ABINITIO 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 DELTAH(double dagger) = 15.8 kcal/mol and DELTAS(double dagger) = -31 cal/(mol.K). The activation parameters for the corresponding addition of lb to 3-phenyl-1,2,4,5-tetrazine (2b) are DELTAH(double dagger) = 16.3 kcal/mol and DELTAS(double dagger) = -31 cal/(mol.K). The (4+2)-cycloaddition of 2b to lb yields a mixture (with the ratios of 90.5:9.5 in dioxane and 94.5:5.5 in acetonitrile) of 3,4-diphenylpyridazine (6b) and 3,5-diphenylpyridazine (6c). Detailed ab initio electronic structure calculations are carried out in order to explain these experimental findings, and the computed electronic wave functions are analyzed with the help of rigorous interpretive tools. At the MP2/6-311G** level, the activation energy for the Diels-Alder cycloaddition of acetylene (1a) to 2a is estimated at 12.6 kcal/mol. The activation energy for the decomposition of the resulting intermediate (4a) to pyridazine (6a) and nitrogen (7a) is only 5.5 kcal/mol at the HF/6-31G* level. At the MP2/6-311G** level, the reaction barrier for the decomposition of 4a is so small that the corresponding transition state cannot be located. The head-to-tail cycloaddition of lb to 2b, which results in 6c as a final product, is preferred to the head-to-head cycloaddition (leading to 6b) by 0.9 kcal/mol at the HF/6-31G* level. However, when the attractive dispersion interactions between the phenyl rings are taken into account, formation of 6b is found to be favored over that of 6c by 4.0 kcal/mol, in agreement with the experimental observations. This unexpected conclusion provides the first documented example of regiospecificity being primarily controlled by the dispersion (attractive van der Waals) interactions.