Regioselective Sulfenylation of α'-CH3 or α'-CH2 Groups of α,β-Unsaturated Ketones with Heterocyclic Thiols

Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

I2/DMSO-Promoted Synthesis of Diaryl Sulfide- and Selenide-Embedded Arylhydrazones

Functionalization and derivatization of arylhydrazones are important in pharmaceutical, medicinal, material, and coordination chemistry. In this regard, a facile I₂/DMSO-promoted cross-dehydrogenative coupling (CDC) for direct sulfenylation and selenylation of arylhydrazones has been accomplished utilizing arylthiols/arylselenols at 80 °C. This method provides a metal-free benign route for the synthesis of a variety of arylhydrazones embedded with diverse diaryl sulfide and selenide moieties in good to excellent yield. In this reaction, molecular I₂ acts as a catalyst, and DMSO is utilized as a mild oxidant as well as solvent to produce several sulfenyl and selenyl arylhydrazones through a CDC-mediated catalytic cycle.

KIO3-Mediated γ-C(sp3)-H Sulfenylation of Enaminones

A metal-free regioselective γ-C(sp³)-H sulfenylation of enaminones with heterocyclic thiols is reported. This transformation is efficient, mild, scalable, and environmentally friendly and tolerates a large variety of enaminones substrates and heterocyclic thiols. The utility of this strategy is demonstrated in a late-stage modification of bioactive natural products and drug derivatives.

N-Triflination of pyrazolones: a new method for N-S bond formation

A simple method, which takes place quickly in 5 min, is developed for the N-triflination of pyrazolones using CF3SO2Na (Langlois reagent) and phenyliodine(iii)bis(trifluoroacetate) (PIFA). This reaction takes place at the imine nitrogen centre instead of the more reactive C4-position, forming a new N-S bond. A variety of pyrazolone derivatives were subjected to the reaction. Unlike the previous reports on sulfenylation or sulfonylation of pyrazolone, wherein the corresponding C-S bond is formed, this new method leads to the formation of the hetero-hetero atom bond (N-S bond) at room temperature.

Assessing Methodologies to Synthesize α-Sulfenylated Carbonyl Compounds by Green Chemistry Metrics

α-Sulfenylated carbonyl compounds are important both as active pharmaceutical ingredients and as intermediates in organic synthesis. Owing to their relevance in synthetic organic chemistry, this Minireview focuses on assessing the most relevant synthetic procedures based on green chemistry metrics. The Minireview starts with the traditional routes and then focuses on more recently developed methodologies. These routes include sulfenylating reagents using organocatalysis, cross-dehydrogenative couplings using in situ halogenations to prevent reactive intermediates in high concentrations, oxidative couplings using terminal oxidants such as DDQ or TEMPO, and redox-neutral couplings using transition metal catalysis. These methodologies have been evaluated on the basis of atom economy, E factor, and the safety and toxicity of the transformations and the solvents used. Besides using green metrics to evaluate these novel methodologies, the synthetic utility is also assessed with regard to the availability of starting materials and the generality of the reactions. This Minireview aims to inspire researchers to apply green assessments to other methodologies and also for them to take measures to increase the greenness of their developed transformations.

KIO3-catalyzed cross dehydrogenative coupling reaction: sulfenylation of phenol and arylamine derivatives in water at room temperature

A metal-free direct sulfenylation of phenol and arylamine derivatives with various heterocyclic thiols and thiones using a cross dehydrogenative coupling protocol in water at room temperature has been developed.