Work is good, descriptions could be better

Details


Summary:

This paper reports a methodology for synthesis of substituted benzimidazole, benzothiazole, and benzoxazole in aqueous media. The method is novel in its approach of utilizing water as the medium, making it fit for a green chemistry protocol. The paper reports the screening of various metal oxides for carrying out these reactions and choosing the nano-CeO2 as the preferred one. The adequacy in amount of catalyst, its recyclability and the probable mechanism of its action have also been explored. A thorough study is reflected in the synthesis of 67 such compounds.

The Reactions of Interest:

Practical observations:

  • The authors do not mention the solubility of the various aldehydes in water. Ortho-phenylenediamine is not soluble in water. So, its reaction with aldehydes does not require water. In fact, it was found to be true for various aldehydes. Simple grinding by mortar-pestle yielded products with nearly the same yields.
  • Another method tested, was the catalysis by Copper nitrate in methanolic medium. It required as little as 5ml of methanol at room temperature, to carry out the reactions with fairly good yields. The amount of copper nitrate needed was about 3 mol% and reaction time about 20 minutes.
  • Catalysis by nano-CeO2 followed the results found by the authors, with good reproducibility for optimization and recyclability parameters as reported in Table 2 and Table 3 respectively.
  • One advantage of using nano-CeO2 over other methods was reflected as “In case of Scheme 1, The reaction of 1,2-phenylenediamine and benzaldehyde in 1:2 ratio gave only 1,2-disubstituted benzimidazoles whereas when this ratio was changed to 1:1 then 2-substituted benzimidazole was obtained with 98% product yield under same reaction conditions (Figs. 3–5).” This method proved to be much more decisive in producing the 2-substituted benzimidazole in such high yields.

Modifications/ Comments:

  • Although using water as the medium is a good one, but grinding methods could also be used without disturbing the nano-CeO2.
  • Catalysis by copper nitrate in methanolic/ aqueous solution can be used as an alternative method.


Comments

Mechanism of action of nano-ceria

Arnab Bhattacharya
(13)
PhD, Inorganic & Computational Chemistry, Tripura University, 4yrs

Three types of reactions are described in the paper.

1. Synthesis of benzothiazoles: The essential mehanism is the dehydrogenation, via aerial oxidation in which the thiazoline intermediate is converted to the thiazole. The thiazole is more stable since it enjoys the aromaticity of the benzothiazole ring.

The mechanism is depicted in two similar papers.

DOI: 10.1039/C4RA07058E

doi:10.3390/molecules17056011

2. Synthesis of benzothiazoles: The mechanism is similar to the previous reaction.

In these two procedures, the role of nanoceria is minimal as far as my understanding goes. It is primarily due to arial oxidation which can be achieved even with one drop of Cu(NO3)2 solution in aqueous medium. The catalyst is to promote the oxidation.

3. Synthesis of 1,2 disbstituted benzimidazoles: In this mechanism, howeverthe role of nano-ceria is quite significant as it gives only 1,2 disubstituted product and not a mixture. The mechanism will be similar to that in the follwing paper. This will also be of your interest since it also employs a nano solid acid catalyst.

Synthesis of 1,2-Disubstituted Benzimidazoles in the Presence of SBA-Pr-SO3H as a Nano Solid Acid Catalyst.

Ziarani et. al, Journal of Nanostructures 2 (2012) 213-220.

At first, the solid acid catalyst protonates the carbonyl group of aromatic aldehyde (2) which then condense with 1,2-phenylendiamine (1) to produce dibenzylidene-1,2-phenylenediamine (4).
In the presence of catalyst, ring closure produces five membered ring (5) which produced 1,2-3 disubstituted benzimidazoles (3) by deprotonation and [1, 3] hydrid shift.

I hope this will be useful to you.

I am not an expert on nano-chemistry, but have tried some reactions in the field.

Interesting observations.

Valery Petrykin
(5)
R&D Director at SuperOx Japan LLC, Nov 2011
PhD, Materials Chemistry and Engineering, Tokyo Institute of Technology, 3yrs

It is a very clear description of the work! I am wondering if you have any comments on the mechanism of the catalysis by CeO2 nanoparticles. Do you work in this field?

I am asking this question because one of the intermediate products in our company is thin film of highly (100) textured CeO2 layer. We can make such films on the non-textured substrates, introduce other rare-earth elements and adjust the film thickness from several nanometers to several microns. 

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