Octaethylporphyrin

Octaethylporphyrin is an organic compound that is a relative of naturally occurring heme pigments. The compound is used in the preparation of models for the prosthetic group in heme proteins. It is a dark purple solid that is soluble in organic solvents. As its conjugate base OEP^2-, it forms a range of transition metal porphyrin complexes. When treated with ferric chloride in hot acetic acid solution, it gives the square pyramidal complex FeCl. It also forms the square planar complexes Ni and Cu.

Contrast with other porphyrins

Unlike complexes of the naturally occurring porphyrins, OEP complexes have four-fold symmetry, which simplifies spectroscopic analysis. In contrast to tetraphenylporphyrin and related analogues, H₂OEP features unprotected meso positions. In this way, it is a more accurate model for naturally occurring porphyrins.

Synthesis

H₂OEP is prepared by condensation of 3,4-diethylpyrrole with formaldehyde. The pyrrole precursor may be synthesised by a Barton-Zard reaction of ethyl isocyanoacetate and 3-nitro-3-hexene; the latter can be generated in situ by elimination of acetic acid from 4-acetoxy-3-nitrohexane. Both the elimination and cyclicization reactions require an equivalent of a non-nucleophilic base such as diazabicycloundecene.