Synthesis and x-ray structure of iron stabilized strained cyclic allenes

Material Information

Synthesis and x-ray structure of iron stabilized strained cyclic allenes Valence isomerization between linear perpendicular and bent planar allene
Oon, Su-Min, 1956- ( Dissertant )
Jones, William M. ( Thesis advisor )
Helling, John P. ( Reviewer )
Palenik, Gus J. ( Reviewer )
Dorsey, John G. ( Reviewer )
Sabin, John R. ( Reviewer )
Place of Publication:
Gainesville, Fla.
University of Florida
Publication Date:
Copyright Date:
Physical Description:
xiii, 138 leaves : ill. ; 28 cm.


Subjects / Keywords:
Anions ( jstor )
Chlorides ( jstor )
Ethers ( jstor )
Hydrogen ( jstor )
Iron ( jstor )
Isomerization ( jstor )
Ligands ( jstor )
Solvents ( jstor )
Thermal decomposition ( jstor )
Writing processes ( jstor )
Allene ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
bibliography ( marcgt )
non-fiction ( marcgt )
thesis ( marcgt )


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 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.
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography: leaves 133-136.
General Note:
General Note:
Statement of Responsibility:
by Su-Min Oon.

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Full Text







To my mother and

in memory of my father


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.


ACKNOWLEDGEMENTS . . . . . . . . .

LIST OF TABLES . . . . . . . . . .

LIST OF FIGURES . . . . . . . . . .

ABSTRACT . . . . . . . . . . ..




* xi~

I INTRODUCTION . . . . . . . . . 1




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
Variable temperature C NMR studies of the
dicarbonyl(h -1,2-cycloheptadiene)(h -
cyclopentadienyl)iron(II) tetrafluoro-
borate . . . . . . . . . 110

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


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 -
(h -cydopentadienyl)triphenylphosphine-
iron(II) . . . . . .... . . 119

Attempted decarbonylation of carbonyl(h -
(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


Separat on of the diastereom r of carbonyl
(h -cyclopentadienyl)(h -(7-ethoxy)cyclo-
heptadien-1-yl)triphenylphosphineiron(II). 124
Synthes s of carbonyl(h -1,2-cycloheptadiene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) tgifluoromethanesulfonate from
carbonyl(h -cyclopentadienyl)(h -(7-
iron(II) . . . . . . . . . 125

Epimerization of carbonyl(h2-1,2-cycloheptene)
(h -cyclopentadienyl)triphenylphosphine
iron(II) hexafluorophosphate in the
presence of triphenylphosphite ...... 125
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
Attempted synthesis of dicarbonyl(h -1,2-
cyclohexadiene)(h -cyclopentadienyl)iron
(II) trifluoromethanesulfonate ...... 127
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


APPENDIX . . . . . . . . . . . 132

REFERENCES . . . . . . . . ... . .133

BIOGRAPHICAL SKETCH . . . . . . . . .. .137



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


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



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


We have synthesized and studied the racemization

process of dicarbonyl(h5-cyclopentadienyl)iron(II) [Fp] and


[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.




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



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


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


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


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

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.


H K t-OBu > 0

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 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.


80 kcal/mole 25 kal/mole

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).


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)

8 4 9

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).







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 C2

I| 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 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.




Unlike the Fp completed allenes, not all platinum
completed allenes are fluxional. The Pt(0) complex of
1,3-diphenylpropadiene (16) is not fluxional.1 Ha and Hb
remain distinct in the IH NMR even at high temperatures.


-H-H Pt


Ph Hb



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
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+

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




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


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


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.



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




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-


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.


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.













04J N

r-4 -HJ







A reasonable mechanism for the decomposition of the

h2-1,2-cycloheptadiene Fp (4a) is as follows.

FpFp Fp


4a 20

Fp Fp+

FpOTF +0


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

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


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
Ph3P I

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-


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-



+ (CH3)30BF4 BF4


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.

OCH3 Fp+

+ Ph3CBF4/PF6 BF4/PF6


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+


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


Scheme II

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

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


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


+ 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
is best referred to the review article by Hoffmann32 for all

the possible products from concerted and stepwise cycloaddi-


The pi-donating strength of a Fp in the homocycloocta-

trienylidene Fp was found to be similar to that of a methoxy
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 +


S- Fp


Scheme IV
Stepwise addition

: Fp -


a z Fp+
,, 0 H+

jZ +

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


Fp Fp+

Z a Fpz

b + Fp Fp+



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

5 29

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




1 H



0+ FplI


\ + 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


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


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 0


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


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


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






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.



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
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 -

/-- 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

H Fp+
H Ph3C+ F


H Fp Fp+


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

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


+ Qui





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




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


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)


Fe R P3 > Fe -

0/C OC R

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
have not been successful.3

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).




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



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-


Anti attack of a nucleophile at a h2-olefin Fp bond is
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
Fp CH3 / 1,.CH3
4R -<

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 H+ CH30O.i

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


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


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






0 0
. 0
4J -H

a, *
e --
o ao



r 0




0 m


0 m



-i Z











0 r


(n 0

U4J *



F4 r





4-) -1


0 04




E zr

4-4 U

.0 H



0 0








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).



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


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.


0C.0 03P j

03P CO


and enantiomer RS

and enantiorner SS

FeN Fe+

0H H HN-
OC O3P'*
03P OC

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.

Fe Fe
/// \
OC CO + P03 hv 03


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


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.


03P3 Fe H 03P FeH
H\ H H


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

03P Fe


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 //

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










I )










[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
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

Fe j /<
OC 03P I + P(O0)3 THF> o/ /

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)


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


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.
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

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


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


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


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.


Fe --*1-- Fe

02P 02?--<>P


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


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


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.