The Challenging Reaction of Cyclopropenones with Benzamide Catalyzed by Rh(III): A Computational Study

The mechanism of the cascade reaction between N-methylbenzamide (1) and diphenylcyclopropenone (2), catalyzed by Cp*Rh(III) (110 °C, dichloroethane), was studied using density functional theory (DFT) calculations. To assemble cyclopentene spiroisoindolenones (3) and water, 1 and 2 coordinate to Rh(III). Metal-catalyzed C–H activation is preferred to cyclopropenone opening by 3.7 kcal mol–1. 1 acts as a proton carrier with both 1 and 1H+ assisting hydrogen transfer steps during the reaction. The κ-O cyclopropenone isomerizes to a η2-C═C bonding mode, the ring opens, and the first C–C bond is formed. Proton transfer from N–H to the vinyl group opens the way for obtaining the C–N bond. Two protonation steps, requiring protons from Cvinyl–H and 1H+, lead to cleavage of the C–O group and formation of water (byproduct). The second C–C bond results from an intramolecular Friedel–Crafts alkylation, in which the closest ortho-phenyl carbon binds the formal carbocation in the C5 indolenone ring. The release of 3 and the regeneration of the catalyst occur after two proton transfers assisted by 1 and 1H+ in an exergonic final step (9.3 kcal mol–1). The highest barrier corresponds to the protonation of the vinylic group during the formation of the first C–C bond (29.1 kcal mol–1).