Controlling Chemo- and Site-Selectivity Through the Formation of Radicals Adjacent to a sp2 Carbon.

Achieving high chemo- and site-selectivity is crucial in organic synthesis. High selectivity directly minimizes undesired by-products, thereby increasing the atom economy and eliminating the wasteful purification steps. While the control of chemoselectivity is precisely achieved by the reactivity variation between two different functional groups, that of site-selectivity is a challenging task. Historically, substrate-directed approaches have played a key role in realizing site-selective transformations. On contrary, modern site-selective approaches are rather dependent on radical chemistry. In this context, transformations via metal-catalyzed SET-induced formation of radical intermediates arguably represent a powerful strategy that has received considerable attention within last two decades. Among the various classes, a radical intermediate adjacent to a Csp 2 center is relatively stabilized compared to the corresponding alkyl radical species, which is attributed to the resonance delocalization of the unpaired electron into the adjacent system. By engaging those radical intermediates, numerous approaches that have been accomplished proceed with amazingly perfect chemo- and site-selectivity. Noteworthy instances include transformations via the formation of carboxylate radicals and α-carbonyl radicals from carboxylic acids, ketones, etc. In this account, we describe developments on this area that have emanated primarily from our original research together with a few instances of closely related works by others.