Photocatalytic anti-Markovnikov addition of carboxylic acids to alkenes: an ionic mechanism under radical conditions

Since the advent of photocatalysis, radical addition to alkenes has become one of the most fruitful processes for building up molecular complexity. Herein we describe a novel approach towards photocatalytic addition to alkenes, in which both new C-O and C-H bonds are formed by an ionic mechanism, rather than the conventional radical mechanism. This occurs due to two consecutive radical-polar crossover events, enabling the generation of both radical cation and carbanion species during the reaction. Such a transformation is demonstrated by the acridine-catalyzed addition of carboxylic acids to styrenes. This unique mechanism allows one to carry out the reaction in the absence of additives and also to reverse the regioselectivity of conventional ionic addition to alkenes.