Visible-Light-Mediated Synthesis of Unsymmetrical Diaryl Sulfides via Oxidative Coupling of Arylhydrazine with Thiol Golam Kibriya, Susmita Mondal, and Alakananda Hajra* Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India * S Supporting Information ABSTRACT: A metal-free visible-light-promoted oxidative coupling between thiols and arylhydrazines has been developed to afford diaryl sulfides using a catalytic amount of rose bengal as photocatalyst under aerobic conditions. A library of unsymmetrical diaryl sulfides with broad functionalities was synthesized in good yields at room temperature. The present methodology is also applicable to benzo[d]thiazole-2-thiols, benzo[d]oxazole-2-thiol, 1H-benzo[d]imidazole-2-thiols, and 1H- imidazole-2-thiol. S ulfur-containing organic scaffolds, particularly diaryl sulfides, have a valuable impact in the biological as well as pharmaceutical field because of their immense response against cancer, malarial, Alzheimer’s, Parkinson’s, HIV, etc. diseases. 1 They serve as the building block of several natural products. 2 Besides, diaryl sulfides show various applications in material sciences. 3 Moreover, ipso-substitution reactions via C-S bond cleavage enhance the utility of aryl sulfides as synthetic intermediates. 4 Over the past few decades, synthesis of organic sulfides has received considerable attention from synthetic organic chemists. The conventional methods for the synthesis of diaryl sulfides involve the cross-coupling reaction between prefunctionalized arenes with a suitable arylsulfur reagent (Scheme 1A). 5 Most of the reported methods require toxic and expensive transition metal (Pd, Cu, Ru, Ir, etc.) catalysts in the presence of ligands under harsh reaction conditions. Recently, arylhydrazine has emerged as an effective coupling partner in various carbon-carbon and carbon-heteroatom bond-forming cross-coupling reactions. 6 It is considered as an environ- mentally benign arylating agent for oxidative cross-coupling reactions, as it releases only nitrogen gas and water as byproducts during C-N bond cleavage. Very recently Zhao et al. reported a Pd(II)-catalyzed oxidative cross-coupling between phenylhydrazine and thioarenes for the synthesis of diaryl sulfides at 100 °C. 7 However, synthesis of unsymmetrical diayl sulfides via oxidative coupling at room temperature as well as in water is rare. Therefore, development of a simple and environmentally benign method for the formation of unsym- metrical diaryl sulfides is highly desirable in organic synthesis. In recent years, visible-light-mediated photoredox catalysis becomes a powerful synthetic tool in organic synthesis. 8 Organic dyes are considered as alternatives of photoredox transition metal catalysts due to their inexpensiveness, synthetic versatility, nontoxicity, and better environmental perspective. 9 In continuation of our research interest in visible- light-promoted C-C and C-heteroatom bond forming reactions, 10 herein we report a visible-light-promoted C-S bond forming reaction for the synthesis of unsymmetrical diaryl sulfides using rose bengal as a photocatalyst under ambient air at room temperature in water (Scheme 1B). We commenced our study using 4-methoxybenzenethiol (1a) and phenylhydrazine hydrochloride (2a) as model substrates employing Na 2 CO 3 (2 equiv) as the base and rose bengal (2 mol %) as the photocatalyst in CH 3 CN under 34 W blue LED at room temperature. Gratifyingly, the oxidative cross-coupling product (3aa) was obtained with a 58% yield within 8 h under aerobic conditions (Table 1, entry 1). Inspired by the initial result, we carried out the reaction under different conditions to optimize the yield and the results are summarized in Table 1. We first screened the effect of different solvents such as 1,2-DCE, DCM, THF, toluene, DMSO, DMF, EtOH, and H 2 O(Table 1, entries 2-9). Among them, a high yield was obtained in H 2 O(Table 1, entry 9). The effect of other organic dyes such as eosin Y, eosin B, and rhodamine 6G Received: November 6, 2018 Scheme 1. Synthesis of Unsymmetrical Diaryl Sulfides Letter pubs.acs.org/OrgLett Cite This: Org. Lett. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.orglett.8b03549 Org. Lett. XXXX, XXX, XXX-XXX Org. Lett. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/28/18. For personal use only.