+
+
4334
Organometallics 1996, 15, 4334-4336
Notes
Asym m etr ic Tr ica r bon ylir on Com p lexa tion of
Cycloh exa d ien e Dir ected by Ch ir a l Ester a n d Am id e
Au xilia r ies
Chi Wi Ong,* Cheng Sheng Huang, and Tsu Hsing Chang
Department of Chemistry, National Sun Yat Sen University, Kaoshiung, Taiwan 804
Received May 17, 1996X
Summary: Coupling of (+)-menthol, (+)-phenylethyl-
amine and (-)-proline methyl ester with 4-methoxy-1-
cyclohexa-2,4-dieneacetic acid gave the chiral ester 1 and
amides 2 and 3, respectively, which were then isomerized
into the 1,3-dienes. The diastereoselection induced by
the chiral ester and amide auxiliaries during complex-
ation with nonacarbonyliron was quite encouraging. The
diastereomeric ratios of the ester- and amide-Fe(CO)3
complexes formed were determined directly from the 1H
NMR integration of observable diastereotopic chemical
shifts.
mediated diastereoselective synthesis of spirocycles.7
The proposed intramolecular delivery of the Fe(CO)3
moiety using chiral ester and amide auxiliaries might
prove more successful for the asymmetric recognition
of the two enantiofaces in cyclohexadiene. In this paper
we describe the first stage of this endeavor and dem-
onstrate the feasibility of using chiral ester and amides
auxiliaries for the synthesis of enantiomerically en-
riched tricarbonyl-cyclohexadiene complexes.
4-Methoxy-1-cyclohexa-1,4-dieneacetic acid was pre-
pared by the Birch reduction of commercially available
(4-methoxyphenyl)acetic acid.8 The Birch reduction
product has been reported to be unstable and required
immediate esterification. We have overcome this prob-
lem by washing the ethereal extract of the Birch product
with a small amount of dilute sodium bicarbonate
solution to remove residual formic acid present, thus
preventing hydrolysis of the enol ether. The 4-methoxy-
1-cyclohexa-1,4-dieneacetic acid, a white solid obtained
in this way, was stable in the refrigerator for months.
A number of chiral alcohols and amines were reacted
with 4-methoxy-1-cyclohexa-2,4-dieneacetic acid by uti-
lizing carbonyl diimidazole as a condensing agent to give
the corresponding chiral ester and amide derivatives
(Scheme 1). Thus (+)-menthol gave the chiral ester 1;
(+)-phenylethylamine and (-)-proline methyl ester gave
the chiral amides 2 and 3, respectively. The unconju-
gated cyclohexa-1,4-dienes were preconjugated into the
respective 1,3-conjugated dienes 1a , 2a , and 3a with
Wilkinson’s catalyst in good yields. Reactions of 1a , 2a ,
and 3a with nonacarbonyldiiron in benzene at room
temperature gave complexes 4-6, respectively, in 40-
50% yield. These yields were considered good for the
complexation reaction. Our next task was to determine
the degree of diastereoselectivity achieved during com-
plexation for the various chiral ester and amide auxil-
iaries.
The development of highly efficient methods for the
construction of chiral tricarbonyliron complexes for
organic synthesis has elicited considerable synthetic
efforts.1-5 Chiral tricarbonyliron complexes are power-
ful starting materials in enantioselective synthesis of
natural products since the metal totally dominates
stereocontrol in the subsequent bond constructions.
The use of chiral auxiliary-directed asymmetric tri-
carbonyliron complexation of acyclic dienes has been
reported to proceed in good yield and high diastereo-
selectivity.2 On the other hand, the level of diastereo-
isomeric enrichment during tricarbonyliron complex-
ation using a chiral alkoxy auxiliary in the important
class of cyclohexadiene ligands has been rather poor and
remains a challenge.3,4 New methods must be devised
to increase the influence of the chiral auxiliary during
tricarbonyliron complexation with the cyclohexadiene
ligands. We here report the first attempt to effect
facially selective complexation of the cyclohexadiene
ligand via an intramolecular transfer of the tricarbon-
yliron group attached to chiral ester and amides aux-
iliaries. This type of approach has not previously been
used in the case of cyclohexadiene ligands and Fe(CO)3.
The 4-methoxy-1-cyclohexa-1,3-dieneacetic acid deriva-
tive was chosen on the basis of the previous proposed
coordination of the carbonyl group to Fe(CO)n during
complexation.6 Furthermore this complex has been
shown to be an important building block for the iron-
The diastereoselectivity (Table 1) may be determined
directly from the complexes 4-6 if diastereotopic chemi-
cal shifts were observed in the 1H NMR spectra.
Another indirect method involved the conversion of the
complexes 4-6 into the known tricarbonyl[methyl
X Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) Birch, A. J .; Stephenson, G. R. Tetrahedron Lett. 1981, 779.
Birch, A. J .; Raverty, W. R.; Stephenson, G. R. Organometallics 1984,
3, 1075.
(2) Pearson, A. J .; Chang, K.; McConville, D. B.; Youngs, W. J .
Organometallics 1994, 13, 4.
(3) Potter, G. A.; McCague, R. J . Chem. Soc., Chem. Commun. 1990,
1172.
(4) Ong, C. W.; Hsu, R. H. Organometallics, 1994, 13, 3952.
(5) Howard, P. W.; Stephenson, G. R.; Taylor. S. C. J . Chem. Soc.,
Chem. Commun. 1988, 1603; 1990, 1182.
(6) Greaves, E. O.; Knox, G. R.; Pauson, P. L. J . Chem. Soc., Chem.
Commun. 1969, 1124; Greaves, E. O.; Knox, G. R.; Pauson, P. L.; Toma,
S.; Sim, G. R.; Woodhouse, D. I. J . Chem. Soc., Chem. Commun. 1971,
257.
(7) Knolker, H. J .; Boese, R.; Hartmann, K. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1678. Knolker, H. J .; Hartmann, K. Synlett. 1991, 428.
(8) Pearson, A. J .; Chandler, M. J . Chem. Soc., Perkin Trans. 1 1980,
2238.
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