Hydrogen Atom Transfer Promoted by Carbon-Centered Biradicals via Energy Transfer Catalysis

ConspectusIn the past decade, visible-light-mediated photocatalysis has emerged as an applicable strategy for the generation of diverse radical species via single electron transfer (SET) and energy transfer (EnT) processes. Within this context, visible-light-mediated hydrogen atom transfer (HAT) has attracted major interest due to its mild and environmentally benign conditions applied in the selective activation of C-H bonds. Strategies employing C- and heteroatom-centered radical species to selectively activate C-H bonds have become versatile tools due to their mildness and good functional group compatibility for synthesizing value-added products. In this regard, a review on C-centered radical-promoted HAT processes was reported by Gevorgyan's group ( Chem. Sci. 2020, 11, 12974, DOI: 10.1039/d0sc04881j), and a review on visible-light-promoted remote C-H functionalization via 1,5-HAT was recently reported by Zhu's group ( Chem. Soc. Rev. 2021, 50, 7359, DOI: 10.1039/d0cs00774a). Compared to N- and O-centered radical-promoted HAT processes, C(sp³)-centered radical-promoted HAT is more challenging and less explored due to the small differences in C-H bond dissociation energies. Additionally, in the realm of C-centered radical-promoted HAT, the generation of C-centered radicals has mostly involved SET processes, while the EnT-mediated C-centered radical-promoted HAT process has been less discussed because of the considerable scarcity of related reports.As a result of the rapid advancement of EnT catalysis in synthetic chemistry, C-C-centered biradical species can be readily generated from C═C double bonds via the EnT process using a photocatalyst under visible-light irradiation. An array of transformations (such as cycloaddition and isomerization) involving these C-C-centered biradical species have been reported. Our group recognized these C-C-centered biradicals as practical initiators of the HAT process in C-H functionalization reactions and devoted considerable effort to this field. Initially, we used a triplet excited allene moiety to realize remote sp³ C-H bond activation successfully. Later, a visible-light-induced triplet biradical HAT reaction of diarylethylenes was disclosed by our group. To further enhance its synthetic applicability, we applied simple styrene derivatives to realize such a novel EnT-mediated HAT process. Detailed control experiments combined with comprehensive density functional theory calculations elucidated the reaction mechanisms of these biradical-mediated HAT reactions along with the subsequent transformations of the obtained products.In addition to the advancements achieved in our group, Bach, Petersen, and others also reported such EnT-mediated HAT processes utilizing aryl acrylamide or aryl acrylate derivatives. Given the rapid progress over the past 5 years and the lack of focused discussion in this area, it is necessary to highlight these reports as a complement to the area of visible-light-mediated HAT via carbon-to-carbon processes. We anticipate that this Account will offer worthwhile insights and serve as guidance for future related research.