An efficient green protocol for the synthesis of 2-aryl substituted benzothiazoles

Good approach - less yields. Better methods reported already



This paper reports the synthesis of 2-arylbenzothiazoles via cyclocondensation of 2-aminothiophenol with various aldehydes. Polyethylene glycol-400 (PEG-400) has been used as both solvent and catalyst for the process. The procedure applied, is easy to reproduce and gives high yields in general. I have some observation and modifications.

The Reaction of Interest:

Practical observations:

  • The reaction scheme 1 and the corresponding Table 1 report high yields for a variety of substituted benzaldehydes. The work however, does not mention the reactions for 2-methyl, 2-chloro, 2-bromo and 2-methoxy benzaldehydes.
  • When the reactions were carried out for the above mentioned substituted benzaldehydes, the yields for 2-methyl and 2-methoxy benzaldehydes were very good at 88%, 86% although the time taken was nearly 60 minutes in each case, which in comparison to the 4-methyl and 4-methoxy benzaldehydes, is expected due to steric hindrance.
  • The reactions for 2-chloro and 2-bromo benzaldehydes produced quite low yields of about 45% in 3 hours. On refluxing for the same time, the yields did not improve much. This is expected due to large size of Cl, Br which plays a significant role in steric hindrance.  
  • The applicability of PEG-400 is very well reproduced with various substituted aldehydes with yields of over 78% in all cases for Run 3, as listed for 4-methoxy group, in Table 2.

Modifications/ Comments:

  • The general procedures for all the reactions are well reproducible. Slight modification in terms of reaction conditions can improve the green chemistry approach.
  • I tried to reproduce the results with a grinding procedure instead of stirring. This procedure involves nearly 15 minutes of time with similar yields of 90%. The amount of PEG-400 needed for the procedure is only 2 ml for 8 mmol of the reactants (emulating scheme 1).

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