The Cope Rearrangement is the thermal isomerization of a 1,5-diene leading to a regioisomeric 1,5-diene. The main product is the thermodynamically more stable regioisomer. The Oxy-Cope has a hydroxyl substituent on an sp3-hybridized carbon of the starting isomer.
The driving force for the neutral or anionic Oxy-Cope Rearrangement is that the product is an enol or enolate (resp.), which can tautomerize to the corresponding carbonyl compound. This product will not equilibrate back to the other regioisomer.
The Oxy-Cope Rearrangement proceeds at a much faster rate when the starting alcohol is deprotonated, e.g. with KH. The reaction is then up to 1017 times faster, and may be conducted at room temperature. Aqueous work up then gives the carbonyl compound.
Mechanism of the Cope Rearrangement
Two transition states are possible, and the outcome of the reaction can be predicted on the basis of the most favorable overlap of the orbitals of the double bond, as influenced by stereoelectronic factors:
Reverse Aromatic Cope Rearrangement of 2-Allyl-3-alkylideneindolines Driven by Olefination of 2-Allylindolin-3-ones: Synthesis of α-Allyl-3-indole Acetate Derivatives
T. Kawasaki, Y. Nonaka, K. Watanabe, A. Ogawa, K. Higuchi, R. Terashima, K. Masuda, M. Sakamoto, J. Org. Chem., 2001, 66, 1200-1204.