19916-89-3Relevant articles and documents
Evidence of a borderline region between E1cb and E2 elimination reaction mechanisms: A combined experimental and theoretical study of systems activated by the pyridine ring
Alunni, Sergio,De Angelis, Filippo,Ottavi, Laura,Papavasileiou, Magdalini,Tarantelli, Francesco
, p. 15151 - 15160 (2007/10/03)
We report a combined experimental and theoretical study to characterize the mechanism of base-induced β-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, AxhDH + DN) or via the concerted loss of a proton and the halide (E2, AND EDN) upon base attack. Experimentally, the Taft correlation for H/D exchange, in OD-/D2O with noneliminating substrates (1-methyl-2-(2-Xethyl)pyridinium iodide), is used to predict the expected values of the rate constants for the elimination reactions with N-methylated substrates and F, Cl, Br as the leaving group. The comparison indicates an E1cb irreversible mechanism with F, but the deviation observed with Cl and Br does not allow a conclusive assignment. The theoretical calculations show that for the N-methylated substrate with a fluoride leaving group the elimination proceeds via formation of a moderately stable carbanion. No stable anionic intermediate is instead found when the leaving group is Cl or Br, as well as for any of the nonmethylated species, indicating a concerted elimination. The methylated substrate with Cl shows however only a moderate increase in reactivity compared to the fluorinated substrate, despite the change in mechanism. Very interestingly, our analysis of the computed two-dimensional potential energy surface for the reaction with a F leaving group indeed evidences the lack of a net distinction between the E1cb and E2 reaction paths, which appear to merge smoothly into each other in these borderline cases.
