A DFT study on enantioselective synthesis of aza-β-lactams via NHC-catalyzed [2+2] cycloaddition of ketenes with diazenedicarboxylates

Recently, N-heterocyclic carbenes (NHCs) have been found to be efficient catalysts for the formal [2 + 2] cycloaddition of aryl(alkyl)ketenes and diazenedicarboxylates to give aza-beta-lactams in good enantioselectivity (up to 91% ee) [X.-L. Huang, X.-Y. Chen, S. Ye, J. Org. Chem. 74 (2009) 7585-7587]. However, it is still ambiguous which step is the enantioselectivity-determining step and what the role of NHC catalysts is in this reaction. In this paper, we have suggested a possible mechanism of the title reaction and then theoretically investigated it in detail using density functional theory (DFT). Fully optimized geometries of reactants, products, transition states and intermediates were obtained at the B3LYP/[6-31G (d, p)/LANL2DZ] level of theory, and the results revealed that this reaction had three steps. Our calculated results indicate that the [2 + 2] cycloaddition step is the enantioselectivity-determining step. Moreover, the frontier molecular orbital (FMO) analysis has been carried out to explain why the NHC catalysts can make the [2 + 2] cycloaddition easier to occur, which is mainly due to that the energy gap of FMOs become narrower under the NHC-catalysis condition. Noteworthy, the results of global reactivity indexes analysis are consistent with those of the FMO analysis. Further calculations show that the solvent effect of dichloromethane has no great influence on enantioselectivity of this reaction. (C) 2010 Elsevier B.V. All rights reserved.