Reduction porphyrins

The electrochemical properties were investigated by CV and SWV. The effects of adding anions to solutions of the porphyrin are summarized in Table 20. Interestingly, the porphyrin reduction waves are not significantly perturbed by any anionic guest, but the cobaltocenium moieties do show cathodic shifts of up to 225 mV with the dihydrogenphosphate anions. 

Porphyrin oxidation Porphyrin reduction Cobaltocenium reduction... 

J. P. Collman, et al, Angew. Chem. Int. Ed. Engl. 1994,33, 1537 (Reduction of protons and oxidation by hydrogen, electrochemically by porphyrins reduction of N2 and catalysis). 

The porphyrin reduction potential is also sensitive to the coordinated metal ion and its oxidation state. For example, the reduction of the Co 4-TRPyP) species is cathodically shifted due to the previous formation of Co (4-TRPyP), while the ring oxidation potentials remain almost invariant at 1.5 V. Although the existence of the M P state was already reported for Ni, Co, and Fe porphyrins, the only case detected in supramolecular porphyrins is for Co (4-TRPyP) and Co (4-TCPyP), occurring around — I.IV, as observed in the spectroelectrochemistry measurements (122, 170). Its formation usually shifts the porphyrin and bpy reduction processes to more negative potentials (Table III). In all cases, the spectroelectrochemistry technique has proved essential for the proper assignment of the redox processes (a typical set is shown in Fig. 18) (38, 118, 119, 170). 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

In the case of porphyrins, reduction of Co11 to Co1 is more difficult. No conditions under which tertiary free radicals would reduce PorCo11 into PorCo1 at spectroscopically detectable levels were identified. 

Snn. If Sn11 salts are reacted with porphyrins one usually obtains SnIV porphyrins, but in rigorously dried and degassed pyridine another complex is obtained with a Soret band close to 460 nm [Whitten (203)]. This could not be demetalated, neither could the metal be exchanged with copper salts. The latter treatment results, however, in formation of SnIV porphyrins. Reduction of SnIV porphyrins with sodium boro-hydride leads to tin porphyrins which can be demetalated by hydrochloric acid, but no 460 nm band is found in the reaction mixture [Fuhrhop (75)]. Thus, the properties of Sn11 porphyrins remain unconfirmed. 

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

The porphyrin ligand can support oxidation states of iron other than II and III. [Fe(I)Por] complexes are obtained by electrochemical or chemical reduction of iron(II) or iron(III) porphyrins. The anionic complexes react with alkyl hahdes to afford alkyl—iron (III) porphyrin complexes. Iron(IV) porphyrins are formally present in the carbene, RR C—Fe(IV)Por p.-carbido, PorFe(IV)—Fe(IV)Por nitrene, RN—Fe(IV)Por and p.-nittido, PorFe(IV)... 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

Porphyrinic co-complexes as novel multinuclear catalysts for the reduction of dioxygen directly to water 97ACR437. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

Other important porphyrins which can be derived from hemin are hematoporphyrin (5) and mesoporphyrin (6). Hematoporphyrin (5) which is commercially available at a relatively low price is sensitive towards acid due to the 1-hydroxyethyl groups, so commercial samples contain only 60 to 70% of hematoporphyrin. Pure hematoporphyrin dimethyl ester which is a racemic and diastereomeric mixture of four stereoisomers can be obtained by esterification with diazomethane and subsequent chromatography on neutral alumina.84 The pure stereoisomers can be prepared by enantioselective reduction of diacetyldeuteroporphyrin dimethyl ester.85a b The... 

Hydroxyalkyl)porphyrins,84-85 I07b 109 easily accessible from porphyrins by Friedel-Crafts acylation and subsequent reduction, undergo a stereoselective Claisen rearrangement with N,N-dimethylacetamide dimethyl acetal. The substitution pattern and the stereochemical arrangement of the derived chlorins match those of naturally occurring chlorins9 (see Section 1.2.1.2.). 

After reduction of the nitro function of the porphyrin, the porphyrinamine intermediate can be reacted with z./l-unsaturated carbonyl compounds to yield porphyrins with a fused pyridine ring, which is formed by Michael addition, imine formation and dehydrogenation. 

This retro-aldol-typc fragmentation is possible because the chlorin chromophore stabilizes the anion formed on loss of the a-oxo acid residue. A related reactivity is observed in the reduction of 3-vinylchlorins, e.g. 24, to 3-ethylporphyrins, e.g. 25, in the presence of hydrogen iodide in acetic acid. The mechanism of this reaction can be represented as a sequence of tautomeric reactions which lead to the completely conjugated porphyrin system.32c-40-54... 

The simplest way for the preparation of bacteriochlorins is the reduction of porphyrins or chlorins with diimide or sodium in pentan-1-ol. The course of these reactions depends upon whether a metal is present in the inner core of the macrotetracycle, or not.3a h... 

Isobacteriochlorins, since they are tetrahydroporphyrins, can be obtained by tetrahydrogena-tion of porphyrins and dihydrogenation of chlorins. However, alkali-metal reduction of porphyrins and metalloporphyrins always gives a mixture of chlorins, bacteriochlorins or isobacteriochlorins.14 The method of choice for the preparation of pure isobacteriochlorins, e.g. 2, is the diimide reduction of zinc(II) chlorins, e.g. l.15a,b... 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

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