Mode II cleavages of carbon-cobalt bonds

The prototypical mode II cleavage of carbon-cobalt bonds is the photolytic decom- 

Subsequent reactions of the methyl radicals produce the stable organic products ethane and methane. Schrauzer et al. [81] have shown that the ratio of ethane to methane is sensitive to the nature of both the equatorial and axial ligands. In addition, studies of methylcobalt photolyses in DjO showed that methane arises via two competing processes direct abstraction of a hydrogen atom from the equatorial ligand system (Eqn. 46, in which Co (Chelate)L represents the cobalt complex minus one hydrogen atom) 

The rate of recombination of methyl radicals and cobaloxime(II) (i.e. the reverse of Eqn. 45) has been measured by flash photolysis and found to be very high k ca. 

5-8 X lO M/sec at 25°C) [82,83]. This presumably explains both the slow rate of photolytic decomposition under strictly anaerobic conditions as well as the marked acceleration of the photolytic decomposition rate in the presence of air due to trapping of the radicals by oxygen. In the latter case the final products are cobalt(III) (i.e. aquocobalamin from methylcobalamin) and formaldehyde, with traces of methanol, methane and formic acid. 

A second example of photolytic mode II cleavage is the photolysis of 5 -deoxya-denosylcobalamin which leads cleanly to cob(II)amin and a 5 -deoxyadenosyl radical, which rapidly cyclizes to give 8,5 -cyclic adenosine [84]. In this case the rate of photolytic decomposition is not substantially affected by the presence of oxygen (presumably due to a much lower rate of recombination of the radical product with the cobalt(II) species) but the products are, of course, altered to aquocobalamin and a mixture of 8,5 -cyclic adenosine and adenosine-5 -carboxaldehyde [85], the ratio of the latter two being dependent on the oxygen concentration. 

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