A theoretical investigation of the Co(CO)4--catalyzed carbonylative ring expansion of N-benzoyl-2-methylaziridine to beta-lactams: reaction mechanism and effect of substituent at the aziridine Calpha atom.

The reaction mechanism of the Co(CO)4--catalyzed carbonylative ring expansion of N-benzoyl-2-methylaziridine to afford N-benzoyl-4-methyl-2-azetidinone and N-benzoyl-3-methyl-2-azetidinone was investigated by using the B3LYP density functional theory methodology in conjunction with the conductor polarizable continuum model/united atom Kohm-Sham method to take into account solvent effects. Computations predict that the most favorable reaction mechanism differs from the experimental proposals except for the nucleophilic ring-opening step, which is the rate-determining one. The regioselectivity and stereospecificity experimentally observed is explained in terms of the located reaction mechanism. The substitution of the methyl group at the carbon alpha of aziridine by the phenyl one gives rise to the obtaining of an only product that corresponds to the CO insertion into the C(substituted)-N bond in accordance with experimental findings. When the ethyl group replaces the methyl one the CO insertion occurs into the two C-N bonds, but the regioselectivity of the process is higher than that of the methyl substituent.