Corrins oxidation

Figure 26.6 Vitamin B12 (a) a corrin ring showing a square-planar set of N atoms and a replaceable H, and (b) simplified stmcture of B12. In view of the H displaced from the corrin ring, the Co-C bond, and the charge on the ribose phosphate, the cobalt is formally in the - -3 oxidation state. This and related molecules are conveniently represented as r...

A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

The application of ESR to the ribonucleotide reductase system indicates that the catalytic intermediate is a Co(I)-species. The appearance of Cob(Il)alamin is probably caused by partial oxidation of the Co(I)-species. In the enzyme bound Co(II)-species the benzimidazole ligand is coordinated, and the corrin ring is bound so tightly that the enzyme produces a unique highly resolved ESR spectrum. 

The corrins and porphyrins are another important class of natural chelator molecules (Figure 2.4). They are thermodynamically very stable and have four nearly coplanar pyrrole rings, the nitrogen atoms of which can accommodate a number of different metal ions in different oxidation states like Fe2+ in haem, Mg2+ in chlorophyll and Co3+ in vitamin B12. 

Vitamin Bjj (8.50, cobalamin) is an extremely complex molecule consisting of a corrin ring system similar to heme. The central metal atom is cobalt, coordinated with a ribofuranosyl-dimethylbenzimidazole. Vitamin Bjj occurs in liver, but is also produced by many bacteria and is therefore obtained commercially by fermentation. The vitamin is a catalyst for the rearrangement of methylmalonyl-CoA to the succinyl derivative in the degradation of some amino acids and the oxidation of fatty acids with an odd number of carbon atoms. It is also necessary for the methylation of homocysteine to methionine. 

Scheme 28 outlines Eschenmoser s model corrin synthesis. The enamide (310) was treated with KCN to give (311), and this gave the thiolactam (312) when treated with phosphorus pentasulfide. Benzoyl peroxide oxidation yielded the disulfide (313), and in the presence of the enamide (310) this gave the bicyclo thio-bridged compound (314). Sulfur extrusion, by this time a standard procedure (Scheme 22), provided the vinylogous amidine (315)... 

Heme b is utilized for formation of hemoglobin, myoglobin, and many enzymes. It reacts with appropriate protein precursors to form the cytochromes c. Heme b is converted by prenylation to heme o405 and by prenylation and oxidation to heme fl.405a The porphyrin biosynthetic pathway also has a number of branches that lead to formation of corrins, chlorins, and chlorophylls as shown schematically in Fig. 24-22. 

Nickel(II) complexes have also been reported with reduced porphyrins, usually referred to as chlorins and corrins. Some nickel(II) complexes with chlorins (406)2883 have been obtained as by-products in the template synthesis of tetraalkylporphyrins. The main difference between [Ni(tmc)] and [Ni(tmp)] (tmc = deprotonated tetramethylchlorin, tmp = deprotonated tetra-methylporphyrin Table 110) is the lack of symmetry in the former complex with respect to the latter. The synthesis and reactivity properties of a number of corrin-nickel(II) complexes have been reported, mostly by Johnson and co-workers.2910-2915 Scheme 62 is a typical example of oxidative cyclization in the presence of a nickel salt.2914... 

Oxidation of natural cobalt(III) corrins by KMn04-pyridine gives 5- or 15-carboxy and 5,15-di-carboxy derivatives.253,259 Acetoxylation of the meso methyl groups has also been reported.260... 

Coproporphyrin I synthesis, 816 Coronands classification, 919 metal ion complexes, 928,938 Corphins, 855 Coninoids, 983 Corrins, 871-888 demetallation, 882 deuteration, 879 electrophilic reactions, 879 metallation, 882 NMR, 878 nucleophilicity, 886 nucleophilic reactions, 879 oxidation, 879 oxidative lactamization, 880 oxidative lactonization, 880 photochemistry, 887 reactions, 879 at metal, 885 rearrangements, 879 redox chemistry, 888 spectra, 877 synthesis, 878 Corroles, 871-888 demetallation, 874 deuteration, 872 hydrogenation, 872 metallation, 874 reactions, 872 at metal, 875 redox chemistry, 876 synthesis, 871 Corticotropin zinc complexes medical use, 966 Cotton effect anils, 717... 

More direct attempts by Eschenmoser and his group to build up corrins at the desired oxidation level have unearthed a wealth of kinetic template cyclization processes. These have in turn developed into techniques which can now be applied to porphinoid compounds as well. A brief summary of the types of template cyclization reactions is relevant here. 

The secocorrin nickel complex (101) can also be induced to undergo an electrochemical cyclization to corrin (100) but only in low yield. The experimental conditions involve a one-electron oxidation, followed by a one-electron reduction.269 The same secocorrin complex (101) is the starting material for two cyclization sequences leading to a didehydrocorrin complex (107) with a chromophore of seven double bonds (Scheme 67).269,270 The most interesting feature of these sequences is the remarkably easy acid-catalyzed ring closure of the secocorrinoid complex (106) to corrin (107). 

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