The aza-Payne rearrangement: A theoretical DFT study of the counter-ion and solvent effects

The aza-Payne reaction mechanism has been studied by theoretical DFT calculations. Three reaction pathways have been considered: two of them result in the formation of a three-membered ring, with either inversion (SNI, experimentally observed) or retention (SNR) at the substituted carbon. The third reaction path results in the formation of a four-membered ring. The mechanisms were first studied in the gas phase with the bare anion (An(-)). The role of the counterion (Li+) was then analysed by study of the reaction with the neutral species (An(-)Li(+)) in the gas phase. Results for the (An(-)Na(+)) system are also presented. Finally, the role of the solvent (THF and H2O) was taken into account with the aid of a cavity model. We found that the SNI mechanism was most favoured in anionic gas phase and for the neutral species in solution. Surprisingly, the SNR mechanism was found to be the most favourable one in the gas phase for neutral species in both cases (Li+ and Na+). These findings are interpreted by consideration of the interactions that may develop between the anion and the counter-ion in transition states and are discussed in the light of experimental data on these systems. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2002).