Light‐Induced Direct Decarboxylative Functionalization of Aromatic Carboxylic Acids

Aryl radicals are important intermediates in organic synthesis. The generation of these reactive species via direct decarboxylation of inexpensive and readily available aromatic carboxylic acids is an attractive goal. However, such a process intrinsically exhibits high energy barriers to overcome, which in consequence usually require precious metal catalysis, stoichiometric oxidants and harsh conditions, suffering from limitations such as poor functional group tolerance and low atom economy. In recent years, visible‐light‐induced photochemical reactions have provided new approaches to address this challenge. Three major strategies have been introduced in this emerging field: 1) one‐pot in‐situ activation of benzoic acids to generate intermediates such as benzoyl hypobromites or hypoiodites; 2) the use of specialized photocatalysts like biphenyl/1,4‐dicyanobenzene to promote decarboxylation through photo‐induced electron transfer or charge transfer processes; 3) photo‐induced LMCT (Ligand‐to‐Metal Charge Transfer) strategy where copper or iron salts coordinate to the carboxylate anion and generate aromatic radicals upon visible light excitation. On the basis of these three strategies, this review will systematically summarize the development of visible‐light‐induced direct decarboxylative functionalization of aromatic carboxylic acids, focusing on the reaction mechanism and substrate scope, and discuss their prospects in organic synthesis.