Metal hydrides radical addition reactions

An organometallic radical may undergo several different types of reactions Scheme 3 illustrates some different reactions of CpM(CO)3 radicals (21) including (i) dimerization (ii) halide abstraction from an alkyl hahde or metal halide (iii) hydrogen atom abstraction from metal hydrides (iv) electron-transfer reduction (v) electron-transfer oxidation and ligand addition (vi) electron-transfer induced disproportionation (see Electron Transfer in Coordination Compounds). 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Inverse isotope effects have been observed for some time with metal hydride reactions, such as tin hydrides, but their observation in transition metal reductions may have a considerable impact on the interpretation of results of catalytic studies. It would appear that a process such as (22) may be involved in reductions with Co(CO)4H, which also show an inverse isotope effect ku/ku = 0.58 for 1,1-diphenylethylene and 0.43 for 9-methylidenefluorene). There is now evidence for radical involvement in the cobalt system.Neither the involvement of radicals, nor an unsymmetrical transition state, however, is required for an inverse isotope effect. For example, the radicals taken to have arisen from H atom addition in... 

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