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 va rious phenacyl esters (PhCOCH2-OCOR) results in C-O bond scission lead ing to the formation of acetophenone (PhCOCH3) and the corresponding c arboxylic acid(RCO2H). Preparative experiments showed that the carboxy lic acids are generated in high or quantitative isolated yields. It is argued that this reaction is initiated by a photoinduced electron tra nsfer from the excited state sensitizer to the phenacyl ester. The lat ter process forms the anion radical of the phenacyl ester which in tur n undergoes rapid C-O bond scission leading to the phenacyl radical an d the corresponding carboxylate anion. This mechanism is supported by the following observations. (1) The phenacyl esters quench fluorescenc e from the sensitizers. (2) Analysis of the redox potentials of the se nsitizer excited states and the substrates shows that the proposed ele ctron transfer step is exergonic by 15-20 kcal/mol. (3) The byproducts are indicative of the proposed ion radical intermediates. In particul ar N-methylaniline is detected when N,N-dimethylaniline is used as a s ensitizer. (4) Competing processes are observed in phenacyl esters who se acid components are themselves labile to single-electron transfer. For example, phenacyl 4-bromophenylacetate showed bromide elimination in competition with deprotection.