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25.2.1.1.1 Method 1: Oxidation of Activated C—H Bonds by Selenium Dioxide

DOI: 10.1055/sos-SD-025-00278

Olpp, T.Science of Synthesis, (200725424.

The oxidation of CH bonds α to carbonyl groups by selenium dioxide is an established method for the generation of 1,2-diketones.[‌18‌‌21‌] 2-Oxoaldehydes are accessible by this method if the α-position is either a methyl group (e.g., 7) or the carbonyl group is an aldehyde (e.g., 9) (Scheme 3). For a high-yielding process the substituent R1 must be inert, i.e. should possess ideally no CH bond in the α-position of the ketone (arenes, hetarenes, and quaternary carbon atoms). If an activated CH bond is present in the product 8 (e.g., in methylglyoxal, R1=Me) the starting material should be used in (large) excess to avoid further oxidation of the product. An excess of selenium dioxide, however, is reported to cause overoxidation to glyoxylic acids.[‌14‌] In the presence of water the reaction is accelerated due to the higher reactivity of selenous acid (H2SeO3) formed from selenium dioxide and water.[‌21‌] This method is generally applicable for the synthesis of aryl and hetaryl glyoxals and even bisglyoxals[‌22‌,‌23‌] have been prepared. However, a limitation is that some nitrogen-containing aryl methyl ketones cannot be oxidized to the corresponding glyoxals: 2-acetamidoacetophenone[‌24‌] and 2-acetylpyridine[‌25‌] are oxidized to glyoxylic acids. This corresponds to the observation that non-nitrogen containing aryl methyl ketones are oxidized to glyoxylic acids by selenium dioxide in the presence of pyridine.[‌26‌] A solid-phase reaction including microwave heating of the reactants on silica gel represents a simple experimental alternative.[‌27‌] A variation using catalytic amounts of a selenium oxidant (diphenyl diselenide) and a stoichiometric quantity of ammonium peroxydisulfate [(NH4)2S2O8] in methanol delivers protected glyoxals directly, namely dimethyl 2-oxoacetals.[‌28‌] As an alternative to selenium dioxide as oxidant, ammonium chlorochromate (NH4CrO3Cl) in dimethylformamide is reported to convert acetophenone (7, R1=Ph) and phenylacetaldehyde (9, R1=Ph) into phenylglyoxal (8, R1=Ph) in 80 and 87% yields, respectively.[‌29‌,‌30‌]

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8 8.8 8,8-eeeeeee/M8M (88:8), eeeeee, 88e 88e [‌88‌]
8 8.8 8,8-eeeeeee, eeeeee, 88e 88e [‌88‌]

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