Water and hydroxide ion pathways in the σ-complexation of superelectrophilic 2-aryl-4,6-dinitrobenzotriazole 1-oxides in aqueous solution. A kinetic and thermodynamic study
作者:Taoufik Boubaker、Alain Pierre Chatrousse、François Terrier、Bahoueddine Tangour、Julian M. Dust、Erwin Buncel
DOI:10.1039/b201731h
日期:——
As part of our continuing studies of the highly electron-deficient nature of nitrobenzofuroxans, nitrobenzofurazans and related heterocycles, we report here a kinetic and thermodynamic study of Ï-complexation for a series of 2-aryl-4,6 -dinitrobenzotriazole 1-oxides (3aâe) over a large pH range in aqueous solution. The reaction series represents a modulation in electrophilic properties of the benzotriazole moiety in formation of the corresponding hydroxy Ï-adducts (4aâe). Analysis of the data has allowed dissection of observed rates into forward (kH2O1, kOHâ2) and reverse (kH+â1, kâ2) rate constants as well as the obtention of pKa values for H2O addition to the benzotriazole moiety. Our results reveal that 3aâe are superelectrophilic compounds with respect to 1,3,5-trinitrobenzene (TNB) as a standard electron-deficient aromatic, but less superelectrophilic compared to 4,6-dinitrobenzofuroxan (DNBF). Some data pertaining to buffer catalysis of the formation and decomposition of the adducts together with solvent deuterium isotope effects for these pathways are also reported. From these results, it is concluded that adduct formation occurs via general base catalyzed water attack: the general bases include notably H2O, HCO3â, CO32â as well as OHâ. This contrasts with the situation for the Ï-complexation of DNBF where HCO3â
and CO32â were found to act as nucleophilic catalysts whereas OHâ functioned as a general base catalyst. This contrasting behaviour provides further evidence that the dinitro-activated carbocyclic ring of the benzotriazoles 3aâe ranks somewhat lower in electrophilic/superelectrophilic properties compared to that in DNBF. Altogether, the results provide a basis for understanding the relationship between the superelectrophilic reactivities, as evidenced by the contrasting kinetic and thermodynamic properties of the systems at hand, and the varied abilities of these substrates to react in pericyclic DielsâAlder reactions.
作为我们对硝基苯并呋喃、硝基苯并呋喃和相关杂环的高度缺电子性质的持续研究的一部分,我们在此报告了一系列 2-芳基-4,6-二硝基苯并三唑 1-氧化物的 β-络合动力学和热力学研究(3a–e) 在水溶液中的较大 pH 范围内。该反应系列代表了苯并三唑部分的亲电子特性在形成相应的羟基α-加合物(4a–e) 过程中的调节。对数据的分析允许将观察到的速率分解为正向 (kH2O1, kOH–2) 和反向 (kH+–1, k–2) 速率常数,以及获得 H2O 添加到苯并三唑部分。我们的结果表明,相对于作为标准缺电子芳香族化合物的 1,3,5-三硝基苯 (TNB),3a–e 是超亲电化合物,但与 4,6-二硝基苯并呋喃 (DNBF) 相比,其超亲电性较低。还报告了一些关于加合物形成和分解的缓冲催化以及溶剂氘同位素对这些途径的影响的数据。从这些结果可以得出结论,加合物的形成是通过一般碱催化的水攻击发生的:一般碱主要包括H2O、HCO3-、CO32-以及OH-。这与 DNBF 的 δ-络合情况形成对比,其中 HCO3-
发现CO32-和CO32-作为亲核催化剂,而OH-作为一般碱催化剂。这种对比行为进一步证明,与 DNBF 相比,苯并三唑 3a-e 的二硝基活化碳环的亲电/超亲电性质稍低。总而言之,这些结果为理解超亲电反应性之间的关系提供了基础,这一点可以通过现有系统的对比动力学和热力学性质以及这些底物在周环狄尔斯-阿尔德反应中反应的不同能力来证明。