Vitamin homolytic cleavage

The replacement of the O—H O bridges with BF2 of BPh2 may affect both the complex geometry [178] and the electron density at the central metal ion [184], providing the opportunity of adjusting the Co—C bond strength towards homolytic cleavage, which is currently accepted to be the first step of the reactions catalyzed by the vitamin B12 coenzyme [185]. 

Methylcobalamin (I, R = —CH3) and the vitamin Bi2 coenzyme (I, R = 5 -deoxyadenosyl) are the only known naturally occurring organometailic compounds. Both are derivatives of vitamin 8t2 (cyanocobalamin, I, R = —CN)and both can be synthesized from Bn but are best prepared from hydroxocobalamin. These and numerous other derivatives of Bn containing a cobalt-carbon bond are known, and provided the cobalt is bonded to a primary carbon atom the complexes are thermally very stable, but always photochemically labile as a result of homolytic cleavage of the cobalt-carbon bond. 

When RX is easily reduced, as in the case of allyl iodides and benzyl bromides, the competing further reduction of the intermediate radical is suppressed and radical reactions such as dimerization, addition to double bonds and aromatic compounds or reaction with anions can be favored. The radical pathway can be also promoted by catalysis with reduced forms of vitamin Bn, cobaloximes or nickel complexes. These react with the alkyl halide by oxidative addition and release the alkyl radical by homolytic cleavage. 

Photo-homolytic cleavage of Co—C bonds has been studied for Vitamin 812 and cobalt(m) complexes with tetra-aza-14-membered macrocyclic ligands. In the latter study, the rate of recombination of the radical with cobalt(ii) is ca. 10 M s at 25 °C. The cobalt(iii) complexes have a low energy threshold to photolysis, >540nm. 

This substituent R can also be CN , (most common commercial form), CH3 or OH . The Co-C bond in the adenosyl cobalamine is weak (29 5 kcal moT or 123 21 kJ moT ) and easily undergoes reversible homolytic cleavage at ambient temperature to give the adenosyl radical RCH2 and the 17-electron Co radical. This cleavage is the basis of the enzymatic mechanisms of vitamin B12 coenzyme. 

Octahedral d, 18 e Co complex containing an L3X corrin ligand, and in apical position, an L benzimidazole ligand and an X ligand bonded to Co by a carbon atom [weak bond 28.6 kcal-mol (119.5 kj-mol ) whose facile homolytic cleavage leads to the adenosyl radical]. Vitamin B12 coenzyme catalyzes radical reactions such as the isomerization of halogenated derivatives (for instance coenzyme A), the methylation of a substrate such as homocysteine and the conversion of ribose to deoxyribose. 

As a model study of methyl cobalamine (methyl transfer) in living bodies, a methyl radical, generated by the reduction of the /s(dimethylglyoximato)(pyridine)Co3+ complex to its Co1+ complex, reacts on the sulfur atom of thiolester via SH2 to generate an acyl radical and methyl sulfide. The formed methyl radical can be trapped by TEMPO or activated olefins [8-13]. As a radical character of real vitamin B12, the addition of zinc to a mixture of alkyl bromide (5) and dimethyl fumarate in the presence of real vitamin B12 at room temperature provides a C-C bonded product (6), through the initial reduction of Co3+ to Co1+ by zinc, reaction of Co1+ with alkyl bromide to form R-Co bond, its homolytic bond cleavage to form an alkyl radical, and finally the addition of the alkyl radical to diethyl fumarate, as shown in eq. 11.4 [14]. 

Our results indicate that the autoreduction cannot occur by a conventional outer sphere mechanism because of the gross mismatch of the electrochemical potentials. Experimental data available at this time are consistent with homolytic iron-carbon bond cleavage which may or may not involve a simultaneous nucleophilic attack on the coordinated cyanide. The homolytic metal-carbon bond cleavage may serve as a model for similar processes reported for vitamin Bi2 (26). 

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