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

Abstract

Herein, we disclose an approach to synthesize tertalkyl 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.

Photocatalytic Decarboxylative Functionalization of Cyclopropenes via Cyclopropenium Cation Intermediates

“…2). [8] In this methodology, the nucleophiles are mostly limited to tetrasubstituted ketene silyl acetals, and double addition is unavoidable. We envisaged that starting from redox-active ester substituted cyclopropenes under photoredox conditions we would be able to generate cyclopropenium cations via oxidation of the intermediate radicals.…”