This is a Huigsen 1,3-dipolar cycloaddition. Followed by acid catalysed elimination of the nitro group as nitrous acid to establish aromaticity.

The exact order of events is difficult to establish without more information. Assuming the tosic acid is necessary for the cycloaddition to occur and that no intermediates are observed by TLC then the nitrostyrene is protonated on one of the oxygens, and undergoes the 1,3-dipolar cycloaddition. The intermediate behaves similarly to a protonated aminal and collapses, releasing nitrous acid. Deprotonation then establishes aromaticity.

An alternative is also given in which tautomerisation of the intermediate occurs first so that the relatively unfavourable elimination of nitrous acid is driven by aromatisation to the protonated 2-H triazolium species which is deprotonated to afford the product.

I think the second process is overall lower in energy but the difference between the two is somewhat rate dependent.

Dear Eric B. J. Harris ,

Thank you very much for your painstaking effort in solving the mechanism. Your effort is appreciable.

Thanks a lot.

 Thid is 1,3-dipolar cycloaddition of azide involving eliminaion of nitro group to get a targetted product.

Hi

The reaction is 1,3-dipolar cycloaddition of azide to nitroalkene simultaneously with protnonation of the nitro group by the aid of p-TSA, followed by elimination of nitrous acid.

Good luck