Exploiting Pnictogen σ/π-Hole Interactions for Visible-Light-Induced Radical Transformations

ConspectusThe direct and selective activation of chemical bonds under mild, operationally simple conditions remains a longstanding pursuit in organic synthesis. Recently, elemental σ/π-hole interactions have emerged as powerful noncovalent tools, enabling new modes of molecular activation. Despite their promise, the application of pnictogen σ/π-hole interactions in photoinduced radical processes is still at a nascent stage.In this Account, we describe our recent efforts in leveraging pnictogen σ/π-hole interactions to facilitate the generation of organic radicals under visible-light irradiation. By capitalizing on the properties of pnictogen σ/π-holes─tuned through careful selection of pnictogen elements, electron-withdrawing substituents, and pnictogen hole acceptors─we have developed general strategies for visible-light-induced radical transformations. The key element of this strategy is the use of pnictogen σ/π-hole interactions to assemble charge-transfer complexes (CTCs), which undergo visible-light-induced single-electron transfer (SET) from an electron donor to the pnictogen center. This process generates either pnictogen-centered radicals or substrate-derived radical species, thereby providing a basis for the rational design of new reagents and catalysts. The main advances can be summarized as follows:(1)We established an efficient, transition-metal-free and photocatalyst-free strategy for the generation of a broad range of radical species─including P(III)-centered, alkyl, carboranyl, fluoromethyl, difluoromethyl, trifluoromethyl, pyridyl, oxyalkyl, <i>d</i>_<i>n</i>-alkyl and methylthio radicals─by using pnictogen σ-hole interactions.(2)On this basis, the scope of pnictogen-hole interaction-enabled photoreactions was further expanded by introducing N-heterocyclic nitrenium (NHN) and N-heterocyclic carbene (NHC) systems. The amphiphilic character and π-hole electron-accepting ability of NHNs promote the formation of photoactive CTCs with suitable electron donors, which, upon single-electron transfer, afford NHN-centered radicals. This approach enables a series of metal-free reductive radical transformations mediated by NHN radicals, including the activation of C-I, C-Br, and activated C-Cl bonds, as well as controlled radical polymerizations. In addition, the combination of NHNs with ligated boryl radicals allows the activation of otherwise inert alkyl chlorides, further broadening the applicability and synthetic utility of this strategy.(3)The concept of pnictogen interactions was further extended to NHC-based photocatalytic systems. NHCs, which are isoelectronic and isostructural analogues of NHNs and possess vacant pπ orbitals, can accept an electron to generate NHC radical anions. These species can act as strong reductants capable of activating a range of inert bonds, including C_aryl-F, C_aryl-N, C_aryl-S, C_aryl-Se, and C_aryl-O bonds.Taken together, these advances underscore the potential of pnictogen σ/π-hole interactions in contemporary radical chemistry.