Methods for the synthesis
of cyclopropanes are critical for drug
discovery, chemical biology, total synthesis, and other fields. Herein,
we report the use of the strong sterically encumbered Lewis acid tris(pentafluorophenyl)borane
as a catalyst for the cyclopropanation of unactivated alkenes using
aryldiazoacetates. The cyclopropane products are synthesized using
10 mol % of the catalyst under mild conditions in up to 90% yield
(8:1 to >20:1 dr). We propose that the reaction proceeds via a
Lewis
acid-activated carbene.
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.
Advancements in main-group catalysis are contingent on our ability to quantify effects that enhance reactivity in these systems. Herein we report the rates of alkylation for several substituted phosphines. We...
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