Halogenated Salt Assisted Cu-Catalyzed Asymmetric 1,4-Borylstannation of 1,3-Enynes: Enantioselective Synthesis of Allenylstannes

An enantioselective 1,4-borylstannation of 1,3-enynes employed a chiral sulfoxide phosphine (SOP)/Cu complex as a catalyst, and the desired products, chiral allenylstannes, were first synthesized by asymmetric catalysis with satisfactory yields and enantioselectivies. In this protocol, a catalytic amount of additive, a halogenated salt, plays a crucial role in the success. Control experiments and theoretical studies disclosed that the four-membered ring transmetallation transition states which were stabilized by a halide anion are the key to yields and stereochemical outcomes.