Oxidations with Sulfur and Selenium

Allylic Oxidation with Selenium Dioxide

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Se allylic oxidation mechanism

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The first step is a cycloaddition similar to the carbonyl ene reaction. The allylic seleninic acid produced in the first step undergoes a [2,3]-sigmatropic rearrangement to reinstate the double bond position. Rapid decomposition of the selenium (II) intermediate leads to an allylic alcohol. Oxidation continues to give the α,β-unsaturated carbonyl product.