Summary:

A convenient, clean and environmentally benign method for ‘‘on water’’ synthesis of benzothiazoles/benzothiazolines has been described. Although previous methods are available in literature for synthesis of such compounds, the ease of operation, use of water as the medium, no additional reagent/catalyst and non-production of any waste are the highlights of this methodology.

The Reaction of Interest:

Practical observations:

• Most of the reactions are reproducible with yields of more than 80% within 3 to 6 hours. I found the highly EDG containing aldehydes to be even quicker with around 1 hour for complete reaction. Entries listed as 12 and 13 in Table 1 were the quickest as replicated by me.
• The advantage of using water as the medium is clearly visible. I found out that most of the reactions proceed smoothly on water and the products are easily separable due to non-solubility in water.
• The applicability of tap water instead of distilled water as the medium could be easily confirmed and I found that the reactions do actually proceed with equal ease in both tap and distilled water. I found both yielding products in the range of 95 to 97% as studied for entry 13 in Table 1.
• The use of liquid aminothiophenol was found to speed up the reaction. The solidified form did not give the desired yields which were only about 60% as studied in case of entry 13 in Table 1.

Recommendations:

• The study was thorough and the products were well characterized. So, no major recommendations are needed in this regard. The only area I would like to comment on is the separation of products with residual water in colloidal form. I found dichloromethane to be a better solvent for separation than diethyl ether and would recommend it.