EXTREME C–C BONDS
Highly strained CYCLOPROPANE-BASED molecules help
chemists refine ideas about chemical bonding
STEPHEN K. RI TTER, C&EN WASHINGTON
THE CARBON-CARBON BOND is argu-ably the most fundamental connection in
all of chemistry. Yet a century after chemists began in earnest to unravel the mysteries of chemical bonding, the intricate
details of C–C bonds are still unfolding.
Three research teams have recently contributed to the growing understanding—
and growing complexity—of C–C bonding
with reports on unique cyclopropane-based compounds.
In one case, Akira Sekiguchi and coworkers at the University of Tsukuba, in Japan,
reported the synthesis of the first examples
of aryl-substituted tetrahedranes (J. Am.
Chem. Soc. 2009, 131, 3172). These compounds exhibit unusual electronic properties made possible by a stabilizing C–C
bond that joins the geometrically strained
tetrahedrane and the aryl substituent.
Tetrahedranes are pyramid-shaped
C compounds that in essence consist of
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four fused cyclopropane rings. Chemists
consider the parent tetrahedrane, C H , to
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be one of the most highly strained organic
molecules yet conceived. The compound
is so exceedingly unstable that chemists
haven’t been able to make it. Sekiguchi’s
group and the group of Günther Maier
of Justus Liebig University, in Giessen,
Germany, have shown that tetrahedranes
supported by bulky substituents such as
tert-butyl or trimethylsilyl are stable.
“Tetrahedrane is very unstable due to its
high cage strain and high-energy C–C bond
orbitals,” notes computational chemist
Yirong Mo of Western Michigan University,
who has studied the bonding in tetrahedrane and
its derivatives. Although
tetrahedrane technically
is a saturated molecule, it
acts like it’s unsaturated,
possibly because of “
three-dimensional aromaticity”
of the cagelike σ-bond network, Mo says.
One way to stabilize the
compound is by forming
a bond between tetrahedrane and a large substitu-
ent, which provides a conduit for electrons
to flow and offers extra stability through a
σ-π hyperconjugation effect. Mo has projected that adding substituents with better
electron-donating abilities to tetrahedranes should enhance hyperconjugation
and thus stability, which could be observed
in the form of shorter C–C bond lengths.
Fulvalenes are a broad class of hydrocarbons consisting of two unsaturated rings
joined through a C=C bond. Although many
examples of fulvalenes with different ring
sizes are now known, the simplest fulvalene,
composed of two cyclopropene rings, has
never been made. Like the parent tetrahedrane, it is too unstable to stand on its own.
Bertrand and coworkers turned to bulky
aromatic ring substituents to lend a helping
hand. At first, they tried to add two triisopropylphenyl groups to a chlorinated cyclopropene, with the idea of coupling two of
the cyclopropene rings to make the triafulvalene. But two triisopropylphenyl groups
are too big to fit around cyclopropene.
The researchers resorted to using just
Li
(CH ) Si
33
F
F
CFC CCH
65 6 5
(CH ) Si
33
Si(CH )
33
(CH ) Si
33
Si(CH )
33
(CH ) Si
33
F
F
EXTENDED SUPPORT Electron-donating perfluorophenyl group helps
stabilize the strained tetrahedrane framework via the central C–C bond (red).
Sekiguchi and coworkers took a crack at
doing that by adding unsaturated groups
to trimethylsilyl-substituted tetrahedrane.
Their initial attempts with phenyl groups
failed because the electron-donating
properties don’t appear to be sufficient.
But when the researchers used fluorinated
phenyl groups, they successfully made aryl-substituted tetrahedranes.
The C–C bond joining a single perfluorophenyl group with the trimethylsilyl-substituted tetrahedrane is 1.450 Å, shorter
than the usual 1.507-Å bond observed in
other examples of sp3-sp2 hybridized bond
systems, Sekiguchi and coworkers note.
When the perfluorophenyl group has an
ethynylphenyl substituent, which supplies
additional electron density into the tetrahe-
drane-aryl system, the bond
is shorter at 1.438 Å, sup-
porting Mo’s prediction.
In a second case of
unique C–C bonding, Guy
Bertrand and coworkers at
the University of California,
Riverside, devised a plan to
synthesize a stable triafulvalene—the smallest and most
strained member of the fulvalene family of compounds
(Angew. Chem. Int. Ed. 2009,
48, 517).
Triafulvalene derivative
one triisopropylphenyl along with a tri-methylphenyl (mesityl) group. With their
bulkily substituted dichlorocyclopropene
in hand, Bertrand and coworkers reduced it
with magnesium metal, which coupled two
of the molecules to form the substituted
triafulvalene derivative they were after.
The crystal structure of the compound
reveals that the C=C bond joining the two
cyclopropene rings is exceedingly short,
even for a double bond, at only 1.303 Å. The
bond could be shorter still perhaps, Bertrand’s team notes, but the bond angles of
the bulky substituents suggest that conjugation between the aromatic rings and the
fulvalene system is weak. In addition, other
C–C bond lengths in the cyclopropene
rings indicate that the π bonding is highly
localized around the C=C bonds.
It’s not surprising then that the substituted triafulvalene is quite reactive,
Bertrand says. At room temperature, water
adds across the central C=C bond to form
the HC–COH adduct. “This is certainly one
of the very rare examples of spontaneous
addition of water to the C=C bond of a hydrocarbon,” Bertrand points out.
In a third case of unusual C–C bonding,
a multinational team of computational
chemists has taken a fresh look at the
bonding in [ 1. 1. 1]propellane, a propeller-shaped C molecule made up of three
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