February 27, 2012 - IMM@BUCT

NEWS OF THE WEEK
F
F
R
F
R
F
F
F
H 2 N +
HN
Aniline
+
F
B
F
F
H 2
F
F
F
Cyclohexyl
derivative
F
– [HB(C 6 F 5 ) 3 ]
F
F
METAL-FREE
HYDROGENATIONS
ORGANIC SYNTHESIS: Lewis acid-base
pairs enable unprecedented reduction
of anilines and other aromatics
IN A CHEMICAL FIRST, an international research
team has developed a metal-free reaction that hydrogenates
aromatic rings to form cyclohexyl derivatives.
The achievement could spark a broader range of
applications for industrial hydrogenations, which are
widely used for processing petroleum and foods.
A metal-free aromatic hydrogenation is surprising,
says team leader Douglas W. Stephan of the University
of Toronto, because it’s exceedingly hard to overcome
the additional stability a molecule gains from aromaticity,
even with the best transition-metal catalysts.
Stephan and his colleagues have done so by using a
chemical construct known as a frustrated Lewis pair,
which Stephan introduced in 2006.
Lewis acid-base adducts are common in chemistry:
An electron-deficient Lewis acid readily shares a Lewis
base’s spare pair of electrons. However, when the Lewis
acid and base have bulky substituents, their ability to
form a close relationship is denied, causing the pair to
become “frustrated.”
But the pair still garners penned-up reactivity, comparable
with that of an organometallic catalyst. Several
research groups have shown that frustrated Lewis pairs
can intercept and split H 2 during an electron tug-of-war
and subsequently hydrogenate compounds such as imines,
silyl ethers, and N-heterocyclic compounds.
Stephan’s group in collaboration with computational
chemist Stefan Grimme at the University of Bonn, in
Germany, tackled the hydrogenation of aromatics by
using B(C 6 F 5 ) 3 as the Lewis acid and various anilines
as the Lewis base ( J. Am. Chem. Soc., DOI: 10.1021/
ja300228a ). When H 2 is added, the frustrated Lewis
pair splits H 2 and then reductively adds hydrogens to
aniline’s aromatic ring to form cyclohexylammonium
borate salts. Stephan says the salts could be easily deprotonated
to release the free cyclohexylamines.
Princeton University’s David W. C. MacMillan , an expert
in metal-free organocatalytic reactions, says the reactivity
of frustrated Lewis pairs “is conceptually really
intriguing” and that the new chemistry “certainly makes
one think differently about the notion of aromatic
hydrogenations. All in all, this work points to the exceptional
fertility of this area for new reactivity discoveries
and for mechanistic explorations.” —STEVE RITTER
STM image shows
a lithography mask
used to incorporate
a phosphorus atom
at center pink spot
and electrical leads
at pink rectangular
sites to create
a single-atom
transistor.
VIDEO ONLINE
NANOELECTRONICS: Device’s
performance bodes well for
quantum computing
IN WORK THAT COULD ADVANCE the development
of quantum computers, researchers have created a
transistor that consists of a single atom positioned
precisely between two electrodes in a silicon
MARTIN FUECHSLE
SINGLE-ATOM
TRANSISTOR
substrate. Quantum computers could perform
some calculations not possible on
current computers, such as solving the
Schrödinger equation for large molecules.
Quantum computing specialist Michelle
Y. Simmons of the University of
New South Wales, in Australia, and
coworkers prepared the transistor.
They used scanning tunneling
microscopy, lithography, and
phosphine chemistry to place, with single-lattice-site
spatial accuracy, an individual phosphorus atom between
electrodes in a silicon device ( Nat. Nanotechnol.,
For the researchers’ take on the significance of
their single-atom transistor, visit cenm.ag/trans.
DOI: 10.1038/nnano.2012.21 ). Such precise positioning
hadn’t been achieved before.
Single-atom transistors could be combined to give
integrated circuits of unprecedented density. But creating
such transistors is painstaking, and the feasibility
of making devices that comprise millions or billions of
them is not yet known. The transistor operates only at
close to absolute zero, also limiting applications for now.
Nevertheless, the phosphorus transistor represents
a step toward quantum computers. Quantum computers
would achieve greater power and speed by encoding
information in qubits, which adopt more states than
just the two (0 and 1) in conventional computers bits.
Precise atom positioning would be required to interrogate
the information in qubits accurately.
Device modeler Asen Asenov of the University of
Glasgow believes the experimentation is “groundbreaking.”
Molecular device fabricator Robert A.
Wolkow of the University of Alberta believes that Simmons’
group and others reported substantially similar
results earlier. Some of the study’s simulations have
technical deficiencies, Asenov adds.
Quantum computing expert Bruce E. Kane of the University
of Maryland notes that the one-atom transistor
is not currently practical for conventional devices, nor
does it carry out quantum operations. But he calls the
work “an experimental and engineering tour de force”
and believes Simmons’ group now has the requisite tools
to begin building quantum computers “that would go beyond
the current state of the art.” —STU BORMAN
WWW.CEN-ONLINE.ORG 8 FEBRUARY 27, 2012