Regioselective Boracarboxylation of α-Substituted Vinyl Arenes

An experimental study of the effect of CO2 pressure on the copper(I)-catalyzed boracarboxylation of α-substituted vinyl arenes is reported. Preliminary experiments using ICyCuCl catalyst (ICy = 1,3-bis(cyclohexyl)imidazol-2-ylidine) showed that boracarboxylation of tert-butylstyrene can be achieved in 30 minutes at modest CO2 pressure (3-6 atm) as opposed to 24-36 hours at atmospheric pressure. Due to increased substrate sterics that allowed catalytic reduction of CO2 to compete kinetically, boracarboxylation of a-methyl styrene, even at modest CO2 pressures, gave poor yields. Low CO2 pressure and SIMesCuCl catalyst (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-4,5-dihydro-2-ylidine) provided moderate to good yields of an electronically and sterically diverse group of quaternary carboxylic acid products featuring a -boronic ester functional group.