The Oxy-Cope Rearrangement

What the oxy-Cope does

The parent Cope rearrangement takes a 1,5-hexadiene and shuffles it into an isomeric 1,5-hexadiene: a σ bond migrates from one place in the chain to another while the two π bonds slide over. It is a clean, thermally allowed pericyclic reaction — but as a straight diene-to-diene isomerization it is an equilibrium, often unfavorable, and it usually needs 200–350 °C to go at a useful rate.

The oxy-Cope is the same [3,3] shift with one decisive modification: a hydroxyl group sits on carbon 3, the carbon flanking the bond that breaks. That single OH changes everything. After the rearrangement, C3 finds itself as part of a new C=C double bond bearing the oxygen — an enol — which instantly tautomerizes to a carbonyl. The carbonyl is a thermodynamic sink the reaction cannot climb back out of, so the oxy-Cope runs to completion in one direction. And when you first pull the proton off that hydroxyl to make an alkoxide, the rearrangement speeds up by a factor that can reach 10¹⁷.

    3-hydroxy-1,5-hexadiene              δ,ε-unsaturated carbonyl

      1   2   3(OH)  4   5   6              O
      C = C — C  —  C — C = C     ──→    C — C — C — C — C = C  (numbering rewired)
            [3,3] shift + tautomerize      (an aldehyde or ketone at what was C3)

The mechanism, arrow by arrow

Number the six-atom backbone C1 through C6. The starting diene has π bonds at C1=C2 and C5=C6, single bonds through the middle, and the OH on C3.

1. Coil into a chair. The floppy open chain folds so that the two terminal carbons, C1 and C6, are brought within bonding distance. This puckered six-membered arrangement is the chair-like transition state — the same geometry that makes the Cope and the Claisen rearrangement so predictable.
2. Six electrons circulate. The reaction is concerted and pericyclic: two electrons from the C1=C2 π bond, two from the C5=C6 π bond, and two from the C3–C4 σ bond all flow around the ring in one loop — three curved arrows chasing each other. The transition state is aromatic (six delocalized electrons, Hückel topology), which is precisely why the barrier is low.
3. The σ bond hops. As the electrons move, the old C3–C4 σ bond breaks and a brand-new C1–C6 σ bond forms. Simultaneously the π bonds migrate: C1=C2 becomes C2=C3, and C5=C6 becomes C4=C5. This is the defining "3,3" bookkeeping — count three atoms from the breaking bond on each side to find the new bond.
4. An enol appears. Because the OH was on C3 and the new π bond is C2=C3, the oxygen now sits on an sp² carbon of a C=C — the molecule is an enol.
5. Tautomerize to the carbonyl. The enol tautomerizes: the O–H proton moves to the adjacent carbon and the oxygen collapses into a C=O. The product is a δ,ε-unsaturated aldehyde or ketone (the remaining C=C is now four and five atoms away from the carbonyl). This step is irreversible and pulls the equilibrium fully toward product.

In the anionic variant, step 1 is preceded by deprotonation. The C3 oxygen becomes an alkoxide (O⁻), and its lone pair donates into the breaking C3–C4 bond throughout the transition state. This extra stabilization of the transition state — not of the ground state — is what collapses the activation barrier and delivers the enormous rate enhancement. After rearrangement you have an enolate (the deprotonated enol), which is simply protonated on workup to give the same carbonyl product.

Why the alkoxide is a rocket

Berson's original neutral oxy-Cope (1964) still needed forcing thermal conditions — typically 150–250 °C — because the neutral OH offers only modest transition-state stabilization. In 1975 David A. Evans and Alan M. Golob showed that if you simply deprotonate the alcohol first, the same rearrangement accelerates by 10¹⁰ to 10¹⁷.

The physical picture: the oxygen sits directly on C3, one of the two carbons of the σ bond that is cleaving. A negatively charged oxygen is a far stronger electron donor than a neutral hydroxyl. Its lone pair overlaps with the developing radical/anionic character at the breaking bond in the transition state, spreading the negative charge and lowering the transition-state energy. Because the effect is on ΔG‡ and not on the starting material, it is pure kinetic acceleration.

Two practical consequences follow:

• Counterion matters. The looser (more dissociated) the ion pair, the faster the reaction: potassium > sodium > lithium. Adding 18-crown-6 to sequester K⁺ frees the "naked" alkoxide and gives the largest rate boosts. This is why the canonical reagent is KH / 18-crown-6 in THF.
• Temperature drops dramatically. A neutral oxy-Cope that requires refluxing decalin (≈190 °C) can, as the potassium alkoxide, be complete in minutes at 0–65 °C.

Reagents, conditions, and a worked recipe

A typical anionic oxy-Cope has two operational stages: make the alkoxide, then let it rearrange.

    3-hydroxy-1,5-diene  ──KH (1.1 eq), 18-crown-6 (0.1–1 eq), THF, 0 °C→rt──→
        potassium alkoxide  ──[3,3] shift, rt–65 °C, minutes──→
            enolate  ──H₃O⁺ workup──→  δ,ε-unsaturated carbonyl

• Base. Potassium hydride (KH, 1.05–1.2 equiv) is the classic choice; KHMDS or KOtBu also work. Lithium bases give a much slower rearrangement because the tight Li–O ion pair suppresses the anionic acceleration.
• Additive. 18-crown-6 (catalytic to stoichiometric) chelates potassium, freeing the alkoxide for maximum rate. Not always needed, but it is the difference-maker for sluggish substrates.
• Solvent. THF or dry DME; anhydrous and under inert gas because KH and alkoxides are moisture-sensitive.
• Temperature. Deprotonate cold (0 °C), then warm to room temperature or gentle reflux; many anionic oxy-Copes are complete within minutes to a couple of hours.
• Workup. Quench cautiously (the enolate and residual KH react with water/acid vigorously), then acidic aqueous workup protonates the enolate to the neutral carbonyl.

Scope, stereochemistry, and the chair

The oxy-Cope inherits the stereochemical logic of every chair-transition-state pericyclic reaction. The molecule prefers the chair over the boat, and substituents prefer pseudo-equatorial positions. That preference lets you predict — and control — the geometry of the new double bond and the configuration of new stereocenters from the geometry of the starting diene. The alcohol stereocenter can relay chirality into the product, which is why the reaction is prized for stereocontrolled synthesis.

Key scope notes:

• Substrate pattern. You need a 1,5-diene with a hydroxyl on C3 — in practice a carbinol bearing a vinyl group and an allylic/alkenyl chain (a 1,2-divinylcycloalkanol is the archetypal ring-expansion substrate; the two alkenes must flank the ring bond that breaks).
• Ring expansion. When the 1,5-diene is embedded in a ring, the [3,3] shift converts a small ring into a larger ring or a ring-fused carbonyl. This is the single most useful application (see below).
• Substituent effects. Groups that stabilize the developing radical/anion character at the breaking bond (aryl, additional alkyl) further lower the barrier; the reaction is remarkably tolerant of remote functionality.
• Suprafacial–suprafacial. Like the Cope and Claisen, the thermal [3,3] shift proceeds suprafacially on both allyl fragments, consistent with the Woodward–Hoffmann rules for a six-electron, Hückel-topology, thermally allowed process.

Oxy-Cope vs. related [3,3] shifts

	Cope	Oxy-Cope (neutral)	Anionic oxy-Cope	Claisen
Substrate	1,5-hexadiene	3-hydroxy-1,5-diene	3-alkoxide-1,5-diene	allyl vinyl ether
Bond broken → formed	C3–C4 → C1–C6	C3–C4 → C1–C6	C3–C4 → C1–C6	C–O → C–C
Transition state	Aromatic chair	Aromatic chair	Aromatic chair	Aromatic chair
Driving force	Weak — equilibrium	Enol → carbonyl trap	Enol → carbonyl trap	C=C → C=O (strong)
Typical temperature	200–350 °C	150–250 °C	0–65 °C	150–250 °C
Relative rate	1	~1 (thermal)	10¹⁰–10¹⁷ faster	fast (O in ring)
Product	Isomeric diene	δ,ε-unsat. carbonyl	δ,ε-unsat. carbonyl	γ,δ-unsat. carbonyl
Reversible?	Yes (equilibrium)	No (carbonyl trap)	No (carbonyl trap)	No (carbonyl trap)

Real-world synthesis: ring expansion

The oxy-Cope's killer application is building medium and bridged rings, which are otherwise notoriously hard to close. The recipe is a two-move combination:

1. Add a vinyl nucleophile to a cyclic ketone that already carries an alkene. Adding vinyllithium or vinylmagnesium bromide (a Grignard reagent) to a 2-vinylcyclohexanone (or another β,γ-unsaturated cyclic ketone) installs a tertiary alcohol carrying a new vinyl group next to the ring's own vinyl — a ready-made 1,2-divinyl carbinol, i.e. a 3-hydroxy-1,5-diene. (The two alkenes must flank the σ bond that breaks, so a plain conjugated 2-cyclohexenone — whose alkene sits one carbon too close — is the wrong precursor.)
2. Run the anionic oxy-Cope. Deprotonate and let the [3,3] shift cleave the ring bond and forge the new one. For a 1,2-divinylcyclohexanol the four vinyl carbons are stitched into the ring, opening the six-membered ring into a ten-membered one (a single-vinyl substrate absorbs only two carbons and expands a six-ring to an eight-ring; a bicyclic framework expands into a larger bridged one), with a fresh carbonyl handle exactly where you want it.

Classic showcases include ring-expansion routes to medium-ring terpenoids and the ten-membered carbocycle of the cockroach sex pheromone periplanone B, where an anionic oxy-Cope stitches the strained macrocycle in a single, stereodefined step. Because the reaction sets both the ring size and a carbonyl in one operation — and transfers stereochemistry through the chair — it collapses several conventional steps into one.

Limitations and side reactions

• β-Hydride and retro-processes. Very hot neutral oxy-Copes can suffer competing retro-ene or radical fragmentation; the anionic version's low temperature largely sidesteps this.
• Base sensitivity of the substrate. The strong base (KH) that unmasks the alkoxide can also deprotonate or epimerize acidic positions elsewhere, or open epoxides and esters — the substrate must survive KH/THF.
• Cis/trans diene geometry. The chair transition state demands that both allyl termini reach each other; conformationally locked or trans-fused systems that can't attain the chair rearrange slowly or via the higher-energy boat.
• Competing [1,3] shift. In some strained systems a [1,3]-carbon shift competes with the [3,3] pathway; ring strain and substitution decide which wins.
• Moisture and oxygen. KH and alkoxides are pyrophoric/moisture-sensitive; rigorous anhydrous technique is required, and quenching must be done slowly into the alkoxide, never the reverse.

Who discovered it, and when

The [3,3] shift itself is named for Arthur C. Cope, who characterized the thermal rearrangement of 1,5-dienes in the early 1940s. The oxy-substituted version was reported by Jerome A. Berson and Merle Jones in 1964, who recognized that placing a hydroxyl at C3 diverts the product to a carbonyl through the enol. The reaction stayed a thermal curiosity until David A. Evans and Alan M. Golob at Caltech, in 1975, discovered that converting the alcohol to its potassium alkoxide accelerates the rearrangement by ten to seventeen orders of magnitude. That anionic acceleration turned a high-temperature oddity into one of the most practical carbon–carbon bond-forming and ring-expanding reactions in synthesis, and the underlying principle — that a negative charge adjacent to a breaking bond can catastrophically lower a pericyclic barrier — became a general design idea in physical organic chemistry.