Transition metal-catalyzed carborane cage B-H activation and functionalization: a mechanistic view

Icosahedral carboranes have extensive potential applications across diverse research fields because these compounds have distinctive properties, including high stability, three-dimensional aromaticity, and high boron density. However, selective cage B-H modification among 10 or 11 chemically similar B-H vertices is an intriguing challenge. Experimental progress in cage B-H functionalization has inspired numerous mechanistic studies of the underlying activation mechanisms and factors dictating selectivity. This review summarizes recent mechanistic studies of transition metal-catalyzed selective functionalization of carboranes, focusing on the activation mechanism of specific B-H bonds and the factors governing regio- and enantioselectivities. Concerted metalation-deprotonation, σ-complex-assisted metathesis, oxidative addition, and metal-carbene insertion are discussed in detail as typical models for B-H bond activation. The transition metal oxidation state and proton acceptor determine the favored activation model. The stability of the B-metal species generated via B-H activation affects the energy level of further functionalization, and thus the selectivity is determined by B-H activation or subsequent functionalization. Therefore, the factors dictating selectivity can be identified by analyzing the transition state properties, providing insights into improving regio- or chiral-at-cage selectivities. The opportunities and challenges for future research are discussed. We highlight strategies of exploiting the unique characteristics of carboranes based on C-H bond activation protocols and single-electron transfer approaches including photo- and electrocatalysis. We expect that this review will provide a framework for understanding the catalytic mechanism, rational catalyst design, and exploration of precise functionalization strategies.