Borane-Catalyzed C–F Bond Functionalization of <i>gem</i>-Difluorocyclopropenes Enables the Synthesis of Orphaned Cyclopropanes

Abstract

Herein, we disclose an approach to synthesize <i>tert</i>-alkyl cyclopropanes by leveraging C-F bond functionalization of <i>gem</i>-difluorocyclopropenes using tris(pentafluorophenyl)borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph₂(C₆D₅)C₃]⁺[(C₆F₅)₃BF]^-. We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α-<i>tert</i>-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.