HSAB theory guiding electrophilic substitution reactions of o-carborane

Abstract

Although the three-dimensional aromatic character of carboranes has been well-known for a long time and their electrophilic substitution reactions have been studied, systematic comparison of the differences and similarities of these reactions with aromatic compounds has not been explored. In the electrophilic substitution reaction, the electrophiles act as Lewis acids to accept electrons, and the aromatic compounds and carboranes act as Lewis bases to donate electrons. Therefore, the HSAB (hard and soft, acids and bases) theory can principally predict the outcome of such reactions. Herein, we systematically compared the reactivity of o-carborane and benzene toward different electrophiles to conclude that hard electrophiles prefer to react with benzene and soft electrophiles prefer o-carborane, which is consistent with the HSAB principle. Directed by this finding, we developed the electrophilic selenylation, oxidation, electrophilic mercuration, and Pd(II)-catalyzed B(9)–H bond halogenation, sulfuration, and amination of o-carboranes. We believe this finding will have a guiding significance for the selective B–H functionalization of carboranes.