Metal Complexes of Octaethylporphin: Preparation, Axial Ligand Substitution and Reduction*

Synthesis and Stereochemistry of Hydroporphyrins

“…In agreement with this concept, the free porphomethene 30a and a series of its metal complexes could be isolated. 28 · 52 · 53 It is further proved by the direct comparison of the two isomeric porphodimethenes obtained by photoreduction of the a,y-dimethylporphyrin 29b. 56 One of them, the a,y-dimethyl-ß,3-porphodimethene 31a, is again stabilized for steric reasons.…”

“…It could be shown that the reaction is quite general for porphyrins, [67][68][69][70] and that various reductants can be used, including hydrazines, sulfhydryl compounds, hydroquinones, and stannous chloride. With ascorbic acid as the reducing agent, the presence of a base is always required, and the reaction occurs via electron transfer from the ascorbate anion, and subsequent proton transfer from the protonated base 70 (28), and, due to its potential implications for photosynthesis, a large amount of work has been focused on this molecule, a metallochlorin. Most of the recent studies are concerned with other porphyrin type compounds, however, be it for use of model systems or for the interest as such in these structures.…”

“…28 -52 · 53 and can be summarized as follows: (a) the reduction step leads normally to the dianion, although with some transition metals (Co, Cu. Ni, Cr) only the monoanion is accessible; (b) upon protonation, four reaction products are generally observed: the metal complexes of the porpho dimethene (6), the chlorin (7), the isobacteriochlorin (12), as well as the metalfree porphyrin.…”

“…Addition of a second mole of methanol then results (with Zn as central metal) in an isomerization of the porphodimethene (6) into a chlorin (4). 26 · 28 The reductive protonation was studied in the OEP series for a variety of central metals by…”

“…The most widely investigated chemical reduction that leads to interrupted systems is the treatment of porphyrins (1) with alkali metals, [46][47][48][49][50] or, better, with aromatic radical anions 26,28 to yield the porphyrin dianions, and the subsequent addition of protic solvent (reductive protonation) or alkylating reagents like alkyliodides (reductive alkylation). The reaction of metalloporphyrins with aromatic radical anions was first investigated by Closs and Closs.…”