Control
of the precise structure of molecules
is of extreme importance in nature,
medicine and fine chemicals, like
fragrances. Developing direct routes
to molecules with just the right shape
is the grand challenge of synthetic
chemistry. Subtle differences in shape,
for example molecules that are exact
mirror-images of each other, yet distinct
due to the configuration of chemical
groups attached to a single carbon,
can have profoundly different biological
effects—one enantiomer may be
a therapeutic drug and the other a
toxin. Therefore, efficient means
to purify one mirror image from the
other are essential to the fine chemicals
and pharmaceutical industries. Olefins
are particularly challenging to resolve;
due to the lack of polar groups there
is no convenient chemical handle to
bind one enantiomer more strongly
than the other (imagine the increase
in difficulty of distinguishing your
right and left hands if you had only
fingers and no thumbs). On the other
hand, the motivation for resolving
olefins is high because they are general
and versatile building blocks in the
synthesis of more complex molecules.
Researchers in the Center for the
Science and Engineering of Materials
at Caltech are taking a unique approach
to this problem, creating catalysts
that will produce polymers that selectively
incorporate one handedness when presented
with a random (racemic) mixture of
chiral molecules. The result is to
create a unique class of polymers
that are semicrystalline with side
groups that are enantio-enriched and
to leave behind an enantio-enriched
pool of olefins—a major step
toward supplying enantio-pure starting
materials for the synthesis of drugs
and fine chemicals.
Professor John Bercaw with co-workers
Endy Min, Cliff Barr and Jeff Byers
have shown that the catalyst above
can have a selectivity of over 15:1
for incorporating a single enantiomer
(S), creating a new class of polymers
while achieving synthetically useful
purities of the unpolymerized olefin
enantiomerically enriched in R.
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