A theoretical study of rhodium(I) catalyzed carbonylative ring expansion of aziridines to beta-lactams: crucial activation of the breaking C-N bond by hyperconjugation.

The regioselectivity and enantiospecificity of the [Rh(CO)2Cl]2-catalyzed carbonylative ring expansions of N-tert-butyl-2-phenylaziridine to yield 2-azetidinone and the lack of reactivity of N-tert-butyl-2-methylaziridine along this process were investigated at the B3LYP/6-31G(d) (LANL2DZ for Rh) theory level taking into account solvent effects. According to our results, the regioselectivity in the ring expansion of N-tert-butyl-2-phenylaziridine and the unreactivity of N-tert-butyl-2-methylaziridine experimentally observed are determined by the different degree of activation of the breaking C-N bond in the initial aziridine-Rh(CO)2Cl complex due to its hyperconjugation interaction with the substituent on the carbon atom. When a phenyl substituent is present its hyperconjugation interaction with the C(alpha)-N bond facilitates the insertion of the metal atom into this bond. On the other hand, when the substituent is a methyl group, a larger stability of the initial complex along with a lower stabilization through hyperconjugation of the TS for insertion of the Rh atom into the C(alpha)-N bond make the ring expansion of N-tert-butyl-2-methylaziridine unviable. The enantiospecificity experimentally observed is also reproduced by our calculations given that the stereogenic center is never perturbed to change its configuration.