Metal-Free Dehydrogenative Double C–H Sulfuration To Access Thieno[2,3-b]indoles Using Elemental Sulfur

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.

Multimodal Acridine Photocatalysis Enables Direct Access to Thiols from Carboxylic Acids and Elemental Sulfur

Development of photocatalytic systems that facilitate mechanistically divergent steps in complex catalytic manifolds by distinct activation modes can enable previously inaccessible synthetic transformations. However, multimodal photocatalytic systems remain understudied, impeding their implementation in catalytic methodology. We report herein a photocatalytic access to thiols that directly merges the structural diversity of carboxylic acids with the ready availability of elemental sulfur without substrate preactivation. The photocatalytic transformation provides a direct radical-mediated segue to one of the most biologically important and synthetically versatile organosulfur functionalities, whose synthetic accessibility remains largely dominated by two-electron-mediated processes based on toxic and uneconomical reagents and precursors. The two-phase radical process is facilitated by a multimodal catalytic reactivity of acridine photocatalysis that enables both the singlet excited state PCET-mediated decarboxylative carbon-sulfur bond formation and the previously unknown radical reductive disulfur bond cleavage by a photoinduced HAT process in the silane-triplet acridine system. The study points to a significant potential of multimodal photocatalytic systems in providing unexplored directions to previously inaccessible transformations.

Three-Component Reaction between 3-Acetylcoumarins, Amines, and Elemental Sulfur: A Designed Approach to 3-Amino-4H-thieno[3,2-c]coumarins

In this work, a series of novel 3-amino-4H-thieno[3,2-c]coumarins were designed and synthesized by a one-pot, catalyst-free, and three-component reaction of 3-acetylcoumarins with amines and elemental sulfur. Readily available starting materials, simple heating conditions, facile installation of a sulfur atom into the molecule using S8 as a sulfur source, acceptable functional group tolerance, and synthetically useful yields are some highlighted benefits of this process.

Base-Promoted Reaction of 4-Chloro-3-vinyl Coumarins, Phenacylpyridinium Bromides, and Elemental Sulfur: A Designed Approach to Thiopyrano[4,3-c]chromen-5(1H)-ones

A designed synthesis of thiopyrano[4,3-c]chromen-5(1H)-ones was developed based on a base-promoted three-component reaction between 4-chloro-3-vinyl coumarins, phenacylpyridinium bromides, and elemental sulfur. Readily available starting materials, mild conditions, chemoselectivity, operational simplicity, and synthetically useful yields are some highlighted advantages of these transformations.

A facile protocol for the preparation of 2-carboxylated thieno [2,3-b] indoles: a de novo access to alkaloid thienodolin

A metal-free strategy, alternative to the known complex cycloaddition reactions, towards 2-carboxylated thieno [2,3-b] indole derivatives has been successfully developed. The novel approach involves as starting materials easy accessible 1,2-diaza-1,3-dienes and indoline 2-thione and requires mild reaction conditions. Furthermore, the easy work-up required makes this method amenable for a one-pot approach as demonstrated in the preparation of thienodolin, a natural product isolated from Streptomyces albogriseolus that exhibits valuable biological properties.

Recent advances in the synthesis of thienoindole analogs and their diverse applications

Thiophene-fused heterocyclic organosulfur systems, especially the thieno[3,2-b]indole moiety have attracted significant attention because they show a wide spectrum of biological activities such as antituberculosis, antitumor, antifungal, antibacterial, and human 5-HT5A receptor binding inhibition. Moreover, they also find applications in material chemistry and chemical engineering. Thus, due to their intriguing properties and applications, researchers are continually attempting to create more effective and environment-friendly methods for their preparation. In this review, we present a complete assessment of the current advances in the field of thieno[3,2-b]indole synthesis.

Electrooxidative double C-H/C-H coupling of phenols with 3-phenylbenzothiophenes: facile access to benzothiophene derivatives

An efficient electrooxidative double C-H/C-H coupling of phenols with 3-phenylbenzothiophene has been developed under external oxidant- and catalyst-free conditions. This strategy could enable the highly tunable access to benzothiophene derivatives and exhibited broad substrate generality under mild conditions. The reaction is likely to proceed via the cross-coupling of the p-methoxylphenol radical and the 3-phenylbenzothiophene radical cation.

Copper-Catalyzed Annulation of Indolyl α-Diazocarbonyl Compounds Leads to Structurally Rearranged Carbazoles

Indolyl α-diazocarbonyl compounds have proven to be effective starting materials for the construction of various 2,3-ring fused indole frameworks. Activation of the diazo functional group under metal catalysis generates a spiro-cyclic indolenine-type intermediate which rearranges to provide two distinct carbazoles upon oxidation. The current study investigates the effects of the catalyst as well as the substituents on the migratory group involved in controlling the selectivity of the rearrangement.

A cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes to access thieno[2,3-b]indoles and benzothiophenes

The first catalyst-free cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes was developed. A series of thieno[2,3-b]indoles and benzothiophenes were smoothly obtained in high to excellent yields. The salient features of the protocol include catalyst-free conditions, an environment-friendly solvent, broad substrate scope, and large-scale synthesis.

Chromatography-Free Multicomponent Synthesis of Thioureas Enabled by Aqueous Solution of Elemental Sulfur

The development of a new three-component chromatography-free reaction of isocyanides, amines and elemental sulfur allowed us the straightforward synthesis of thioureas in water. Considering a large pool of organic and inorganic bases, we first optimized the preparation of aqueous polysulfide solution from elemental sulfur. Using polysulfide solution, we were able to omit the otherwise mandatory chromatography, and to isolate the crystalline products directly from the reaction mixture by a simple filtration, retaining the sulfur in the solution phase. A wide range of thioureas synthesized in this way confirmed the reasonable substrate and functional group tolerance of our protocol.

Elemental-Sulfur-Incorporated Cyclizations of Pyrrolidines Leading to Thienopyrroles

We report, herein, the synthesis of thieno[3,2-b]pyrroles from the direct oxidative [4 + 1] cyclization of 2-alkynyl pyrrolidines with elemental sulfur. This transformation likely originates from electrophilic attack at the β-position of pyrrolidine followed by an intramolecular thienannulation to deliver the desired product. Mechanistic investigation suggests that the present reaction involves the formation of dihydrothieno[3,2-b]pyrrole as an intermediate.

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

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.

The synthesis of N,N′-disulfanediyl-bis(N′-((E)-benzylidene)acetohydrazide) from (E)-N′-benzylideneacetohydrazide and S8

Herein we report an oxidative coupling reaction for N–S/S–S bond formation from (E)-N′-benzylideneacetohydrazide and S₈ to furnish substituted N,N′-disulfanediyl-bis(N′-((E)-benzylidene) acetohydrazide). It provides a direct approach for the synthesis of disulfides with good yields.

Herein we report an oxidative coupling reaction for N–S/S–S bond formation substituted N,N′-disulfanediyl bis(N′-((E)-benzylidene)acetohydrazide). It provides a direct approach for the synthesis of disulfides with good yields.

Cascade cyclization versus chemoselective reduction: a solvent-controlled product divergence

A convenient controllable cascade cyclization and partial reduction of enones for the divergent construction of two types of valuable compounds including polysubstituted thiophenes and saturated ketones are developed.

Brønsted acid-promoted thiazole synthesis under metal-free conditions using sulfur powder as the sulfur source

A Brønsted acid-promoted sulfuration/annulation reaction for the one-pot synthesis of bis-substituted thiazoles from benzylamines, acetophenones, and sulfur powder has been developed. One C–N bond and multi C–S bonds were selectively formed in one pot. The choice of the Brønsted acid was the key to the high efficiency of this transformation under metal-free conditions.

A Brønsted acid-promoted protocol for the synthesis of 2,4-disubstituted thiazoles from benzylamines, acetophenones, and sulfur powder under metal-free conditions is described.