Radical photolytic generation

The radical is generated by photolytic decomposition of di-/-butyl peroxide in methylcy-clopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcyclopropane ... 

The great advantage of reactions like Scheme 33 and 34, as compared with the direct attachment of a photola-bile group to the polymer (see Scheme 24) is that in the former systems only polymer bound radicals are formed upon photolysis, whereas in the latter, additionally isolated small radicals are generated. Therefore, less homopolymer is produced in the photolytic step following reactions 33 and 34. 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

Another free-radical arylation method consists of the photolysis of aryl iodides in an aromatic solvent. Yields are generally higher than in 14-17 or 14-21. The aryl iodide may contain OH or COOH groups. The mechanism is similar to that of 14-17. The aryl radicals are generated by the photolytic cleavage ArI AR + T. The reaction has been applied to intramolecular arylation (analogous to the Pschorr reaction). A similar reaction is photolysis of an arylthallium bis(trifluoroacetate) (12-21) in an aromatic solvent. Here too, an unsymmetrical biaryl is produced in good yields. ... 

Herranz, M.A., et al., Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes. Journal of the American Chemical Society, 2007.130(1) p. 66-73. 

G. Ferraudi, Inorg. Chem. 17, 2506 (1978) generates methyl radicals photolytically. 

Alkyl radicals have also been prepared by reaction of alkylbromides with photolytically generated Re(CO)5 (from Re2(CO)io) [17], photolysis of cobalt-alkyl complexes [20], photolysis of AIBN [17, 21, 22] or thermolysis of TEMPO adducts [23]. 

There are many excellent books and reviews on the structure and reactions of secondary radical ions generated in radiolytic and photolytic reactions. Common topics include the means and kinetics of radical ion production, techniques for matrix stabilization, electronic and atomic structure, ion-molecule reactions, structural rearrangements, etc. On the other hand, the studies of primary radical ions, viz. solvent radical ions, have not been reviewed in a systematic fashion. In this chapter, we attempt to close this gap. To this end, we will concentrate on a few better-characterized systems. (There have been many scattered pulse radiolysis studies of organic solvents most of these studies are inconclusive as to the nature of the primary species.)... 

The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

The photo-initiated addition process appears to have general applicability, although it can require extensive photolysis times [194-196]. Indeed, photolytic generation of RF- from RFI has been the method used to add Rf- to C60 and C70, not for synthetic purpose, but to examine epr spectra of the resulting radical species [197-199]. A good comprehensive review of the early work on thermal and photochemically-induced free radical addition reactions to olefins can be found in Sosnovsky s book [60]. 

For all production methods and especially for direct photolytic methods, care needs to be taken so that the radicals are generated, or rapidly converted, into a thermal distribution as relaxation of electronic or vibrational states into the monitored ground state would complicate the kinetics. The advantages of pulsed lasers as light sources are well documented [9]. 

Instead of the sensitizer, a photolytically generated halogen atom can also add to the olefin and produce a radical, which may then rotate about the double bond ... 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

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