Solvolysis of substituted benzyl azoxyarenesulfonates:The IUPAC name for benzyl azoxyarenesulfonate is N-benzyl-N′-arylsulfonyloxydiazene N-oxide. characterisation of the transition state and the selectivity of benzylic intermediates in 50% aqueous 2,2,2-trifluoroethanolElectronic supplementary information (ESI) available: synthesis and analytical data of azoxyarenesulfonates. See http://www.rsc.org/suppdata/p2/b1/b106887n
作者:Isobel M. Gordon、Howard Maskill
DOI:10.1039/b106887n
日期:2001.11.1
Thirteen substituted benzyl azoxyarenesulfonates have been prepared and rate constants for solvolysis of twelve have been measured in 50% (v/v) aqueous 2,2,2-trifluoroethanol over a range of temperatures from which activation parameters and rate constants at 25 °C have been determined. A six-point Hammett correlation for substituents in the benzylic electrofuge, with azoxytoluene-p-sulfonate as the common nucleofuge, gives ρ(σ+) =
−3.27. With 4-methylbenzyl as a common electrofuge, four substituents in the azoxybenzenesulfonate nucleofuge give ρ(σ) = 1.07. Comparison with model reactions indicates a synchronous concerted rate-limiting fragmentation for these substrates. With the less reactive 3-chlorobenzyl as electrofuge, ρ for substituents in the nucleofuge is only ca. 0.7 which indicates a lower degree of charge development in the arenesulfonate
in the transition state for these reactions. Product analyses have been carried out for compounds of high, intermediate, and low reactivity. These indicate a reactivity–selectivity relationship for capture of the substituted benzylic electrophiles by water and 2,2,2-trifluoroethanol. Substituted benzaldehydes are formed from the less reactive substrates, indicating trapping of the first-formed benzylic carbenium ion by nitrous oxide and subsequent elimination of nitrogen and a proton; but yields are low and decrease as the substituted benzyl cation becomes increasingly stable.
我们制备了 13 种取代的苄基偶氮苯磺酸盐,并测量了其中 12 种在 50% (v/v) 2,2,2- 三氟乙醇水溶液中一定温度范围内的溶解速率常数,据此确定了 25 °C 时的活化参数和速率常数。以偶氮甲苯对磺酸盐为普通核苷酸,苄基电泳中取代基的六点哈米特相关性得出 ρ(σ+) =-3.27。以 4-甲基苄基作为普通电解质,偶氮苯磺酸酯核苷酸中的四个取代基给出了 ρ(σ) = 1.07。与模型反应的比较表明,这些底物发生了同步的协同限速破碎。用反应性较低的 3-氯苄基作为电助剂时,核助剂中取代基的 ρ 仅为 0.7 左右,这表明在这些反应的过渡态中,异壬磺酸盐的电荷发展程度较低。针对高、中、低反应活性的化合物进行了产物分析。这些分析表明,水和 2,2,2-三氟乙醇捕获取代的苄基亲电体存在反应活性-选择性关系。从反应性较低的底物中生成了取代的苯甲醛,这表明氧化亚氮捕获了第一种生成的苄基羰离子,随后消除了氮和一个质子;但是产量很低,而且随着取代的苄基阳离子变得越来越稳定,产量也在降低。