Heck-type coupling vs. conjugate addition in phosphine–rhodium catalyzed reactions of aryl boronic acids with α,β-unsaturated carbonyl compounds: a systematic investigation

Abstract

The competition between Heck-type coupling and conjugate addition in phosphine–rhodium catalyzed reactions of aryl boronic acids with α,β-unsaturated carbonyls has been systematically investigated in a toluene–H₂O biphasic system. Aside from the intrinsic nature of rhodium and the enolization of carbonyls, the phosphine supporting ligand on rhodium, the ratio of aryl boronic acid to α,β-unsaturated carbonyl and the pH value of the aqueous phase were found to affect the competition significantly. Highly selective rhodium-based catalyst systems have therefore been developed for both Heck-type coupling and conjugate addition by synergistically tuning the supporting ligand, the boronic acid to olefin ratio and other reaction conditions. Conjugate addition with selectivity >99% and Heck-type coupling with selectivity of up to 100%, 98% and 84% for acrylates, acrylamides and methyl vinyl ketone, respectively, could be achieved in the rhodium-catalyzed reactions of aryl boronic acids with α,β-unsaturated carbonyls using the corresponding optimized rhodium-based catalyst systems.