Microwave-Assisted, Metal- and Solvent-Free Synthesis of Diaryl Thioethers from Aryl Halides and Carbon Disulfide in the Presence of [DBUH]⁺[OAc]⁻

Abstract

Aryl sulfides and their derivatives have found widespread applications in biological science, medicinal chemistry and materials chemistry. [1][2][3] Therefore, C-S bond formation is of great importance and has attracted much attention in recent years. Conventionally, C aryl -S bond formation requires high reaction temperatures and the use of toxic and high-boiling polar solvents. [4][5][6] A series of metal catalysts including Pd, 7 Cu, 8 Fe, 9 Co 10 and Ni 11 has been employed to promote this transformation under relatively mild conditions. However, the existing methods still suffer from some drawbacks. For example, the direct use of thiols is environmentally unfriendly due to their foul smell, volatility and toxicity. In addition, they are prone to undergo oxidative homocoupling to form disulfides. To address these issues, different thiol surrogates such as thiourea, 12 potassium thiocyanate, 13 potassium ethyl xanthogenate, 14 potassium thioacetate, 15 metal sulfides 16 and carbon disulfide 17 have been employed. Green solvents have also been successfully used for C aryl -S bond formation reactions. 12,18 These protocols, however, still require high reaction temperatures and long reaction times as well as the use of metal catalysts. Thus, the development of an efficient and environmentally benign process for C aryl -S bond formation is still in demand and remains a challenge.Recently, Zheng et al. reported a highly efficient way to capture CS 2 by employing an ionic liquid ([DBUH] + [OAc] -). 19 This kind of ionic liquid can not only activate CS 2 but also shows promising reusability as a solvent. Inspired by this work, we envisioned whether this ionic liquid could be applied to C aryl -S bond formation where CS 2 was used as a thiol surrogate. Here we report a microwave-assisted direct coupling between aryl halides and carbon disulfide in ionic liquid [DBUH] + [OAc] -(Scheme 1). Results and discussionInitially, 1-iodo-4-nitrobenzene (1a) was selected as the model substrate to optimise the reaction conditions (Table 1). The solvent exhibited a strong influence on this transformation. Aprotic organic solvents including toluene, acetonitrile, ethyl acetate and dichloromethane afforded moderate to good yields of the product 2a (Table 1, entries 2-5), whereas reaction in polar protic solvents, such as methanol, provided only 20% yield of product 2a (Table 1, entry 6). The reaction did not take place in the presence of water (Table 1, entry 7). Encouragingly, Microwave-assisted, metal-and solvent-free synthesis of diaryl thioethers from aryl halides and carbon disulfide in the presence of [DBUH] + [OAc] -