Ni 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 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 -based pathway that involves fast homolytic cleavage of the C-Br bond by an alkyl-Ni complex, followed by radical coordination to Ni that determines the observed regioselectivity.