An easy, green, efficient and simple for synthesis of derivatives of benzimidazole-, benzothiazole- and benzoxazole-2-thiol has been described. Although previous methods are available in literature for synthesis of such compounds, this method reports the use of copper sulfate as a catalyst. The authors have reported both heating and ultrasonic irradiation as energy sources and presented a comparison between the two. The article is well presented, but I have some comments about the procedure itself.
The Reaction of Interest:
For the preparation of the title compounds, the authors have mentioned 2 conditions. In the section “General Procedure for Preparation of Compounds” the condition (i) is mentioned as “To a solution of potassium isopropyl xanthate (1.5 mmol) in water (5 ml) was added CuSO4 (10 mol%) followed by 2-aminophenols or 2-aminothiophenol or 1,2-phenylenediamines (1 mmol). The mixture was stirred at 90 °C for an appropriate time according to Table 4.”
The condition (i) is mentioned as “To a solution of potassium isopropyl xanthate (1.5 mmol) in water (5 ml) was added CuSO4 (10 mol%) followed by 2-aminophenols or 2-aminothiophenol or 1,2-phenylenediamines (1 mmol). The reaction mixture was irradiated at room temperature with ultrasound for an appropriate time according to Table 4.”
The authors also reported the comparison of solvents for their effectiveness as “Next, we investigated the effect of solvent on the synthesis of benzoxazole-2-thiol. Among three solvents tested, water yielded the best results, glycerol gave a good yield of the products and solvent such as THF was ineffective (Table 2).”
Practical observations and Recommendations:
- With reference to Table 4, I found the general trend to be reproducible in both water and glycerol while heating, but the results for microwave irradiation were not tested.
- I tried to replicate the reactions using copper nitrate and Ferric hydroxide as catalysts as replacements for copper sulfate. The results were promising both in terms of time taken and energy consumed. Entries 11 and 12 in Table 4 were the ones which show significant improvement with copper nitrate at room temperature. Ferric hydroxide was instrumental in reducing the time for entry 8.
- I would recommend using water as solvent.