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

Abstract

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 (I2/K2S2O8), inorganic bases, strong organic acids, iodine, I2/triphenyl phosphine (PPh3), N‐chlorosuccinamide (NCS), N‐bromosuccinamide (NBS), potassium iodate (KIO3), I2/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.

Recent advances in selective mono-/dichalcogenation and exclusive dichalcogenation of C(sp2)–H and C(sp3)–H bonds

“…1). The carbon-chalcogen bond formation represents one of the most powerful methods for synthesizing organochalcogen compounds and it has been achieved mainly through cross-coupling, addition reactions and C-H chalcogenation, utilizing diverse chalcogen sources [14][15][16][17][18][19] under transition metal-catalyzed/mediated, [20][21][22][23][24][25][26] metal-free, [27][28][29][30][31] photocatalyzed, [32][33][34][35][36] electrochemical, 37 organocatalyzed 38,39 or enzymatic conditions. 40 As one of the most commonly employed protocols, transition-metal-catalyzed/mediated crosscouplings generally employ organic (pseudo)halides as reaction partners, 19,[41][42][43][44] however, recent years have also witnessed the increasing use of other sources such as organic boronic acids, carboxylic acids, amines, phenols/alcohols, aldehydes, or esters as the coupling partners.…”

Pd-catalyzed site-selective C(sp2)–H chalcogenation of amino acids and peptides using a picolinamide auxiliary

“…[70][71][72][73][74][75] Therefore, the synthesis of chalcogenides has been an immense subject of study. In this regard, several metal-mediated traditional cross-coupling strategies [76][77][78][79][80][81][82][83] and metal-free protocols [84][85][86][87][88][89] have used typical approaches to assemble chalcogenides. However, most of the methods suffer from the inevitable need to perform pre-functionalization of the starting materials, limited substrate scope and harsh reaction conditions.…”

Mechanism and Origin of Enantioselectivity in Bifunctional Squaramide‐Catalyzed α‐Thiolation of Azlactones

“…[1,2] In this context, the formation of the sulfide group has the potential to enhance the biological activity of a given compound. [3,4] Moreover, the advent of novel thiolation methodologies, especially those employing electrophilic sulfur sources, [5,6] recently presented great development, growingly appearing as an alternative to classic protocols associated to the formation of the sulfide group. [7][8][9] Azlactones, which consist of five-membered ring heterocycles presenting a diversity of reactive sites, appear as promising building-blocks in organic synthesis.…”