2396-03-4Relevant articles and documents
EPR and spin-trapping investigation of free radicals from the reaction of 4-methoxybenzenediazonium tetrafluoroborate with melanin and melanin precursors
Reszka, Krzysztof J.,Chignell, Colin F.
, p. 7752 - 7760 (1993)
The interaction of synthetic DOPA melanin (DM) and its precursors (catechols and phenols) with 4-methoxybenzenediazonium tetrafluoroborate (4-MeO-PhN2BF4) has been studied using EPR spectroscopy and the spin-trapping technique. We found that DM, catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyhydrocinnamic acid, 3,4-dihydroxyphenylalanine, and 6-hydroxydopamine all react with 4-MeO-PhN2+ through a one-electron-transfer process which gives rise to an aryl radical (4-MeO-Ph.) derived from the diazonium compound and to radicals from melanin and from the catechol(amine)s. The formation of aryl radicals is an autocatalytic process. To explain the autocatalysis, we postulate a mechanism in which the key step is the formation of o-quinones. In aerated solutions the aryl radicals react with oxygen, which leads to oxygen consumption. The reaction was found to be order 1, 0.5, and 0.35 with respect to 4-MeO-PhN2+, catechol, and oxygen concentration, respectively. Phenol, 4-hydroxyanisole, and tyrosine do not reduce 4-MeO-PhN2+ unless they are activated by the enzyme tyrosinase. In the presence of tyrosinase, tyrosine produces the most efficient reducing agent. This indicates that the conversion of phenols to o-dihydroxybenzene derivatives by tyrosinase is essential for aryl radical formation from 4-MeO-PhN2+. These observations substantiate the ability of hydroquinones and semiquinone radicals to promote the homolysis of diazonium salts to generate aryl radicals. Such reductive activation of diazonium compounds may be pertinent to their biological, mutagenic, and carcinogenic action.
Reductive Fragmentation of 9,9-Diarylfluorenes. Concurrent Radical Anion and Dianion Cleavage. Electron Apportionment in Radical Ion Fragmentations
Walsh, Thomas D.
, p. 1511 - 1518 (2007/10/02)
Both Radical anions and dianions of 9,9-diarylfluorenes cleave an aryl ring after reduction by alkali metals or naphtalenide radical anions in ether solvents.The relative amount of cleavage through each intermediate depends on the alkali metal cation, the solvent, and the presence or absence of 18-crown-6-ether.The tendency for dianion cleavage parallels that for disproportionation of radical anions to dianions and neutral hydrocarbons.Radical anion fragmentation is proposed to proceed via heterolytic cleavage in which electron flow is in the direction which offsets the charge distribution in the radical ion.In the present case, this initially affords 9-arylfluorenyl radical and aryl anion, which subsequently indergo electron exchange to form the more stable 9-arylfluorenyl anion and aryl radical.
Free-Radical Reductions of Arenediazonium Ions in Aqueous Solution. IV. Kinetics of Reactions of para-Substituted Diazonium Ions with Benzyl Alcohol, Isopropyl Alcohol And Methanol
Packer, John E.,Heigway, Christopher J.,Miller, Helen M.,Dobson, Brian C.
, p. 965 - 977 (2007/10/02)
Electron-withdrawing substituents are shown to increase the chain length of free-radical hydrodediazoniation reactions, but the actual reaction step causing the substituent effect depends on the relative rates of propagation and termination reactions.With benzyl alcohol as reducing agent the rate of the slow propagation step is increased, while with isopropyl alcohol the rate of the termination step is decreased.Rate constants for some reactions of radicals with diazonium ions are reported, and the nature of some of these reactions and their implication foran understanding of the homolysis of aromatic diazo compounds are discussed.
