1-(4-Dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole: Synthesis, X-ray crystallography and density functional theory calculations

The product of the reaction between o-phenylenediamine and 4-dimethylaminobenzaldehyde, mp=179°, is unambiguously shown to be 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole by X-ray crystallography. It crystallises in the monoclinic space group P21/c, with a=18.327(5) Å, b=6.318(15) Å, c=18.204(4) Å, β=110.73(2)°, R1=0.0647. Quantum chemical calculations [density functional theory, B3LYP/6-31G(d)] are used to propose a reaction mechanism for the rearrangement of the initially formed bis-Schiff base via a two-step process involving cyclization (TS1, ΔG≠=32 kcal mol–1) to a tetrahedral intermediate (3, ΔGreact=30 kcal mol–1). A formal (1,3)-shift (TS2, ΔG≠=38 kcal mol–1 with respect to 3) then yields in a strongly exothermic reaction (ΔGreact=−17 kcal mol–1) the rearranged product, 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole (2).