In recent years, visible light photoredox catalysis has become a powerful alternative to existing methodologies for generating transient radical species via outer-sphere mechanisms. Consequently, numerous well-known chemistry could be interwoven with visible light photoredox catalysis, thus unlocking novel enabling techniques for both C-C and C-X bond formation under exceptionally mind conditions with broad substrate scope and functional group tolerance.
Firstly, we aimed at developing a visible light-mediated atom transfer radical cyclization of unactivated alkyl iodide. Due to the large mismatch in reduction potential, unactivated alkyl iodide are challenging substrates to directly reduce with visible light photocatalysts. The successful development of this transformation and preliminary mechanistic studies challenge the perception that a canonical photoredox catalytic cycle is being operative. The careful control experiments revealed that a rather elusive exciplex between tertiary amine and alkyl iodide might come into play to facilitate the interaction of the substrate and photocatalyst via electron transfer or energy transfer. This protocol operated under mild conditions and exhibits excellent chemoselectivity profile while avoiding parasitic hydrogen atom transfer pathways.
In 2017, we described a redox-neutral intermolecular dicarbofunctionalization of styrenes with inert CO2 at atmospheric pressure and carbon-centered radicals. At the outset of this work, the synthesis of valuable phenylacetic acids with CO2 remained confined to single bond formation with relatively simple backbones. Our protocol took advantage of photoredox catalysis to generate open-shell species that add across the double bond, setting the stage for generating a benzyl anion via single-electron transfer prior CO2 insertion. In this manner, this protocol offers the opportunity of triggering a double C–C bond-formation with concomitant CO2 insertion, and in the absence of any Ni catalyst or sophisticated ligand backbone, thus exploiting a previously unrecognized opportunity that complements existing catalytic carboxylations.
Our last venture into photoredox catalysis focused on the functionalization of native sp3 C-H bonds. we have unlocked a modular photochemical platform for forging C(sp3)–C(sp2) and C(sp3)–C(sp3) linkages from abundant alkane sp3 C–H bonds as functional handles using the synergy between nickel catalysts and simple, cheap and modular diaryl ketones. This method is distinguished by its wide scope that is obtained from cheap catalysts and starting precursors, thus complementing existing inner-sphere C–H functionalization protocols or recent photoredox scenarios based on iridium polypyridyl complexes. Additionally, such a platform provides a new strategy for streamlining the synthesis of complex molecules with high levels of predictable site-selectivity and preparative utility. Mechanistic experiments suggest that sp3 C–H abstraction occurs via HAT from the ketone triplet excited state, thus offering a new technique for bond-forming reactions within the metallaphotoredox arena.