Metal-free thiolation of sulfonyl hydrazone with thiophenol: synthesis of 4-thio-chroman and diarylmethyl thioethers

A simple, efficient, and transition metal-free approach was developed for accessing 4-thio-substituted chroman and diarylmethyl thioethers from sulfonyl hydrazones. This protocol provides straightforward access to a class of diarylmethane derivatives with good to excellent yields. This operationally simple protocol exhibited good tolerance for labile functional groups, providing biologically relevant chemical libraries. This safe and feasible route is applicable to the large-scale synthesis of 4-thio-substituted chromans, which are of great synthetic interest because of their further reaction potential.

Visible-Light-Induced Regioselective C-H Sulfenylation of Pyrazolo[1,5-a]pyrimidines via Cross-Dehydrogenative Coupling

A visible-light-induced cross-dehydrogenative methodology has been developed for the regioselective sulfenylation of pyrazolo[1,5-a]pyrimidine derivatives. Rose bengal, blue LEDs, KI, K₂S₂O₈, and DMSO are all essential for this photocatalytic transformation. The protocol is applicable for the synthesis of a library of 3-(aryl/heteroaryl thio)pyrazolo[1,5-a]pyrimidine derivatives with broad functionalities. The selectivity and scalability of the methodology have been also demonstrated. Moreover, the efficiency of this strategy for sulfenylation of pyrazoles, indole, imidazoheterocycles, and 4-hydroxy coumarin has been proven. The mechanistic investigation revealed the radical-based mechanism and formation of diaryl disulfide as a key intermediate for this cross-dehydrogenative coupling reaction.

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.

Electrochemical Synthesis of Polysubstituted Sulfonated Pyrazoles via Cascade Intermolecular Condensation, Radical-Radical Cross Coupling Sulfonylation, and Pyrazole Annulation

Electrochemical synthesis of polysubstituted sulfonated pyrazoles from enaminones and sulfonyl hydrazides was established under metal-free, exogenous-oxidant-free, and mild conditions. By judicious choice of different electrochemical reaction conditions, NH₂-functionalized enaminones or N,N-disubstituted enaminones can react with aryl/alkyl sulfonyl hydrazides to afford tetra- or trisubstituted sulfonated pyrazoles in moderate to good yields, respectively. The gram-scale electrochemical transformation demonstrated the efficiency and practicability of this synthetic strategy. In addition, the sulfonated NH-pyrazole can be obtained via the dissociation of the N-tosyl group. Mechanistic studies reveal that the electrochemical cascade reaction synthesis of polysubstituted sulfonated pyrazoles proceeded via the sequence of intermolecular condensation, radical-radical cross coupling sulfonylation, and pyrazole annulation.

Recent Advances of Green Catalytic System I2/DMSO in C–C and C–Heteroatom Bonds Formation

Developing a green, practical and efficient method for the formation of C–C and C–Heteroatom bonds is an important topic in modern organic synthetic chemistry. In recent years, the I2/DMSO catalytic system has attracted wide attention because of its green, high efficiency, atomic economy, low cost, mild reaction conditions and it is environment-friendly, which is more in line with the requirements of sustainable chemistry. Heteroatom-containing compounds have shown lots of important applications in pharmaceutical synthesis, agrochemicals, material chemistry and organic dyes. At present, the I2/DMSO catalytic system has been successfully applied to the synthesis of various heteroatom-containing compounds. The C–C and C–Heteroatom bonds have been formed efficiently, which has been proved to be a green and mild catalytic system. In this review, the research achievements of the I2/DMSO catalytic system in the formation of C–C and C–Heteroatom bonds from 2015 to date are described, and the research area is prospected. This review attempts to reveal the general law of iodine catalysis and lay a foundation for the design of new reactions.

The journey of C–S bond formation from metal catalysis to electrocatalysis

This perspective describes the journey of C–S bond constructions starting from transition metal catalysis through oxidant catalysis, photocatalysis and very recently employed electrocatalysis by using various sulfur surrogates.

Visible Light-Driven and Singlet Oxygen-Mediated Photochemical Cross-Dehydrogenative C3-H Sulfenylation of 4-Hydroxycoumarins with Thiols Using Rose Bengal as a Photosensitizer

A visible light (white light-emitting diode/direct sunlight)-driven photochemical synthesis of a new series of biologically interesting 3-(alkyl/benzylthio)-4-hydroxy-2H-chromen-2-ones has been achieved through a cross-dehydrogenative C₃-H sulfenylation of 4-hydroxycoumarins with thiols at ambient temperature in the presence of rose bengal in acetonitrile under an oxygen atmosphere. The notable features of this newly developed method are mild reaction conditions, energy efficiency, metal-free synthesis, good to excellent yields, use of low-cost materials, and eco-friendliness.

Metal-Free, Acid/Phosphine-Induced Regioselective Thiolation of p-Quinone Methides with Sodium Aryl/Alkyl Sulfinates

A simple and efficient method for the regioselective thiolation of p-quinone methides with sodium aryl/alkyl sulfinates has been established using an acid/phosphine-induced radical route under transition-metal-free conditions. A broad range of sodium aryl/alkyl sulfinates and p-quinone methides (p-QMs) are compatible for the reaction, giving the expected products with good to excellent yields. Control experiments were also performed to gain insights into the generation mechanism of thiyl radicals and hydrogen-atom transfer process. This protocol provides a safe and feasible way for the formation of carbon-sulfur bonds.

Recent Advances in Iodine-Promoted C-S/N-S Bonds Formation

Sulfur-containing scaffold, as a ubiquitous structural motif, has been frequently used in natural products, bioactive chemicals and pharmaceuticals, particularly C-S/N-S bonds are indispensable in many biological important compounds and pharmaceuticals. Development of mild and general methods for C-S/N-S bonds formation has great significance in modern research. Iodine and its derivatives have been recognized as inexpensive, environmentally benign and easy-handled catalysts or reagents to promote the construction of C-S/N-S bonds under mild reaction conditions, with good regioselectivities and broad substrate scope. Especially based on this, several new strategies, such as oxidation relay strategy, have been greatly developed and accelerated the advancement of this field. This review focuses on recent advances in iodine and its derivatives promoted hybridized C-S/N-S bonds formation. The features and mechanisms of corresponding reactions are summarized and the results of some cases are compared with those of previous reports. In addition, the future of this domain is discussed.

Synthesis of 4-chalcogenyl pyrazoles via electrophilic chalcogenation/cyclization of α,β-alkynic hydrazones

A facile method for the synthesis of 4-chalcogenylated pyrazoles has been developed via electrophilic chalcogenation/cyclization of α,β-alkynic hydrazones. The cyclization of α,β-alkynic aldehyde hydrazones could be induced by using either sulfenyl chloride or the S-electrophiles generated in situ from the reaction of NCS and arythiol. The developed method was successfully applied to the synthesis of the sulfenyl analogue of celecoxib.

Metal-free C–S coupling of thiols and disulfides

A literature overview on C–S coupling reactions using thiols or disulfides as sulfur surrogates under metal-free conditions is presented. Reagents for the transformations include polyvalent iodines, peroxides, tert-butyl nitrite (TBN), DDQ, and aerial oxygen, among others.

Synthetic Strategy for Pyrazolo[1,5-a]pyridine and Pyrido[1,2-b]indazole Derivatives through AcOH and O2-Promoted Cross-dehydrogenative Coupling Reactions between 1,3-Dicarbonyl Compounds and N-Amino-2-iminopyridines

An efficient method has been developed for the synthesis of uniquely substituted pyrazolo[1,5-a]pyridine and pyrido[1,2-b]indazole derivatives, which involves acetic acid and molecular oxygen promoted cross-dehydrogenative coupling reactions of respective β-ketoesters and β-diketones (like ethyl acetoacetate, ethyl benzoylacetate, methyl propionylacetate, acetylacetone, dimedone, 1,3-cyclohexanedione, and 1,3-cyclopentanedione) with N-amino-2-iminopyridines. The proposed tentative mechanism involves formal acetic acid-promoted oxidative C(sp3)-C(sp2) dehydrogenative coupling followed by dehydrative cyclization under a catalyst-free condition within high atom economy processes.

Sustainable Synthetic Approach for (Pyrazol-4-ylidene)pyridines By Metal Catalyst-Free Aerobic C(sp2)-C(sp3) Coupling Reactions between 1-Amino-2-imino-pyridines and 1-Aryl-5-pyrazolones

A novel, metal catalyst-free, and efficient method has been developed for the synthesis of (pyrazol-4-ylidene)pyridine derivatives. The process involves dehydrogenative coupling of 1-amino-2-imino-pyridines with 1-aryl-5-pyrazolone derivatives utilizing O2 as the sole oxidant. The new method benefits from a high atom economy, efficiency, and substrate scope, as well as the simplicity of reaction and product purification procedures.

Cross-Dehydrogenative C-H/S-H Coupling Reactions

Carbon-sulfur bond formation represents a key step in the synthesis of thioethers, which are a common structural motif in many pharmaceutically compounds. The direct cross-dehydrogenative coupling of C-H/S-H bonds has become a powerful tool for C-S bond formation. As these coupling reactions avoid pre-functionalization of the starting materials, they are more atom-economical, practical, and environmentally friendly than traditional cross-coupling reactions. In this review, we will highlight the most important developments in this novel and interesting research arena with the emphasis on the mechanistic aspects of the reactions. The review is divided into three major sections: (1) C(sp³)-H/S-H bonds coupling reactions; (2) C(sp²)-H/S-H bonds coupling reactions; and (3) C(sp)-H/S-H bonds coupling reactions.

Transition-metal-free KI-catalyzed regioselective sulfenylation of 4-anilinocoumarins using Bunte salts

An efficient and eco-friendly protocol for the KI-catalyzed regioselective sulfenylation of 4-anilinocoumarins with Bunte salts was established under metal-free conditions.

Metal- and photocatalyst-free visible-light-promoted regioselective selenylation of coumarin derivatives via oxidation-induced C–H functionalization

A visible-light-promoted approach for the regioselective selenylation of 4-amino substituted coumarins has been initially realized under metal- and photocatalyst-free conditions at room temperature.

Metal-Free Synthesis of 2-Aminobenzothiazoles via Iodine-Catalyzed and Oxygen-Promoted Cascade Reactions of Isothiocyanatobenzenes with Amines

In this paper, a highly efficient and sustainable synthesis of 2-aminobenzothiazoles through the cascade reactions of isothiocyanatobenzenes with primary or secondary amines by using iodine as a catalyst and oxygen as an oxidant is presented. Mechanistically, the formation of the title compounds involves the in situ formation of the required benzothiourea intermediate followed by its intramolecular cross dehydrogenative coupling of a C(sp²)-H bond and a S-H bond. To our knowledge, this should be the first example in which 2-aminobenzothiazoles are efficiently prepared from simple and cheap isothiocyanates and amines under metal-free conditions by using iodine as a catalyst and molecular oxygen as an oxidant with water as the byproduct. Compared with literature protocols, this method eliminates the use of ortho-halo-substituted precursors, expensive transition-metal catalysts, and hazardous oxidants.

Direct intramolecular amination of tryptophan esters to prepare pyrrolo[2,3-b]indoles

A metal-free iodine-catalyzed intramolecular amination has been developed for the practical synthesis of pyrrolo[2,3-b]indoles from readily available tryptophan esters. The transformation has been applied to a wide array of substrates and can be performed on gram scale under very mild conditions.

Iodine-catalyzed cascade annulation of alkynes with sodium arylsulfinates: assembly of 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives

An efficient and environmentally benign iodine-catalyzed cascade cyclization has been achieved to afford functionalized 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives.

Cs2CO3-promoted cross-dehydrogenative coupling of thiophenols with active methylene compounds

α-Sulfenylation of carbonyl compounds has been achieved via Cs₂CO₃-promoted aerobic cross-dehydrogenative coupling of thiophenols with active methylene compounds.

Iodine-catalyzed sulfenylation of pyrazolones using dimethyl sulfoxide as an oxidant

An iodine catalyzed sulfenylation of pyrazolones with a diverse range of heterocyclic thiols, heterocyclic thiones and disulfides has been described using dimethyl sulfoxide as an oxidant, which is an inexpensive, readily available and green oxidant.