- 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
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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.
- Alunni, Sergio,De Angelis, Filippo,Ottavi, Laura,Papavasileiou, Magdalini,Tarantelli, Francesco
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p. 15151 - 15160
(2007/10/03)
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- Photoinduced Alkoxylation of 2-Vinylpyridinium Ion
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Photoirradiation of 2-vinylpyridine in acidic methanol afforded methyl 2-(2-pyridyl)ethylether in a high yield.Reactions in acetic ethanol and 2-propanol also provided the corresponding alkoxyl derivatives along with a considerable amount of 2-ethylpyridine.It was suggested that photoinduced intramolecular charge-shift from the pyridinium ion moiety into the vinyl group initiates the regioselective nucleophilic addition of alcohol.
- Ishida, Akito,Uesugi, Tatsumi,Takamuku, Setsuo
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p. 1580 - 1582
(2007/10/02)
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- SYNTHESIS OF FUNCTIONALLY SUBSTITUTED PYRIDINES AND DIPYRIDYL-SUBSTITUTED COMPOUNDS WITH THE AID OF LOW-VALENCE COMPOLEXES
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The possibility of the preparation of oxygen, nitrogen, and sulfur-containing pyridines and dipyridyl-substituted compounds by the cyclocotrimerization of substituted propionitriles with acetylene under the influence of a Co(2-ethylhexanoate)2-Al(C2H5)3 catalyst was demonstrated.It was established that the nature of the heteroatom in substituted propionitriles has virtually no effect on the direction of the reaction.
- Selimov, F. A.,Khafizov, V. R.,Dzhemilev, U. M.
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p. 290 - 295
(2007/10/02)
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