The photochemistry of 3- and 4-acetylphenylacetic acids (6 and 7) has been studied in aqueous solution. This work is a continuation of research efforts aimed at understanding the structural effects on the efficacy for benzyl carbanion photogeneration via photodecarboxylation. The nitro group (at the 3- and 4-positions) is known to be an exceptionally good activating group on the benzene ring because of its enhanced electron-withdrawing effect in the excited triplet state for photodecarboxylation and the related photo-retro-aldol type process. It is shown in this work that the acetyl group is an equally good activating group for the photodecarboxylation. Thus, the photochemistry of 6 and 7 parallels much of what was observed for the corresponding nitrophenylacetic acids 1 and 2. Both 6 and 7 photodecarboxylate efficiently (Φd = 0.60 and 0.22, respectively, at pH 7) via the carboxylate form, to give observable (by laser flash photolysis) benzyl carbanion or related intermediates. The meta isomer 6 displays an acid-catalyzed pathway for photodecarboxylation at pH < 3 and along with its enhanced overall reactivity, is consistent with a meta effect of the acetyl group. Triplet state reactivity is inferred from sensitization and laser flash photolysis experiments. Based on the results of this work, the acetyl group may now be viewed as an "enhanced" electron-withdrawing group (in the excited state) when attached to a benzene ring (i.e., in acetophenone derivatives) that is capable of inducing ionic reactions, which is not the traditional photochemistry that is expected from such compounds.Key words: photodecarboxylation, acid catalysis, acetophenone, photogenerated carbanion, enol, enolate.
3-和4-乙酰基苯乙酸(6和7)的光化学在水溶液中进行研究。这项工作是为了理解结构对苯甲基负离子光发生的效力的影响而进行的研究工作的延续。硝基团(在3-和4-位置)被认为是苯环上的一个异常好的活化基团,因为它在激发的三重态中具有增强的电子吸引效应,对于光解羧反应和相关的光-返醛反应类型有影响。本工作表明,乙酰基团对于光解羧同样是一个良好的活化基团。因此,6和7的光化学与相应的硝基苯乙酸1和2观察到的许多现象类似。在pH 7下,6和7均通过羧酸形式有效地进行光解羧(Φd = 0.60和0.22),以产生可观察到的(通过激光闪光光解)苯甲基负离子或相关中间体。对于pH < 3,间位异构体6显示出光解羧的酸催化途径,并且由于其增强的整体反应性,与乙酰基团的间位效应一致。三重态反应性是通过感光和激光闪光光解实验推断出来的。根据这项工作的结果,当连接到苯环(即在乙酰苯酮衍生物中)时,乙酰基团现在可以被视为一个“增强”的电子吸引基团(在激发态中),能够诱导离子反应,这与传统的预期不同。这些化合物所期望的光化学不同。关键词:光解羧反应,酸催化,乙酰苯酮,光生负离子,烯醇,烯醇酸。