Glycine enolates:: The effect of formation of iminium ions to simple ketones on α-amino carbon acidity and a comparison with pyridoxal iminium ions
Glycine enolates:: The effect of formation of iminium ions to simple ketones on α-amino carbon acidity and a comparison with pyridoxal iminium ions
复制标题
DOI：
10.1021/ja078006c
复制
发表时间：
2008-02-13
影响因子：
15
通讯作者：
Richard, John P.
中科院分区：
文献类型：
--
作者：
Crugeiras, Juan;Rios, Ana;Richard, John P.
通过H-1 NMR分析确定了在D2 O中的平衡常数，用于从甘氨酸甲酯到丙酮的加成和从甘氨酸到苯乙醛酸的加成形成亚胺/亚胺离子。一阶速率常数，也由1H NMR测定，报告的氘交换溶剂D2 O和α-氨基碳的甘氨酸甲酯和甘氨酸的酮和布朗斯台德碱的浓度增加的存在下。这些速率和平衡数据被用来计算二阶速率常数去质子化的DO-和酮加合物的α-亚氨基碳的布朗斯台德碱。丙酮和甘氨酸甲酯之间以及乙醛酸苯酯和甘氨酸之间形成的亚胺离子估计分别使母体碳酸去质子化的pK（a）为21和29时降低7个单位和15个单位。亚胺离子形成对苯乙醛酸和5 ′-脱氧吡哆醛（DPL）的影响[Toth，K.;理查德，J. P. J. Am。Soc.2007，129，3013-3021]对甘氨酸的碳酸性的影响是类似的。然而，DPL是甘氨酸去质子化的比苯乙醛酸酯好得多的催化剂，因为氨基酸和DPL转化为反应性亚胺鎓离子的热力学驱动力特别大。
Equilibrium constants in D2O were determined by H-1 NMR analyses for formation of imines/ iminium ions from addition of glycine methyl ester to acetone and from addition of glycine to phenylglyoxylate. First-order rate constants, also determined by 1H NMR, are reported for deuterium exchange between solvent D2O and the alpha-amino carbon of glycine methyl ester and glycine in the presence of increasing concentrations of ketone and Bronsted bases. These rate and equilibrium data were used to calculate second-order rate constants for deprotonation by DO- and by Bronsted bases of the alpha-imino carbon of the ketone adducts. Formation of the iminium ion between acetone and glycine methyl ester and between phenylglyoxylate and glycine is estimated to cause 7 unit and 15 unit decreases, respectively, in the pK(a)'s of 21 and 29 for deprotonation of the parent carbon acids. The effect of formation of iminium ions to phenylglyoxylate and to 5'-deoxypyridoxal (DPL) [Toth, K.; Richard, J. P. J. Am. Chem. Soc. 2007, 129, 3013-3021] on the carbon acidity of glycine is similar. However, DPL is a much better catalyst than phenylglyoxylate of deprotonation of glycine, because of the exceptionally large thermodynamic driving force for conversion of the amino acid and DPL to the reactive iminium ion.