Direct B–H Functionalization of Icosahedral Carboranes <i>via</i> Hydrogen Atom Transfer
Hongyuan Ren, Ping Zhang, Jingkai Xu, Wenli Ma, Deshuang Tu, Changsheng Lu, Hong Yan
Abstract
The efficient and selective functionalization of icosahedral carboranes (C 2 B 10 H 12 ) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B–H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B–H functionalization of icosahedral carboranes via B–H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B–H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B–H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B–H heterolytic mechanism in the traditional strategies.