ACYL SUBSTITUENT EFFECTS ON THIOHEMIACETAL EQUILIBRIA
ACYL SUBSTITUENT EFFECTS ON THIOHEMIACETAL EQUILIBRIA
复制标题
DOI：
10.1021/ja00505a030
复制
发表时间：
1979-01-01
影响因子：
15
通讯作者：
BYERS, LD
中科院分区：
文献类型：
--
作者：
KANCHUGER, MS;BYERS, LD
测定了一系列醛与两种硫醇(谷胱甘肽，pKa=9.1，对硝基苯酚，pAa=4.4)形成硫代半缩醛的平衡常数。硫代半缩醛反应和醛水合反应对醛上的吸电子取代基同样敏感。硫代半缩醛形成的平衡常数As(其中谷胱甘肽是硫醇)与Taft极性取代基常数有关：logK$=1.6 5*+1.4 1，/·=0.986。P*(90%置信度)的值间估计为1.47-1.83。硫代半缩醛与对硝基硫酚的生成也与谷胱甘肽的Taft方程有很好的相关性。硫代半缩醛的相对稳定性与硫醇的碱性无关。然而，对于所有被测试的醛，硫代半缩醛在醛水合物上的稳定度为4.3(±0.2)千卡/摩尔(25℃)。硫代半缩醛阴离子生成的p*值估计为3.0。这为用过渡态与阴离子四面体中间体的相似性来解释酰基转移到硫醇酸盐的速率的p*值提供了一个有用的指标。
The equilibrium constants for thiohemiacetal formation between a series of aldehydes and two thiols (glutathione, pKa= 9.1, and p-nitrothlophenol, pAa= 4.4) were measured. Thiohemiacetalformation and aldehyde hydration are equally sensitive to electron-withdrawing substituents on the aldehyde. The equilibrium constant for thiohemiacetal formation, As (where glutathione is the thiol), is related to the Taft polar substituent constant: log K $= 1.65*+ 1.41,/·= 0.986. The inter-val estimatorof p*(at 90% confidence) is 1.47-1.83. Thiohemiacetal formation with p-nitrothiophenol is also correlated rea-sonably well with* by the same Taft equation obtained with glutathione. The relative stabilitiesof thiohemiacetals are independent of the basicity of the thiol. For all thealdehydes tested, however, the thiohemiacetal is stabilized over the aldehyde hydrate by 4.3 (±0.2) kcal/mol (25 C). The p* value for formationof the thiohemiacetal anion is estimated to be 3.0. This pro-vides a useful index for interpreting the p* value obtained for the rate of acyl transfers to thiolates in terms of the resemblance of the transition state to the anionic tetrahedral intermediate.