Relative Reactivity of Three Oxygen Lone Pairs of an Α-Nucleophile in SN2 Reactions

The SN2 reaction of H2C=NO− with CH3Cl was investigated at the AM1, STO-3G, and 6-31 + G* levels. Three modes of approach of CH3Cl to the oxygen of the nucleophile were examined: syn and anti to the lone pair on the nitrogen in the molecular plane, and an approach along the π system of the oximate anion. The AM1 method differed markedly from the two abinitio methods, both in the reactivity order of the three geometries as well as in the relative stabilities of the products. The reactivity order calculated at the STO-3G and the 6-31 + G* levels was syn > π > anti (AM1 order: anti > syn > π). Product stability order calculated by the two abinitio methods was syn > anti > π (AMI order: anti > syn > π). Excellent agreement was obtained between percent bond elongations at the transition state predicted by the equation suggested by Shaik, Schlegel, and Wolfe, and the activation barriers calculated at the 6-31 + G* level. The relevance of the calculations with respect to the α-effect is also discussed. Keywords: oxygen lone pairs, aximate, SN2, abinitio.