peri-Dimethylamino substituent effects on proton transfer at carbon in α-naphthylacetateesters: a model for mandelate racemase
作者:Richard J. Delley、Subhajit Bandyopadhyay、Mark A. Fox、Constanze Schliehe、David R. W. Hodgson、Florian Hollfelder、Anthony J. Kirby、AnnMarie C. O'Donoghue
DOI:10.1039/c1ob06525d
日期:——
The rate constants for exchange of hydrogen for deuterium at the α-CH2 positions of 8-(N,N-dimethylaminonaphthalen-1-yl)acetic acid tert-butyl ester 1 and naphthalen-1-ylacetic acid tert-butyl ester 2 have been determined in potassium deuteroxide solutions in 1â:â1 D2Oâ:âCD3CN, in order to quantify the effect of the neighbouring peri-dimethylamino substituent on α-deprotonation. Intramolecular general base catalysis by the (weakly basic) neighbouring group was not detected. Second-order rate constants, kDO, for the deuterium exchange reactions of esters 1 and 2 have been determined as 1.35 à 10â4 Mâ1 sâ1 and 3.95 à 10â3 Mâ1 sâ1, respectively. The unexpected 29-fold decrease in the kDO value upon the introduction of a peri-dimethylamino group is attributed to an unfavourable steric and/or electronic substituent effect on intermolecular deprotonation by deuteroxide ion. From the experimental kDO values, carbon acid pKa values of 26.8 and 23.1 have been calculated for esters 1 and 2.
在 1:â1 D2Oâ:âCD3CN 中的氘氧钾溶液中,测定了 8-(N,N-二甲基氨基萘-1-基)乙酸叔丁酯 1 和萘-1-基乙酸叔丁酯 2 δ-CH2位置上氢与氘交换的速率常数,以量化邻近的近二甲基氨基取代基对δ-质子化的影响。没有检测到邻近基团(弱碱性)对分子内一般碱催化作用的影响。酯 1 和酯 2 的氘交换反应的二阶速率常数 kDO 分别为 1.35 Ã 10â4 Mâ1 sâ1 和 3.95 Ã 10â3 Mâ1 sâ1 。引入一个近二甲基氨基后,kDO 值意外地降低了 29 倍,这是因为氘代氧化物离子对分子间去质子化作用产生了不利的立体和/或电子取代基效应。根据实验 kDO 值,计算出酯类 1 和 2 的碳酸 pKa 值分别为 26.8 和 23.1。