Efficient Generation of C-S Bonds via a By-Product-Promoted Selective Coupling of Alcohols, Organic Halides, and Thiourea

Unconventional approaches for synthesis of 2-substituted benzothiazoles

Benzothiazoles are among the most important heterocycles in pharmaceuticals, natural products, and functional materials. Hence, developing direct and sustainable routes by replacing hazardous/toxic thiol reagents with nontoxic reagents for constructing 2-substituted benzothiazoles is of utmost importance. In this review, we have highlighted many synthetic methods for the synthesis of 2-substituted benzothiazoles from different starting materials under transition metal catalysis or metal-free systems.

Iodine-mediated synthesis of indolyl-1,3,4-thiadiazole amine derivatives and their DFT analysis

An effective iodine-mediated one-pot three-component strategy for the construction of indolyl-1,3,4-thiadiazole amines has been described. The method involved the reaction of tosylhydrazine with indole-3-carboxaldehyde and ammonium thiocyanate as a non-toxic source of sulphur under metal-free conditions. The developed protocol included readily available substrates, shorter reaction time, use of low-toxic thiocyanate, and benzylation with a broad substrate scope, making it a viable approach for multiple applications. Furthermore, DFT analysis was conducted using the Gaussian 09 package.

TM-free full utilization of S atoms: synthesis of thioethers from disulfides and quaternary ammonium salts

Disulfides are commonly used as alternatives of thiols; however, the full utilization of both S atoms of disulfides under TM-, oxidant/reductant-free conditions is still challenging. In this study, an efficient synthesis of thioethers from disulfides and quaternary ammonium salts under TM-, oxidant/reductant-free conditions has been developed. Both S atoms of disulfides can be transformed into thioether products, thus improving sulfur resource utilization. The method can be easily extended to the synthesis of valuable alkyl dithiocarbamates and can be readily scaled up to the gram scale, showing good practicality value.

Temperature-Controlled Chemoselective Couplings of Alkyl Halides with Disulfides

An unprecedented, transition metal-free S-alkylation of disulfides with alkyl halides is developed for the first time, providing an efficient and green synthesis of thioethers and even thioesters. Notably, this new method allows the full utilization of both sulfur atoms of disulfides under chemical reductant-free conditions and can be easily scaled up in gram scale, showing good practical value. Control experiments suggested that water, unprecedentedly, serves as the terminal reductant of the whole reaction. Moreover, controllable N-alkyl thiopyridones can be readily obtained at a much lower reaction temperature from the same starting materials, suggesting a kinetic control of N-alkylation vs a thermodynamic control of the S-alkylation process may be involved in the reaction. Interestingly, a halo anion-mediated alkyl group migration from N-alkyl thiopyridones to S-alkyl pyridyl thioethers was observed, suggesting that the reaction may undergo a new and novel reaction mechanism.

Synthesis of Unsymmetrical Trisulfides from S-Substituted Sulphenylthiosulphates

Trisulfide unit is widely found in natural products and has garnered attention due to the unique pharmacological and physiochemical properties. However, despite limited progress, widely applicable approaches for constructing unsymmetrical trisulfides have so far remain scarce. In this work, an easy-to-prepare, solid-state and scalable reagent, S-substituted sulphenylthiosulphate, has been developed for the divergent synthesis of unsymmetrical trisulfides. Alkyl electrophile substrates, including bromides, chlorides, iodides and tosylates, with diverse substituents are smoothly converted to the corresponding trisulfides with S-substituted sulphenylthiosulphates and thiourea as another sulfur source. Furthermore, the late-stage modification of drug molecules was successfully achieved through this method.

Metal-Free Electrochemical Reduction of Disulfides in an Undivided Cell under Mass Transfer Control

Electroorganic synthesis is generally considered to be a green alternative to conventional redox reactions. Electrochemical reductions, however, are less advantageous in terms of sustainability, as sacrificial metal anodes are often employed. Divided cell operation avoids contact of the reduction products with the anode and allows for convenient solvent oxidation, enabling metal free greener electrochemical reductions. However, the ion exchange membranes required for divided cell operation on a commercial scale are not amenable to organic solvents, which hinders their applicability. Herein, we demonstrate that electrochemical reduction of oxidatively sensitive compounds can be carried out in an undivided cell without sacrificial metal anodes by controlling the mass transport to a small surface area electrode. The concept is showcased by an electrochemical method for the reductive cleavage of aryl disulfides. Fine tuning of the electrode surface area and current density has enabled the preparation of a wide variety of thiols without formation of any oxidation side products. This strategy is anticipated to encourage further research on greener, metal free electrochemical reductions.

One-pot synthesis of thioethers from indoles and p-quinone methides using thiourea as a sulfur source

Thiourea is an inexpensive and user friendly sulfur reagent that acts as a sulfur source. A simple and efficient protocol has been developed to access thioethers by reacting indoles with p-quinone methides using thiourea as the sulfur source. In our experiments, the reaction apparently proceeded through an S-(3-indolyl)isothiuronium iodide intermediate and subsequent generation of indolethiol that attacked the 1,6 position of p-quinone methides to give desired thioethers in good to excellent yields.

One-pot thiol-free synthetic approach to sulfides, and sulfoxides selectively

A facile and efficient thiol-free one-pot method for direct synthesis of sulfides and sulfoxides under green conditions without using any metal catalyst is reported. For this purpose, we used benzyl bromides as starting materials in the presence of potassium thioacetate (PTA) and Oxone® which are low-cost, and readily accessible chemicals. This method is highly compatible with a variety of functional groups and delivered a series of sulfides, bis-sulfides, and sulfoxides in good yields. The selective formation of sulfoxides over sulfones is discussed via a mechanism.

A facile and efficient thiol-free one-pot method for direct synthesis of sulfides and sulfoxides under green conditions without using any metal catalyst is reported.

Water oxidation by Brønsted acid-catalyzed in situ generated thiol cation: dual function of the acid catalyst leading to transition metal-free substitution and addition reactions of S-S bonds

An unprecedented water oxidation reaction by a small organic molecule, i.e., the thiol cation generated in situ by Brønsted acid-catalyzed heterolytic cleavage of S-S bond of a disulfide, is observed...

Efficient Construction of 5H-1,4-Benzodiazepine Derivatives by a Catalyst-Free Direct Aerobic Oxidative Annulation Strategy

A catalyst-free direct aerobic oxidative annulation reaction of 2-aminobenzylic amines and α-hydroxy ketones efficiently afforded versatile 5H-1,4-benzodiazepine derivatives by employing air as economic and green oxidant under mild conditions. Interestingly, solvent was found to be crucial to the reaction, so that by using acetic acid as the best solvent an efficient and practical method could be achieved, requiring no catalysts or additives at all. This method tolerates a wide range of 2-aminobenzylic amines and α-hydroxy ketones and could be scaled up to multigram synthesis and directly applied in one-step synthesis of the pharmaceutically active N-desmethylmedazepam derivatives, revealing the potential of this new method in the synthesis of 5H-1,4-benzodiazepine skeleton-based pharmaceuticals and chemicals.

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.

Synthesis of difluoromethyl and deuterium-labeled difluoromethyl thioethers from aliphatic electrophiles

A one-pot difluoromethylthiolation of alkyl electrophiles with thiourea and diethyl bromodifluoromethylphosphonate is described. The transition-metal-free approach, readily available reagents, and mild conditions provide a practical way for the synthesis of difluoromethyl thioethers. By changing the "H" source to the most commonly used "D" sources CD₃OD and D₂O, this strategy enables efficient synthesis of SCF₂D-substituted molecules in good yields with high levels of D incorporation.

Selective synthesis of pyridyl pyridones and oxydipyridines by transition-metal-free hydroxylation and arylation of 2-fluoropyridine derivatives

An efficient protocol for the construction of various pyridyl pyridone and oxydipyridine derivatives through a hydroxylation and arylation tandem reaction of 2-fluoropyridines is reported. Under simple transition-metal-free conditions, the reaction provided a series of products in good to excellent yields, and their structures were confirmed by crystal diffraction analysis. Furthermore, the controlling effect of 6-position substituents on the highly selective synthesis of pyridone and oxydipyridine was studied.

Selective construction of alkaloid scaffolds by alcohol-based direct and mild aerobic oxidative Pictet–Spengler reactions

Tetrahydro-β-carboline and β-carboline alkaloid scaffolds can be selectively obtained by direct aerobic oxidative Pictet–Spengler reactions of tryptamines with alcohols using TBN/TEMPO as the catalysts and oxygen as the oxidant under mild conditions.

Promoting Effect of Crystal Water Leading to Catalyst-Free Synthesis of Heteroaryl Thioether from Heteroaryl Chloride, Sodium Thiosulfate Pentahydrate, and Alcohol

It is observed the crystal water in sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O) can promote its multicomponent reaction with heteroaryl chlorides and alcohols, providing a facile, green, and specific synthesis of unsymmetrical heteroaryl thioethers via one-step formation of two C-S bonds under catalyst-, additive-, and solvent-free conditions. Mechanistic studies suggest that the crystal water in Na₂S₂O₃·5H₂O is crucial in generating the key thiol intermediates and byproduct NaHSO₄, which then catalyzes the dehydrative substitution of alcohols with thiols to afford thioethers.

Efficient Synthesis of Quinazolinones by Transition-Metal-Free Direct Aerobic Oxidative Cascade Annulation of Alcohols with o-Aminoarylnitriles

A mild and atom-economic method was developed for direct and efficient synthesis of quinazolinones through a transition-metal-free aerobic oxidative cascade annulation reaction of widely available o-aminoarylnitriles and alcohols. Air could be employed as an effective oxidant under mild conditions, generating water as the only byproduct. Possibly owing to the "cesium effect", the water-soluble base CsOH was found to be crucial in all key steps of the reaction mechanism. Because a wide range of substrates can be used to prepare substituted quinazolinones without contamination by transition-metal residues, this method may be of interest for application in pharmaceutical synthesis. Possible reaction paths were also proposed according to control reactions.

Substrate Self-Assisted Secondary Bond Activation of Allylic Alcohol in a Tsuji-Trost Reaction Revealed by NMR Methods

The first experimental evidence for the palladium-catalyzed secondary bond activation of allylic alcohols in a Tsuji-Trost reaction was provided by NMR methods, such as variable-temperature ¹H NMR, diffusion-ordered spectroscopy (DOSY), Job's method, ¹H NMR titration, and nuclear Overhauser enhancement spectroscopy (NOESY). The experimental results revealed that the substrate self-assisted activation of allylic alcohols is probably performed via a 1:1 binding six-membered-ring complex, which are formed by the formation of the secondary bonds, the hydrogen bond and P···O noncovalent bond between allylic alcohol and phosphonium ylide.

K₂S₂O₈-Promoted Aryl Thioamides Synthesis from Aryl Aldehydes Using Thiourea as the Sulfur Source

Thiourea as a sulfur atom transfer reagent was applied for the synthesis of aryl thioamides through a three-component coupling reaction with aryl aldehydes and N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAC). The reaction could tolerate various functional groups and gave moderate to good yields of desired products under the transition-metal-free condition.

Cesium carbonate-promoted synthesis of aryl methyl sulfides using S-methylisothiourea sulfate under transition-metal-free conditions

In the presence of cesium carbonate, an efficient synthesis of aryl methyl sulfides by the reactions of aryl halides with commercially available S-methylisothiourea sulfate is developed. This odourless and highly crystalline solid can be used as the substitute for malodorous methanethiol. The gram-scale reaction also proceeds smoothly without the use of column chromatography separation. Similarly, 2-(dimethylamino)ethylthio and cyclopropylmethylthio groups can be easily introduced into the aromatic rings from the corresponding S-[2-(dimethylamino)ethyl]isothiourea dihydrochloride and S-cyclopropylmethylisothiourea hydrobromide. The possible reaction mechanism is proposed. It is believed that this route to aryl alkyl sulfides is well competitive with currently known methods due to its wide substrate scope, excellent yields, easy operation and transition-metal-free conditions.

Specific N-Alkylation of Hydroxypyridines Achieved by a Catalyst- and Base-Free Reaction with Organohalides

A specific N-alkylation of 2-hydroxypyridines is achieved by reacting with organohalides under catalyst- and base-free conditions. The observed HX-facilitated conversion of pyridyl ether intermediates to 2-pyridone products may account for the success and specific N-alkylation of the reaction under the unexpectedly simple conditions. This new reaction may provide a useful alternative for the synthesis of 2-pyridones and analogous structures because of its >99% N-selectivity, relatively broad scopes of both substrates, and no mandatory use of catalysts and bases.

Efficient dehydrative alkylation of thiols with alcohols catalyzed by alkyl halides

Alcohols can be efficiently converted into the useful thioethers by a transition metal- and base-free dehydrative S-alkylation reaction with thiols or disulfides by employing alkyl halides as the effective catalyst. This simple and efficient method is a green and practical way for the preparation of thioethers, as it tolerates a wide range of substrates such as aryl and alkyl thiols, as well as benzylic, allylic, secondary, tertiary, and even the less reactive aliphatic alcohols.