Tris(pentafluorophenyl)borane-Catalyzed Cyclopropanation of Styrenes with Aryldiazoacetates

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.