Transition Metal-Free Sulfenylation of C-H Bonds for C-S Bond Formation in Recent Years: Mechanistic Approach and Promising Future

Transition metal-free sulfenylation of C-H bonds for C-S bond formation has recently emerged as sustainable protocols for the functionalisation of various molecules. Researchers have extensively developed such protocols for the construction of biologically relevant sulfur scaffolds. There has been a gradual shift from metal-catalyzed to metal-free C-S bond formation methodologies, because the latter offer environmentally benign and inherently safe access to novel synthetic routes in organic chemistry. The present review offers a dynamic discussion on recent advances in transition metal-free C-S bond formation via C-H bond functionalization using different surrogate sulfenylating reagents. The review has comprehensively been devoted to radical and ionic mechanistic approaches. Some simple and eco-friendly reagents for sulfenylation viz. tertbutyl hydrogen peroxide (TBHP), hydrogen peroxide (H2O2), iodine/potassium persulphate (I-2/K2S2O8), inorganic bases, strong organic acids, iodine, I-2/triphenyl phosphine (PPh3), N-chlorosuccinamide (NCS), N-bromosuccinamide (NBS), potassium iodate (KIO3), I-2/bovine serum albumin (BSA), tetrabutyl ammonium iodide (TBAI)/HBr and graphene oxide along with their mechanistic approach has been detailed in this review. Moreover, modern methods for the C-S bond formation i. e., biocatalyst, electrochemical and visible light induced sulfenylation, and traditional sulfenylation methodologies have also been incorporated.