Investigation of the solvent effect, regioselectivity, and the mechanism of the cycloaddition reaction between 2-chlorobenzimidazole and benzonitrile oxide

This theoretical study, performed using density functional theory at the B3LYP/6-311+G(d,p) level, shows that the most likely route to obtain 3-phenyl[1,2,4]oxadiazolo[4,5-a]benzimidazole from the reaction of 2-chlorobenzimidazole with benzonitrile oxide implies the presence of anionic species. A concerted [3+2] cycloaddition reaction on the imidoyl group of 2-chlorobenzimidazole is not possible. The presence of a polar protic solvent (MeOH) favors the reaction. The analysis of the Wiberg indices shows that the transition states are earlier in MeOH than in THF or gas phase. Finally, topological analysis of the electron localization function indicates that the formation of the N4-C3 and C5-O bonds and the breaking of the C5-Cl bond in the preparation of 3-phenyl[1,2,4]oxadiazolo[4,5-a]benzimidazole are marked for each by the presence of an asynaptic basin V(Asyn) and proceed through tetrahedral intermediates, indicating the nonconcerted nature of the mechanism. These results are in good agreement with the experimental results.