Iron-Promoted Three-Component 2-Substituted Benzothiazole Formation via Nitroarene ortho-C–H Sulfuration with Elemental Sulfur

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.

Base-Catalyzed Chalcogenative Annulation of N-Maleimido O-Aminobenzyl Alcohol with Elemental Sulfur/Selenium: Access to 1,4-Sulfa-/Selena-zepanes

We reported an unprecedented chalcogen element (Se, S) insertion reaction with functionalized aminomaleimide to assemble medium-sized nitrogen-containing organic selenium/sulfur catalyzed by bases. During these [5/6 + 1] tandem annulations, a variety of low-valent inorganic chalcogenides exhibited excellent compatibility, providing a wide scope of structurally diverse 1,4-selenazepanes (17 examples, 60-71% yields), 1,4-benzothiazepines (20 examples, 74-86% yields), and 1,4-thiazines (6 examples, 31-68% yields). The characteristics of this transformation are high atomic economy, formation of two C-Se or C-S bonds in one step, and avoiding the use of unstable and toxic selenium/sulfur reagents under mild conditions.

Base-Promoted [4 + 1 + 1] Multicomponent Tandem Cycloaddition of Ortho-Substituted Nitroarenes, Aldehydes, and Ammonium Salts To Access 2,4-Substituted Quinazoline Frameworks

We report a base-promoted, metal-free multicomponent tandem reaction, involving a [4 + 1 + 1] cycloaddition process between ortho-substituted nitroarenes, aldehydes, and ammonium salts. Modifying the substituents on the nitroaromatic compounds effectively provides structurally diverse 2-substituted and 4-alkenylquinazolines with good to excellent yields (77%-90% and quinazoline 51 examples) and high tolerance for various inorganic ammonium salts (13 examples, such as NH3·H2O, NH4Cl, and NH4HF2). A new method for constructing 2,4-substituted quinazoline compounds with high selectivity from simple nitrogen source compounds was developed, and the reaction can be scaled up to a gram scale. Additionally, this method also facilitates the preparation of organic molecules with photophysical properties, offering new insights into the further transformation of quinazolines.

Magnetically recoverable catalysts for efficient multicomponent synthesis of organosulfur compounds

This manuscript introduces a groundbreaking study on the development and application of magnetically recoverable catalysts for the efficient multicomponent synthesis of organosulfur compounds. Capitalizing on the unique advantages of magnetic recovery, these catalysts streamline the synthesis process, offering an innovative solution that marries efficiency with environmental sustainability. By facilitating the multicomponent reaction of key precursors in the presence of sulfur sources, the catalysts enable the straightforward synthesis of various valuable organosulfur compounds, crucial in numerous pharmaceutical, agricultural, and material science applications. Key findings demonstrate a significant enhancement in reaction yields and selectivity and the remarkable ease with which the catalysts can be recovered and reused, thereby reducing both waste and operational costs. Magnetic catalysts, often based on magnetic iron nanoparticles, facilitate rapid and efficient reactions under mild conditions, offering superior atom economy, reduced solvent use, and the potential for scalable processes. Additionally, magnetically separating the catalysts from the reaction mixture enables multiple recycling cycles, reducing waste and operational costs. The review also discusses the mechanistic insights, challenges, and recent advancements in this field alongside future directions for developing more robust and versatile magnetic catalytic systems. This research embodies a significant step forward in the field of catalysis, highlighting the potential of magnetically recoverable catalysts to revolutionize the synthesis of complex molecules. Future perspectives discussed in the manuscript focus on expanding the scope of these catalysts to broader applications, optimizing catalyst design for enhanced performance, and further aligning chemical synthesis processes with the principles of green chemistry. This review covers the literature from 2010 to the end of 2024, and it encompasses the different one-pot protocols for synthesizing various heterocyclic organosulfur compounds based on magnetically recoverable catalysts.

Visible-Light-Promoted Cascade Coupling of 2-Isocyanonaphthalenes with Elemental Sulfur and Amines to Construct Naphtho[2,1-d]thiazol-2-Amines

A visible-light-induced strategy has been explored for the synthesis of naphtho[2,1-d]thiazol-2-amines through ortho-C-H sulfuration of 2-isocyanonaphthalenes with elemental sulfur and amines under external photocatalyst-free conditions. This three-component reaction, which utilizes elemental sulfur as the odorless sulfur source, molecular oxygen as the clean oxidant, and visible light as the clean energy source, provides a mild and efficient approach to construct a series of naphtho[2,1-d]thiazol-2-amines. Preliminary mechanistic studies indicated that visible-light-promoted photoexcitation of reaction intermediates consisting of thioureas and DBU might be involved in this transformation.

Synthesis of 2-benzyl benzoxazoles and benzothiazoles via elemental sulfur promoted cyclization of styrenes with 2-nitrophenols and N,N-dialkyl-3-nitroanilines

Herein we report a method for affording 2-benzyl benzoxazoles from substituted styrenes and 2-nitrophenols. The success of this method relies on the use of simple reagents, namely elemental sulfur and DABCO. A combination of identical reagents was utilized for the annulation of styrenes with N,N-dialkyl-3-nitroanilines to afford 2-benzyl benzothiazoles. Overall, benzoxazoles and benzothiazoles bearing useful functionalities such as halogens, amines, and heterocyclic groups were isolated in moderate to good yields. Our methods are a rare example of divergent transformations of substituted nitroarenes towards 2-benzyl benzoxazoles and benzothiazoles.

Iron(III)-Catalyzed Regioselective Synthesis of Electron-Rich Benzothiazoles from Aryl Isothiocyanates via C-H Functionalization

We report herein a direct synthetic route for the preparation of 2-arylbenzothiazoles using aryl isothiocyanates and electron-rich arenes. The synthetic route involves triflic acid promoted addition of the arenes to aryl isothiocyanates followed by FeCl₃-catalyzed C-S bond formation via C-H functionalization. The approach provides the advantage of synthesis of benzothiazoles without the conventional use of aryl aldehyde/carboxylic acid precursors employing the less expensive iron(III) catalyst.

Control of selectivity in the preparation of 2-substituted benzoazoles by adjusting the surface hydrophobicity in two solid-based sulfonic acid catalysts

A series of metal-free tandem reactions for the synthesis of pharmaceutically important 2-substituted benzoazoles from isothiocyanates and 2-aminothiophenol under catalyst-free conditions in the presence of Et-PMO-Me-PrSO₃H (1a) and SBA-15-PrSO₃H (1b) as solid acids were carried out in a highly selective way under solvent free conditions. A significant selectivity changeover toward either 2-mercaptobenzoxazole or 2-aminobenzoazole derivatives could be achieved by changing the employed catalyst from the relatively hydrophobic material 1a to the more hydrophilic catalyst 1b. This simple experimental procedure with a novel selective approach toward benzoazoles accompanied by green and reusable catalysts could be considered as an alternative to the existing methods for the synthesis of 2-substituted benzoazole derivatives.

CuBr2-Catalyzed Annulation of 2-Bromo-N-Arylbenzimidamide with Se/S8 Powder for the Synthesis of Benzo[d]isoselenazole and Benzo[d]isothiazole

A CuBr₂-catalyzed annulation of 2-bromo-N-arylbenzimidamide with selenium/sulfur powder for the synthesis of benzo[d]isoselenazole and benzo[d]isothiazole in generally good yields was investigated. This synthetic strategy features good substrate scope and functional group tolerance. Furthermore, the corresponding products could be converted into N-aryl indoles via rhodium^III-catalyzed ortho C-H activation of the N-phenyl ring, providing an efficient approach for axial aromatic molecules.

Halogen-Free π $\upi$ -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

In π $\upi$ -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a π $\upi$ -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (VOC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and VOC s of more than 0.84 V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing π $\upi$ -conjugated polymers to achieve highly efficient and solution-processable OPVs.

Selective oxidative β-C-H bond sulfenylation of tetrahydroisoquinolines with elemental sulfur

In this article, a convenient and efficient KIO₃-promoted oxidative sulfenylation at the β-position of tetrahydroisoquinolines and subsequent aromatization in the presence of elemental S₈ is presented. The reaction proceeds with moderate to good yields via a double C-S formation process. A wide range of structurally diverse 4-sulfenylisoquinolines/3-sulfenylpiperidine were synthesized with excellent functional group tolerance and high efficiency.

A Four-Component Reaction for the Synthesis of Thienopyrrolediones under Transition Metal Free Conditions

A three-starting-material four-component reaction strategy is described to construct thienopyrrolediones (TPDs) from the simplest raw materials, elemental sulfur, aldehydes, and β-ketoamides, under transition metal free conditions. Compared with traditional multistep reaction sequences, this process is simple, efficient, environmentally friendly, and atom-economic and has laid the foundation for further development of an easily synthesized TPD unit.

Synthesis of 2-Arylbenzothiazoles from Nitrobenzenes, Benzylamines, and Elemental Sulfur via Redox Cyclization

A sustainable advanced synthetic method was developed based on redox cyclization for the synthesis of 2-arylbenzothiazoles in good to excellent yields from readily available nitrobenzenes, benzylamines, and elemental sulfur without the use of transition-metal catalysts. This method is remarkable: nitro group reduction, a benzylamine redox reaction, sulfuration, condensation, and cyclization, all proceed in a single step to generate a heterocycle. It is also highly atom-economical and does not require any external oxidizing or reducing agents.

Elemental Sulfur-Promoted Benzoxazole/Benzothiazole Formation Using a C═C Double Bond as a One-Carbon Donator

An efficient method to assemble diverse benzoxazoles/benzothiazoles in good yields was developed via oxidative cyclization with 2-aminothiophenols or 2-iodoanilines as raw materials. In this protocol, elemental sulfur was used as the effective oxidant and C atoms on the C═C double bond were introduced as a one-carbon donator.

Mo-Catalyzed One-Pot Synthesis of N-Polyheterocycles from Nitroarenes and Glycols with Recycling of the Waste Reduction Byproduct. Substituent-Tuned Photophysical Properties

A catalytic domino reduction-imine formation-intramolecular cyclization-oxidation for the general synthesis of a wide variety of biologically relevant N-polyheterocycles, such as quinoxaline- and quinoline-fused derivatives, and phenanthridines, is reported. A simple, easily available, and environmentally friendly dioxomolybdenum(VI) complex has proven to be a highly efficient and versatile catalyst for transforming a broad range of starting nitroarenes involving several redox processes. Not only is this a sustainable, step-economical as well as air- and moisture-tolerant method, but also it is worth highlighting that the waste byproduct generated in the first step of the sequence is recycled and incorporated in the final target molecule, improving the overall synthetic efficiency. Moreover, selected indoloquinoxalines have been photophysically characterized in cyclohexane and toluene with exceptional fluorescence quantum yields above 0.7 for the alkyl derivatives.

Sulfur-Promoted Redox Cyclization of 2,2′-Dinitrodiphenyl Disulfides and Benzylamines

This study aims to develop green and sustainable advanced synthetic methods based on redox cyclization. Elemental sulfur is shown to promote the efficient synthesis of 2-substituted benzothiazoles from 2,2′-dinitrodiphenyl disulfides and benzylamines via redox cyclization in the absence of transition-metal catalysts and solvents. The 2-substituted benzothiazoles are obtained in good to excellent yields. This synthetic methodology is highly atom-economical and does not require any external oxidizing and/or reducing agents.

Metal-catalyzed C-S bond formation using sulfur surrogates

Sulfur-containing compounds are present in a wide range of biologically important natural products, drugs, catalysts, and ligands and they have wide applications in material chemistry. Transition metal-catalyzed C-S bond-forming reactions have successfully overcome the obstacles associated with traditional organosulfur compound syntheses such as stoichiometric use of metal-catalysts, catalyst-poisoning and harsh reaction conditions. One of the key demands in metal-catalyzed C-S bond-forming reactions is the use of an appropriate sulfur source due to its odor and availability. The unpleasant odor of many organic sulfur sources might be one of the reasons for the metal-catalyzed C-S bond-forming reactions being less explored compared to other metal-catalyzed C-heteroatom bond-forming reactions. Hence, employing an appropriate sulfur surrogate in the synthesis of organosulfur compounds in metal-catalyzed reactions is still of prime interest for chemists. This review explores the recent advances in C-S bond formation using transition metal-catalyzed cross-coupling reactions and C-H bond functionalization using diverse and commercially available sulfur surrogates. Based on the different transition metal-catalysts, this review has been divided into three major classes namely (1) palladium-catalyzed C-S bond formation, (2) copper-catalyzed C-S bond formation, and (3) other metal-catalyzed C-S bond formation. This review is further arranged based on the different sulfur surrogates. Also, this review provides an insight into the growing opportunities in the construction of complex organosulfur scaffolds covering natural product synthesis and functional materials.

NaOH-Mediated Direct Synthesis of Quinoxalines from o-Nitroanilines and Alcohols via a Hydrogen-Transfer Strategy

A NaOH-mediated sustainable synthesis of functionalized quinoxalines is disclosed via redox condensation of o-nitroamines with diols and α-hydroxy ketones. Under optimized conditions, various o-nitroamines and alcohols are well tolerated to generate the desired products in 44-99% yields without transition metals and external redox additives.

Transition-Metal-Free Synthesis of 2-Substituted Benzothiazoles from Nitrobenzenes, Methylheteroaryl Compounds, and Elemental Sulfur, Based on Nitro-Methyl Redox-Neutral Cyclization

Greener and more sustainable chemical processes are required to address increasing environmental pollution and depletion of natural resources. This paper aims to develop greener and more sustainable modern synthetic chemical processes using redox-neutral cyclization. Redox-neutral cyclization has been shown to promote the efficient synthesis of 2-substituted benzothiazoles from easily available nitrobenzenes, methyl-heteroaryl compounds, and elemental sulfur in the absence of transition-metal catalysts. The 2-substituted benzothiazoles were obtained in reasonable yields through the sulfuration of electron-deficient C-H bonds with elemental sulfur. This synthetic methodology also affords a high atom economy without the use of any external oxidizing and/or reducing reagents.

Recent Advances in the Synthesis and Applications of 2-Arylbenzothiazoles

This review firstly covers the applications of 2-arylbenzothiazoles as amyloid imaging agents, antitumor agents, and organic luminescent materials. Then we review the recent advances in the synthesis of 2-arylbenzothiazole derivatives. On the one hand, we introduce the approaches for construction of the 2-arylbenzothiazole core, including the following categories: (i) classic condensation of 2-aminothiophenols, (ii) direct arylation of benzothiazoles, (iii) intramolecular cyclization of thiobenzanilides, and (iv) tandem cyclization of anilines/ nitroarenes­ with elemental sulfur or sulfides. On the other hand, the transition-metal-catalyzed direct C–H functionalizations of 2-aryl­benzothiazoles are also involved in this review.

1 Introduction

2 Applications of 2-Arylbenzothiazoles

3 Construction of the 2-Arylbenzothiazole Core

4 Synthesis 2-Arylbenzothiazoles via Direct C–H Functionalization

5 Conclusion

Current advances in the synthetic strategies of 2-arylbenzothiazole

Benzothiazole is a privileged scaffold in the field of synthetic and medicinal chemistry. Its derivatives and metal complexes possess a gamut of pharmacological properties and high degree of structural diversity that has proven it vital for the investigation for novel therapeutics. The 2nd position of benzothiazole is the most active site that makes 2-arylbenzothiazole as felicitous scaffolds in pharmaceutical chemistry. The extensive significance of benzo-fused heterocyclic moieties formation has led to broad and valuable different approaches for their synthesis. This review deals with the synthetic approaches developed so far for the synthesis of 2-arylbenzothiazoles. Moreover, this article abridges the publications devoted to the synthesis of this moiety over the last 6 years. This study gives a current precis of research on the fabrication of 2-arylbenzothiazoles through different synthetic pathways and shall be helpful for researchers and scientists who are working in this field to make more potent biologically active benzothiazole-based drugs.

Synthesis of 2-Arylbenzothiazol-5-amines from N,N-Dialkyl-3-nitroanilines

We report a simple method for coupling of N,N-dialkyl-3-nitroarenes, elemental sulfur, and activated sp3 C–H bonds in 2-methylazaarenes or arylacetic acids to afford derivatives of 2-arylbenzothiazol-5-amines. Only DABCO base was required, and many heterocycles such as imidazoles, oxazoles, quinolines, and thiophenes were compatible with the reaction conditions. Our approach offers a simple route to useful substituted thiazol-5-amines from commercially available nitroarenes.

Recent Advances in Sulfur-Containing Heterocycle Formation via Direct C–H Sulfuration with Elemental Sulfur

The synthesis of sulfur heterocycles via the construction of C–S bonds has received considerable attention due to their biological value and extensive pharmaceutical application. While diverse sulfurating agents have been developed over the past few decades, in this regard, elemental sulfur, with advantages of low toxicity, odorless nature and chemical stability, has great potential for the construction of diverse sulfur heterocycles through its direct incorporation into the target molecules in a concise way. Direct functionalization of inert C–H bonds can shorten the number of reaction steps and minimize the amount of waste formed. Hence, heteroannulations via direct C–H sulfuration is considered to be an attractive strategy for the synthesis of sulfur heterocycles. In the last few years, a vast array of concise systems have been reported for the synthesis of some valuable sulfur heterocycles such as thiophenes, thienoindoles, thienothiazoles, thiazoles, benzothiazoles, and thiadiazoles through direct C–H sulfuration/annulations with elemental sulfur. These are discussed in detail in this review.

1 Introduction

2 Thiophenes

3 Thienoindoles

4 Thienothiazoles

5 Other Fused Thiophenes

6 Thiazoles

7 Benzothiazoles

8 Thiadiazoles

9 Others

10 Summary and Outlook

Catalyst- and Additive-Free Method for the Synthesis of 2-Substituted Benzothiazoles from Aromatic Amines, Aliphatic Amines, and Elemental Sulfur

Under catalyst- and additive-free conditions, a novel, convenient, environmentally friendly method was developed for the synthesis of 2-substituted benzothiazoles via the three-component one pot reaction from aromatic amines, aliphatic amines, and elemental sulfur. The reaction achieves double C-S and one C-N bond formations via cleavage of two C-N bonds and multiple C-H bonds. Furthermore, the mechanism research shows that DMSO acts as an oxidant in the cyclization reaction.

New synthesis of 2-aroylbenzothiazoles via metal-free domino transformations of anilines, acetophenones, and elemental sulfur

A new synthesis of 2-aroylbenzothiazoles via iodine-promoted domino transformations of anilines, acetophenones, and elemental sulfur was demonstrated. The highlights of this tandem synthesis are (1) easily available anilines and acetophenones as feedstock; (2) transition metal-free conditions; (3) inexpensive, nontoxic, easy handling, and abundant elemental sulfur as a building block. This synthetic strategy would complement the existing methods in the synthesis of this important heterocyclic scaffold. To our best knowledge, the formation of 2-aroylbenzothiazoles from simple anilines, acetophenones, and elemental sulfur was not previously reported in the literature.

A new synthesis of 2-aroylbenzothiazoles via iodine-promoted domino transformations of anilines, acetophenones, and elemental sulfur was demonstrated.

Three-component synthesis of 1,4-benzothiazines via iodide-catalyzed aerobic C–H sulfuration with elemental sulfur

Five to Six: Beyond the well-established thiazole formation from elemental sulfur, this method provides the first access to the corresponding six-membered N,S-heterocyclic products via direct functionalization of multiple C–H bonds.

Transition metal-free α-Csp3-H oxidative sulfuration of benzyl thiosulfates with anilines to form N-aryl thioamides

A metal-free approach to N-aryl thioamides from Bunte salts and anilines in DMSO has been developed. This method tolerated a wide range of functional groups on the aromatic ring, providing an ideal way to N-aryl thioamides in good to excellent yields from cheap and easily available starting materials. A plausible mechanism was also proposed based on the X-ray single crystal diffraction, NMR and MS analyses of DMSO-concerning intermediates.

Copper-Catalyzed Redox Coupling of Nitroarenes with Sodium Sulfinates

A simple copper-catalyzed redox coupling of sodium sulfinates and nitroarenes is described. In this process, abundant and stable nitroarenes serve as both the nitrogen sources and oxidants, and sodium sulfinates act as both reactants and reductants. A variety of aromatic sulfonamides were obtained in moderate to good yields with broad substrate scope. No external additive is employed for this kind of transformation.

Thiol substrate-promoted dehydrogenative cyclization of arylmethyl thiols with ortho-substituted amines: a universal approach to heteroaromatic compounds

A thiol substrate-promoted one-pot synthesis of a library of heteroaromatic compounds was developed.