DFT study of cycloaddition reaction of isothiocyanates with diazoazoles to 4-imino-4H-pyrazolo[5,1-d] [1,2,3,5]thiatriazines

The mechanism of reaction of isothiocyanates with diazopyrazoles to 4-imino-4H-pyrazolo[5,1-d] [1,2,3,5]thiatriazines was investigated. It was suggested that the reaction proceeds via a pseudopericyclic cycloaddition with activation barrier of similar to 31 kcal mol(-1) (B3LYP/6-31G(*)). The mechanism was confirmed by NBO and ACID schemes. The question of regioselectivity was also explored. It was shown that an acyl substituent on the isothiocyanate led to a stabilization of the final product due to S... O interaction. (c) 2007 Wiley Periodicals, Inc.