Homolytic and heterolytic radical cleavage in the Kolbe reaction -: Electrochemical oxidation of arylmethyl carboxylate ions
Homolytic and heterolytic radical cleavage in the Kolbe reaction -: Electrochemical oxidation of arylmethyl carboxylate ions
复制标题
DOI：
10.1016/s0022-0728(00)00365-x
复制
发表时间：
2001-02-16
影响因子：
4.5
通讯作者：
Savéant, JM
中科院分区：
文献类型：
--
作者：
Andrieux, CP;Gonzalez, F;Savéant, JM
对一系列化合物的循环伏安响应进行了分析，并在少数情况下分析了产物的分布，由此得出了芳甲酸根离子氧化脱羧化的机理。在所有情况下，第一电子的去除和导致二氧化碳形成的键的断裂是连续的而不是协同的过程。在大多数情况下，未配对的电子位于酰氧基的羧基氧上，该自由基会经历快速的均相裂解。反应受第一个电子步骤的动力学控制。生成的烷基自由基离电极表面很近，在有时间进行二聚反应之前就被氧化了，因此只产生碳阳离子衍生的产物(非科尔贝反应)。当羧酸盐离子的芳香族部分更容易氧化时，如4-二甲氨基苄基和9-蒽甲基衍生物，酰氧基具有两性离子性质，裂解较慢，遵循异解机制(涉及分子内解离电子转移)。这就是运动控制逐步过渡到解理步骤的原因。较慢的切割速度也是形成大量二聚体和碳阳离子衍生产物的原因。(C)2001 Elsevier Science B.V.保留所有权利。
The mechanism of oxidative decarboxylation of arylmethyl carboxylate ions is derived from the analysis of the cyclic voltammetric responses of an extended series of compounds, and, in a few selected cases, of product distribution. In all cases, the removal of the first electron and the cleavage of the bond that results in the formation of CO2, are successive rather than concerted processes. In a majority of cases, the unpaired electron is located on carboxyl oxygen of the acyloxy radical which undergoes a fast homolytic cleavage. The reaction is kinetically controlled by the first electron step. The resulting alkyl radical is formed so close to the electrode surface that it is oxidized before having time to dimerize thus yielding exclusively carbocation-derived products (non-Kolbe reaction). When the aromatic portion of the carboxylate ion is easier to oxidize, as with the 4-dimethylamino-benzyl and 9-anthracenyl-methyl derivatives, the acyloxy radical has a zwitterionic character, the cleavage is slower and follows a heterolytic mechanism (involving an intramolecular dissociative electron transfer). This is the reason that the kinetic control passes progressively to the cleavage step. The slower cleavage is also the cause of the formation of a substantial amount of dimer together with carbocation-derived products. (C) 2001 Elsevier Science B.V. All rights reserved.