Tetradehydrocorrin complexes

Presumably the biosynthetic pathways of the porphyrins and corrins are related to some extent, and the direct template cyclization of a tetrapyrrole has now been described to give a mixture of 10 diastereoisomers of tetradehydrocorrin complexes (equation 54).271 In the pursuit of chemical and biosynthetic similarities and differences between corrins and porphyrins, it is necessary to... 

The stronger diamagnetic shielding of the resonances of the meso-protons in the NMR spectrum of [Co(CN)2(A2ODC)] with respect to the corresponding tetradehydrocorrin complex is consistent with the existence of an extended it conjugation. 

The general electronic structure and the coordination and reaction behaviors of the bisdehydrocorrin complexes are discussed here in reference to those of the corrinoid and tetradehydrocorrin complexes. 

The use of metal ions as templates for macrocycle synthesis has an obvious relevance to the understanding of how biological molecules are formed in vivo. The early synthesis of phthalocyanins from phthalonitrile in the presence of metal salts (89) has been followed by the use of Cu(II) salts as templates in the synthesis of copper complexes of etioporphyrin-I (32), tetraethoxycarbonylporphyrin (26), etioporphyrin-II (78), and coproporphyrin-II (81). Metal ions have also been used as templates in the synthesis of corrins, e.g., nickel and cobalt ions in the synthesis of tetradehydrocorrin complexes (64) and nickel ions to hold the two halves of a corrin ring system while cycliza-tion was effected (51), and other biological molecules (67, 76, 77). 

The reaction affords a complex mixture of products containing the dia-stereoisomers of a Ni(II) C,D-tetradehydrocorrinate. In addition to this major product, other nickel corrinoids have been isolated from the reaction mixture containing Ni(AODC), B,C,D-Ni(AXDC) and A,C,D-Ni(AXDC) which surprisingly are derived from dehydrogenation of the parent complex, despite the anaerobic conditions of the reaction. 

B,C,D-Ni(AXDC) fills the last gap in the corrin family and its preparation has been reported by Monforts [76, 77]. This compound has been obtained by cyclization of a Ni complex of the linear tetrapyrrole reported in Fig. 28. The synthetic pathway is similar to that reported for tetradehydrocorrins a cyano group is utilized to protect the 1-position and then removed by alkaline hydrolysis. Cyclization may be acid catalyzed at room temperature, or accomplished thermally at 240-260 °C, with similar yields. 

Fig.31a-c. The structural formulae of the cobalt complexes of a) cobinamide, b) l-I9-dimethyl-A,D-tetradehydrocorrin, and c) 1,19-di-methyloctadehydrocorrin... 

Dicyanocobyrinic acid heptamethyl ester (52) undergoes oxidation at the a-bridge by molecular oxygen in the presence of ascorbic acid to form the wew-hydroxy-lactone (53) the dicyanocobyrinic acid lactone (54a) can be transformed into a variety of /S-meso-substituted derivatives, e.g. (54b), and reduction of the bromo- and iodo-lactones (54b R = Br or I) led to dicyanocobyrinic acid heptamethyl ester (52)/ Nickel complexes of 5-methyl derivatives of 1,19-dimethyl-octadehydrocorrins have been prepared, and the synthesis of l,19-diethoxycarbonyl-octamethylplatinum(ll) tetradehydrocorrin bromide by oxidative ring closure of the corresponding 1,19-diethoxycarbonyl-bilatriene in the presence of platinum chloride has been reported. 

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