Ab initio quantum-mechanical and experimental mechanistic studies of Diels-Alder reactions between unsubstituted and phenyl-substituted acetylenes and 1,2,4,5-tetrazines

1,2,4,5-tetrazine (2a) undergoes thermal addition to phenylacetylene (1b) in dioxane with ΔH * =15.8 kcal/mol and ΔS * -31 cal/(mol-K). The activation parameters for the corresponding addition of 1b to 3-phenyl-1,2,4,5-tetrazine (2b) arc ΔH * =16.3 kcal/mol and ΔS * =-31 cal/(mol-K). The (4+2)-cycloaddition of 2b to 1b yields a mixture (with the ratios of 90.5:9.5 in dioxane and 94.5:5.5 in acetonitrile) of 3,5-diphenylpyridazine (6c) and 3,5-diphenylpyridazine (6c)