Cobalt corrin, alkylation

For the homolytic mode of formation of the Co - C bond in coenzyme Bi2 (2) the structure [51] and reactivity of cob(II)alamin (23) gave crucial information. The radicaloid 23 has a pentacoordinated Co(II) center and is considered to fulfill all the structural criteria of a highly efficient radical trap (see Fig. 10), since its reactions with alkyl radicals occur with negligible restructuring of the DMB-nucleotide coordinated cobalt-corrin moiety [51]. From this it is understandable that the remarkably high reaction rate of 23 with alkyl radicals (such as the 5 -deoxy-5 -adenosyl radical) and the diastere-ospecificity for the reaction to occur at the j8-face, are both consistent and explainable due to the structure of cob(II)alamin. The coordination of the DMB-nucleotide in 23 controls the (a/j8)-diastereoface selectivity (in both a kinetic and thermodynamic sense) in alkylation reactions at the Co(II) center. 

With incomplete cobalt-corrins the situation is again more complex, with two diastereoisomeric alkylation products often formed [74,84,134]. In spe-... 

Two major mechanisms have to be taken into consideration for the alkylation of Co -corrins. The classical mechanism of a bimolecular nucleophilic substitution reaction at carbon (the Co -corrin acts as a nucleophile) leads to /3-aUcylated Co -corrins with high diastereoselectivity. Secondly, an electron transfer-induced radical process (where the Co -corrin acts as a one-electron reducing agent) may also lead to cobalt alkylation. The observed formation of incomplete a-aUcylated Co -corrins under kinetically controlled conditions has been proposed to occur via this path. The high nucleophilic reactivity of Co -corrins and their diastereoselective nucleophilic reaction on the ( upper ) /3-face are not increased by the nucleotide function on the ( lower ) a-face rather they appear to be an inherent reactivity of the corrin-bound tetracoordinate Co -center. Among the organometallic B12 derivatives prepared to date, neopentylcobalamin, benzylcobalamin, and... 

The key role in the experimental study of cis and trans effects in cobalt(III) complexes has been played by the corrinoid group of complexes and by SO and the alkyl ligands. The data relating to the corrin-oids will be summarized and used as a basis for a discussion of the other... 

In several biological reactions the oxidation state is not changed but rearrangements occur. Vitamin B, an alkyl-cobalt(III) complex of a substituted corrin, is a cofactor of enzymes catalyzing 1,2-carbon rearrangements. Another important reaction is the conversion of citrate to isocitrate in the citric acid cycle catalyzed by the enzyme acotinase containing a FeS-cluster. 

The combination of this radical with the Bi2r species generates a new alkyl cobalamin with the substrate as ligand. We have thus accomplished a transalkylation of the cobalt atom. How does the Co—C bond become activated toward homolytic cleavage It is believed that the presence and proper orientation of the propionamide side chains on the corrin ring are responsible for the ease of the enzymatic system, possibly by some distortion of the corrin (269). Support for this hypothesis comes from the fact that hydrolysis of a side chain to the corresponding acid results in an inactive coenzyme Bi2 molecule. It is not known why nature proceeds by homolytic fission, a unique situation in coenzyme chemistry ... 

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