DFT Studies on the Dirhodium-Catalyzed [3 + 2] and [3 + 3] Cycloaddition Reactions of Enol Diazoacetates with Isoquinolinium Methylide: Mechanism, Selectivity, and Ligand Effect

The reaction mechanisms
of dirhodium-catalyzed [3 + 2] and [3 + 3] cycloaddition between enol
diazoacetate and isoquinolinium methylide have been studied in detail
using density functional theory and a solution-phase translational
entropy model. The reaction starts with the formation of a metallic
carbene intermediate first, from which two competing reaction channels
of [3 + 2] and [3 + 3] cycloaddition take place. For CAT1-catalyzed reactions, the calculated activation free energy barriers
for [3 + 3] and [3 + 2] cycloaddition reactions are 14.3 and 16.0
kcal mol–1, respectively, which is in good agreement
with the ratio of products. Both the steric and electronic effects
have been considered for CAT2- and CAT3-catalyzed
reactions, with which the ratio of products has also been rationalized.