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

The photooxidation of various benzyl methyl ether derivatives (XPhCH2OMe, w ith X = H, 4-MeO; 4-Me, 3-Cl, 4-CF3 and 3-CF3) sensitized by 2,4,6-tripheny lpyrylium tetrafluoroborate in MeCN, in the presence of oxygen, was investi gated by product analysis, fluorescence quenching and quantum yield determi nations. The reaction is suggested to proceed through a benzylic radical ca tion intermediate, formed by electron transfer from the ether to the sensit izer. Besides the back electron transfer reaction, which gives the reactant again, the primarily formed radical cation is deprotonated and the resulta nt a-methoxybenzyl radical reacts through two different paths to yield. ben zaldehydes and methyl benzoates in a ratio that depends on the ring substit uent. Fluorescence quenching and quantum yield studies have pointed out tha t (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.