Castro Enrique A, Arellano David, Pavez Paulina, Santos José G
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
J Org Chem. 2003 Aug 8;68(16):6192-6. doi: 10.1021/jo034538+.
The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Brønsted-type plot of slope beta = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Brønsted-type plots of slopes beta = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T(-)) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i). Substitution of S(-) by O(-) in the intermediate T(-) destabilizes this species. (ii). The change of DNPO in T(-) to DNPS also destabilizes this intermediate. (iii). Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv). The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.
在25.0℃、离子强度为0.2M（KCl）的水中，对一系列酚类与O-甲基-O-2,4-二硝基苯基硫代碳酸酯（MDNPTOC）、O-苯基-O-2,4-二硝基苯基硫代碳酸酯（PDNPTOC）和O-乙基2,4-二硝基苯基二硫代碳酸酯（EDNPDTC）的反应进行了动力学研究。在酚过量于底物的情况下，所有反应均呈现准一级动力学，且对酚氧阴离子为一级反应。EDNPDTC的反应显示出斜率β = 0.67的线性布朗斯特型图，表明是协同机理。另一方面，MDNPTOC和PDNPTOC的酚解反应分别呈现斜率β = 0.27和0.28的线性布朗斯特型图，这与逐步机理一致，即阴离子四面体中间体（T(-)）的形成是速率决定步骤。通过将所研究反应的动力学和机理与类似反应进行比较，得出以下结论：（i）。中间体T(-)中S(-)被O(-)取代会使该物种不稳定。（ii）。T(-)中DNPO变为DNPS也会使该中间体不稳定。（iii）。底物的非离去基团中MeO被PhO取代对动力学没有影响，可能是由于电子效应和空间效应的补偿。（iv）。四面体中间体中的氨基变为苯氧基会使中间体不稳定。
