Crafty Cobalt Polyborylations

Precious-metal catalysts such as iridium and rhodium are widely used to promote C–H borylation reactions, in which a boronate group is installed in place of a hydrogen atom. The aryl boronate product is a useful intermediate that can in turn be used in cross-coupling and other reactions to complete functionalization of the C–H bond. Although functionalizing aryl ring carbons with groups such as boron pinacolate is well-known, less common is achieving addition of the boronate to benzylic carbons in molecules such as toluene. W. Neil Palmer, Paul J. Chirik, and colleagues of Princeton University have now devised a method for adding not just one boronate group to toluene’s methyl group but an unprecedented two or three, depending on the ratio of reactants, amount of catalyst, and reaction time. In addition, their approach works on branched alkylarenes, which has not been accomplished before (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b12249). The chemistry is made possible by new α-diimine cobalt dialkyl and bis(carboxylate) catalysts the Princeton team has created.