Borane-Catalyzed C-F Bond Functionalization of gem -Difluorocyclopropenes Enables the Synthesis of Orphaned Cyclopropanes.

Herein, we disclose an approach to synthesize tert -alkyl cyclopropanes by leveraging C-F bond functionalization of gem -difluorocyclopropenes using tris(pentafluorophenyl)borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph 2 (C 6 D 5 )C 3 ] + [(C 6 F 5 ) 3 BF] - . We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α- tert -alkyl centers (63-93% yield). The regioselectivity of the addition to cyclopropenium ions is controlled by their steric and electronic properties and enables access to 3,3-bis(difluoromethyl)cyclopropenes in short order. The resulting cyclopropene products are readily reduced to the corresponding orphaned cyclopropanes under hydrogenation conditions. Quantum chemical calculations reveal the nature of the C-F bond cleavage steps and provide evidence for catalysis by boron and not silylated oxonium ions, though Si-F bond formation is the enthalpic driving force for the reaction.