Transition metal mediated functionalization of o-carboranes

Abstract

Carboranes are a class of boron hydride clusters in which one or more of the BH vertices are replaced by CH units. Unlike small boranes, carboranes are kinetically and thermodynamically very stable as well as relatively chemically inert, which are often called three-dimensional relatives of benzenes. They are finding many applications in medicine as boron neutron capture therapy (BNCT) agents, in nanomaterials/supramolecular design as building blocks, and as ligands for transition metals [1]. However, their unique structures make derivatization difficult, which limits their application scope. To this end, there is a need to develop new methodologies for the functionalization of carboranes. In view of the widespread use of transition metals in synthetic chemistry, we initiated a research program to develop transition metal-mediated/catalyzed synthetic methods for cage C–H/B–H functionalization of carboranes. This short article highlights the recent development and addresses the problems in this growing area. Earlier experimental results show that the M–Ccage (Ccage: hypervalent carbon) σ bond in transition metal-carboranyl complexes are generally inert toward various electrophiles, and hence significantly different from traditional M–C (C: tetravalent carbon) bonds [2]. This can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the non-traditional M–Ccage bonds, a series of transition metal-o-carboryne (ocarboryne = 1,2-dehydro-o-carborane) complexes have been prepared as the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M–Ccage bonds with electrophiles (Chart 1). In fact, metal-o-carboryne complexes are important intermediates for the preparation of a large class of cage C–H functionalized carboranes. For instance, the nickel-oChart 1 o-Carborane and its derivatives.