Protecting groups that can be removed through photochemical electron transfer: Mechanistic and product studies on photosensitized release of carboxylates from phenacyl esters

Photolysis of electron-donating photosensitizers in the presence of various phenacyl esters (PhCOCH2-OCOR) results in C-O bond scission leading to the formation of acetophenone (PhCOCH3) and the corresponding carboxylic acid(RCO2H). Preparative experiments showed that the carboxylic acids are generated in high or quantitative isolated yields. It is argued that this reaction is initiated by a photoinduced electron transfer from the excited state sensitizer to the phenacyl ester. The latter process forms the anion radical of the phenacyl ester which in turn undergoes rapid C-O bond scission leading to the phenacyl radical and the corresponding carboxylate anion. This mechanism is supported by the following observations. (1) The phenacyl esters quench fluorescence from the sensitizers. (2) Analysis of the redox potentials of the sensitizer excited states and the substrates shows that the proposed electron transfer step is exergonic by 15-20 kcal/mol. (3) The byproducts are indicative of the proposed ion radical intermediates. In particular N-methylaniline is detected when N,N-dimethylaniline is used as a sensitizer. (4) Competing processes are observed in phenacyl esters whose acid components are themselves labile to single-electron transfer. For example, phenacyl 4-bromophenylacetate showed bromide elimination in competition with deprotection.