Formation of pyrazol-1,3,4-thiadiazoles through 1,3-dipolar cydoaddltlons of 3-thioxo-[1,2,4]-triazepin-5-one with nitrilimines: An experimental and computational study

Abstract

In this work the experimental results and the computational study of the title compounds and some ancillary compounds are reported. Two bicyclic pyrazol-1,3,4-thiadiazole derivatives were synthezised by reaction between 6-dimethyIaminomethyIene-3-thioxo-[1,2,4]-triazepin-5-one 1 and several nitrilimines 2a-f to give corresponding spirocycloadducts 3a-f, which undergo a rapid rearrangement leading to the new bicyclic compounds, 4a-f and 5a-f. These obtained bicyclic products were characterized by 1H and 13C NMR spectroscopy and finally by X-ray crystallography. Theoretical calculations hawe been carried out using DFT methods to rationalize the formation of the two new bicyciic compounds. Two reaction types are involved in the formation of the compounds 4a-f and 5a-f. The first one is a 1,3-dipolar cycloaddition (13DC) reaction between 1 acting as dipolarophile and 2a-f as dipoles. The results indicate that the cycloaddition between 1 and 2g, as model of 2a-c, takes place wia a high asynchronous bond-formation process. The regioseiectivity obtained from the calculations is in complete agreement with the formation of the unique spirocycloadducts 3a-f. The second reaction leading to the formation of the final products is a domino process that is initiated by the quick and Irreversible cleavage In a catalytic acid enwironment of triazepenic ring. Copyright © 2008 John Wiley & Sons, Ltd.