Preparation, structure and addition reactions of 4- and 5-aminoimidazoles

Abstract

Catalytic reduction of 5-nitroimidazoles 4 in dioxane solution gives 5-aminoimidazoles 2 in good yield. The derivatives 2d–f were isolated as stable, crystalline compounds which undergo slow decomposition on exposure to air. In a similar manner, solutions of 4-aminoimidazoles 1 were generated from the corresponding 4-nitroimidazoles 3 but attempts to isolate the amines were unsuccessful. The amines 1 and 2 are conveniently generated in situ and are used preparatively without isolation. With aryl isocyanates, aryl isothiocyanates, and diketene, 4- and 5-aminoimidazoles, 1 and 2, give N-addition products whereas with dimethyl acetylenedicarboxylate, the C-addition products 11 and 15 are obtained. Thermal cyclisation of these adducts 11 and 15 gives imidazo[4,5-b]pyridin-5(4H)-one derivatives 12 and 16. AM1 calculations for simple 4- and 5-aminoimidazoles, 1 and 2 and 4- and 5-nitroimidazoles, 3 and 4 are reported. Molecular geometries, enthalpies of formation, dipole moments, and ionisation potentials are analysed and compared with experimental values. A Frontier Orbital Analysis of electrophilic addition reactions of 5-aminoimidazoles 2 is described and used to rationalise the preference of reagents for N- or C-addition.