Catalytic Enantioselective S_N2'-Type Reaction for Axial Allene Chirality

Optically active compounds are abundant in nature and are of fundamental interest for organic, medicinal, and material chemistry. Catalytic transformation of readily available racemic compounds into the same value-added enantiomeric products is one of the most efficient and cost-effective protocols. Herein, an S_N2'-type enantioselective reaction of racemic propargylic alcohols with aryl boronic acids (both are readily available) under a rhodium catalyst empowered by a new phosphoramidite-olefin bidentate ligand affording tetrasubstituted enantioenriched allenes has been unveiled. A pair of diastereomeric vinylic rhodium species with an allylic hydroxyl group has been identified and characterized by ESI-MS and TWIM-MS. Based on the X-ray diffraction data of the catalyst and other mechanistic studies, it is confirmed that the reaction is a conceptually new approach of parallel path enantio-convergent transformation, which is different from the classic dynamic kinetic resolution and dynamic kinetic asymmetric transformations.