Binding Catalysis and Inhibition by Metal Ions and Protons in the Enolization of Phenylacetylpyrazine
作者:Anne-Gaelle Collot、Michael Courtney、Dara Coyne、Stephen E. Eustace、Rory A. More O’Ferrall
DOI:10.1021/jo802650s
日期:2009.5.1
A study of the enolization of phenylacetylpyrazine (PzCOCH2Ph) catalyzed by acid, base and metal ions in aqueous solution shows, unusually, that metal ions are more effective catalysts than protons, e.g., for zinc kZn/kH = 600. Such behavior contrasts with that of the structurally related phenacylpyridine (PyCH2COPh) for which kZn/kH = 0.0065. To interpret this difference, equilibrium constants for
对酸,碱和金属离子在水溶液中催化的苯乙酰基吡嗪(PzCOCH 2 Ph)烯醇化的研究表明,与众不同的是,金属离子比质子更有效,例如,对于锌k Zn / k H = 600。行为与结构相关的苯并吡啶(PyCH 2 COPh)的行为相反,后者的k Zn / k H= 0.0065。为了解释这种差异,已经测量或估计了苯基乙酰基吡嗪的互变异构以及质子和金属离子与其酮互变异构体和烯醇酸根阴离子的结合的平衡常数,并与现有的苯并吡啶进行了比较。互变异构常数K E = 1.2×10 -3(p K E = 2.9)通过结合分别通过酮互变异构体的碘化或溴化以及松弛度较差的烯醇测量的烯醇化的正向和反向速率常数得出。对于酮互变异构体,NMR测量得出N-质子化为ap K a = -0.90 ,分光光度测量得出p K电离为烯酸酯阴离子的a = 11.90。对于烯醇,从用于瞬态烯醇反应物的酮化和吸光度测量的pH曲线获得了单质子化和双质子化的p