Photohomolysis Reactions

Photohomolysis reactions of cobalt(III) pseudohalide complexes can be used to effect photoreduction to cobalt(II) complexes. Thus, intramolecular photoelectron transfer in the complexes Co(CN)5N3 and Co(NH3)5N3 leads to oxidation of the azide ion to the azide radical, and reduction of the cobalt(III) center to cobalt(II). Evidence for the initial formation of the azide radical comes from the photolysis of solutions containing Co(CN)5N3 and iodide ion, when the iodine anion radical I2 is observed in the solution. This formation of I2 results from the photochemical generation of the azide radical, which then oxidizes the iodide ion to an iodine atom (Scheme 2.1). Subsequently, spin trapping experiments with phenyl-N-t rf-butyl nitrone has been used to verify the formation of azide radicals from the photolysis of Co(CN)5N and Co(NH3)sN3 / ... 

The preparation of a variety of vinylic functionalized C-glycosides involves a photochemical reaction between telluroglycosides with aryl alkynes at 120 °C. Glycosyl radicals were initially formed by photohomolysis of the C-Te bond and readily added to the alkyne a vinyl telluride (e.g., 49) was finally obtained by a further attack of RTe- (Scheme 3.32) [82],... 

Another important source of perturbation of a chemical system is light, such as a laser flash. The irradiation can cause a rapid photochemical reaction, such as photohomolysis of a single bond. The reverse, thermal reaction will then regenerate the reactant(s). This method differs from the other relaxation methods mentioned above in that the relaxation process brings the system back to its initial state rather than to a new equilibrium. The amount of energy deposited with a flash is often large enough to temporarily perturb even an irreversible thermal system, which makes this technique applicable to both reversible and irreversible reactions. Flash photo-lytic methods are a subject of a later chapter and will not be dealt with here. 

The photolysis of the dimethyl complex Os(CH3)2(CO)4 yields methane, but no ethane. This reaction occurs by an initial photohomolysis of an Os-C bond. A similar loss of an alkyl group has been observed in the UV irradiation of MnR(CO)s (R = CH3, C6H5). Bond homolysis occurs in these photoreactions of MnR(CO)s as evidenced by the trapping of the alkyl and manganese pentacarbonyl radicals that are formed by 2,3,5,6-tetramethyl nitrosodurene -... 

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