Regioselectivity of Pd-catalyzed o-carborane arylation: a theoretical view

The mechanism and the regioselectivity of Pd-catalyzed B(4)-H arylation and B(4,5)-H diarylation for o-carborane with bidentate directing groups were investigated. The complete catalytic cycle involves Pd-mediated concerted metalation-deprotonation, oxidative addition, counterion exchange, and reductive elimination, where counterion exchange is critical. The reductive elimination is the rate- and regioselectivity-determining step for the whole catalytic cycle. B(3)-Arylation is unfavorable because the steric repulsion between the substituent group on C(2) and the metal moiety would lead to significant distortion of o-carborane and would result in a higher activation energy for reductive elimination. The distortion of o-carborane can be clearly observed from the change of its aromaticity in the corresponding transition states by the calculation of nucleus-independent chemical shifts. Moreover, counterion exchange with carboxylate before reductive elimination significantly increases the positive charge of palladium, which leads to a lower activation free energy for the sequential step.