DFT Studies on Ligand Controlled Highly Selective Copper-Catalyzed Borylations of Allenes

The mechanism of the Cu-catalyzed borylations of allenes with bis (pinacolato) diboron has been studied by DFT calculations. The possible pathways involve two key steps, borylcupration and gamma-protonolysis. Computed results show that the gamma-protonolysis step is not only the rate-limiting step but also determines the regioselectivity. When employing monophosphine P(p-MeOC6H4)(3) as the ligand, the electronic and steric effects are both effective in the control of regio- and stereoselectivity; however, for sterically hindered bulky bisphosphine ligand BIPHEP, it is the steric hindrance that reverses the regioselectivity of the gamma-protonolysis process.