NiI Catalyzes the Regioselective Cross-Coupling of Alkylzinc Halides and Propargyl Bromides to Allenes

We describe the unprecedented formation of allenes by Ni-catalyzed cross-coupling of propargyl bromides with alkylzinc halides. The reaction regioselectivity is complementary to the previously reported formation of propargyl-coupled compounds. Experiments support the formation of Ni-I complexes as the active species and the participation of radical intermediates. Kinetic studies showed that the reaction is first order with respect to the electrophile, zero-order with respect to the nucleophile (fast transmetalation), and one-half order with respect to the metal catalyst. Mechanistic studies support a bimetallic Ni-I-based pathway that involves fast homolytic cleavage of the C-Br bond by an alkyl-Ni-I complex, followed by radical coordination to NiI that determines the observed regioselectivity.