Oxy-Cope rearrangement: practice and mechanism
As in the Cope rearrangement the C3–C4 bond breaks. The OH on C3 becomes part of an enol, which tautomerises to the carbonyl group: an unsaturated carbonyl compound forms.
Example
Like the Cope rearrangement but with OH at C3. As the alkoxide (KH) it is enormously accelerated. An enol or enolate forms first and tautomerises to the carbonyl compound, which makes the reaction practically irreversible.
Mechanism step by step
- 1
KH deprotonates the OH group at C3 to the alkoxide; this speeds up the rearrangement up to 10¹⁷-fold (anionic oxy-Cope).
- 2
[3,3]-sigmatropic rearrangement: C3–C4 breaks, C1–C6 forms. C3 becomes an enolate carbon.
- 3
On aqueous workup the enolate is protonated and tautomerises to the aldehyde or ketone.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The enol is only the first product. It tautomerises at once to the more stable keto or aldehyde form.
Often confused with: Cope rearrangement · Mild oxidation with PCC · Acid-catalysed dehydration (E1)
Frequently asked questions
Why is the oxy-Cope irreversible?
The enol tautomerises to the far more stable carbonyl compound, which no longer has a 1,5-diene with OH, so a reverse reaction is impossible.
What is it used for?
For ring expansions: 1,2-divinylcyclohexanol gives a ten-membered ring. This builds medium rings that are otherwise hard to make.