Rhodium‐Catalyzed 1,5‐Sigma Migratory Ring Expansion of <i>gem</i> ‐Difluorodienyl Cyclopropanes and Cyclobutanes

Rhodium-catalyzed 1,5-sigma migratory ring expansion (SMRE) of common dieneylcyclopropanes (DECPs) and dieneylcyclobutanes (DECBs) fails. But replacing the terminal CH₂ group with CF₂ group for both DECPs and DECBs, the resulting substrates of gem-difluorodienyl cyclopropanes (df-DECPs) and gem-difluorodienyl cyclobutanes (df-DECBs) can achieve this reaction, which has broad reaction scope and high efficiency to give seven- and eight-membered carbocycles with a carbonyl group. The keys to the success of this reaction are: oxidative cyclometalation becomes easier with the help of two F atoms; the metallacycle intermediate can detour the sluggish reductive elimination by defluorination to generate Rh carbene intermediate (defluorocarbenation process), which can undergo easier carbene migratory insertion and β-H elimination, giving rise to the final products. Mechanistic studies via experiments and quantum chemical calculations support the proposed mechanism. The gem-difluoro ene in the substrates of df-DECPs and df-DECBs of the present SMRE reaction can be regarded as a ketene surrogate, and this strategy can be exploited in many transition-metal-catalyzed reactions in the future.