Eco-friendly synthesis of 2-substituted benzothiazoles catalyzed by cetyltrimethyl ammonium bromide (CTAB) in water

Innovative but has errors which affects reproducibility

Details


Summary:

This paper reports a CTAB-catalyzed method developed for the synthesis of 2-substituted benzothiazoles by the condensation of 2-aminothiophenol with aldehydes (RCHO: R = Alkyl, Aryl, Heteroaryl, 2-Arylformyl). Use of cetyltrimethyl ammonium bromide (CTAB) in an “on water” one-pot procedure without using any additional organic solvents and oxidants, are the highlights of this methodology.

The Reactions of Interest:

Three types of benzothiazole synthesis are reported in this paper

Synthesis of 2-alkyl benzothiazoles

Synthesis of 2-aryl benzothiazoles

Synthesis of 2-arylformylbenzothiazoles

Practical observations:

  • As listed in Table 1, I tried the reactions and found the claim of the authors about the effectiveness of 5 mol% CTAB to be reproducible with 85% yield. Increasing the amount of CTAB to 20 mol% does not result in any significant improvement in yield.
  • The Tables 2, 3 and 4 list the synthesis of 2-alkyl, 2-aryl and 2-arylformylbenzothiazoles respectively. I found all the reactions to be yielding in the range of 75 to 92%. The reaction times for alkyl aldehydes was considerably high (about 10 hours) as compared to aryl and arylformyl aldehydes.

Comments/Recommendations:

  • The authors have mentioned that “It is noteworthy that the present protocol is superior
    to the previous method for the synthesis of 3a by heating in DMF at 100
    °C catalyzed by 50mol% of molecular iodine21 with the yields of 31% (Table 2, entry 1). Moreover, the
    2-styrylbenzothiazole can react well with 2-aminothiophenol in high yield in short time (Table 2, entry 5)
    ”.

I would recommend a comprehensive study on other reaction conditions relevant to such reactions before pointing out a specific advantage.

“2-styrylbenzothiazole can react well with 2-aminothiophenol” is factually incorrect. The benzothiazoles are produced by reaction of aldehydes with 2-aminothiphenol.

  • The application of CTAB is not necessary for synthesising 2-alkyl, 2-aryl benzothiazoles. I would recommend the readers to follow an article by Chakraborti et. al. (Green Chem., 2007, 9, 1335–1340) which describes synthesis of 2-substituted benzothiazoles/thiazolines in water in the absence of any acid/base catalyst. The positive side of this paper however, is the first-time synthesis of 2-arylformylbenzothiazoles from the condensation of 2-aminothiophenol with arylformyl aldehyde. The proposed mechanism does support it. In this context, application of CTAB catalyst is very useful.


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