Mechanism of One-Electron Oxidation of β-, γ-, and δ-Hydroxyalkyl Sulfides. Catalysis through Intramolecular Proton Transfer and Sulfur−Oxygen Bond Formation

Abstract

The mechanism of photoinduced electron transfer between sulfur-containing alcohols and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by five hydroxyalkyl sulfides, 2-(methylthio)ethanol (2-MTE), 2,2‘-dihydroxydiethyl sulfide (2,2‘-DHE), 3-(methylthio)propanol (3-MTP), 3,3‘-dihydroxydipropyl sulfide (3,3‘-DHP), and 4-(methylthio)butanol (4-MTB), with varying numbers of OH groups and varying locations with respect to the sulfur atom, were determined to be in the range (3.3−4.8) × 109 M-1 s-1 for neutral solutions. The intermediates identified were the CB ketyl radical anion (CB•-), the CB ketyl radical (CBH•), and intermolecularly (S∴S)-bonded radical cations. An additional absorption band at approximately 400 nm in the transient spectra for some of the hydroxyalkyl sulfides was assigned to the intramolecularly (•S−O)-bonded species (only for hydroxyalkyl sulfides which could adopt a five-membered ring structure). The spectra of appropriate (S∴S)+ and (•S−O) intermediates for the hydroxyalkyl sulfides were determined from complementary pulse radiolysis studies in acid and neutral aqueous solutions of the hydroxyalkyl sulfides, respectively. The observation of ketyl radical anions and intermolecular (S∴S)-bonded radical cations of the hydroxyalkyl sulfides was direct evidence for the participation of electron transfer in the mechanism of quenching. Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of formaldehyde formation from the steady-state measurements were determined. The values of these quantum yields indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is a minor photochemical pathway (with a contribution of ∼5−25% depending on the numbers of OH groups). Competing processes of proton transfer and back electron transfer within the CT complex gave significant contributions to these yields. Detailed mechanisms for the CB-sensitized photooxidation of sulfur-containing alcohols are proposed, discussed, and compared with that for the •OH-induced oxidation. One striking feature of the mechanisms is that there is a catalytic influence of neighboring groups on the radical reaction pathways during one-electron oxidation of the hydroxyalkyl sulfides in comparison to comparable reactions of nonsubstituted alkyl sulfides. Support for the mechanisms came in part from an analysis of observed solvent isotope effects on radical quantum yields.

Effect of Molecular Interactions on Electron‐Transfer and Antioxidant Activity of Bis(alkanol)selenides: A Radiation Chemical Study

“…In case of SePOH and SeBOH the yield of HCHO was negligible (< 0.005 mmol J À1 )a nd was less than 2%, of the total COH radical. This unusually lower yield of HCHO in SeEOH as compared to SuEOH suggests facile formation and higher stabilityo ft he (> Se\Se <) + radical as comparedt o( > S\S <) + .I nf act it was earlier reportedb yB obrowski et al that even at 0.1 m SuEOH, no (> S\S <) + radical was observed at neutralp H. [29] The other products generated from the decay of a-{bis(hydroxyl alkyl)}selenomethine radicalc ould not be quantified. The detailso ft he reaction mechanism are depicted in Scheme 2.…”

“…Also, the higher covalent radius in seleniumr educes steric congestion at the selenium center as compared to sulfur.T hese unique features allow the selenium atom to readily form the (> Se\Se <) + radicalt oan extent that other intermediates like (> Se\OH) and( > SeC + )a re not observed. For the same reasons, sulfur analogues do not show (> S\S <) + radical formation even at ac oncentrationo f 0.1 m. [29] The quantum chemical calculations support these observations, where the calculated enthalpy change for formation of (> Se\Se <) + -type radicali se xothermic, whilei ti se ndothermicfor (S\S <) + radicals.…”

“…In fact it was earlier reported by Bobrowski et al. that even at 0.1 m SuEOH, no (>S∴S<) + radical was observed at neutral pH . The other products generated from the decay of α‐{bis(hydroxyl alkyl)}selenomethine radical could not be quantified.…”

Oxidation studies of a novel peptide model N-acetyl-3-(methylthio)propylamine

“…The lone electron pairs are necessary to form the three-electron sulfur-nitrogen bonds under consideration. Such threeelectron bonding can also be achieved with the participation of the carbonyl oxygen atom [16,31,32], however, this would require the formation of a thermodynamically and kinetically [33] less favorable seven-membered ring compared to a five-membered ring with the nitrogen atom. On the other hand, it has been recently reported [32] that such (SO) + formation gains the energy benefit of 0.04 eV in cyclic Met-Met dipeptides.…”

Factor analysis of transient spectra. Free radicals in cyclic dipeptides containing methionine

“…2, in addition to the traditional (S;O) ? reference spectrum [27], were used in spectral resolutions. However, conductivity weighting [15,16] was added for spectral resolutions in the time region between 25 and 50 ls.…”