Polyoxometalate-Supported Pd(II)-Catalyzed B(9)–H Nitration of <i>o</i>/<i>m</i>-Carboranes

Nitro compounds are essential chemicals that serve as critical building blocks and intermediates in the synthesis of various pharmaceuticals, agrochemicals, and advanced materials. Consequently, nitration is a fundamental transformation in organic chemistry. Electrophilic aromatic substitution is one of the most important types of nitration reactions. Carboranes, a class of boron–carbon clusters characterized by their three-dimensional aromaticity, can undergo electrophilic substitution reactions. However, traditional electrophilic nitration conditions typically result in hydroxylated carboranes rather than the desired nitro compounds. Herein, we present a practical synthetic method for B(9)-NO2-o/m-carboranes via Pd(II)-catalyzed B(9)–H activation using KNO2 as the nitrating reagent, facilitated by phosphomolybdic acid (PMA). The success of this transformation hinges on the steric hindrance, strong Bro̷nsted acidity, and weak nucleophilicity of PMA, which enhance the electrophilicity of the palladium catalyst and significantly suppress the formation of B–O coupling products. Reduction of the nitro group using LiAlH4 provided 9-NH2-m-carborane, and further derivatization of 9-NO2-m-carborane and 9-NH2-m-carborane produced a series of B(9)-functionalized m-carboranes.