Application of α-Keto Acids in Metal-Free Photocatalysis

Abstract

As a carboxylic acid, α-keto acid can be used as an acylating reagent by decarboxylation in organic synthesis. However, traditional methods often need transition metal catalysts or high temperatures, which limits the application of α-keto acid to a certain extent. In recent years, organic transformations induced by visible light have attracted extensive attention due to its mild conditions and simple operations. Herein, the recent advances of metal-free photocatalytic application of α-keto acids are summarized, which mainly include acylation reactions and cyclization reactions. Keywords α-keto acid; visible light; metal-free catalysis; acylation 羧酸是一类重要的化学原料, 具有易得、低毒和稳 定等优点, 在有机合成中得到了广泛应用. 其中, α-酮 酸是一类特殊的羧酸, 通常在脱羧偶联反应中作为酰基 化试剂(图 1) [1,2] . 但是, 传统方法往往使用过渡金属催 化剂或者需要高温等苛刻的反应条件. 因此, 开发更加 绿色、温和的脱羧偶联新方法具有重要意义.近年来, 可见光促进的有机合成有效避免了传统有 机反应的苛刻条件, 已经成为当前的研究前沿和热点领 域 [3][4][5][6][7][8][9][10][11][12][13] . 通过可见光促进反应的策略, 人们开发了许多 新型高效的有机合成方法 [14][15][16][17][18][19][20][21][22][23][24][25] . 特别值得注意的是, 雷 爱文等 [26] 由于贵金属光催化剂的价格昂贵, 且产物在纯化的 过程中容易产生金属残留等问题, 近年来, 人们发展了 多种无金属的有机光催化剂用于可见光诱导 α-酮酸类 化合物的化学转化. 图 1 中列举了近年来光催化 α-酮酸 脱羧的有机光催化剂, 包括曙红 B (eosin B)、罗丹明 B (rhodamine B)和孟加拉玫瑰红(rose bengal)等. 本文综 述了无金属光催化剂作用下, 可见光诱导 α-酮酸类化合 物的脱羧反应的最新进展, 主要包括: (1) α-酮酸在酰化 反应中的应用, (2) α-酮酸在环化反应中的应用, (3)其他 反应.

Recent advances in the photocatalytic synthesis of aldehydes

“…21 Since the AgNO 3 -catalyzed oxidative decarboxylative acylation reaction of pyridines with α-keto acids was reported by Fontana and Minisci's group in the 1990s, 22 the application of α-keto acids as acyl radical precursors has been widely studied. 23 As the simplest α-keto acids, glyoxylic acid is expected to be an ideal formylation reagent via decarboxylation coupling, which could directly introduce an aldehyde group after the decarboxylative reaction. 24…”

Catalyst-Free Cascade Annulation of Enaminones and Aryl Diazonium Tetrafluoroboronates for Cinnoline Synthesis and the Anti-Inflammatory Activity Study

“…Particularly, employing enaminones as building blocks has been identified with highly broad application in the construction of aromatic cycles, including both heteroaryls and carboaryls . In this context, we hypothesized that it is possible to develop a more convenient and atom-economical method for the synthesis cinnolines by employing enaminone as the main building block without using transition-metal catalysts . Herein, we report a catalyst- and additive-free method for cinnoline synthesis via the reactions of enaminones and aryl diazonium tetrafluoroboronates, providing easy and efficient accesses to diverse 3-acyl cinnolines (Scheme B).…”

Photoredox Neutral Decarboxylative Hydroxyalkylations of Heteroarenes with α-Keto Acids

“…Besides, Kanai et al and our group have independently developed photoredox neutral Minisci hydroxyalkylation directly using aldehydes with basically thiophosphoric acid (TPA) and LiBr as hydrogen atom transfer (HAT) agents, respectively (Scheme b). Alternatively, α-keto acids are stable and readily available compounds and have been widely employed as acylating reagents with only the CO 2 byproduct formed with the recent development of mild photoredox acylation of N-heteroarenes (Scheme c) . We envision that the α-keto acids, with acyl radicals formed by decarboxylation, would be used for the Minisci-type hydroxyalkylation of N-heteroarenes under a suitable photoredox neutral catalytic system.…”