Ab initio and DFT modeling of stereoselective deamination of aziridines by nitrosyl chloride

摘要

The stereochemical course of the deamination of cis-2,3-dimethylaziridine by nitrosyl chloride was investigated at the QCISD/6-31G(d) level. Calculations reveal that the reaction takes place in two steps. In the first step, the reactants form a pre-reactive complex, followed by a spiro-type bicyclic transition state, which on dissociative cycloelimination gives the N-nitrosoaziridine intermediate. In the second step, this intermediate undergoes cycloreversion through a slightly asynchronous concerted transition state to form an alkene with the same stereochemistry, which is in total agreement with experiment. In the whole reaction, the denitrosation step is found to be rate-determining. For comparison, geometry optimizations and energies were also obtained at the B3LYP/6-31G(d) level. It was found that the B3LYP energy results differed significantly from the QCISD ones. To analyze the reason for this difference, B3LYP calculations were repeated by varying the contribution of exact exchange in the Becke functional. With respect to the QCISD results, it has been shown that the functional with 0% exact exchange yields the best activation barriers, whereas the functional with 30% exact exchange is the most suitable one to carry out the complexation and reaction energy calculations. (C) 2004 Wiley Periodicals, Inc.