Photo-oxidative dealkylation of alfa- alkylbenzyl methyl ethers induced by titanium dioxide in acetonitrile.

TiO2-sensitised photo-oxidation of -alkylbenzyl methyl ethers in deaerated CH3CN gives the expected corresponding ketone, whereas in an aerated medium a dealkylation compound (methyl benzoate) is obtained as the principal product, with respect to the ketone, or the exclusive product (when the alkyl group is tert-butyl). The product analysis and distribution, together with a qualitative estimate of the relative reactivity, suggest that the mechanism in deaerated CH3CN is the same as that previously hypothesized for non--alkylated benzyl ethers; in particular, the carbonyl compound should be formed through oxidation of the benzylic radical (obtained by deprotonation of the cation radical) to the corresponding cation. To justify the formation of methyl benzoate in an aerated medium, a reasonable hypothesis is that the -alkyl--methoxybenzylperoxy radical (obtained from the competitive attack of oxygen on the benzylic radical) undergoes a dealkylation process; in particular, this intermediate, as a tertiary peroxy radical, could form a dimer that evolves into the corresponding oxy radical, giving the ester through a -scission process. The general mechanism suggested in aerated medium was confirmed by evaluating the relationship between the ester/carbonyl (E/C) molar ratio and the adiabatic ionization potential of the benzylic radical intermediate. To evaluate the influence of medium heterogeneity, the E/C ratio data were compared with those obtained from an electron-transfer photosensitised [by 9,10-dicyanoanthracene (DCA)] oxidation in a homogeneous phase. A significant confirmation of the mechanism, in both deaerated and aerated media, was obtained by the reaction performed from 4-methoxy--ethylbenzyl methyl ether (as a model) in the presence of H2 18O, sensitised by either TiO2 or DCA.