Cope rearrangement template

3-Methylhexa-1,5-diene rearranges to hepta-1,5-diene; with an alkoxide at C3 the anionic oxy-Cope gives an enolate and then a ketone.

Mechanism step by step

The all-carbon [3,3]-sigmatropic shift of a 1,5-diene, and the anionic oxy-Cope

1. Cope rearrangement – σ bond C3–C4 breaks as C1–C6 forms
2. Anionic oxy-Cope – An alkoxide at C3 (from KH) speeds the shift up to 10¹⁷-fold
3. Protonation – The enolate gives the ketone on work-up

Key points

• Reversible: the equilibrium favours the more substituted (more stable) diene.
• Chair transition state: the stereochemistry of the product is predictable.
• Oxy-Cope products tautomerise to carbonyl compounds, so they are irreversible.
• Same [3,3] pattern as the Claisen rearrangement, with C in place of O.

Drawn with 63 atoms, 13 arrows, 5 shapes – every one editable in MolDraw.

Sources

• Cope & Hardy, J. Am. Chem. Soc. 1940, 62, 441 Original paper
• Evans & Golob, J. Am. Chem. Soc. 1975, 97, 4765 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook