Ab initio molecular dynamics simulations of elimination reactions in water solution: exploring the borderline region between the E1cb and E2 reaction mechanisms.

We report a theoretical study, based on ab initio molecular dynamics simulations in water solution, of the mechanism of base-induced beta-elimination reactions in systems activated by the pyridyl ring, with halogen leaving groups. The systems investigated represent borderline cases, where it is uncertain whether the reaction proceeds via a carbanion intermediate (E1cb, A(xh)D(H) + D(N)) or via the concerted loss of a proton and the halide (E2, A(N)D(E)D(N)) upon base attack. Recent theoretical and experimental evidence points toward the lack of a net distinction between the E1cb and E2 reaction paths, which seem to merge smoothly into each other in these borderline cases (Alunni, S.; De Angelis, F.; Ottavi, L.; Papavasileiou, M.; Tarantelli, F. J. Am.Chem. Soc. 2005, 127, 15151-15160). In this study, we explore the dynamics on the potential energy surface for the reaction of 2-(2-fluoroethyl)-1-methyl pyridinium with OH- by means of Car-Parrinello simulations in water solution. Our results indicate that the reaction mechanism effectively evolves through the potential energy region of the carbanion: the carbon-fluoride bond breaks only after the carbon-hydrogen bond, confirming the conclusions of a recently reported study of the potential energy surface for this system.