14
TAMING THE VINYL CARBOCATION
by BETHANY HALFORD
IN THE REALM OF REACTIVIT Y,
only react to form one of two possible
The chemists think the area is still
THE VINYL CARBOCATION IS A
enantiomers in a C–H insertion
rich for exploration. Sigman, who is
BEAST SO FLEETING, SO KEEN
reaction. The chemists liken the
currently chair of the Department
TO COMBINE OR REARRANGE,
catalyst to an enzyme in that it is able
of Chemistry at the U, says the
THAT MANY HAVE DOUBTED
to exert stereocontrol over a highly
information they’ve gleaned could
THAT IT CAN BE TAMED TO
reactive intermediate.
help their team redesign the catalysts to be more practical. The chemists
CREATE MOLECULES IN A
Houk and Sigman then modeled the
would also like to expand their
behavior computationally to verify
substrates. In this work, the C–H
But by confining a vinyl carbocation
that the reaction is indeed going
insertion was an intramolecular
within a bulky catalyst, chemists
through the vinyl carbocation.
transformation, in which the vinyl
have now shown they can coax this
“Now that chemists know
cation and the C–H bond were
wildly reactive intermediate to insert
these intermediates
in the same molecule. They’d
itself into a carbon-hydrogen bond
can be harnessed,”
like to see the work extended
in which one enantiomer of a chiral
Sigman says, “they
to intermolecular reactions—in
product is preferentially produced.
can be creative
which the vinyl cation
about what
and the C–H bond
The enantioselective insertion comes
kind of
it inserts into are on
from the collaboration of researchers
molecules
different molecules. <
in Hosea M. Nelson’s group at the
they make
California Institute of Technology,
with this
Matthew S. Sigman’s group at the
type of
University of Utah, and K. N. Houk’s
reaction.”
STEREOSELECTIVE WAY.
group at the University of California, Los Angeles.
"The idea that carbocations
Excerpted with permission from Chemical & Engineering News. Copyright ©2022 by the American Chemical Society. The article, “Chemists tame the vinyl carbocation,” was first published in C&EN on December 8, 2022 and appeared in Volume 100, Issue 44.
After screening dozens of catalysts,
as reactive as unstabilized vinyl
the chemists found a family of what’s
cations could be engaged in highly
Image credit: Science; Confined to an organic
called imidodiphosphorimidate
enantioselective reactions would have
catalyst (space-filling model), this vinyl
organocatalysts designed by its
seemed unlikely until recently," says
carbocation substrate (stick model) is poised to
developer to serve as general
Harvard University's Eric N. Jacobsen,
react stereoselectively.
catalysts for a variety of asymmetrical
who also develops new reactions. The
transformations. This family was
work represents "a significant advance
able to create and confine the vinyl
in asymmetric catalysis with highly
cation in such a way that it would
reactive intermediates."
CATA LYST
| Department of Chemistry
