Recent advances in radical chemistry proceeding through pro-aromatic radicals

This review illustrates pioneering work and recent advancements on the use of pro-aromatic compounds as carbon- and heteroatom-radical precursors. Pro-aromatic systems such as 1,4-cyclohexadienes, dihydronicotinamides, 1,4-dihydropyridines, 4-substituted Hantzsch esters, pyridinium salts, and benzothiazolines are easily transformed to pro-aromatic radicals and can be used to realize hydrogen atom or radical functional group transfer chemistry. 1,4-dihydropyridines and dihydronicotinamides form complexes with suitable acceptors, and these donor-acceptor complexes engage in intramolecular electron transfer leading to radical pairs that contain a pro-aromatic moiety. We highlight the underlying principle for the generation of reactive radicals in these systems that is gaining aromaticity as the driving force for the fragmentation of the pro-aromatic radicals. Moreover, many pro-aromatic reagents contain weak activated C–H bonds, qualifying them as good H-atom donors in C-radical reductions. Studies from our laboratory along with recent developments from other groups are classified with respect to the structure of the pro-aromatic radical precursor.