Site-Selective B─H Activation via HAT Toward Xanthyl-closo-Carboranes as Bench-Stable Precursors of Organosulfur Boron Clusters.

In this work, a light mediated direct regioselective B─H functionalization to afford unexplored xanthyl-closo-carboranes is described by a nitrogen-centered radical (NCR)-mediated hydrogen atom transfer (HAT) process. To directly activate a B─H bond in an icosahedral carborane, which contains 10 B─H with very similar electronic properties, represents a challenging goal. The reaction is applied to several mono and disubstituted o- and m-carboranes affording, in discrete to good yields, boryl xanthates which act as novel bench-stable and versatile platforms for the functionalization of the carborane cage with sulfur-containing functionalities: alkyl- and aryl sulfides, thiols, thioesters and sulfonyl chlorides are readily obtained. The reaction has also been scaled up and an analogue of the pesticide Chlorbenside has been synthesized. EPR studies, additionally to deuteration experiments, confirm the formation of an NCR as the promoter of a HAT process to the most electron-rich boron vertex with the lowest B─H bond dissociation energy (BDE), to produce a B(9)-centered radical. A deep computational study contributes to the mechanism proposal.