How a Chromium Tricarbonyl Complex Catalyzes the [3 + 2] Cycloaddition Reaction of N-Substituted Phenylnitrones with Styrene: A Molecular Electron Density Theory Analysis

The reaction mechanisms of [3 + 2] cycloaddition (32CA) between two N-substituted phenylnitrones (NPPN and NtBPN) and styrene (STY) in the presence of a chromium tricarbonyl complex (Cr(CO)3) have been studied within the framework of molecular electron density theory at the MPWB1K/6-311G(d,p) level. Activation energy analysis reveals that these 32CA reactions take place with high activation barriers due to their non-polar character and with the exo/ortho approach as a favorable reaction path. The activation energy of TS1 series (NPPN) is about 5-6 kcal/mol lower than that of the TS2 series (NtBPN), due to the steric hindrance of tert-butyl at the TS2 structures. Coordination of chromium tricarbonyl to each reactant to form the corresponding complex slightly increases the nucleophilic and electrophilic character of NPPN:Cr, NtBPN:Cr, and STY:Cr, which can be considered a strong nucleophile and electrophile able to participate in a polar 32CA reaction. This polar character in the presence of the chromium tricarbonyl complex was verified by the higher (up to 0.20e) values of global electron density transfer for Cr(CO)3-coordinated transition states. Electron localization function analysis along the exo/ortho reaction path reveals the non-concerted mechanism in the formation of a new bond with the initial appearance of a C-C single bond followed by the O-C one.