Preparation of cyclohex-2-enone

Cyclohexanone is converted into its lithium enolate with LDA at −78 °C, and the enolate reacts with phenylselenenyl chloride to give 2-(phenylselanyl)cyclohexanone.

Oxidation with hydrogen peroxide at room temperature forms the selenoxide, which spontaneously eliminates benzeneselenenic acid (PhSeOH) through the five-membered transition state. Cyclohex-2-enone is obtained under much milder conditions than with classical bromination and dehydrobromination.