Oxidation of Ketones to a-Dicarbonyl Compounds

Methyl and methylene groups adjacent to carbonyl groups are easily oxidized to carbonyls to yield a-keto aldehydes or a-diketones. The reagent of choice is selenium dioxide or selenious acid. The reaction is catalyzed by acids and by acetate ion and proceeds through transition states involving enols of the carbonyl compounds [518]. The oxidation is carried out by refluxing the ketone with about 1.1 mol of selenium dioxide in water, dilute acetic acid, dioxane, or aqueous dioxane [517]. The byproduct, black selenium, is filtered off, but small amounts of red selenium sometimes remain in a colloidal form and cannot be removed even by distillation of the product. Shaking the product with mercury [523] or Raney nickel [524] takes care of the residual selenium. The a-dicarbonyl compounds are yellow oils that avidly react with water to form white crystalline hydrates (equations 407 and 408). 

Refluxing 2-acetylmesitylene with selenium dioxide in aqueous dioxane for 5 h yields 82.5% of mesitylglyoxal as a yellow oil [576], The reaction is applicable to acetophenones 520, 678, 1001, to deoxybenzoin, its homologues, and their derivatives 517, 5i8] and to cyclic ketones [50 ]. a-Dicarbonyl compounds are produced in good yields (equation 409). 

The oxidation of methylene groups to carbonyls is especially easy if the methylenes are flanked by two carbonyl groups. The methylene group in barbituric acid is oxidized to the keto group by chromium trioxide either directly [555] or after reaction with benzaldehyde [554] (equation 410). 

Oxidations of a-methyi and a-methyiene groups are also effected by dimethyl sulfoxide in the presence of aqueous hydrobromic acid [1001] or hydrogen bromide [1002] (equation 409). 

Heating 1,3-indanedione with dimethyl sulfoxide and anhydrous hydrogen bromide at 70-90 C, removing the resulting dimethyl sulfide by distillation, and heating the residue for 1 h at 100 °C with 0.3 N hydrochloric acid furnish ninhydrin in 80-82% yield [1002], 

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