Corrins metal complexes

Carboxy derivatives of seco-corrin metal complexes can be synthesized as shown in Scheme 17. Using the nickel(II) complex (86), treatment with acetic acid/triethylamine in toluene at 110° gave a 56% yield of the nickel corrin (87), along with about 20% of the decarboxylated uncyclized material (88). This latter observation suggested that the ring closure step precedes decarboxylation in accord with this it was shown that the intermediate nickel 19-carboxy-... 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

A crystal structure of a cationic corrin salt has been analyzed.251 The macrocycle is significantly ruffled and the pyrrole ring A is tipped out of the mean plane. The two inner protons are placed on N-21 and N-23. The deviation from the mean plane is small in the metal complexes.252... 

The metal complexes found in geological media are based on four classes of ligand the porphyrins, the simplest being porphin (19), dihydroporphyrins (chlorins) (20), tetrahydropor-phyrins (21) and the corrins (22). These in turn are generally thought to arise from the breakdown products of the chlorophylls, haem-type centres, especially the cytochromes, and related complexes such as vitamin B12 and nirrins, associated perhaps with metal enzymes in methanogenic and allied bacteria. 

The first example reported in the literature is the cyclization of dihydrobilin to octadehydrocorrin [51-54]. The reaction is catalyzed by the presence of nickel or cobalt salts. As in the case of corrole and its metal complexes such ring closure reaction has been carried out in alcoholic solution, it is oxidative and base catalyzed. It has been demonstrated that the formation of the corrin ring is part of an equilibrium where the oxidative ring closure is coupled with a reductive ring opening reaction [55]. 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

In such a large subject, this article can only focus on certain aspects, namely those that involve complexation with inorganic substrates. We only consider the synthetic macrocycles, with emphasis on transition metal complexation. Aza, oxa, and, to a lesser extent, thia and phospha macrocycles are also covered. The naturally occurring porphyrins, corrins, corphins, chlorins, and phthalocyanins, as well as the cyclodextrins, are not included. Because of the general complexity of macrocychc systems and the resulting complicated systematic names, commonly used abbreviations or simplified names will be employed. This review will encompass the synthesis, thermodynamics, stmcture, and applications of macrocychc ligands. 

An impressive example of the possibilities of metal complex photochemistry is the substantiation of the critical step in corrin synthesis reported by Eschenmoser 145> Ring closure is brought about by antara-facial cycloisomerization of a secocorrinoidic palladium complex by photochemical 1.16-hydrogen shift. 

Liquid-phase catalysts are close models to enzymes and can be a gentle alternative method of destruction of halogenated hydrocarbons. Transition metal complexes, in particular metal porphyrins, corrins and phthalocyanines, have been studied in homogeneous abiotic aqueous systems as potential remediation catalysts, but further identification of degradative products is necessary, since innocuous products must result if synthetic catalysts are to be used effectively. Moreover, the implementation of homogeneous catalysts is still impractical because of problems with separating the catalyst in principle these can be overcome by immobi-... 

In the corrin structure, the n system is interrupted between C-1 and C-19 and the macrocycle is not aromatic. C-1 (and C-19) is chiral and resolution of the ( ) isomers is possible. The naturally occurring corrin has the (IR) configuration, i.e. the Me group is on the a side of the ring. A crystal structure of a cationic corrin salt has been analyzed. The macrocycle is significantly ruffled and the pyrrole ring A is tipped out of the mean plane. The two inner protons are placed on N-21 and N-23. The deviation from the mean plane is small in the metal complexes. ... 

Atom Transfer Atom Transfer (AT) takes place typically in the case of d7 complexes, which abstract the halogen atom from RX. The radical formed combines then with a second metal [193, 194]. A classical example of this mechanism is the hydrodehalogenation with cyanocobaltates(II) (see Section 18.2.1) [8, 9], but an analogous pathway was suggested recently for the Co(II) corrin-catalyzed dechlorination of CC14 in the presence of S2 /cysteine as reductant (Eqs. (11)—(12))... 

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. 

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