol To the title compound (0.33 g/ 0.63 mmole) dissolved in 10 ml of pentane at 0°C was added TMSOTF (0.2 ml; 0.63 mmole). An orange precipitate fell out of solution immediately. The precipitate was allowed to settle and the pentane above was removed via a cannula. The precipitate was washed twice (15 ml each) with cold (-78°C) pentane and the wash removed via a cannula. The precipitate was then cooled to -78 °C and a slurry of sodium carbonate in cold ethanol (-78 °C) was added. The reaction mixture was allowed to warm to room temperature over a 2 hr period. The reaction mixture was adsorbed onto alumina and eluted down a 1 1/2" x 1" alumina column (neutral, grade II) first with hexane followed by benzene. A red band eluted by benzene was collected and gave 0.17g (50% yield) of the h 1 (6-ethoxy)cyclohexen-l-yl Fpp as a red oil. IR (KBr) 1905s cm" 1 ; 1 K NMR (100 MHz, CDC1 3 ) 1.17 (t,3H), 1.24-2.00 (b,6H), 3.27 (s,lH), 3.50 (m,2H), 4.35 (d, 5H, 3 J pH =0 . 97Hz ) , 5.04 (t,lH), 7.21 (s,15H); 13 C NMR (25 MHz, CDC1 3 > 16.14(-CH 3 ), 18.96, 30,12, 30.95 (C3-C5) , 63 . 99 (-OCH 2 ~) , !4.12(C6), 84.65(Cp), 139 . 38 (C2 , 3 J p =7 . 33Hz) , 14 5.08 (CI, 2 J pc =2 6.86Hz) , 127 . 54-137 . 77 (PPh 3 ) ,
PAGE 144
131 222.44 (CO, J pc =32.95Hz) ; Anal, calcd. for c 32 H 33°2 P l Fe l C,71.64; H,6.20; Found: C,71.78; H,6.26.
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APPENDIX X-RAY CRYSTAL STRUCTURE OF CARBONYL (h -1 , 2-CYCLOHEPTADIENE) (h -CYCLOPENTADIENYL) TRIPHENYLPHOSPHINEIRON (II) HEXAFLUOROPHOSPHATE Crystal data ; [C H^OPFe] + PF~, M=650.4, triclinic, space group PI, a=9.998(3)A, b=10 . 801 (3) A, c=14 . 675 (4) A, 0=91.40(2)°, 8=107.32(2)°, Y =101 . 24 (2) ° , V=1478 . 1 (6) A 3 , D =1.46 gem" 3 , F(000)=668, p (Mo Ka)+7.0 cm" . Nicolet R3m diffractometer, 4406 reflections [1.5 <20< 47.0°]. 3743 observed with Fo > 3a (Fo) . The structure was solved by direct methods (SOLV included in SHELXTL system) and refined using the 'blocked cascade* least-square method. 380 parameters refined: coordinates and anisotropic thermal parameters of non-H atoms, and isotropic thermal factors of H-atoms in 7-membered ring, and a scale factor. The final R and R (w=l/a 2 ) values are 0.052 and 0.042, respectively, w The atomic coordinates for this work are available on request from the Director of Cambridge Crystallographic Data Center, University Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW. 132
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REFERENCES 1. W.R. Moore, R.D. Bach; J. Am. Chem. Soc. 94 , 3148 (1972). 2. P.D. Gardner, E.T. Marquis; Tetrahedron Lett. 25, 2793 (1966) . 3. C. Wentrup, G. Gross, A. Maquestian, R. Flammang; Angew. Chem. Int. Ed. Engl. 2_2, 542 (1983). 4. G. Krow, "Top. Stereochem." Ed. E.L. Eleil, N.L. Allinger, Wiley-Interscience , New York, 1970, Chap. 5, pg. 31. 5. R.O. Angus, Jr., M.W. Schmidt, R.P. Johnson; J. Am. Chem. Soc, 107, 532 (1985). 6. The C.-C 2 -C, angle was found to be 168.0°. J.L. Juche, J.C. Damlano, P. Crabbe, C. Cohen-Addad, J. Lajzerowicz; Tetrahedron, 3_3' 961 (1977) . 7. W.R. Moore, W.R. Moser; J. Am. Chem. Soc, 92^, 5469 (1970). 8. P.W. Dillon, G.R. Underwood; J. Am. Chem. Soc, 9j5, 779 (1974) . 9. a) A.T. Bottini, F.P. Carson, R. Fitzgerald, K.A. Frost, II.; Tetrahedron, 28, 4883 (1970). b) A.T. Bottini, L.L. Hilton, J. Plott; Tetrahedron, _3_i' 1997 (1975) . 10. M. Balci, W.M. Jones; J. Am. Chem. Soc, 102 , 7608 (1980) 11. R. Winchester, Ph.D. Dissertation, Univ. of Florida (1985). 12. a) B.L. Shaw, A.J. Stringer; Inorg. Chim. Acta Rev. 7, 1 (1973) . b) F.L. Bowden, R. Giles; Coord. Chem. Rev., 20, 81 (1976). c) S. Otsuka, A. Nakamura; Adv. Organomet. Chem., 14, 265 (1976). 13. B. Foxman, D. Mars ten, A. Rosen, S. Raghu, M. Rosenblum; J. Am. Chem. Soc, 99, 2160 (1977). 133
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134 14. F. Manganiello, Ph.D. Dissertation, Univ. of Florida (1982) . 15. J. P. Visser, R.E. Ramakers; J.C.S. Chem. Commun. , 178 (1972) . 16. A.C. Cope, W.R. Moore, R.D. Bach, H.S. Winkler; J. Am. Chem. Soc, 92, 1243 (1970). 17. a) M.J.S. Dewar; Bull. Soc. Chim. Fr. , 1_8, C79 (1951). b) J. Chatt, L.A. Duncanson; J. Chem. Soc, 2339 (1953) . 18. T.L. Jacobs; "Transition Metal Complexes of Allenes" , Chap. 4 in Vol II of the Chemistry of Allenes (ed. S.L. Landor) Academic Press, New York, 1982, 277. 19. S. Otsuka, A. Nakamura, K. Tani; J. Orgonometal. Chem. 14, P30 (1968) . 20. K. Vrieze, H.C. Volger, M. Gronert, A. P. Pratt; J. Organometal. Chem., 1£, P19 (1969). 21. K. Okamoto, Y. Kai, N. Yasuoka, N. Kasai; J. Organometal. Chem., 65, 427 (1974) and references cited therein. 22. a) E.L. Hoel, G.B. Ansell, S. Leta; Organometallics , 5, 585 (1986). b) L.N. Lewis, J.C. Huffman, K.G. Caulton; J. Am. Chem. Soc, 102, 403 (1980). 23. B.M. Foxman; J.C.S. Chem. Commun., 221 (1975). 24. The h -isobutylene Fp complex is commonly used for such a thermal exchange but the h -propene Fp complex was found to work just as well. W.P. Giering, M. Rosenblum; J.C.S. Chem. Commun., 441 (1971). 25. J. Celebuski, G. Monro, M. Rosenblum; Organometallics, 5, 256 (1986) . 26. C.P. Casey, W.H. Miles, J. Tukada; J. Am. Chem. Soc, 106 , 2924 (1985) . 27. Tetraphenylborate anion is known to be susceptible to electrophilic ipso attack. P. Legzdins, D.T. Martin; Organometallics, 2, 1785 (1983). 28. a) B.L. Shaw, A.L. Stringer; Inorg. Chim. Acta Vev. , 1_, 1 (1973). b) M.S. Lupin, J. Powell, B.L. Shaw; J. Chem. Soc. A, 1687 (1966).
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135 29. K. Vrieze, H.C. Volger, A. P. Pratt; J. Organometal. Chem. , 21, 467 (1970) . 30. S. Otsuka, A. Nakamura; "Acetylene and Allene Complexes," Adv. Organomet. Chem., 1A, 265 (1976). 31. For a brief description of spin saturation transfer technique, see: B. Carpenter; " Determination of Organic Reaction Mechanisms, " Wiley-Interscience, New York, 1984, pg. 168. 32. H.M.R. Hoffmann; Angew. Chem. Int. Ed. Engl., 23, 1 (1984). 33. M.D. Radcliffe, Ph.D. Dissertation, University of Florida (1982) . 34. A S-alkyl shift to a Fp-carbene carbon has been reported recently. R.S. Bly, G.S. Silverman; Organometallics, 3, 1765 (1984). 35. T.G. Traylor, H.J. Berwin, J. Jerkunica, M.L. Hall; Pure Appl. Chem., 30_, 599 (1973) and references cited therein. 36. A. Cutler, D. Ehmholt, W.P. Giering, P. Lennon, S. Raghu, A. Rosan, M. Rosenblum, J. Tancrede, D. Wells; J. Am. Chem. Soc, 9_8, 3495 (1976). 37. a) S.R. Su, A. Wojcicki; J. Organometal. Chem., 21_, 231 (1971). b) P. Reich-Rohrwig, A. Wojcicki, Inorg. Chem., 13_, 2457 (1974). c) K. Nicholas, S. Raghu, M. Rosenblum; J. Organometal. Chem., 7_8, 133 (1974). 38. a) D.L. Reger, E. Mintz , L. Lebioden; J. Am. Chem. Soc, 108 , 1940 (1986). b) D.L. Reger, E. Mintz; Organometallics, 3, 1759 (1984). c) T.C. Flood, K.D. Campbell; J. Am. Chem. Soc, 1£6, 2853 (1984). d) T.C. Flood, K.D. Campbell, H.H. Downs, S. Nakanishi; Organometallics, U, 1590 (1983). e) H. Brunner, B. Hammer, I. Bernal, M. Draux; Organometallics, 2, 1595 (1983). f) D.F. Shriver: J. Am. Chem. Soc, 102 , 5093 (1980). 39. T.C. Forschner, A.R. Cutler; Organometallics, _4, 1247 (1985) . 40. See, for example: S.G. Davis; " Organotransition Metal Chemistry: Applications to Organic Synthesis" ; Pergamon Press: New York, 1982, pg. 128.
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136 41. For a brief description of the principle of mcroscopic reversibility, see: J. March; " Advanced Organic Chemistry, 3rd. Ed .," John Wiley & Sons, New York, 1985, pg. 189. 42. K. Stanley, M.C. Baird; J. Am. Chem. Soc, 97, 6598 (1975) . See references cited therein for other R-S conventions . 43. T.G. Attig, P. Rich-Rohrwig , A. Wojcicki; J. Organomet. Chem. , 51, C21 (1973) . 44. J.I. Seeman, S.G. Davies; J. Am. Chem. Soc, 107 , 6522 (1985) . 45. a) G.J. Baird, J. A. Bandy, S.G. Davies, K. Prout; J. Chem. Soc, Chem. Commun. , 1202 (1983). b) G.J. Baird, S.G. Davies, R.H. Jones, K. Prout, P.J. Warner; J. Chem. Soc, Chem. Commun., 745 (1984). 46. Molecular mechanics calculations were performed by Mr. Paul Hanna utilizing MMX Version 1.0 of Gilbert and Gajewski, Indiana University. Mr. Hanna' s assistance is appreciated. 47. This allenic hydrogen resonance is buried beneath the ring methylene resonances. Its approximate chemical shift was measured by H NMR from the appropriately deuterated compound. 48. Representative examples are a) D.L. Hughes, A.L.J. Pombeiro, C.J. Pickett, R.L. Richards; J. Chem. Soc, Chem. Commun, 992 (1984). b) B.M. Foxman; Ibid., 221 (1975). c)N. Yasuoka, M. Morita, Y. Kai, N. Kasai; J. Organomet. Chem., 90, 111 (1975). 49. D.L Thorn; Organometallics , A, 192 (1985). 50. F.J. Manganiello, S.M. Oon, M.D. Radcliffe, W.M. Jones; Organometallics, 4, 1069 (1985) . 51. D. Dombek, R.J. Angelici; Acta Inorg. Chim. , 1_, 345 (1973). 52. B.M. Mattson, W.A.G. Graham; Inorg. Chem., 20_, 3186 (1981) .
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BIOGRAPHICAL SKETCH The author was born on September 6, 1956, in Singapore. His family later moved to Malaysia where he grew up and became a citizen. He received his early education at Sam Tet School in Ipoh. He left Malaysia before completing his Sixth Form studies and came to Furman University in South Carolina. He graduated from Furman in June 1979 with a B.S. in chemistry. The author also received a M.S. in polymer science and engineering from the University of Massachusetts in September 1980. He came to the University of Florida in September 1980 and studied under Dr. W.M. Jones. The author enjoys cooking and is often told that he is an excellent cook and baker. He also considers organic chemistry a great culinary experience. His hobbies include scuba diving, sailing, downhill skiing and bicycling. He also holds a private pilot license and prefers flying to jumping out of perfectly good airplanes. He loves classical music, theater, ballet and opera and is a piano student. The author is a big brother with the Big Brothers and Big Sisters of Gainesville and supports the manatees in Florida. 137
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138 The author has accepted a postdoctoral position with Dr. Michael Doyle of Trinity University in San Antonio, Texas.
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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. -\ William M. JonesX Chairman Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. John F. Helling Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Professor of Chemistry
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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. &
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UNIVERSITY OF FLORIDA 3 1262 08553 3585
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