Question

An $\mathbf{\alpha }\mathbf{,}\mathbf{\beta }$-unsaturated carbonyl compound can be prepared by a reaction known as a selenenylation–oxidation reaction. A selenoxide is formed as an intermediate. Propose a mechanism for the reaction.

Short answer

Mechanism of selenenylation-oxidation

Step-by-step solution

Step-by-Step Solution Step 1: Selenenylation-oxidation reaction

The addition of selenium-containing compound is called ad selenylation.

Selenium is electrophilic. Therefore, a nucleophilic carbanion can attack the selenium atom. The product thus formed is oxidized to form the $\mathrm{\alpha },\mathrm{\beta }$-carbonyl compound.

The oxidation reaction is carried out using hydrogen peroxide, a stronger oxidizing agent than oxygen.

The oxidizing character of hydrogen peroxide is due to the presence of two oxygen atoms. The single bond between the two oxygen atoms is weak and can easily be fragmented into two OH bonds.

Selenoxide

Compounds containing a Se=O bond are called selenoxides. The electronegativity difference between oxygen and selenium is very high; therefore, the bond is highly polar.

Due to the high polarity of the Se-O bond, it lacks the pi-bond character and exists more in ionic form. This decreases the stability of the compounds. Therefore, Selenoxide undergoes elimination or rearrangement reactions to convert into its more stable forms.

Synthesis of α,β-carbonyl compounds using selenium

The reaction is initiated by a base that abstracts the carbonyl compound's alpha hydrogen. The carbanion thus formed attacks the selenium atom eliminating the halide group. This reaction is called selenenylation.

The product formed in selenenylation is oxidized using hydrogen peroxide, during which the lone pair on Selenium attacks the oxygen atom removing a hydroxide oxide. An ${}^{+}\mathrm{S}-\mathrm{OH}$bond is a bond from which the proton is removed.

The ${}^{+}\mathrm{S}-{\mathrm{O}}^{-}$bond thus formed is called the selenoxide bond and is highly reactive. The negatively charged oxygen atom abstracts proton, which causes the breakage of the Se-C bond, forming $\mathrm{\alpha },\mathrm{\beta }$-carbonyl compound.

The mechanism of the reaction can be given as:

Mechanism of selenenylation-oxidation