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

Abstract

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.

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

“…7 In addition, we reported the useful synthesis of 2-substituted benzothiazoles from nitrobenzenes, methylheteroarenes, and elemental sulfur without the use of a transition-metal catalyst (Scheme 1d). 8,9 However, because of the relatively low reactivity of the electron-deficient C-H bonds, direct sulfuration of these compounds with elemental sulfur remains a challenge.…”

“…Similar reactivity has been reported in the literature. 5,8 The product 3fa was obtained from 4-hydroxynitrobenzene (1f) in good yield (entry 6). Surprisingly, 3fa was also obtained from 4nitroanisole (1g) in moderate yield (entry 7).…”

“…Based on the experimental results and the literature, [6][7][8]13 we proposed a possible reaction mechanism (Scheme 5). First, nitrobenzene (1a) was subjected to a 6-electron reduction to form aniline (8), and two equivalents of 2a reacted to form 4 and The reaction of 4 with sulfur afforded 5. The reaction of 5 with 8 afforded 7 and 2a.…”

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

“…The iron-promoted synthesis of 2-substituted benzothiazoles from nitrobenzenes, elemental sulfur, and benzyl alcohol, 6 or their synthesis from nitrobenzenes, elemental sulfur, and methylheteroarenes (or phenylacetic acids) in the absence of a transition-metal catalyst have been reported. 7,8…”

“…The iron-promoted synthesis of 2-substituted benzothiazoles from nitrobenzenes, elemental sulfur, and benzyl alcohol, 6 or their synthesis from nitrobenzenes, elemental sulfur, and methylheteroarenes (or phenylacetic acids) in the absence of a transition-metal catalyst have been reported. 7,8 •2 HCl SYNTHESIS0 0 3 9 -7 8 8 1 1 4 3 7 -2 1 0 X Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart 2021, 53, 2485-2493 Further, Nguyen et al used 2,2′-dinitrodiphenyl disulfides and phenylacetic acids as starting materials for the elegant synthesis of 2-substituted benzothiazoles in the presence of a Fe/S catalyst (Scheme 1, b). 9,10 However, the use of phenylacetic acid is limited for general chemists.…”