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, Wako1. 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 bonds2. 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.
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