Enantioselective Copper-Catalyzed Vinyl Boron-Wittig Reaction to Form Allenes.

The development of general methods for the enantioselective synthesis of axially chiral molecules remains a longstanding challenge in organic synthesis. Among these, allenes represent a particularly important class of motifs due to their unique structural and electronic properties, yet their catalytic enantioselective construction remains underdeveloped. Here we report a copper-catalyzed enantioselective vinyl boron-Wittig reaction that enables the efficient synthesis of chiral allenes from readily available 1,1-diborylalkenes and aldehydes. This transformation proceeds under mild conditions using earth-abundant copper catalysts and readily accessible ligands, and exhibits a broad substrate scope, encompassing both aryl- and alkyl-substituted diborylalkenes as well as aromatic and aliphatic aldehydes. The method delivers di- and tri-substituted chiral allenes in good yields, with high enantioselectivity and excellent functional-group tolerance, and is applicable to structurally complex substrates derived from natural products and pharmaceuticals. The resulting functionalized allenes can be readily diversified, highlighting the synthetic utility of this approach. Mechanistic studies support a pathway involving regioselective transmetalation, enantioselective 1,2-addition, and a copper-promoted B-O elimination, thereby establishing a new paradigm for allene synthesis through a catalytic boron-Wittig-type reaction.