When Light Meets Nitrogen-Centered Radicals: From Reagents to Catalysts.Acc Chem Res. 2020 05 19; 53(5):1066-1083.AC
Nitrogen-centered radicals (NCRs) are a versatile class of highly reactive species that have a longer history than the classical carbon-based radicals in synthetic chemistry. Depending on the N-hybridization and substitution patterns, NCRs can serve as electrophiles or nucleophiles to undergo various radical transformations. Despite their power, progress in nitrogen-radical chemistry is still slow compared with the popularity of carbon radicals, and their considerable synthetic potential has been largely underexplored, which is, as concluded by Zard, mainly hampered by "a dearth of convenient access to these species and a lack of awareness pertaining to their reactivity".Over the past decade, visible-light photoredox catalysis has been established as a powerful toolbox that synthetic chemists can use to generate a diverse range of radical intermediates from native organic functional groups via a single electron transfer process or energy transfer under mild reaction conditions. This catalytic strategy typically obviates the need for external stoichiometric activation reagents or toxic initiators and often enables traditionally inaccessible ionic chemical reactions. On the basis of our long-standing interest in nitrogen chemistry and catalysis, we have emphasized the use of visible-light photoredox catalysis as a tactic to discover and develop novel methods for generating NCRs in a controlled fashion and synthetic applications. In this Account, we describe our recent advances in the development of visible-light-driven photoredox-catalyzed generation of NCRs and their synthetic applications.Inspired by the natural biological proton-coupled electron transfer (PCET) process, we first developed a strategy of visible-light-driven photoredox-catalyzed oxidative deprotonation electron transfer to activate the N-H bonds of hydrazones, benzamides, and sulfonamides to give the corresponding NCRs under mild reaction conditions. With these reactive species, we then achieved a range of 5-exo and 6-endo radical cyclizations as well as cascade reactions in a highly regioselective manner, providing access to a variety of potentially useful nitrogen heterocycles. To further expand the repertoire of possible reactions of NCRs, we also revealed that iminyl radicals, derived from O-acyl cycloalkanone oxime esters, can undergo facile ring-opening C-C bond cleavage to give cyanoalkyl radicals. These newly formed radical species can further undergo a variety of C-C bond-forming reactions to allow the synthesis of diverse distally functionalized alkyl nitriles. Stimulated by these studies, we further developed a wide variety of visible-light-driven copper-catalyzed radical cross-coupling reactions of cyanoalkyl radicals. Because of their inherent highly reactive and transient properties, the strategy of heteroatom-centered radical catalysis is still largely underexplored in organic synthesis. Building on our understanding of the fundamental chemistry of NCRs, we also developed for the first time the concept of NCR covalent catalysis, which involves the use of in situ-photogenerated NCRs to activate allyl sulfones, vinylcyclopropanes, and N-tosyl vinylaziridines. This catalytic strategy has thus enabled efficient difunctionalization of various alkenes and late-stage modification of complex biologically active molecules.In this Account, we describe a panoramic picture of our recent contributions since 2014 to the development and application of the visible-light-driven photoredox systems in the field of NCR chemistry. These studies provide not only efficient methods for the synthesis of functionally rich molecules but also some insight into the exploration of new reactivity or reaction modes of NCRs.