Castro Enrique A, Bessolo Jorge, Aguayo Raul, Santos José G
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
J Org Chem. 2003 Oct 17;68(21):8157-61. doi: 10.1021/jo0348120.
The reactions of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO(2): 1, 2, and 3, respectively) with a series of secondary alicyclic amines (SAA) were subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions were followed spectrophotometrically by monitoring the release of 4-nitrobenzenethiolate anion at 420-425 nm. Under excess amine, pseudo-first-order rate constants (k(obsd)) are obtained for all reactions. The plots of k(obsd) vs [SAA] at constant pH are linear with the slope (k(N)) independent of pH. The statistically corrected Brønsted-type plots (log k(N)/q vs pK(a) + log p/q) for the reactions of 1 and 2 are nonlinear with slopes at high pK(a), beta(1) = 0.27 and 0.10, respectively, and slopes at low pK(a), beta(2) = 0.86 and 0.84, respectively. The Brønsted curvature is centered at pK(a) (pK(a)(0)) 10.0 and 10.4, respectively. The reactions of SAA with 3 exhibit a linear Brønsted-type plot of slope 0.81. These results are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T(+/-)). For the reactions of 1 and 2, there is a change in rate-determining step with amine basicity, from T(+/-) breakdown to products at low pK(a), to T(+/-) formation at high pK(a). For the reactions of 3, breakdown to products of T(+/-) is rate limiting for all the SAA series (pK(a)(0) > 11). The increasing pK(a)(0) value as the substituent in the acyl group becomes more electron withdrawing is attributed to an increasing nucleofugality of SAA from T(+/-). The greater pK(a)(0) value for the reactions of SAA with 1, relative to that found in the pyridinolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5), is explained by the greater nucleofugality from T(+/-) of the former amines, compared to isobasic pyridines, and the greater leaving ability from T(+/-) of 2,4-dinitrophenoxide relative to 4-nitrobenzenethiolate.
对一系列仲脂环胺（SAA）与S-4-硝基苯基4-X-取代硫代苯甲酸酯（X = H、Cl和NO₂，分别为1、2和3）的反应，在44 wt%乙醇-水体系中、25.0℃且离子强度为0.2 M（KCl）的条件下进行了动力学研究。通过在420 - 425 nm处监测4-硝基苯硫酚阴离子的释放，用分光光度法跟踪反应进程。在胺过量的情况下，得到了所有反应的准一级速率常数（k(obsd)）。在恒定pH下，k(obsd)对[SAA]的作图呈线性，斜率（k(N)）与pH无关。1和2反应的经统计校正的布朗斯特型作图（log k(N)/q对pK(a) + log p/q）是非线性的，在高pK(a)时斜率分别为β(1) = 0.27和0.10，在低pK(a)时斜率分别为β(2) = 0.86和0.84。布朗斯特曲率分别以pK(a)（pK(a)(0)）10.0和10.4为中心。SAA与3的反应呈现斜率为0.81的线性布朗斯特型作图。这些结果与通过两性离子四面体中间体（T(+/-)）的逐步机理一致。对于1和2的反应，随着胺碱性的变化，速率决定步骤发生改变，在低pK(a)时是T(+/-)分解为产物，在高pK(a)时是T(+/-)形成。对于3的反应，T(+/-)分解为产物是所有SAA系列的速率限制步骤（pK(a)(0) > 11）。随着酰基中取代基吸电子能力增强，pK(a)(0)值增大，这归因于SAA从T(+/-)中离去能力的增强。SAA与1反应的pK(a)(0)值比2,4-二硝基苯基苯甲酸酯吡啶解反应中观察到的（pK(a)(0) = 9.5）更大，这是因为与等碱性吡啶相比，前者胺从T(+/-)中的离去能力更强，且2,4-二硝基苯氧基相对于4-硝基苯硫酚阴离子从T(+/-)中的离去能力更强。
