Asymmetric synthesis of α-chiral β-hydroxy allenes: copper-catalyzed γ-selective borylative coupling of vinyl arenes and propargyl phosphates†
Copper-catalyzed enantioselective coupling of vinyl arenes with bis(pinacolato)diboron (B2pin2) and propargylic phosphates is presented. The protocol affords a facile route to enantioenriched α-branched allenes with a versatile pinacolboronate group at the β-position with high enantioselectivity up to 98 : 2 er. In the presence of a copper catalyst complexed with a chiral bisphosphine ligand, catalytic assembly of α-chiral β-hydroxy allenes was accomplished through highly selective γ-addition of a borylalkyl copper species to propargyl substrates, followed by oxidation. Catalyst-controlled divergent cyclization reactions of the resulting allenes led to concise syntheses of enantioenriched dihydropyran and dihydrofuran rings without any loss of chiral purity.