Imidazolyl radicals

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

The flash photolysis of hexaarylbiimidazole produces imidazolyl radicals " which have been shown to be more nearly planar than the parent dimers ort/io-substituents in the aryl rings decrease the radical stability. The radicals oxidize electron-rich substrates by rapid electron abstraction from tertiary amines, iodide, and metal ions, and by hydrogen abstraction from phenols, mercaptans, secondary amines, and active methylene com-pounds." " Studies have been made of the photooxidation of /euco-triphenyl-methane dyes by these radicals." " ... 

An example of the use of the 5-imidazolyl radical has been described by Bowman (90JCS(P1)919). Radicals were generated from the respective halo derivatives, using either an electron-transfer method (Na in NH3 f-BuOH)... 

Aldabbagh (04T8065) studied the intermolecular radical addition of 5- and 2-imidazolyl radicals to different aromatic solvents, and an example is outlined in Scheme 63. 

For further imidazolyl radicals, see [72Tanl], [66Mayl]. °) Calculations. 

Flash Photolysis of a Substituted Hexaarylbiimidazole and Reactions of the Imidazolyl Radical. 

R.L. Cohen [19] examined the effect of substitution on the rate of disappearance of 2 in reaction (4). His selection of compounds was designed to emphasize steric variations in 2. A plot of Cohen s rate constants vs. —Ep/2 values for various imidazole-imidazolyl radical... 

Thus flash photolysis of a substituted hexaarylbiimidazole and reactions of the imidazolyl radical [25] was employed to study the rate of reaction of 2-(o-chlorophenyl)-4,5-diphenylimidazolyl radical with additives in various solvents. Evidence based on measured rate constants, including kinetic deuterium isotope effects, prove that the rate-determining step in the reaction L " " aromatic amine is an electron exchange reaction at the amino nitrogen, while in the reaction L + hydroquinone the rate-determining step is hydrogen abstraction. 

The biimidazole-sensitized photooxidation of leuco triphenyknethane dyes [15], specifically of tris(2-methyl-4-diethylaminophenyl) methane by photogenerated 2-(o-chlorophenyl)-4,5-diphenyl-imidazolyl radical (L ) was studied by flash photolysis. An electron-exchange reaction involving L occurs at an unprotonated amino nitrogen of the leuco dye and is responsible for the first oxidation step. Subsequent reactions do not involve the L radical and depend only on the structure of the leuco dye and environmental effects. The influence of pH on both the course and rate of the dye-forming reaction was investigated. 

Photolysis of HABIs as a result of UV-irradiation leads to radicals that abstract an electron from the leucodye salt, which then disproportionates to form at least one dye molecule. Visible light absorbed by the quinone converts it into a reduced derivative, which can react with the imidazolyl radical as the latter is formed. Reaction between these species is very rapid and prevents the radical from interacting with the leucodye salt. Hence, no color is formed. The quinone reduction product also interacts with the cellulosic binder, converting the initially light yellow coating to colorless. Initial studies of a preferred quinone/H-donor combination employed Carbowax. In time amine derivatives were found to be more effective. 

The imidazolyl radicals may recombine to form the starting dimer. Considering the rather large molecular weight of o-Cl-HABI (MWt 660) or TCTM-HABI (MWt. 848) the bulky radicals may not move much in a viscous medium and show a tendency to recombine. This ability to recombine is responsible for the stability of HABI based imaging systems. 

In an efficient photopolymerization system, the excited photoinitiator L-L must be sufficiently energetic and long-lived to decompose spontaneously or to interact with a second component to produce the active free radicals. Once formed, the free radical R- (here the imidazolyl radical L- reacts with the chain transfer agent, e.g., 2-mercaptobenzoxazole, the monomer M as in thermal polymerization, undergoing propagation, chain transfer, and termination steps ... 

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