Axially chiral allenylboranes: catalytic asymmetric synthesis by palladium-catalysed hydroboration of but-1-en-3-ynes and their reaction with an aldehyde

Reaction of but-1-en-3-ynes (CH2CR–CCH: R = H, n-C5H11) with catecholborane in the presence of a palladium catalyst bearing a chiral monodentate phosphine ligand, (S)-(–)-MeO-MOP, give optically active (3-substituted-1,2-butadienyl)-1,3,2-benzodioxaborolanes [Me(R)CCCH(BO2C6H4)], the reaction of which with benzaldehyd proceeded with syn attack to give the corresponding optically active but-3-ynyl alcohols of up to 61% enantiomeric excess (e.e.).