QUANTUM-CHEMICAL INVESTIGATION OF AZOLES. 1. ALTERNATIVE ELECTROPHILIC SUBSTITUTION MECHANISMS IN 1,2- AND 1,3-AZOLES

Quantum-chemical calculations were performed for the molecular structures of 1,2-azoles (pyrazole, isoxazole, isothiazole), 1,3-azoles (imidazole, oxazole, thiazole), and the corresponding intermediates of electrophilic substitution reactions (with protons as the model electrophiles): azolium ions, bipolar ions (ylides/carbenes), cationic σ-complexes, as well as activation energy values were calculated for the decomposition of ylides. The calculations were performed for gas phase and aqueous solutions according to the B3LYP method in a 6-31G(d) basis set, with corrections for the zero-point vibration energy. The solvation effects were taken into account by using the overlapping spheres model (IEFPCM). The results of the calculations explained some features of electrophilic substitution in azoles according to two alternative mechanisms: the classical addition-elimination with cationic σ‑complex intermediates, and the mechanism of elimination-addition that involves ylides (carbenes) as key intermediates. How to Cite Belen'kii, L. I.; Nesterov, I. D.; Chuvylkin, N. D. Chem. Heterocycl. Compd. 2014 , 49 , 1611. [ Khim. Geterotsikl. Soedin. 2013 , 1739.] For this article in the English edition see DOI 10.1007/s10593-014-1412-8