A HELICAL POLYPHENYLACETYLENE
457
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As a reference, the copoly(1/PSPA) was also used as a cat-
alyst for the alkylation of benzaldehyde and showed a lower
optical yield (16.1% ee) than copoly(1/2) containing a similar
amount of unit 1, as shown in Table 1, Nos.1 and10. The
higher optical yield using copoly(1/2) might be caused by
the tighter and more rigid chiral helical cis-cisoid conforma-
tion of copoly(1/2) making the chiral environment in
copoly(1/2) more stable, and therefore, the chirality could
be transferred more efficiently during the alkylation of benz-
aldehyde resulting in the higher enantioselectivity. The rela-
tively loose and soft chiral helical cis-transoidal conformation
of copoly(1/PSPA) made the chiral environment surrounding
the active sites (amino alcohol groups’ positions) unstable,
which caused the lower optical yield than that using co-
poly(1/2). All the polymer catalysts could be easily and effec-
tively recovered by precipitating the reaction mixture in
methanol with more than 80% recovery yields.
In summary, novel copolyphenylacetylenes having achiral
amino alcohol pendent groups (copoly(1/2)), whose chiral
structures arise solely from the one-handed helical conforma-
tion, was successfully prepared by the helix-sense-selective
copolymerization of two achiral phenylacetylenes, i.e., 1
having an achiral amino alcohol residue and 2 having two hy-
droxyl groups, using a chiral catalytic system ([Rh(nbd)Cl]2/
(S)- or (R)-phenylethylamine). The chiral helical polymers
were successfully used as the chiral ligand to catalyze the
enantioselective diethylzinc addition to benzaldehyde in
higher enantioselectivities than a chiral polymer having its
chiral structures both in the main chain and the side groups.
Since the sign of the chirality and %ee of the products of the
alkylation catalyzed by copoly(1/2)s was controlled by the
sign of the chirality and %ee of the catalyst, a controllable
enantioselectivity of this polymeric chiral catalyst was found.
In addition, the polymer catalysts could be recovered by
precipitation after the reaction. To the best of our knowledge,
this is the first example of an asymmetric catalyst for
the enantioselective alkylation by one-handed helical
polyphenylacetylenes without any other chiral moieties as a
chiral ligand for the catalyst.
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SUPPORTING INFORMATION
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Additional supporting information may be found in the
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Chirality DOI 10.1002/chir