Reversible Hydrogen Additions to Reduced Porphyrins

Although this article deals with electron transfers with metalloporphyrins and generally does not cover the chemical reactions of the porphyrin ligand, it is felt that one exception has to be made and that the protonation of the methine bridges of porphyrin n anions should be discussed. This reaction invariably occurs when the reduction of porphyrins is carried out in protic media and is usually fully reversible with a rise in the redox potential of the solution, very much like electron addition and removal [Cfoss (35), Inhoffen (99, 101, 102, 103), Kiselev (113), Mauzerall (129, 130), Peychal-Heiling (147, 148), Seely (156), Shablya (157), Shulga (158), Sidorov (159), Woodward (213, 214)]. 

Rearrangement of metallophlorins to air-stable chlorins has been observed in various instances [e.g. (Buckler (20), Closs (35), Fuhrhop (75)]. The further reduction to chlorinphlorins of natural chlorophyll derivatives occurs exclusively on the /3 bridge, which again proves the special vulnerability to nucleophilic attack of methine bridges between pyrrole and pyrrolenine units [Inhoffen (101, 102, 103)]. 

The behavior of the porphyrin ligand in irreversible chemical reactions follows the same trends, as exemplified by the two extreme cases of the magnesium porphyrins, where the ligand n system is extraordinarily nucleophilic, and tinIV porphyrins, where it reacts as an electrophile. 

The typical reactions of the porphyrin ligand can be compared to those of aromatic compounds, whereas interruption of the macrocyclic conjugation path at one methine bridge leads to a more polyene-type reactivity pattern of the resulting phlorins. 

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