Unexpected hydroxylamine-induced ring-closure reactions of meso-tetraphenylsecochlorin bisaldehyde

Reaction of meso-tetraphenylsecochlorin bisaldehyde with hydroxylamine results in the formation of the known meso-tetraphenyl-2-nitroporphyrin and the novel meso-tetraphenylimidazolophorphyrin. The products are the result of two diverging pathways of the presumed intermediate monooxime monoaldehyde that are unusual and surprising, but fully rationalized. The structures of both products were confirmed by spectroscopic techniques and single crystal X-ray diffractometry. This reaction represents the first reaction in which a pyrrole in a porphyrin was formally replaced by an imidazole moiety. The optical properties of the free base and metalloimidazoloporphyrin under neutral and acidic conditions are discussed. Further, an alternative synthesis of the imidazoloporphyrin Ni(II) based on meso-tetraphenyl-1-formyl-chlorophin is presented.