Hock rearrangement

What is Hock rearrangement?

The Hock rearrangement, also known as the Hock cleavage, is a reaction that was first reported by Hock and Schrader in 1936. The reaction involves the protic or Lewis acid-promoted rearrangement of hydroperoxides containing unsaturated units attached to the carbon bearing the hydroperoxide group. This leads to the formation of oxycarbonium ions, which are then attacked by water, resulting in the cleavage of the C-C bond and the formation of two carbonyl compounds.

Hock rearrangement - general reaction scheme - Hock cleavage
Hock rearrangement

R1, R2, R3, R= H, alkyl, aryl (see list of acronyms)

The Hock rearrangement is frequently observed in lipids and fatty acids, where allylic and dienylic hydroperoxides are formed upon radical hydrogen abstraction in the presence of air or oxidation by singlet oxygen (1O2). While the Hock rearrangement is typically promoted by acid, it can also occur in the absence of an added acid, such as when the ene reaction solution is injected onto a gas chromatography column for product isolation. Additionally, some Hock rearrangements occur at temperatures substantially below room temperature. The migratory aptitude of groups in this reaction generally follows the order:

cyclobutyl > aryl > vinyl > hydrogen > cyclopentyl ≈ cyclohexyl >> alkyl

Depending on the substrate structure and choice of Lewis acid, dialkyl peroxide and ozonides may decompose via the Hock rearrangement path. Furthermore, the silver ion adducts of hydroperoxides and cyclic peroxides also undergo this rearrangement to give aldehydes and epoxides.

Finally, the intermediate oxycarbonium ion may undergo β-proton elimination instead of attacking by a water molecule, resulting in the formation of a product of divinyl ether.

References

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