DFT studies of β-elimination reactions in water solution with different bases: Theory vs experiment

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• 作者：Edoardo Mosconi ; Filippo De Angelis ; Leonardo Belpassi ; Francesco Tarantelli ; Sergio Alunni
• 关键词：Elimination reaction ; DFT ; pK_a ; Transition state
• 刊名：Journal of Molecular Structure: THEOCHEM
• 出版年：2010
• 出版时间：30 January 2010
• 年：2010
• 卷：940
• 期：1-3
• 页码：103-114
• 全文大小：2257 K

文摘

We describe a theoretical investigation of a prototype β-elimination reaction in systems activated by the pyridine ring. The reactions investigated, the acetohydroxamic-induced and OH⁻-induced β-elimination with the nitrogen protonated N-[2-(2-pyridyl)ethyl]quinuclidinium substrate, offers a unique opportunity to test computational methodologies for the study of β-elimination reactions in solution since for this system detailed kinetic experimental data have been obtained. We calculated the pK_as of the acetohydroxamic acid and of the substrate, for which experimental estimates are available. We then thoroughly characterized the reactive free-energy profile. Our study establishes that the reaction proceeds via a quasi reversible E1cb mechanism for the reaction induced by acetohydroxamate base, and irreversible E1cb mechanism for the reaction induce by OH⁻ base, involving a stable carbanion intermediate. Except for a discrepancy in the reproduction of the pK_a of the acetohydroxamic acid and of the H₂O, the calculated free-energy profile is in excellent agreement with the experiment, showing the general reliability of the present approach.