Synthesis and properties of 4H-imidazoles

Abstract

The photolysis of 1-vinyltetrazoles to give 1H-imidazoles is extended to the synthesis of isolable nonaromatic 4H-imidazoles when the vinyl group is terminally disubstituted. Thus photolysis of the 2-phenyltetrazole (7a), prepared from isobutyraldehyde, at 254 nm in light petroleum gives the 4,4-dimethylimidazole (12a). Similarly the cyclohexenyltetrazole (10) gives the tetrahydro-3aH-benzimidazole (13). Photolysis of the 2-methyltetrazole (7b) in dilute light petroleum solution at 0 °C gives 2,4,4-trimethylimidazole (12b) and photolysis of (7b) and (7c) in methanol gives methanol adducts (15) of the corresponding 4H-imidazoles. 5-Unsubstituted 4H-imidazoles are susceptible to nucleophilic attack at C-5; thus (12a) gives the hydrate (16) on adsorption on alumina, and the dihydroimidazoles (15a) and (17) respectively with methanol and methylmagnesium iodide. On heating, the 4,4-dimethylimidazole (12a) rearranges quantitatively to the 4,5-dimethylisomer (19) by successive [1,5] methyl and hydrogen shifts.