Pb(II)-Promoted Amide Cleavage: Mechanistic Comparison to a Zn(II) Analogue

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• 作者：Eric S. Elton ; Tingting Zhang ; Rajeev Prabhakar ; Atta M. Arif ; Lisa M. Berreau
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：October 7, 2013
• 年：2013
• 卷：52
• 期：19
• 页码：11480-11492
• 全文大小：703K
• 年卷期：v.52,no.19(October 7, 2013)
• ISSN：1520-510X

文摘

Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) chelate ligand, [(epppa)Pb(NO₃)₂] (4-NO₃) and [(epppa)Pb(ClO₄)₂] (4-ClO₄), were prepared and characterized. In the solid state, 4-NO₃ exhibits 魏⁵-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO₃ is a 1:1 electrolyte with a coordinated NO₃^{鈥?/sup>, whereas 4-ClO₄ is a 1:2 electrolyte. Treatment of 4-ClO₄ with 1 equiv Me₄NOH路5H₂O in CH₃CN:CH₃OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO₄)₂ (3-ClO₄). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO₄ and 3-ClO₄ as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO₄ and 3-ClO₄ differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO₄; second-order behavior for 3-ClO₄), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO₄, and the properties of an initial intermediate isolated from each reaction upon treatment with Me₄NOH路5H₂O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO₄ and 3-ClO₄, indicate that the Zn(II)-containing 3-ClO₄ initially undergoes amide deprotonation upon treatment with Me₄NOH路5H₂O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO₄ instead directly forms a neutral amide-containing, epppa-ligated Pb(II)鈥揙H/Pb(II)-OCH₃ equilibrium mixture upon treatment with Me₄NOH路5H₂O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO₄ is nucleophilic attack of the Pb(II)-OCH₃ moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)鈥揙H/Pb(II)-OCH₃ mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO₄.}