The diboron compound bis(pinacolato)diboron reacts with aromatic heterocycles or simple arenes to an arylboronate ester with iridium catalyst [IrCl(COD)] (a modification of Crabtree's catalyst) and base 4,4′-di-tert-butyl-2,2′-bipyridine in a C-H coupling reaction for example with benzene:
In one modification the arene reacts using only a stoichiometric equivalent rather than a large excess using the cheaper pinacolborane:
Unlike in ordinary electrophilic aromatic substitution (EAS) where electronic effects dominate, the regioselectivity in this reaction type is solely determined by the steric bulk of the iridium complex. This is exploited in a meta-bromination of m-xylene which by standard AES would give the ortho product: