Riley oxidation is an organic reaction in which selenium dioxide oxidizes an enolizable α-methyl or α-methylene group adjacent to the carbonyl group of an aldehyde or ketone. The α-carbon is converted into a carbonyl group, usually producing a 1,2-dicarbonyl compound. ^[1,5]

The reaction is named after British chemist Harry Lister Riley. Riley, John Frederick Morley, and Norman Alfred Child Friend reported the oxidation of aldehydes and ketones with selenium dioxide in 1932. 

General Reaction

The oxidation of an α-methylene carbon next to a carbonyl group can be represented as: ^[1,3,5]

R¹–CO–CH₂–R² –[SeO₂]→ R¹–CO–CO–R²

When the substrate is a methyl ketone, the α-methyl group is converted into an aldehyde group:

R–CO–CH₃ –[SeO₂]→ R–CO–CHO

The original carbonyl group remains in the molecule, while the adjacent carbon becomes a second carbonyl carbon.

These equations show the overall organic transformation; the selenium-containing products may vary with the reaction conditions.

Example

Acetophenone is oxidized to phenylglyoxal, an α-keto aldehyde. ^[3]

C₆H₅–CO–CH₃ –[SeO₂]→ C₆H₅–CO–CHO

Mechanism

A commonly proposed mechanism for Riley oxidation involves enol formation and selenium-containing intermediates. Experimental evidence supports the involvement of β-ketoseleninic-acid-type species. The pathway shown below is simplified, particularly with respect to proton-transfer steps. ^[2,4,5]

Step 1: Keto–Enol Tautomerism

The aldehyde or ketone exists in equilibrium with a small amount of its enol tautomer. The carbon–carbon double bond of the enol is electron-rich.

Step 2: Formation of the C–Se Bond

The electron-rich enol attacks the electrophilic selenium center, forming a C–Se bond at the α-carbon.

Step 3: Reorganization of the Selenium Intermediate

Proton transfer and reformation of the carbonyl group produce a β-ketoseleninic-acid-type intermediate. This intermediate then undergoes dehydration and reorganization, producing another selenium-containing species that can react with water.

Step 4: Formation of the 1,2-Dicarbonyl Compound

Reaction with water followed by cleavage of the C–Se bond converts the α-carbon into a carbonyl group. The 1,2-dicarbonyl compound and reduced selenium-containing material are formed.