The Role of Aromatic Radical Cations and Benzylic Cations in the 2,4,6-Triphenylpyrylium Tetrafluoroborate Photosensitized Oxidation of Ring-Methoxylated Benzyl Alcohols in CH2Cl2 Solution

A steady-state and laser flash photolysis (LFP) study of the TPPBF4-photosensitized oxidation of ring-methoxylated benzyl alcohols has been carried out. Direct evidence on the involvement of intermediate benzyl alcohol radical cations and benzylic cations in these reactions has been provided through LFP experiments. The reactions lead to the formation of products (benzaldehydes, dibenzyl ethers, and diphenylmethanes) whose amounts and distributions are influenced by the number and relative position of the methoxy substituents. This behavior has been rationalized in terms of the interplay between the stabilities of benzyl alcohol radical cations and benzyl cations involved in these processes. A general mechanism for the TPPBF4-photosensitized reactions of ring-methoxylated benzyl alcohols has been proposed, where the α-OH group of the parent substrate acts as the deprotonating base promoting α-C−H deprotonation of the benzyl alcohol radical cation (formed after electron transfer from the benzyl alcohol to TPP*) to give a benzyl radical and a protonated benzyl alcohol, precursor of the benzylic cation. This hypothesis is in contrast with previous studies, where formation of the benzyl cation was suggested to occur from the neutral benzyl alcohol through the Lewis acid action of excited TPP+ (TPP*).