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Figure 1: A scandium catalyst (green spheres, center) can catalyze the addition of a phenylsilane (orange structure) to anisole (blue structure) to give cleaner products, using fewer reagents, than achieved previously. Credit: 2012 Zhaomin Hou |
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In chemistry, downsizing can have positive attributes.
Reducing the number of steps and reagents in synthetic reactions, for example,
enables chemists to boost their productivity while reducing their environmental
footprint. This type of ‘atom economy’ could soon improve, thanks to a new
rare-earth metal catalyst developed by Zhaomin Hou and colleagues at the RIKEN
Advanced Science Institute, Wako. Their catalyst makes it simpler to modify
aromatic carbon–hydrogen (C–H) bonds with silicon-bearing silyl ligands—a
reaction step critical to pharmaceutical and materials science manufacturers alike.
Silicon, which is less electronegative than carbon or
hydrogen atoms, can significantly alter the electronic characteristics of an
organic molecule. Replacing the hydrogen atoms of an aromatic C–H group with
silyl groups has emerged as an important strategy in industrial-scale chemical
synthesis because these substituents can tune molecular reactivity, enabling
construction of elaborate chemical frameworks.
Chemists normally use transition metals such as platinum or
rhodium to catalyze aromatic silylation reactions. But to achieve high
conversions, these catalysts need to be mixed with additional hydrogen acceptor
reagents, which can generate unwanted waste products, including alkanes.
Hou and colleagues have pioneered studies into rare-earth
metals, such as scandium, which have different catalytic properties to
transition metals. Recently, they found that ‘half-sandwich’ scandium
complexes, bonded on one side by a flat organic ring, showed unique activity
and selectivity in the presence of carbon double bonds. This made
investigations of unsaturated aromatic molecules a natural next step.
When the researchers mixed a methoxy–benzene compound called
anisole with the half-sandwich scandium catalyst and a phenylsilane, they found
that the silyl group substituted onto the aromatic ring with excellent
selectivity and yields (Fig. 1). Furthermore, the catalyst did not require
hydrogen acceptor reagents, and generated only H2 gas as a by-product. Hou
notes that this reaction is highly advantageous in terms of atom economy.
X-ray and spectroscopic measurements revealed that the
working form of the catalyst, which contained a pair of ‘bridging’ hydrogen
atoms, activated the reaction by coordinating the anisole’s methoxy group to
the rare-earth metal. According to Hou, this relatively strong interaction
directs silylation to occur almost exclusively at the position adjacent to the
methoxy unit on the aromatic ring—a 'regioselectivity' that outshines that of
transition metal catalysts, whose weak oxygen–metal interactions often produce
an undesirable mix of silylation isomers.
The team will continue to explore new approaches to
improving catalytic sustainability and selectivity by tapping into the
extraordinary properties of rare-earth metals.
SOURCE – RIKEN
