Metal-carbon bond homolytic cleavage

For a review of homolytic cleavage of carbon-metal bonds, see Barker, P.J. Winter, J.N. in Hartley Patai, Ref. 129, p. 151. 

A widely accepted mechanism of coenzyme Bir dependent rearrangements encompasses, as the initial step, the homolytic cleavage of the carbon-cobalt bond to generate the 5 -deoxyadenosyl radical. The thermodynamic and kinetic aspects of this and related processes involving the homolysis of transition metal-carbon bonds are discussed. 

When polymerization occurs at a transition metal-carbon bond, this type of complexing weakens the metal-carbon bond and facilitates homolytic cleavage (202) with migration of the polymer radical to one... 

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). 

Electrochemical oxidations of alkylmetal donors are irreversible in all cases studied owing to rapid homolytic cleavage of the metal-carbon bond at the cation-radical stage (Eq. 3) ... 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

Barker, P. J., Winter, J. N., Homolytic Cleavage of Metal-Carbon Bonds, Chem. Met.-Carbon Bond 2 [1985] 151/218. 

The subsequent experimental verification of these results indeed proved that IPrAu-alkyl compounds start to decompose only at temperature of about 180 °C. In analogy with organomercury compounds, this decomposition could be based on a homolytic metal-carbon bond cleavage. 

Ultraviolet activation of the complexes hexacarbonylbis( / 5-cyclopen-tadienyl)dimolybdenum and -ditungsten (135) has been studied in detail (136-138). In addition to homolytic cleavage of the metal-metal bonds, loss of carbon monoxide has also been observed. The products of the photochemical reactions of [(t/5-CH3C5H4)M(CO)3]2 (M = Mo, W) with the dienes la- 1c, lg and It and 1,3,5-cycloheptatriene (68a) differ markedly from those obtained from the thermal reaction of [(f/s-C5Hs)Mo(CO)B]2 (n = 2,3) with dienes (139,140). 

Besides the /3-H elimination mechanism, chain transfers to aluminum and to the monomer can occur. Also, /3-alkyl abstraction has to be considered. The homolytic cleavage of the transition metal—carbon a-bond should only play a minor role (decomposition of the catalyst). 

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