Richard O. C. Norman

Electron spin resonance studies. Part XLVI. Oxidation of thiols and disulphides in aqueous solution: formation of RS˙, RSO˙, RSO2˙, RSSR–, and carbon radicals

Radicals which mediate in the oxidation of thiols with the TiIII–H2O2 couple and CeIV and of disulphides with the former reagent, in aqueous solution at room temperature, have been studied by e.s.r. spectroscopy. The e.s.r. evidence establishes that signals with g ca. 2·01 and 2·005 are those of sulphinyl (RSO˙) and sulphonyl (RSO2˙) radicals, respectively. Radicals of both these types are detected in almost all instances; thiyl radicals (RS˙) cannot be detected directly but their involvement is demonstrated by spin-trapping methods; and two dithiols yield spectra with g ca. 2·013 which are attributed to cyclic disulphide radical-anions. Reaction mechanisms consistent with the e.s.r. data and earlier product studies are suggested.

Electron spin resonance studies. Part 61. The generation and reactions of the t-butoxyl radical in aqueous solution

The t-butoxyl radical has been generated in aqueous solution from the reaction between TiIII and ButOOH in a flow system. Evidence is presented which indicates that, although the fragmentation of ButO· to Me· and acetone is rapid under these conditions (k > 106 dm3 mol–1 s–1), competing addition reactions (e.g. to vinyl ethers, furan) and abstraction reactions (with alcohols) can be characterized. ButO· is shown to be electrophilic, like ·OH, in its reactions, but with prop-2-en-1-ol, unlike ·OH, it undergoes abstraction rather than addition. Changes in the behaviour of t-butoxyl at very low pH are attributed to the formation and reaction of the protonated counterpart ButOH+˙.

Electron spin resonance studies. Part L. Reactions of alkoxyl radicals generated from alkyl hydroperoxides and titanium(III) ion in aqueous solution

A variety of alkoxyl radicals has been generated in aqueous solution by the one-electron reduction of alkyl hydroperoxides by titanium(III) ion, and their behaviour has been studied by e.s.r. spectroscopy in conjunction with a rapid-flow system. Although the alkoxyl radicals themselves are not directly detectable by e.s.r., both the adducts which they form with the spin-trap CH2:NO2– and the carbon-centred radicals into which they are transformed can be monitored; in this way, a novel 1,2-shift [e.g. PrO·→·CH(OH)Et], as well as intramolecular 1,5-hydrogen transfer, intramolecular addition to an olefinic bond, and fragmentation, have been demonstrated. Estimates have been obtained for the rate constants for some of these processes (e.g. that for the 1,2-shift mentioned is ca. 107 s–1). and the behaviour of alkoxyl radicals in aqueous and non-aqueous media has been compared.

Electron spin resonance. Part 67. Oxidation of aliphatic sulphides and sulphoxides by the sulphate radical anion (SO4–˙) and of aliphatic radicals by the peroxydisulphate anion (S2O82–)

E.s.r. experiments employing in situ photolytic decomposition of the peroxydisulphate anion (S2O82–) have been carried out to study the reaction of SO4–˙ with aliphatic sulphides and sulphoxides. For the former, ‘dimer’ radical cations (R2SSR2+˙) are detected; these are evidently generated via direct one-electron oxidation of the sulphide (to give firstly R2S+˙). The more complex behaviour of the latter [involving, for example, the reaction of Me2S(O) to give ˙CH2S(O) Me (at low pH), Me˙, and MeSO2·] is interpreted in terms of the initial formation of an (undetected) radical cation R2S(O)+˙, subsequent competing reactions of which include deprotonation and hydration (followed by fragmentation).Flow-system studies of the generation of radicals with ˙OH (from TiIII/H2O2) in the presence of S2O82– provide evidence that the latter is an effective oxidant for both unsubstituted and α-oxygen-substituted alkyl radicals. The rate of oxidation is increased by α-Me and α-OH or -OR groups [e.g. for Me˙ the rate constant is 3.3 ×104 dm3 mol–1 s–1; for ˙CH2OH, the value is 1.3 × 105 dm3 mol–1 s–1].

Electron spin resonance studies. Part XXXIX. A kinetic investigation of the role of radical reduction in metal ion–hydrogen peroxide systems

Reduction of carbonyl-conjugated radicals by TiIII and FeII in aqueous solution has been investigated by e.s.r. spectroscopy and pulse radiolysis. Kinetic data have enabled the importance of such reactions in metal ion–hydrogen peroxide systems to be assessed. For example, reduction by TiIII at pH 1 is negligible under typical flow system conditions whereas at pH 7 reduction by TiIII–EDTA can provide an important pathway for radical termination. The dependence of reduction rates on radical structure has been investigated.

The reactions of alkynes, cyclopropanes, and benzene derivatives with gold(III)

The reaction of gold(III) compounds with alkynes, arenes, and cyclopropanes have been studied. With the first two, the products can be accounted for in terms of electrophilic attack by the metal on the unsaturated centre. The reaction with alkynes bears a close resemblance to the mercury(II)-catalysed hydration but leads to higher conversions. Aromatic compounds react to give chloroarenes, the isomer distribution being consistent with initial metallation of the ring followed by replacement of the dichlorogold group by a chlorine atom. However, isolation of the intermediate arygold compounds did not prove possible. Cyclopropanes were not oxidised to the expected 1,3-adducts; instead 1,2-adducts resulting from initial gold(III)-catalysed isomerisation to an alkene were obtained.

Electron spin resonance studies. Part XLIII. Reaction of dimethyl sulphoxide with the hydroxyl radical

A kinetic analysis is given of the reaction between the hydroxyl radical (generated from the TiIII–H2O2 couple in a continuous flow system) and dimethyl sulphoxide in aqueous acid at ambient temperature; steady-state concentrations of both Me˙ and MeSO2˙ have been monitored. Evidence is presented that the former radical arises from addition of ˙OH to the sulphoxide followed by rapid decomposition of the intermediate species Me2SO2˙. There was no evidence for the formation of ˙CH2S(O)Me. Rate constants for the reaction between hydroxyl and Me2SO, for that between Me˙ and MeSO2H, and for the dimerisation of methyl are estimated as 5 × 109, ca. 106, and 5·6 × 109 l mol–1 s–1, respectively.

Electron spin resonance studies. Part 64. The hydroxyl radical-induced decarboxylation of methionine and some related compounds

Spin-trapping e.s.r. experiments employing both MeNO2(in conjunction with generation of ·OH from the TiIII–H2O2 couple in a flow system) and ButNO (in conjunction with the photolytic decomposition of H2O2) confirm that reaction of ·OH with methionine, S-methylcysteine, and some related compounds effects oxidative decarboxylation. It is proposed that the reaction proceeds via the sequential formation of an hydroxyl adduct at sulphur, a sulphur-centred radical-cation, and a (cyclic) sulphuranyl radical in which the carboxylate function becomes bonded to sulphur.

Electron spin resonance studies. Part XXX. One-electron reduction of carbonyl-containing compounds by the radicals ·CO2H, ·CO2–, and ·CMe2·NH2

The following carbonyl compounds undergo one-electron reduction by the radicals ·CO2H, ·CO2–, or ·CMe2·NH2 to give species which have been characterised by their e.s.r. spectra; acetaldehyde, propionaldehyde, acetone, glyoxal, biacetyl, pyruvaldehyde, ethyl pyruvate, pyruvic acid, furmaric acid, maleic acid, and diethyl maleate. The dependence of the spectra on pH reveals the occurrence of acid-base equilibria for some of the radicals, and there is also evidence that some of the radicals undergo interconversion between tautomeric forms; the behaviour of fumaric and maleic acid, and the differences between the two compounds, are particularly notable in these respects.

Electron spin resonance studies. Part 62. Acid-catalysed interconversion of β-hydroxyalkyl radicals: radical-cation intermediates

Evidence is presented that isomerism of β-hydroxyalkyl radicals (e.g., the conversion of ·CH2CMe2OH into ·CMe2CH2OH) can be brought about in acid solution and that reaction occurs via the formation and hydration of a radical-cation. The reaction rates for loss of OH–(e.g.·CHMeCEt2OH > ·CH2CHMe2OH > ·CH2CHMeOH) are correlated with the stabilities of the radical-cations as measured by the ionization potentials of the corresponding alkenes.