TBHP/KI-Promoted Annulation of Anilines, Ethers, and Elemental Sulfur: Access to 2-Aryl-, 2-Heteroaryl-, or 2-Alkyl-Substituted 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.

Microwave-accelerated and benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with cyclic alkanes under metal-free conditions

A novel and efficient method for preparing exocyclic indan derivatives, with this method involving benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with simple cyclic alkanes under microwave irradiation, has been developed. The presented approach showed advantages of simple conditions, an environmentally friendly protocol, good functional-group tolerance, and high yields of products.

HOAc catalyzed three-component reaction for the synthesis of 3,3'-(arylmethylene)bis(1H-indoles)

An efficient HOAc catalyzed three-component reaction of 2-(arylethynyl)anilines with arylaldehydes has been achieved, which leads to the generation of 3,3'-(arylmethylene)bis(1H-indoles) with good to excellent yields and high regioselectivity under transition-metal-free conditions. Four new C-C and C-N bonds were effectively formed in a one-pot procedure. Subsequent research on the reaction mechanism indicated that the reaction likely involved the processes of intramolecular cyclization and cascade intermolecular dehydration condensation.

Synthetic Approaches to Biologically Active C-2-Substituted Benzothiazoles

Numerous benzothiazole derivatives are used in organic synthesis, in various industrial and consumer products, and in drugs, with a wide spectrum of biological activity. As the properties of the benzothiazole moiety are strongly affected by the nature and position of substitutions, in this review, covering the literature from 2016, we focus on C-2-substituted benzothiazoles, including the methods of their synthesis, structural modification, reaction mechanisms, and possible pharmacological activity. The synthetic approaches to these heterocycles include both traditional multistep reactions and one-pot atom economy processes using green chemistry principles and easily available reagents. Special attention is paid to the methods of the thiazole ring closure and chemical modification by the introduction of pharmacophore groups.

Electrochemical Oxidative Difunctionalization of Alkenes to Access α-Oxygenated Ketones

Dioxygenation of alkenes was developed by the combination of electrochemical synthesis and aerobic oxidation, leading to easy accessibility of α-oxygenated ketones in an eco-friendly fashion. Using air as the oxygen source and the absence of transition metals were the critical features of this protocol. A wide range of alkenes and N-hydroxyimides were found to be compatible and provided α-oxygenated ketones in moderate to high yields.

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-Free" Nanoassemblies of AIEE-ICT-Active Pyrazine Derivative: Efficient Photoredox System for the Synthesis of Benzimidazoles

Supramolecular nanoassemblies of an AIEE-ICT-active pyrazine derivative (TETPY) having strong absorption in the visible region and excellent transportability have been utilized as an efficient photoredox catalytic system for the synthesis of a variety of benzimidazoles having electron-withdrawing/electron-releasing/aliphatic groups under "metal-free" conditions. The reaction protocol involves the successful harvesting of visible light by TETPY assemblies to catalyze the coupling of o-phenylenediamine/substituted diamines and substituted aromatic/heterocyclic/aliphatic aldehydes under aerial conditions using mixed aqueous media as the reaction solvent. TETPY assemblies could activate aerial oxygen to generate superoxide for completing the vital proton abstraction step without the need for any external metal/base/oxidant. Moreover, all the products are purified by recrystallization from organic solvents. The TETPY assemblies also exhibited high efficiency in catalyzing the synthesis of 2-substituted benzothiazoles and quinazolines in excellent yields.

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