Substituent effects on o-carborane reactivity with cyclic (alkyl)(amino)carbene

Abstract

The functionalization of carboranes, particularly at boron vertices, is a topic of extensive research. Here, we report the reactivity of ortho-carboranes (oCb) with cyclic (alkyl)(amino)carbene (CAAC) - a transformation that, surprisingly, has not been previously reported. We found that the substitution at the carbon atoms of the oCb strongly influences the reaction outcome. Unsubstituted oCb (H at carbon) reacts with CAAC via carbene insertion into the C−H bond. However, when the carbon atoms of the oCb are substituted, the reaction is dictated by the electronic nature of these substituents: electron-withdrawing groups (EWG) promote the formation of the nido-type products via the nucleophilic attack by CAAC at the B3 (or B6) position, whereas electron-donating groups (EDG) favor insertion of CAAC into the B−H bonds at these positions. The explanation of this reactivity pattern, along with the proposed mechanism of these processes, is supported experimentally and by density functional theory (DFT) computations.