Greener Approaches to Radical Reactions Involving Chalcogens: From Homolytic Substitution to Photocleavage

Background: Homolytic substitution at higher order chalcogens is no doubt a useful tool for radical based synthetic methodologies. Intramolecular variants of these reactions have received considerable attention in recent years and are a reliable route to prepare interesting molecules exhibiting a diverse range of biological activities. Methods: This review highlights tin-fee and photo-initiated routes to homolytic substitution at chalcogens. The use of thiohydroximate esters as precursors for radicals makes intramolecular homolytic substitution reactions easily initiated using near UV light. Intermolecular variants of these reactions offer to the synthetic practitioner a convenient route to a number of different radicals whose reactivity can be utilized to form interesting, non-chalcogen containing molecules. Evolving from the latter method are unique tellurium based radical precursors that do not undergo homolytic substitution but rather produce synthetically useful radicals by using near UV light. Conclusions: Employing these reactions in synthesis provides a greener alternative to some traditional radical based methods eliminating the requirement for chemical initiators and toxic chain carriers. These versatile methods afford atom and chemical efficient processes that can be performed using low cost white light sources.