the optical resolution step from commercially available
starting materials.2b We decided to explore alternative chiral
groups which are easily accessible and provide efficient chiral
environments comparable to or, preferably, more effective
than the binaphthyl structures. Herein, we describe highly
screw-sense selective polymerization of 1 by means of chiral
phenylpalladium initiators 4-6 (Figure 1), in which the
phenyl groups are substituted with optically active o-
substituents such as aminocarbonyl, oxazoline, and imida-
zoline groups derived readily from the corresponding chiral
amine precursors.
First of all, optically active o-(N-substituted aminocarbo-
nyl)iodobenzenes 7 were prepared from o-iodobenzoic acid
with commercially available optically active prim-amines and
then subjected to palladation with a palladium(0)-dimeth-
ylphenylphosphine complex generated from Cp(π-allyl)-
palladium with 3 equiv of the phosphine (Scheme 2). The
Scheme 2
Figure 1. Chiral organopalladium initiators (R*PdIL2) used in the
previous studies (3) and in this study (4-6).
demonstrate that the helical sense of polymer main-chain
consisting only of sp2-hybridized carbons can be effectively
induced by a sole screw-sense determining group at the
polymer end.3,4
resultant o-(aminocarbonyl)phenylpalladium(II) complexes
were reacted with 1 molar equiv of 3,6-di-p-tolyl-1,2-
diisocyanobenzene (1a) to afford quinoxalinylpalladium(II)
complexes 4a-f bearing o-(N-alkylaminocarbonyl)phenyl
substituents.
In the presence of chiral initiators 4a-f (1/40 equiv) thus
prepared, polymerization of 1,2-diisocyanobenzene 1b was
carried out at room temperature (Scheme 3). After consump-
The remarkable stereocontrol in the asymmetric polym-
erization may be largely due to the ability of the binaphthyl
skeleton to create a highly effective asymmetric environment
as has been established in a variety of asymmetric reactions5
or chiral recognition processes.6 Although some simple
binaphthyl derivatives are now readily available in enan-
tiopure forms, preparation of optically active binaphthalene
derivatives with more elaborate structures often meets with
synthetic difficulties. In particular, the preparation of our
binaphthyl-based initiators requires several steps including
Scheme 3
(3) The screw-sense control by a sole chiral end group was demonstrated
in nickel-catalyzed polymerization of monoisonitrile. Kamer, P. C. J.; Nolte,
R. J. M.; Drenth, W. J. Am. Chem. Soc. 1988, 110, 6818.
(4) Screw-sense control of conjugated polymers by other means have
been reported. By chiral side chains, see: Takei, F.; Yanai, K.; Onitsuka,
K.; Takahashi, S. Chem. Eur. J. 2000, 6, 983. Ramos, E.; Bosch, J.; Serrano,
J. L.; Sierra, T.; Veciana, J. J. Am. Chem. Soc. 1996, 118, 4703. Yashima,
E.; Maeda, Y,; Okamoto, Y. J. Am. Chem. Soc. 1998, 120, 8895. By
incorporation of configurationally stable axial chiralities in the main chains,
see: Dai, Y.; Katz, T. J. J. Org. Chem. 1997, 62, 1274. Gin, M. S.;
Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121,
2643. By interaction of polymer side chains with optically active small
molecules, see: Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y. J.
Am. Chem. Soc. 1996, 118, 9800. Yashima, E.; Matsushima, T.; Okamoto,
Y. J. Am. Chem. Soc. 1997, 119, 6345. By chirality at the propagating living
termini, see: Deming, T. J.; Novak, B. M. J. Am. Chem. Soc. 1992, 114,
7926.
(5) (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (b)
Hayashi, T. Acc. Chem. Res. 2000, 33, 354. (c) Shibasaki, M.; Sasai, H.
Top. Stereochem. 1999, 22, 201.
(6) (a) Kyba, E. B.; Koga, K.; Sousa, L. R.; Siegel, M. G.; Cram, D. J.
J. Am. Chem. Soc. 1973, 95, 2692. (b) Review: Stoddart, J. F. Top.
Stereochem. 1987, 17, 207.
tion of 1,2-diisocyanobenzene 1b, the polymerization was
quenched by NaBH4, giving 2b in moderate-to-good isolated
yields (Table 1). With initiators (S)-4a-d having N-(R-
substituted ethyl)aminocarbonyl groups, right-handed helical
polymers with low screw-sense excesses (se) (5-26%) were
formed (entries 1-4). The selectivity was significantly
improved by use of (S)-4e having the (S)-N-[R-(cyclohexyl)-
ethyl]aminocarbonyl group, giving 2b with 70% se (entry
5). L-Valine-derived initiator 4f resulted in the formation of
2b with low se (entry 6).
We then turned our attention to cyclic chiral groups such
as oxazoline and imidazoline for further improvement of the
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Org. Lett., Vol. 4, No. 3, 2002