TiO2-photocatalyzed reactions of some benzylic donors

TiO 2 -photocatalyzed oxidation of toluene ( 1a ), benzyltrimethylsilane ( 1b ), and 4-methoxybenzyltrimethylsilane ( 1c ) has been carried out in acetonitrile under oxygen, under nitrogen, and in the presence of electrophilic alkenes under various conditions (using Ag 2 SO 4 as electron acceptor, adding 2.5% H 2 O, changing solvent to CH 2 Cl 2 ). Benzyl radicals, formed via electron transfer and fragmentation, are trapped. A good material balance is often obtained. The overall efficiency of the process depends on the donor E ox , on the rate of fragmentation of the radical cation, and on the acceptor present (Ag + is an efficient oxidant, an electrophilic alkene a poor one, O 2 is intermediate). With ring-unsubstituted benzyl derivatives 1a and 1b , oxidative fragmentation occurs mainly close to the catalyst surface. The benzyl radicals form at a high local concentration and give benzaldehyde under O 2 , bibenzyl under N 2 and dibenzylated derivatives by attack on the alkenes (acrylonitrile, fumaronitrile, maleic acid). In this case, using CH 2 Cl 2 –O 2 enhances the yield of benzaldehyde. With methoxylated 1c , however, the radical cation migrates into the solution before fragmentation and, therefore, the free benzyl radical is formed. This radical in part is oxidized to the cation, giving a considerable amount of benzylacetamide (or of the alcohol with water), and in part is trapped by the alkenes. The last reaction is less efficient in this case and yields monobenzyl derivatives.Key words: photocatalysis, oxidation, alkylation.