DIELS-ALDER TRANSITION STRUCTURES OF HETERO-DIENOPHILE ADDITION TO 4H-PYRAZOLE CALCULATED BY AB-INITIO METHODS

Transition structures for the hetero Diels-Alder additions of ethylene , formaldehyde, formaldimine, cis-diazene, trans-diazene, and nitrosyl hydride to 4H-pyrazole were obtained by ab initio molecular orbital c alculations. The geometries of the transition structures and the react ants were optimized at the RHF/3-21G level of theory, whereas the ener gies were evaluated at the MP2/6-31G level on the RHF/3-21G geometrie s. The transition structures are for a concerted mechanism, and they a re fully synchronous with symmetrical dienophiles. The new forming bon ds are shorter and the asynchronicity is higher than in the case of th e corresponding all-carbon diene. Also, the activation energy for the reaction of 4H-pyrazole with ethylene is higher and with cis-diazene i s lower compared to analogous reaction with cyclopentadiene. An except ional energy difference (18.8 kcal mol(-1)) was predicted for the exo/ endo cis-diazene addition to 4H-pyrazole.