Coupled oxidation of iron tetraarylporphyrins as a synthetic tool for linear tetrapyrroles

Abstract

Coupled oxidation of iron tetraphenylporphyrins was studied as a tool for preparation of various linear tetrapyrroles. We obtained two linear tetrapyrrole products, biladienone with the meso carbon retained and bilindione with one of the meso carbons eliminated. Biladienone was isolated as a 15-hydroxy derivative. Acid catalyzed elimination of water from biladienone yielded bilatrienone. Bilindione was converted to the zinc complex of 5-oxaporphyrin by reacting it with acetic anhydride and zinc acetate. The zinc complex of 5-oxaporphyrin was a labile electrophile, and readily reacted with a weak nucleophile such as methanol or ethanol. The nucleophilic ring cleavage was used to prepare a variety of bilinone derivatives with substituent at the terminal pyrrole ring. The zinc complex of 5-oxaporphyrin fluoresced red light with a quantum yield of 7%. Free base bilinone showed solvatochromism by forming proton-transferred hydrogen bonding with aprotic amide, which bears similarity to the red to far-red photochromism of phytochromes. The zinc complex of biladienone bearing long alkyl chains was used as an active layer of molecular switch, where flexible framework of linear tetrapyrrole and self-assembling properties lead to 103 fold electric current reduction by application of an external electric field. The zinc complex of biladienone showed allosteric binding to amines, where allosteric behavior was caused by its dimerization.