Synthesis of 1,2-Bis- and 1,1,2-Tris-Borylalkanes under Transition Metal-Free and Solvent-Free Conditions

Multi(boronate) esters are useful building blocks in modern chemical synthesis. Herein, we have developed an efficient, transition metal- and solvent-free method for the regioselective boration of alkenes and alkynes. The alkali metal Lewis base (NaOMe)-mediated reactions, using bis(pinacolato)diboron (B2pin2) as the boron reagent, resulted in the diboration of alkenes at room temperature and the triboration of alkynes at 60 °C to produce synthetically useful alkyl 1,2-bis(boronate) esters and 1,1,2-tris(boronate) esters, respectively, in excellent yields and with high regioselectivity. This environmentally benign protocol demonstrates a broad substrate scope and good functional group tolerance for alkenes and alkynes. The gram-scale reaction further highlights the practical usefulness of the method. The proposed reaction pathway has been evaluated based on stoichiometric reactions and DFT studies.