Riley Oxidation
The Riley oxidation is the selenium dioxide-mediated oxidation of methylene groups to alpha ketones and at the allylic position of olefins.
Riley Oxidation
Reaction Mechanism
The oxidation begins with attack by the enol tautomer at the electrophilic selenium center. Then, a molecule of water is lost, and a second equivalent of water attacks the alpha position. In the final step, selenic acid is released to deliver the 1,2-dicarbonyl product.
The alkene with an allylic hydrogen and the enophile react to generate a new σ-bond with migration of the ene bond and a 1,5 hydrogen shift. Then a 2,3-Sigmatropic rearrangement occurs through an envelope-like transition state, giving rise to an allylselenite ester that delivers the (E)-allylic alcohol product upon hydrolysis.
Experimental Procedure
To a solution of the ketone (85.3 μmol, 1.0 eq) in 1,4-dioxane (9 mL) was added selenium dioxide (20.0 eq) in one portion at 23 ºC in a pressure tube. The resulting suspension was stirred vigorously and heated at 100 ºC. After 7 h, heating was ceased, and the reaction mixture was diluted with diethylether. The suspension was filtered through a short pad of Celite, washed with diethylether, and the filtrate was concentrated. The residue was purified by flash column chromatography on silica gel to yield the desired product (70% yield).
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