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 degrees, is unambiguously shown to be 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole by X-ray crystallography. It crystallises in the monoclinic space group P2(1)/c, with a=18.327(5) angstrom, b=6.318(15) angstrom, c=18.204(4) angstrom, beta=110.73(2)degrees, R-1=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, Delta G(not equal) = 32 kcal mol(-1)) to a tetrahedral intermediate (3, Delta G(react)=30 kcal mol(-1)). A formal (1,3)-shift (TS2, Delta G(not equal),6 = 38 kcal mol(-1) with respect to 3) then yields in a strongly exothermic reaction (Delta G(react)=-17 kcal mol(-1)) the rearranged product, 1-(4=dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole (2). (c) 2006 Elsevier B.V. All rights reserved.