Photochemistry of α-diketones. II. Some photo-oxidation reactions

The photochemical behaviour of 1,1,4,4-tetramethyl-2,3-dioxotetralin (I) in the presence of oxygen has been thoroughly investigated in the solvents methanol and benzene. Products which are formed as a result of the presence of oxygen include a,a?-o-phenylenediisobutyric anhydride (II), a,a?-o-phenylenediisobutyric acid (III), the peroxy acid lactone (XXI), the lactone (XXV), a-(o-isopropenyl)phenylisobutyric acid (XXVII), and the half-ester methyl hydrogen a,a?-o- phenylenediisobutyrate (XXVI). An attempt to prove that the reported preparation of the anhydride (II) from (I) by the action of sodium ethoxide in ethanol-ether in the presence of air was in fact a photochemical oxidation failed when it was found that no anhydride could be isolated under the reported conditions. The major product was 2-ethoxycarbonyl-2-hydroxy-1,1,3,3-tetramethylindane (XXXII, R = C2H5), formed by a benzilic ester rearrangement.     A less thorough investigation of the photo-oxidation of camphorquinone, benzil, 3,3,5,5-tetramethyl-1,2,dioxocyclopentane, and phenanthrene-9,10-quinone is also reported.     A mechanism is suggested for the formation of anhydrides by photo- oxidation of a-diketones. Studies with benzil and dibenzoyl peroxide indicate that the intermediate formation of diacyl peroxides cannot account for the production of acids (or diacids from cyclic a- diketones) when a-diketones are photolytically oxidized.