Angewandte
Chemie
DOI: 10.1002/anie.200803719
Chiral Catalysts
Helical Poly(quinoxaline-2,3-diyl)s Bearing Metal-Binding Sites as
Polymer-Based Chiral Ligands for Asymmetric Catalysis**
Takeshi Yamamoto and Michinori Suginome*
In memory of Yoshihiko Ito
A helix is one of the simplest and best-organized chiral
structural motifs, being widely found not only in molecules,
but in natural and artificial materials. Naturally occurring
helical macromolecules, such as DNA, RNA, and polypep-
tides, adopt one of the two helical senses (right or left) and
play vital biological roles, which depend on their three-
dimensional chiral structure. Helical synthetic polymers have
also gained increasing interest on the basis of recent progress
in asymmetric polymer synthesis, which has enabled the
selective construction of unnatural macromolecular architec-
Figure 1. A system for catalytic asymmetric synthesis using optically
tures with nonracemic helical structures.[1] Efficient induction
active, single-handed helical polymers as a chiral catalyst.
of the main chain helical sense to polymers, such as
poly(methacrylate)s,[2]
poly(isocyanate)s,[3]
poly(isocya-
nide)s,[4] poly(acetylene)s,[5] and polyguanidines,[6], has been
achieved. In the unnatural macromolecular world, even
achiral monomers, which have no stereogenic centers, can
form helical structures in which either a left- or right-handed
helical sense is prevalent.
systems that enable higher selectivity and catalyst efficiency
for the development of practical chiral polymer catalysts.
The conditions for synthetic helical polymers to serve as
practical chiral catalysts in asymmetric synthesis include:
a) very high purity for a one-handed screw sense; b) a highly
stable helical structure, even in solution, with no racemization
or denaturation; c) modifiable side chains onto which cata-
lytically active sites can be introduced without affecting the
helical structure. Most nonracemic helical polymers are
unable to fulfill all of these requirements, leading to the
paucity of highly effective polymer catalysts for asymmetric
synthesis.
Poly(quinoxaline-2,3-diyl)s are a unique class of helical
polymers prepared by living polymerization of o-diisocyano-
benzenes.[12] They feature an exceptionally robust helical
structure, which shows no change even at 808C in solution for
several days.[13] The most striking feature of these polymers is
the high screw-sense excess, which relies on the chiral
terminal group derived from the chiral initiator for the
asymmetric living polymerization.[14] With these unique
characteristics, we have studied the application of the
polyquinoxaline scaffold to new chiral polymer catalysts.
Herein, we report the asymmetric synthesis of new quinoxa-
line polymers with a coordinating group on a side chain, and
their use as chiral ligands for transition-metal catalysts. The
new monodentate polymer-based ligands are used in palla-
dium-catalyzed asymmetric hydrosilylation of styrenes,
affording a remarkable level of asymmetric induction.
For the synthesis of phosphine-substituted polymers, we
prepared the o-diisocyanobenzene 1, bearing a diphenylphos-
phine oxide group, as the monomer for the living polymer-
ization (Scheme 1). To obtain good solubility and to simplify
the structural analysis of the polymer, we copolymerized 1
with “spacer monomer” 2, which has no coordinating group
and thus served as the framework of the helical structure. The
One of the important functions of chiral biopolymers is to
undertake biocatalyses, in which highly enantioselective
reactions take place on the chiral architecture. It seems
likely that unnatural polymer scaffolds can be tailored exhibit
the same properties by virtue of the freedom of molecular
design, which allows the possibility of using either a left- or
right-handed helix and the introduction of metallic elements
that usually are not contained in natural biosystems
(Figure 1).[7] The use of chiral helical polymers as scaffolds
for chiral reaction environments was first realized with
poly(methylmethacrylate)-based chiral polymers,[8,9] followed
by the use of other helical polymers as chiral catalysts.[10]
However, the degree of asymmetric induction and catalyst
efficiency in these systems were still moderate in comparison
with existing low-molecular-weight chiral catalysts and poly-
mer catalysts into which low-molecular-weight chiral catalysts
are embedded.[11] It is highly desirable to find new polymer
[*] T. Yamamoto, Prof. Dr. M. Suginome
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2722
E-mail: suginome@sbchem.kyoto-u.ac.jp
[**] This work was supported by Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and
Technology (Japan) and the Ogasawara Foundation for the Promo-
tion of Science and Engineering.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 539 –542
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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