Differential Dihydrofunctionalization: A Dual Catalytic Three-Component Coupling of Alkynes, Alkenyl Bromides, and Pinacolborane.

A new method for differential dihydrofunctionalization of terminal alkynes enables the synthesis of allylic boronate esters through reductive three-component coupling of terminal alkynes, alkenyl bromides, and pinacolborane. The transformation is promoted by cooperative action of a copper/palladium catalyst system and results in hydrofunctionalization of both π-bonds of an alkyne. The synthesis of allylic boronate esters can be accomplished in the presence of a wide range of functional groups, including, esters, nitriles, alkyl halides, sulfonyl esters, acetals, protected terminal alkynes, aryl halides, and silyl ethers. Mechanistic experiments reveal the importance of subtle ligand effects on the performance of the palladium co-catalyst.