Emerging trends in photochemical activation of elemental sulfur for C-S bond formation

Elemental sulfur (S₈) has emerged as a versatile and sustainable sulfur source, as it circumvents the inherent problems related to pre-functionalized sulfur precursors, and hence the development of efficient synthetic strategies employing elemental sulfur (S₈) as a sulfur source is highly desirable. Elemental sulfur is odorless, non-toxic, inexpensive, and available in abundance as an industrial byproduct, making it an ideal sulfur source in alignment with the green chemistry principles, which emphasize atom economy and waste minimization. The inherent reactivity of elemental sulfur is effectively harnessed, employing various synthetic strategies for the construction of C-S bonds, thus circumventing the need for thiols and sulfonyl precursors. Integration of sulfur with other sustainable protocols like metal-free strategies, multi-component or cascade reactions, base-promoted protocols, electrochemical methods, and visible-light-driven processes has further strengthened the importance of sulfur as a greener alternative. In this context, we summarize the recent developments of the integration of the photochemical process with elemental sulfur. Upon photoexcitation, sulfur undergoes ring-opening and generates reactive intermediates, including thiyl radicals or polysulfide radical species, through photo-redox, photo-hydrogen atom transfer (HAT), and photocatalytic cleavage pathways. This review summarizes a mechanistic overview of photochemical activation of sulfur in the early stage of development, focusing on modes of activation, reactive intermediates, and reaction scope.