Photo-oxidative fragmentation of some α-alkyl substituted 4-methoxybenzyl alcohols and methyl ethers sensitized by TiO2

The TiO2-catalyzed photo-oxidation of some alpha-alkyl substituted 4-methoxybenzyl alcohols and ethers [4-MeOPhCH(OR)Y] has been investigated in MeCN, in the presence of Ag2SO4. When Y=t-Bu, the intermediate radical cation undergoes exclusive C-alpha-H bond cleavage (leading to the corresponding ketone) if R = Me, but predominantly C-alpha-C-beta bond cleavage (leading to 4-MeOPhCHO) if R=H. The C-alpha-H bond cleavage path is also the major one when Y= i-Pr and R=H. For the substrates where Y=CH(OMe)Me or CH(OH)Me, exclusive C-C bond cleavage is observed when R is either H or Me. A similar situation holds when the photo-oxidations are carried out in the presence of oxygen, however a change in the outcome of the reaction has been observed for the substrates where R=Me and Y=t-Bu or CH(OMe)Me, which both form 4-MeOC6H4CO2Me as the exclusive product. These results are interpreted on the basis of the structural effects on the fragmentation reactions of the intermediate radical cations, the stability of the formed carbon radicals and carbocations, as well as the possible reactions of the former with oxygen.