Leaving group effects in thiolester alkaline hydrolysis. Part 1. A keten-mediated (E1 cB) pathway for basic hydrolysis of S-acetoacetylcoenzyme A and analogues
作者:Kenneth T. Douglas、Norbert F. Yaggi
DOI:10.1039/p29800001037
日期:——
ylcysteamine follows a kinetic ionisation curve with an inflexion corresponding to the pK of this ester as determined by spectrophotometric and electrometric titrations. The rate constant at high pH was shown to follow a Brønsted relationship with βL.G.–1.13, where βL.G. is the slope of a plot of the logarithm of the rate constant versus the pKa of the conjugate acid of the leaving group. This, and
在水性介质中,已经研究了一系列离去基团取代的乙酰硫基乙酸酯(CH 3 COCH 2 COSR)的碱性水解。N-乙酰基-S-乙酰乙酰半胱胺的水解遵循动力学电离曲线,其弯曲度对应于该酯的p K(通过分光光度法和静电滴定法测定)。高pH下的速率常数显示出与βL.G呈布朗斯台德关系。–1.13,其中βL.G. 是速率常数对数与p K a的对数图的斜率离去基团的共轭酸的量。这以及速率比较,活化参数和动力学溶剂同位素效应的其他证据表明,E 1 cB水解机理涉及通过烯酮过渡态使酯烯醇盐离子发生单分子塌陷。S-乙酰乙酰辅酶A也通过该机理在碱中水解。通过这种非分子过程,可以直接比较ArS和ArO离开组的驱逐速率。对于p K L.G.的离去组 如图10所示,氧阴离子离去CA。比硫醇盐阴离子快1或2个数量级;对于p K L.G. 6.0,氧气比硫的优势是103 –10 4折。在直接的结构比较中,PhS的离去速度是PhO的32倍。空间释放的在贡献ë