REACTION OF IMIDAZOLES WITH ETHYLENE AND SINGLET OXYGEN - AN AB-INITIO THEORETICAL-STUDY

The structures of the three possible tautomers of imidazole were optim ized with the MP2/6-31G theoretical model, and their relative energie s were compared. All possible transition structures for the Diels-Alde r reaction between the imidazole tautomers as dienes and ethylene and oxygen as dienophiles were generated at the RHF/3-21G and RHF/6-31G l evel of ab initio molecular orbital calculations. The activation energ ies were also evaluated at the MP3/6-31G//RHF/6-31G* level. One trans ition structure for the ethylene addition to the most stable imidazole isomer was also generated at the MP2/6-31G level, and single point e nergies calculated on the structure were compared to the ones obtained on RHF/3-21G and RHF/6-31G geometries. The structures correspond to concerted but not necessarily synchronous cycloadditions. The synchron icity of the reactions is discussed on the basis of the bond orders of the new forming bonds in the transition states. Also, on the basis of the bond orders qualitative predictions of the reactivity of the diff erent imidazole tautomers can be made. The quantitative order of the r eactivity was obtained by comparing the activation energies for the et hylene and oxygen additions to the imidazole tautomers. It was conclud ed that imidazole is not a particularly suitable diene for cycloadditi on reactions because its HOMO energy is too low and LUMO energy is too high for efficient molecular orbital overlap. This is due to the arom atic stability of the imidazole ring and the presence of the nitrogen lone pair orbital very close to the reactive centers which increases t he energy of the transition structures through repulsions of the fille d orbitals. Because of this interaction it is predicted that the produ cts of the cycloaddition reactions, if formed, will be from imidazole tautomer 1.