Selective Photoinduced Oxidation of Benzylic Methylen Groups through UV Irradiation in Presence of Ferric Chloride

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Abstract
Selective oxidation of benzylic methylene groups through UV irradiation in acetone/water/FeCl3mixtures is reported. This method applied to a series of model compounds, provides an easy access to the corresponding 1‐oxo derivative. Thus, tetralin gives a 100% yield of 1‐tetralone, indane leads to 60% of 1‐indanone, and diphenylmethane is oxidized to benzophenone with 80% yield. However, under the same conditions, alkyl‐substituted aromatic hydrocarbons such as toluene, ethyl‐ and propylbenzene lead to low yields of aldehydes or ketones. Isochromane furnishes a mixture of two substances which can be interconverted, namely the expected l‐isochromanone (9%) and the corresponding hydroxy acid (23%). We failed to apply the method to nitrogen heterocycles containing benzylic groups such as 1,2,3,4‐ and 5,6,7,8‐tetrahydroquinoline as they do not react due to the formation of complexes which precipitate from the solutions.