Pincer ligands boost Kumada coupling

Nickel catalysts with pincer-like ligands can efficiently and selectively promote a coupling reaction between two aromatic rings, even for low reactivity reagents.

The pincer ligands can help less reactive carbon atoms to unite

The complexes, synthesised by Zhong-Xia Wang and Li Wang at the University of Science and Technology of China, in Hefei, act as catalysts for the Kumada cross coupling reaction, a well-established way to make carbon-carbon bonds. This reaction, between an aryl Grignard reagent and an aryl halide to create a biaryl structure, is usually catalysed by palladium or nickel complexes with N-heterocyclic carbenes and phosphorous-containing compounds as ligands.

However, catalytic systems for unactivated electrophilic substrates remain a synthetic challenge. The Hefei duo has made a series of nickel catalysts with different pincer-like tridentate amido ligands. The catalysts work with less reactive reagents, like para-methoxy aryl chloride and sterically congested reactants, and show good selectivity.

'These amido pincer ligands contain different co-ordinate atoms [either N or P] and have different steric hindrance,' said Wang. 'The nickel complexes with N,N,N- ligands and N,N,P- ligands exhibit better activity than those complexes with P,N,P- ligands.' In most cases, the catalytic activity declines as the steric hindrance of the ligands increases.

'The catalytic activities of our catalysts are not the highest reported,' said Wang, 'but it proves the potential of amido pincer ligands as a new catalyst system for Kumada coupling.' The team now plan to optimise the structure of the ligands to improve their activity.

Alison Stoddart