Regioselective Cu-Catalyzed Hydroboration of 1,3-Disubstituted-1,3-Dienes: Functionalization of Conjugated Dienes Readily Accessible through Ene-Yne Metathesis

A regioselective Cu-catalyzed formal hydroboration of 1,3-disubstituted-1,3-dienes is reported. Highly substituted 1,3-dienes produced through catalytic ene-yne metathesis provided substrates for the regioselective hydroboration with variable substituents conveniently introduced from a 1-alkene and a terminal alkyne. For the achiral reaction, the diphosphine 1,2-bis(diphenylphosphino)benzene (dppbz) was used as the ligand. For the enantioselective reaction, EtDuPhos was used as the diphosphine ligand, and good enantioselectivites were obtained in most cases. Control studies showed that the stereoisomeric purity of the 1,3-diene and the Cu(I) source were key variables that led to the highest regioselectivity. In situ NMR experiments and deuterium quenching confirmed the presence of an allyl copper intermediate, which did not exchange with other 1,3-dienes. A proposed model for the regioselectivity is based on the addition of Cu-B to the 1,3-diene where steric effects in the 3,4-addition are more severe than those in the favored 1,2-addition mode. The products were further functionalized and were employed in a dual-catalytic cross-coupling reaction.