CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Abstract

Abstract— The aerobic dye‐sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para‐substituted 2,6‐di‐t‐butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s̀+ gives ρ ‐ 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo‐chemically generated singlet oxygen with 2,4,6‐triphenylphenol gives 2,4,6‐triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6‐triphenylphenol with singlet oxygen. A charge‐transfer mechanism for quenching of singlet oxygen by phenols is proposed.

Solvent effect on the reactivity of monosubstituted phenols towards singlet molecular oxygen (O2(1Δg)) in alkaline media

“…The R-constant was defined for molecules such as phenolates and anilinium ions possessing lone electron pairs and substituents conjugated with the reaction site, as in the case with our para-substituted phenolates. In 1978, Thomas and Foote (4) reported one such treatment (Hammett plot) involving cr+ values for a series of trisubstituted phenols (molecular species) in organic solvents. The substituents were highly electron-releasing alkyl derivatives and the 02('A,) interaction was, in practice, entirely due to physical quenching.…”

“…[4] are the kinetic rate constants and the quantum yield of 0,('A,) generation by the dye sensitizer (4,).…”

Kinetic study of the quenching reaction of singlet oxygen by α-, β-, γ-, δ-tocotrienols, and palm oil and soybean extracts in solution

“…As reported in previous works, 29,34,35) the k Q AO (S) value for the reaction of 1 O 2 with an AO was determined by Equation (3).…”

Ab initio study of the mechanism of singlet‐dioxygen addition to hydroxyaromatic compounds: Negative evidence for the involvement of peroxa and endoperoxide intermediates

“…This mechanism is consistent with the fact that the rate constant of the hydroperoxide-formation process increases with decreasing the ionization potential of a hydroxyaromatic compound. 5,7 However, in our aforementioned study, 22 we concluded that for 1,2-dihydroxyethene the electron-transfer intermediate has an energy of about 35 kcal/mol in nonpolar solvents. In that study we proposed a very similar mechanism with an initial hydrogenatom transfer (Fig.…”

“…2). [5][6][7]15 A mechanism similar to that of the ene reaction and involving a hydrogen-atom transfer from a putative linear intermediate 20 is not reliable, because of a too large hydrogen-oxygen distance. The mechanism proposed by Saito et al 5 involves an initial formation of 1,4-endoperoxides (Fig.…”