N-Hydroxyphthalimide/Cobalt(II) Catalyzed Low Temperature Benzylic Oxidation Using Molecular Oxygen

A variety of (substituted) aryl glyoxylates is formed in good to excellent yield under very mild conditions by direct oxidation of the corresponding arylacetic esters or mandelic acid esters with molecular oxygen and N-hydroxyphthalimide/cobalt(II) acetate as catalyst. Heteroaromatic analogs are more difficult to oxidize with this system. The effect of substitution in the aromatic ring of N-hydroxyphthalimide on the oxidation of ethylbenzene has been studied. Electron withdrawing substituents accelerate the oxidation of ethylbenzene and promote the formation of acetophenone. Electron donating substituents lead to decreased rates of oxidation and enhance the selectivity for 1-phenylethanol.