Kinetic investigation of the phenolysis of phenyl 4-nitrophenyl and phenyl 2,4-dinitrophenyl carbonates
作者:Enrique A. Castro、Mauricio Angel、Paulina Pavez、José G. Santos
DOI:10.1039/b103397m
日期:2001.11.29
The reactions of phenyl 4-nitrophenyl carbonate (PNPC) and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in water at 25.0 °C, ionic strength 0.2 mol dm−3 (KCl). Under phenoxide or total phenol excess over the substrate all these reactions obey pseudo first-order kinetics and are first order in phenoxide. The Brønsted-type plots for the nucleophilic rate constants are linear with slopes 0.61 and 0.49 for the phenolysis of PNPC and PDNPC, respectively. The magnitude of these slopes and the absence of curvature in the Brønsted plot at pKa
= 7.1 for the PNPC reactions are consistent with concerted mechanisms (one step) for both reaction series. PDNPC is more reactive than PNPC toward phenoxide nucleophiles; this can be explained by the presence of a second nitro group in PDNPC, which (i) leaves its carbonyl carbon more positively charged than that of PNPC, making the former a better electrophile, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The larger nucleophilic rate coefficients found in this work relative to those obtained in the concerted phenolysis of 4-nitrophenyl and 2,4-dinitrophenyl methyl carbonates is explained by a stronger electron withdrawal from PhO compared to MeO. Comparison of the concerted phenolysis of PNPC with the stepwise reactions of quinuclidines with the same substrate indicates that substitution of a quinuclidino group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilises the intermediate.
苯基 4-硝基苯酯 (PNPC) 和苯基 2,4-二硝基苯酯 (PDNPC) 与一系列均相酚盐阴离子的反应在水中的动力学研究在 25.0 °C,离子强度 0.2 mol dm−3(KCl)的条件下进行。在酚盐或总酚过量的情况下,这些反应均遵循拟一级动力学,并对酚盐呈现一级反应。亲核速率常数的布仑斯惕线性图的斜率分别为 0.61 和 0.49,对应于 PNPC 和 PDNPC 的酚解反应。这些斜率的值以及在 pKa = 7.1 时 PNPC 反应在布仑斯惕图中无曲率,与这两种反应系列协同机制(一步)一致。PDNPC 相对于 PNPC 对酚盐亲核试剂具有更高的反应性;这可以由 PDNPC 中的第二个硝基解释,(i)使得羰基碳比 PNPC 具有更多正电荷,从而成为更好的亲电试剂,以及 (ii) 使得 2,4-二硝基苯氧基比 4-硝基苯氧基成为更好的离去基团。本文发现的大于协同酚解反应中的 4-硝基苯甲酸酯和 2,4-二硝基苯甲酸酯甲酯的亲核速率常数,可由PhO相比MeO更强的电子抽取效应解释。将 PNPC 的协同酚解与相同底物的奎宁环的逐步反应进行比较,表明在偶极四面体中间体中用苯氧基取代奎宁环基团会极大地破坏中间体的稳定性。