Mechanism of the oxidation of benzylic ethers photosensitized by a 2,4,6-triphenylpyrylium salt

Abstract

The photooxidation of various benzyl methyl ether derivatives (XPhCH₂OMe, with X = H, 4-MeO, 4-Me, 3-Cl, 4-CF₃ and 3-CF₃) sensitized by 2,4,6-triphenylpyrylium tetrafluoroborate in MeCN, in the presence of oxygen, was investigated by product analysis, fluorescence quenching and quantum yield determinations. The reaction is suggested to proceed through a benzylic radical cation intermediate, formed by electron transfer from the ether to the sensitizer. Besides the back electron transfer reaction, which gives the reactant again, the primarily formed radical cation is deprotonated and the resultant α-methoxybenzyl radical reacts through two different paths to yield benzaldehydes and methyl benzoates in a ratio that depends on the ring substituent. Fluorescence quenching and quantum yield studies have pointed out that (i) the singlet-excited state of the acceptor is mainly involved and (ii) the competition between deprotonation and back electron transfer reaction is regulated by the nature of the substituent. Hydrogen peroxide, detected together with the organic products, has been suggested to be formed in the TPP⁺ regeneration process through the involvement of oxygen and the acidity of the medium.