Transition-metal-catalyzed
addition of aryl halides across carbonyls
remains poorly developed, especially for aliphatic aldehydes and hindered
substrate combinations. We report here that simple nickel complexes
of bipyridine and PyBox can catalyze the addition of aryl halides
to both aromatic and aliphatic aldehydes using zinc metal as the reducing
agent. This convenient approach tolerates acidic functional groups
that are not compatible with Grignard reactions, yet sterically hindered
substrates still couple in high yield (33 examples, 70% average yield).
Mechanistic studies show that an arylnickel, and not an arylzinc,
adds efficiently to cyclohexanecarboxaldehyde, but only in the
presence of a Lewis acid co-catalyst (ZnBr
2
).