Mechanistic Basis for Divergent Stereoselectivity in Intramolecular Ni-Catalyzed Cross-Electrophile Coupling Reactions

Nickel-catalyzed asymmetric coupling reactions are known to proceed through both stereospecific and stereoablative mechanisms. Herein, we report a mechanistic investigation of a previously reported Ni-catalyzed cross-electrophile coupling reaction that proceeds via divergent mechanisms with different stereochemical outcomes. Depending on the benzylic substituent, from the same organonickel intermediate, competing rates of transmetalation or single electron transfer shunt the reaction to either pathway. For one substrate, the formation of an arylzinc intermediate dominates. This intermediate subsequently participates in intramolecular Negishi coupling, which is an enantiospecific reaction. For another substrate, single electron transfer outcompetes transmetalation, triggering a sequential reduction mechanism, a stereoablative pathway.