WHY IS TETRAZOLE NOT PRACTICAL AS A DIENE IN DIELS-ALDER REACTIONS ANAB-INITIO THEORETICAL-STUDY

An ab initio theoretical study of the reactivity of tetrazole tautomer s, following the example of the ethylene cycloaddition reactions, was undertaken. The geometries of the reactants, the transition structures , and the products for the three tetrazole tautomers were generated at RHF/3-21G, RHF/6-31G*, and MP2/6-31G*. The energies were evaluated u p to the MP4/6-31G//MP2/6-31G* level. The relative stabilities of the tetrazole tautomers were studied, and their relative reactivities wer e predicted from their frontier molecular orbitals (FMO), the charge t ransfer between the reactants in the transition structures, the extent of the reaction calculated from the bond orders, and the electronic l ocation of transition state calculated from the fraction of the overal l charge transferred in the transition state. It was demonstrated that FMO and the charge transferred in transition states are not reliable methods for evaluation of relative reactivity of tetrazole tautomers. However the percentage of charge transferred, and the percentage of re action advance can be useful when the energies of reactants and produc ts are available. The calculated activation energies predict that the most reactive tetrazole tautomer is the non-aromatic 1T, while the lea st reactive is aromatic 3T. It was predicted that the cycloaddition re action with tautomer 1T is experimentally achievable but other side re actions with either tautomer 1T or product 1TS might prevent the accum ulation of the primary cycloaddition product in the reaction mixture.