The Cope and Claisen Rearrangements

Heat a 1,5-diene to 200 °C in a sealed tube. With no catalyst, no added reagent, and no light, it quietly reorganizes itself. It forms a new 1,5-diene with different atoms bearing the double bonds. Heat an allyl vinyl ether instead, and it becomes a γ,δ-unsaturated carbonyl compound. Again there is total control over which carbon ends up bonded to which. This often gives a single stereochemical outcome out of four possible diastereomers. These are the Cope and Claisen rearrangements. They are the two most important members of the [3,3]-sigmatropic family. They are among the most mechanistically transparent reactions in organic chemistry. Nothing ionizes, and nothing radical is generated. Six electrons simply flow around a six-membered loop in a single concerted step. The reason these reactions matter goes well beyond thermodynamic curiosity. A [3,3]-shift builds a new carbon–carbon (Cope) or carbon–carbon (Claisen, via an allyl vinyl ether) bond. Its transition state has a geometry so well defined — almost always a chair. Because of this, the relationship between starting alkene geometries and new stereocenters is completely predictable. This turns what looks like a "rearrangement" into a genuine bond-forming strategy. Chemists deliberately construct 1,5-dienes or allyl vinyl ethers for this purpose. Their only job is to flip into the thermodynamically preferred product. That product carries exactly the stereochemistry a synthesis requires. The Ireland–Claisen and Johnson–Claisen variants are industrial-strength versions of this idea. They are used to install quaternary and acyclic stereocenters. These are difficult to make any other way. The big picture is that both rearrangements share the same orbital logic. This is the same logic that governs electrocyclic ring closures and Diels–Alder cycloadditions. Picture a cyclic array of six p-orbitals and six electrons. The transition state is aromatic in character (a Hückel system). It is thermally allowed suprafacially by the Woodward–Hoffmann rules. The [3,3] label itself is inherited machinery. So is the suprafacial/antarafacial bookkeeping that explains why heat, not light, drives it. The point of this article is to show concretely, atom by atom, how that machinery works. It focuses on these two specific and enormously useful reactions. It also shows how synthetic chemists have learned to bend the chair transition state to their will. This article builds the mechanism from first principles using curved arrows and orbital symmetry. It works through the aromatic transition-state model that explains rate and stereochemistry together. It derives the chair-vs-boat energy preference and the resulting stereochemical predictions explicitly. It then applies all of this to three worked examples. These span a textbook Cope rearrangement, an Ireland–Claisen used in real total synthesis, and a tandem anionic oxy-Cope problem of competition difficulty.

Key takeaways

The Cope rearrangement interconverts two 1,5-hexadienes via a [3,3]-sigmatropic shift: one σ-bond breaks, one σ-bond forms, and two π-bonds shift position, all in one step.
The Claisen rearrangement is the same shift applied to an allyl vinyl ether (O in place of one CH₂), converting it irreversibly into a γ,δ-unsaturated carbonyl compound.
Both go through a cyclic six-electron, six-atom transition state that is thermally allowed suprafacial–suprafacial, exactly analogous to the Diels–Alder transition state.
The transition state strongly prefers a chair geometry over a boat, typically by 5–10 kcal/mol, which makes the stereochemical outcome predictable from the alkene geometries.
Substituent effects (Ireland enolate geometry, oxy-anion acceleration, aromatic-ring re-aromatization in the aromatic Claisen) let chemists control rate, regiochemistry, and stereochemistry independently.
Key idea: a [3,3]-sigmatropic rearrangement is a six-electron pericyclic reaction whose chair-like transition state converts alkene/enol-ether geometry directly into product stereochemistry, with no intermediates to scramble the information.

Where this lives today

The Ireland–Claisen rearrangement is a standard tool in total synthesis for setting acyclic, 1,2- and 1,3-stereocenters with predictable relative configuration — it appears throughout polyketide and macrolide synthesis (erythromycin-class antibiotics, discodermolide-type cytotoxic natural products) precisely because propionate-derived natural products are built from repeating methyl-bearing stereocenters that this rearrangement installs directly and reliably. The Johnson–Claisen (orthoester Claisen) variant, which generates the vinyl ether component in situ from an allylic alcohol and a trialkyl orthoacetate, is widely used industrially because it avoids strong base and low temperatures, making it practical on large scale for fragrance and flavor synthesis as well as pharmaceutical intermediates. The aromatic Claisen rearrangement (allyl aryl ether to ortho-allylphenol) remains a textbook method for installing carbon substituents ortho to a phenol, relevant in the synthesis of natural phenolic and lignan-type structures. The anionic oxy-Cope rearrangement, because of its enormous rate acceleration, allows [3,3]-shifts to be run under mild conditions (often 0 °C to room temperature) compatible with sensitive functional groups elsewhere in a complex molecule, and is a workhorse step in the synthesis of complex polycyclic terpenoids and steroids, frequently combined in tandem with subsequent intramolecular reactions (Michael additions, aldol cyclizations) exploiting the freshly generated enolate. More broadly, the underlying orbital-symmetry logic connects directly to Suprafacial and Antarafacial Sigmatropic Shifts and to the general classification scheme of Sigmatropic Rearrangements: Order and Nomenclature, and the same chair/boat, substituent-mapping analysis that predicts Cope and Claisen stereochemistry is exactly the kind of reasoning invoked when planning a synthesis via Retrosynthetic Analysis: Disconnections and Synthons — recognizing a target's 1,2- or 1,3-stereorelationship as the "output" of a hidden [3,3]-shift is a standard, powerful retrosynthetic disconnection.

:::faq[What exactly does "[3,3]" mean in [3,3]-sigmatropic rearrangement?] The two numbers count atoms along each of the two chains extending from the breaking σ-bond to the atom where the new σ-bond forms, on each side. In a [3,3]-shift the migrating σ-bond breaks between atoms labeled "1" on each side, and the new σ-bond forms between the atoms three positions away on each chain (atom 3 and atom 3′), counting the original σ-bonded atoms as position 1. For both the Cope and Claisen rearrangements this means: start at the breaking C–C (or C–O) bond, count outward through the allylic system on each side, and the new bond forms exactly three atoms out on both sides — hence "[3,3]," distinct from a [1,3]-shift (bond migrates to an adjacent, not three-atoms-away, position) or a [2,3]-shift (as in the Wittig rearrangement).

Ask the tutor: How would you predict the product diastereomer of an Ireland–Claisen rearrangement starting from an (Z)-configured allylic alcohol combined with the (E)-silyl ketene acetal, using the chair transition-state model?

The physical picture

Both rearrangements involve a molecule with two multiple bonds separated by a single σ-bond and one intervening sp³ (or, in Claisen, an oxygen-bearing) atom on each side — precisely the connectivity required for a [3,3]-shift. Number the atoms 1–2–3–3′–2′–1′ so that the breaking σ-bond is C3–C3′ (Cope) or C3–O3′ (Claisen), and the two π-systems are C1=C2 and C1′=C2′. As the old σ-bond breaks, a new σ-bond forms between C1 and C1′, and the two π-bonds slide over by one position, becoming C2=C3 and C2′=C3′. All of this happens in a single transition state: no discrete diradical, no discrete ion pair, no ylide. The six atoms and six electrons involved (two π-bonds, one σ-bond) arrange themselves into a six-membered ring shape at the transition state, with the σ-bond that is breaking and the σ-bond that is forming both lying roughly parallel to the plane containing the two π-systems, so that continuous orbital overlap is possible all the way from starting material to product. The key geometric question is what shape does that six-membered transition-state ring take — chair or boat — because this single choice is what makes the stereochemistry predictable. Just as cyclohexane strongly prefers a chair over a boat because of eclipsing (torsional) interactions and 1,3-diaxial strain, the pericyclic transition state prefers a chair-like arrangement of the six atoms for exactly analogous steric reasons, even though the "bonds" in this ring are partly formed/breaking rather than fully single bonds. The energy difference between chair and boat transition states in the parent Cope rearrangement is about 5–6 kcal/mol, which at typical reaction temperatures (150–250 °C) translates into overwhelming (>98%) preference for whichever product the chair pathway delivers. Because the chair transition state is rigid and well-defined, the relative disposition of substituents on C1, C1′ (or wherever stereocenters end up) in the product is dictated entirely by (a) the E/Z geometry of the starting alkenes and (b) whether those substituents sit pseudo-equatorial or pseudo-axial in the chair. Pseudo-equatorial placement is strongly preferred (again for steric reasons, just as in cyclohexane), so a substituent's fate is fixed the moment you decide it will occupy the equatorial-like position in the lowest-energy chair.

:::svg[The [3,3]-sigmatropic array: atom numbering and electron flow in the Cope rearrangement]

Figure

Chair vs boat transition states: substituent fate in the Cope rearrangement

For the Claisen rearrangement of an allyl vinyl ether, the same six-membered chair applies, but position 3′ is replaced by oxygen: the breaking bond is C3–O, and the product is an enol that immediately tautomerizes to the much more stable carbonyl compound. This tautomerization is what makes the Claisen rearrangement thermodynamically and often kinetically irreversible — you trade a C–O σ-bond and a C=C π-bond for a C–C σ-bond and a C=O π-bond, and the C=O π-bond is much stronger. The Cope rearrangement, by contrast, connects two 1,5-hexadienes of comparable stability and is genuinely an equilibrium; it only goes to useful completion when the product is made more stable by ring strain relief (oxy-Cope, divinylcyclopropane), aromatization, or an anionic accelerant.

Figure

Claisen rearrangement: allyl vinyl ether to γ,δ-unsaturated carbonyl

The core ideas

The unifying statement, inherited from orbital-symmetry analysis of pericyclic reactions, is that a [3,3]-sigmatropic shift is a six-electron, suprafacial-suprafacial process, thermally allowed by the Woodward–Hoffmann rules:

[3,3]_{s,s}: \quad \text{thermally allowed (6 electrons, Hückel topology, suprafacial on both components)}

where: the two subscripts $s,s$ mean the σ-bond migrates across the same face of each π-system it interacts with — geometrically realized as the chair (or boat) six-membered transition state, not as some twisted Möbius array. The energetics of the rearrangement are captured by a standard Eyring-type expression for the rate constant, with the crucial point being that the activation entropy is large and negative because six atoms must be organized into one tight, ordered transition state:

k = \frac{k_B T}{h}\, e^{-\Delta G^{\ddagger}/RT}, \qquad \Delta G^{\ddagger} = \Delta H^{\ddagger} - T\Delta S^{\ddagger}

where: $k_B$ is Boltzmann's constant, $h$ is Planck's constant, and $\Delta S^{\ddagger}$ for the parent Cope rearrangement is about $-13$ to $-14$ cal/(mol·K) — strongly unfavorable, consistent with a single, highly ordered six-membered cyclic transition state and no molecularity change (unimolecular throughout). The chair/boat energy difference, which is the single number that makes stereopredictions possible, is expressed as a simple free-energy gap:

\Delta\Delta G^{\ddagger}_{\text{boat–chair}} \approx 5\text{–}6\ \text{kcal/mol at 25–250 °C, giving } K_{\text{eq(chair vs boat pathway)}} \gg 100:1

where: this ratio is obtained from $\Delta\Delta G^{\ddagger} = -RT\ln(k_{\text{chair}}/k_{\text{boat}})$ , and it is large enough that even at 250 °C (523 K) the chair pathway dominates by well over 99%.

Derivation AMechanism 1 — The Cope rearrangement of 3-methyl-1,5-hexadiene, chair TS, with explicit stereochemical bookkeeping

Goal. Show, arrow by arrow, how a single enantiomer of a substituted 1,5-hexadiene passes through a chair transition state to give one diastereomer of product predictably, and verify the outcome against the alternative (boat) pathway.

Step 1 — Identify the σ-bond and number the array. Take (S)-3-methyl-1,5-hexadiene: CH₂=CH–CH(CH₃)–CH₂–CH=CH₂. Label the atoms C1=C2–C3(–CH₃)–C4–C5=C6; here the migrating σ-bond is C3–C4, and the two π systems are C1=C2 and C5=C6 (renaming C4,C5,C6 as C1′,C2′,C3′ for symmetry with the general scheme: C3–C3′ ≡ C3–C4). Six atoms, six π/σ electrons: C1, C2, C3, C3′(=C4), C2′(=C5), C1′(=C6).

Step 2 — Draw the chair conformation of the six-atom array. Arrange C1–C2–C3–C3′–C2′–C1′ in a chair, exactly as you would draw cyclohexane, with the methyl substituent on C3 pointing pseudo-equatorial (the lower-energy choice by roughly 1.5–2 kcal/mol relative to pseudo-axial, just as in a cyclohexane ring).

Step 3 — Push the arrows. One curved arrow moves from the C3–C4 σ-bond into the new π-bond forming at C2–C3 (i.e., electron density flows toward C2=C3). A second arrow moves from the C1=C2 π-bond to form the new σ-bond C1–C1′. Symmetrically, on the other allyl fragment, the C4–C5 (≡C3′–C2′) bond becomes the new π-bond C2′=C3′, and the C5=C6 π-bond electrons flow to help form the same new C1–C1′ σ-bond from the other direction. All six electrons move in a closed loop, matching the aromatic six-membered Hückel array required for the thermally allowed pathway.

Step 4 — Read off the product connectivity. The new σ-bond is C1–C1′ (old terminal CH₂ carbons are now directly bonded), and the new π-bonds are C2=C3 and C2′=C3′ — i.e., the double bonds have moved inward by one position on each side, and the methyl group, formerly on the sp³ carbon C3, is now on an sp² alkene carbon (C3, now part of C2=C3). The product is 4-methyl-1,5-heptadiene-type skeleton correctly renumbered as 3-methyl-1,5-hexadiene's degenerate partner — for the parent (unsubstituted at C3) this rearrangement is degenerate (product = starting material), which is exactly why the parent Cope rearrangement was first detected using deuterium labeling, not by any change in gross structure.

Step 5 — Check the chair vs. boat prediction explicitly. In the chair TS, the pseudo-equatorial methyl on C3 ends up controlling the newly forming alkene geometry: because the C2–C3 bond becomes a π-bond and C3 was sp³ with the methyl equatorial, the methyl group is delivered specifically cis or trans (E or Z) on the new alkene depending on which face of the chair it occupied. Running the same substrate through the boat TS instead would deliver the opposite alkene geometry; since $\Delta\Delta G^{\ddagger}_{\text{boat-chair}} \approx 5$ –6 kcal/mol, the chair product dominates by a factor

\frac{k_{\text{chair}}}{k_{\text{boat}}} = e^{\Delta\Delta G^{\ddagger}/RT} = e^{5500/(1.987\times 523)} \approx e^{5.3} \approx 200

at a representative Cope temperature of 250 °C (523 K), confirming that the chair-derived alkene geometry dominates the product mixture (>99% chair-derived diastereomer).

Go deeper · Derivation B · 2 minMechanism 2 — The Ireland–Claisen rearrangement: enolate geometry controls product stereochemistry

Derivation BMechanism 2 — The Ireland–Claisen rearrangement: enolate geometry controls product stereochemistry

Goal. Show how deprotonating an allylic ester to a specific enolate geometry (E or Z) and silylating it converts the Claisen rearrangement into a fully stereospecific carbon–carbon bond-forming reaction, used constantly in total synthesis to set adjacent stereocenters.

Step 1 — Form the ester. Start from an allylic alcohol, e.g. (E)-crotyl alcohol esterified with propanoic acid to give the allylic propanoate ester: CH₃CH₂C(=O)–O–CH₂–CH=CH–CH₃ (E-alkene). This ester contains the allyl vinyl ether pattern once it is converted to its enolate (the C=O becomes a vinyl ether-like enolate C=C–O⁻).

Step 2 — Deprotonate stereoselectively (kinetic enolization). Treat with LDA (a bulky, non-nucleophilic base) at low temperature (–78 °C) in THF, optionally with HMPA. Under Ireland's model, kinetic deprotonation through a Zimmerman–Traxler-like six-membered TS (the base removing the α-proton while the ester carbonyl oxygen coordinates lithium) gives, in THF alone, predominantly the Z(O)-enolate (using the convention where the OLi and the α-substituent are cis); adding HMPA to disrupt the chelated TS flips the selectivity to favor the E(O)-enolate. This is the crucial "dial": enolate geometry is chosen at will by choice of solvent/additive.

Step 3 — Trap the enolate as the silyl ketene acetal. Add TMSCl (or TBSCl) to trap the lithium enolate before it can equilibrate, locking in the enolate geometry as a silyl ketene acetal: CH₃CH=C(OTMS)–O–CH₂–CH=CH–CH₃. This is now a genuine allyl vinyl ether (the "vinyl" part is the ketene acetal C=C–OTMS), perfectly set up for a [3,3]-shift.

Step 4 — Warm to trigger the [3,3]-shift. On warming to room temperature or slightly above, the silyl ketene acetal undergoes the same six-electron, chair-preferring [3,3] rearrangement as any Claisen system: the σ-bond that breaks is C(allylic)–O, the new σ-bond that forms is C(α to former carbonyl)–C(terminal allylic carbon), and the two π-systems both shift by one position. Curved arrows: one arrow from the O–CH₂ σ-bond becomes the new π-bond of the forming carbonyl (after aqueous workup hydrolyzes the silyl enol ether); one arrow from the ketene acetal C=C moves to form the new C–C σ-bond; one arrow from the allylic C=C moves to become the new alkene.

Step 5 — Map enolate + alkene geometry onto product stereochemistry via the chair. Build the chair with the (Z)- or (E)-silyl ketene acetal geometry and the (E)-crotyl alkene geometry both placed to minimize 1,3-diaxial-like interactions — both large groups pseudo-equatorial. For the (Z)-ketene acetal combined with the (E)-allylic alkene, the chair places the α-methyl (destined to be the new stereocenter α to the carboxylic acid) and the ring-forming carbon substituents in a defined relative sense, delivering the syn (or anti, depending on the exact convention used for that specific system) diastereomer of the 2-methyl-3-pentenoic acid product essentially exclusively (typically >95:5 dr). Flipping to the (E)-ketene acetal (via HMPA) while keeping the same allylic alkene geometry flips the diastereomer obtained, because the same chair now places the methyl on the opposite face. This is the operational heart of the Ireland–Claisen: enolate geometry, chosen by solvent, dictates product diastereomer, chosen by chair-TS geometry — a fully predictive, non-empirical design principle.

Worked examples

Example 1 (foundational): Degeneracy and the deuterium-labeling proof for the Cope rearrangement of 1,5-hexadiene. The parent Cope rearrangement, CH₂=CH–CH₂–CH₂–CH=CH₂ (1,5-hexadiene) rearranging to "itself," is a textbook case precisely because the starting material and product are constitutionally identical — the [3,3]-shift interconverts C1–C6 with C6–C1 and C3–C4 σ-bond with a new C1–C6 σ-bond, but by the molecule's own symmetry this looks like doing nothing. To prove the rearrangement is real (rather than assuming no reaction occurs because nothing changes), Doering and Roth (1962) synthesized 3,4-dideuterio-1,5-hexadiene, in which C3 and C4 each carry a deuterium in place of one hydrogen. Heating this labeled compound at 225 °C causes the label to migrate: deuteriums that started at the central (sp³, C3/C4) positions end up, after rearrangement, at the terminal vinyl (C1/C6) positions, because the [3,3]-shift moves the σ-bond from C3–C4 to C1–C6 while sliding the π-bonds inward. Monitoring this isotopic scrambling by NMR gave a clean first-order rate constant,

k_{225^\circ\text{C}} \approx 1.7\times10^{-5}\ \text{s}^{-1}, \qquad \Delta H^{\ddagger} \approx 33.5\ \text{kcal/mol}, \qquad \Delta S^{\ddagger} \approx -13.8\ \text{cal/(mol}\,\cdot\,\text{K)}

The large negative $\Delta S^{\ddagger}$ is exactly what a single, highly ordered cyclic six-membered transition state predicts (compare to a bimolecular Diels–Alder cycloaddition, which has comparably large negative activation entropy for the same reason — two independent fragments becoming one rigid ring in the transition state). As a limiting-case check: if the rearrangement instead proceeded through two independent, freely rotating allyl radicals (a stepwise homolysis/recombination pathway), $\Delta S^{\ddagger}$ would be expected to be strongly positive for the bond-breaking step (increased rotational freedom on generating two radical fragments) — the observed strongly negative value rules this stepwise mechanism out and confirms the concerted, cyclic, chair-like six-membered pathway.

Example 2 (applied): An Ireland–Claisen rearrangement used to set a quaternary-adjacent stereocenter in synthesis. Consider the propanoate ester of (E)-hex-3-en-1-ol substituted so that the α-carbon of the ester already bears a methyl group (i.e., a 2-methylpropanoate ester of a secondary allylic alcohol), a common motif when building propionate-derived natural product fragments (e.g., polyketide synthesis). Deprotonation with LDA/THF at –78 °C selects the chelated, Zimmerman–Traxler-like transition state and delivers the Z(O)-silyl ketene acetal preferentially (typically 90:10 to >95:5 Z:E depending on ester substitution), trapped in situ with TBSCl. Warming the ketene acetal to 60–110 °C (Ireland–Claisen conditions are milder than the classical aromatic Claisen because the silyl ketene acetal is a more electron-rich, more reactive dienophile-like component) drives the [3,3]-shift through the chair transition state with the newly forming C–C bond, the α-methyl group, and the alkene substituent all arranging pseudo-equatorially. The result is the 2,3-syn (or anti, by the specific convention for that system) diastereomer of the carboxylic acid product with a newly formed C–C bond and two contiguous stereocenters, obtained in a single pot, in high diastereomeric ratio, and with absolute stereochemical predictability from the enolate geometry alone — no chiral catalyst required, because the chirality transfer is a purely geometric consequence of the chair TS. This exact strategy (Ireland–Claisen, chair TS, chirality transfer from an existing allylic stereocenter or from enolate geometry) appears repeatedly in the synthesis of macrolide antibiotics and polyketide natural products, where 1,2- and 1,3-relationships between methyl-bearing stereocenters must be set with high fidelity.

Example 3 (exam/competition level): Anionic oxy-Cope rearrangement — rate acceleration and its origin. A 3-hydroxy-1,5-hexadiene (a 1,5-hexadiene bearing an OH at C3) undergoes the ordinary, neutral Cope rearrangement only sluggishly, requiring high temperatures. But deprotonating the C3-OH to the alkoxide with KH (often with 18-crown-6 to fully dissociate the potassium counterion and maximize charge on oxygen) accelerates the same [3,3]-shift by a factor of $10^{10}$ to $10^{17}$ relative to the neutral alcohol or the parent hydrocarbon — the "anionic oxy-Cope" effect (Evans, 1980). The mechanistic reason: after the shift, the alkoxide at C3 becomes an enolate oxygen (conjugated to the newly formed C2=C3 π-bond), which is dramatically more stable than an isolated, non-conjugated alkoxide. Because the transition state is "product-like" enough to feel much of this extra stabilization (an early reflection of the eventual enolate resonance), the activation barrier drops steeply — this is a rate acceleration driven by transition-state stabilization from developing negative-charge delocalization, exactly analogous to how an adjacent carbanion-stabilizing group accelerates other pericyclic and non-pericyclic reactions. Set up concretely: 1-phenyl-1-(1-hydroxy-allyl... more simply, take 3-hydroxy-1,5-hexadien-3-yl system where C3 bears OH, a phenyl group, and is flanked by a vinyl group, i.e., 1-phenyl-1-vinyl-2-propenylcarbinol-type substrate PhC(OH)(CH=CH₂)(CH₂–CH=CH₂). Treating with KH/18-crown-6 in THF generates the potassium alkoxide; the [3,3]-shift proceeds through the same chair transition state used throughout this article, with the developing enolate oxygen occupying the position that best conjugates with the newly forming alkene. The product, after aqueous workup, is the corresponding δ,ε-unsaturated ketone (the enolate tautomerizes/protonates to the carbonyl compound) with the phenyl group and the new alkene geometry set by the chair. A rough Hammett-type / Eyring estimate: if $\Delta G^{\ddagger}_{\text{neutral}} \approx 33$ kcal/mol at 250 °C and the anionic acceleration lowers this by, say, 20 kcal/mol (a typical experimental magnitude for well-stabilized cases), the rate ratio is

\frac{k_{\text{anionic}}}{k_{\text{neutral}}} = e^{\Delta\Delta G^{\ddagger}/RT} = e^{20{,}000/(1.987\times 523)} \approx e^{19.3} \approx 2\times10^{8}

which already illustrates, with entirely non-round numbers, why anionic oxy-Cope rearrangements routinely proceed at or below room temperature where the neutral version would require hours at 200–300 °C. As a limiting-case check: if the alkoxide is instead tightly ion-paired to a small, hard cation (Li⁺ without crown ether), much of the negative charge is quenched by ion pairing rather than being available to stabilize the developing enolate, and the acceleration factor drops by several orders of magnitude relative to the fully dissociated K⁺/18-crown-6 case — consistent with the "free alkoxide" origin of the effect.

Limiting cases, scope & common traps

Situation	What happens	Why
Parent 1,5-hexadiene, no substituents	Cope rearrangement is degenerate (product = starting material)	Only detectable via isotopic (D) labeling; proves mechanism without changing gross structure
Allyl vinyl ether (Claisen)	Irreversible, drives to γ,δ-unsaturated carbonyl	Enol → ketone/aldehyde tautomerization is strongly downhill (C=O >> C=C in bond strength)
3-hydroxy-1,5-hexadiene, deprotonated (anionic oxy-Cope)	Rate acceleration of $10^{10}$ – $10^{17}$	Developing enolate at C3 stabilizes the transition state; charge delocalization, not simple inductive effect
cis,cis-1,5-cyclodecadiene / divinylcyclopropane	Cope proceeds at or below room temperature	Ring strain in starting material is relieved dramatically in the ring-expanded product (thermodynamic driving force)
Aromatic Claisen (allyl phenyl ether)	[3,3]-shift to ortho-allylphenol, followed by tautomerization/[1,3]-shift if both ortho positions are blocked	Re-aromatization (rearomatized dienone → phenol) is the thermodynamic driving force; a formal [1,3] shift can follow if direct tautomerization is blocked
Boat transition state forced (e.g., by ring constraints in medium rings)	Alternative, sometimes opposite, diastereomer obtained	Ring size can make the chair geometrically inaccessible; always check whether the substrate can actually adopt a chair before predicting stereochemistry
Ireland–Claisen, THF vs. THF/HMPA	Enolate geometry flips Z(O) ↔ E(O)	HMPA disrupts chelated (Ireland) TS, changing kinetic deprotonation face

The chemist behind it

Origins · 1 minLudwig Claisen and how this was discovered

Ludwig Claisen

1851–1930 · German organic chemistry, late 19th–early 20th century, era of classical structural organic synthesis

Ludwig Claisen was a master of practical, bench-driven organic chemistry working in an era before quantum mechanical rationalizations of pericyclic reactions existed at all — Woodward and Hoffmann's orbital-symmetry treatment was seventy years in Claisen's future. He discovered both the base-mediated ester condensation that bears his name and, separately in 1912, the thermal rearrangement of allyl vinyl ethers and allyl aryl ethers to carbonyl compounds and ortho-allylphenols respectively. Claisen had no mechanistic tool beyond careful product analysis, yet he correctly identified the connectivity change (allyl group migrating from oxygen to carbon) purely from isolating and characterizing products — the six-membered cyclic transition-state picture, and its Woodward–Hoffmann justification, were retrofitted decades later once orbital symmetry arguments became available. Arthur Cope, working independently at MIT in the 1940s, discovered the analogous all-carbon rearrangement of 1,5-dienes, and it was only in the 1960s (Doering and Roth's deuterium-labeling kinetics) that the concerted, single-transition-state nature of both reactions was placed on rigorous mechanistic footing.

History

- 1912 — Ludwig Claisen reports the thermal rearrangement of allyl vinyl ethers and allyl aryl ethers.

- 1940 — Arthur C. Cope reports the rearrangement of 1,5-dienes (the "Cope rearrangement").

- 1962 — W. von E. Doering and W. R. Roth use deuterium labeling and kinetics to establish the concerted, chair-preferring mechanism of the Cope rearrangement.

- 1965–1970 — Woodward and Hoffmann's orbital-symmetry rules place [3,3]-sigmatropic shifts on a rigorous theoretical footing as thermally allowed suprafacial-suprafacial processes.

- 1972–1976 — Robert Ireland develops the Ireland–Claisen rearrangement (silyl ketene acetal Claisen), giving synthetic chemists direct control of enolate geometry and hence product diastereomer.

- 1980 — David Evans reports the anionic oxy-Cope rearrangement, demonstrating rate accelerations of many orders of magnitude via alkoxide-driven transition-state stabilization.

Why is heat required and not light, given that six π-electrons are involved just like some photochemical reactions?

Orbital symmetry analysis shows that a [3,3]-sigmatropic shift with suprafacial–suprafacial geometry (the geometrically accessible chair or boat pathway) is thermally allowed because it has six electrons in a Hückel (untwisted) topology — the same reasoning that makes thermal Diels–Alder cycloadditions (also six electrons, Hückel, suprafacial-suprafacial) allowed under heat rather than light. Photochemical excitation would instead favor an antarafacial component somewhere in the array, which is geometrically very difficult to achieve for a simple acyclic 1,5-diene or allyl vinyl ether, so photochemical [3,3]-shifts of this type are rare compared to their thermal counterparts.

Can the Cope rearrangement be catalyzed, or does it always require high temperature?

The neutral, all-carbon Cope rearrangement typically requires 150–250 °C because the activation enthalpy (around 33 kcal/mol for the parent system) is intrinsically high — there is no low-energy catalytic pathway analogous to acid/base catalysis for simple addition reactions, since no bond is fully ionic at any point. However, chemists have learned to accelerate it dramatically by changing the substrate rather than adding a catalyst: generating an alkoxide adjacent to the rearranging system (anionic oxy-Cope) can lower the barrier by 15–20+ kcal/mol, and building in ring strain that is relieved by the rearrangement (as in divinylcyclopropane-to-cycloheptadiene rearrangements) can bring the reaction down to room temperature or below.

What is the difference between the Ireland–Claisen and the Johnson–Claisen rearrangements?

Both convert an allylic alcohol into a functionalized carboxylic acid derivative via a [3,3]-shift, but they build the required vinyl ether component differently. The Ireland–Claisen esterifies the allylic alcohol, then deprotonates the ester to a lithium enolate (with controllable E/Z geometry) and traps it as a silyl ketene acetal before the thermal [3,3]-shift. The Johnson–Claisen instead condenses the allylic alcohol directly with a trialkyl orthoacetate under mild acid catalysis, generating a mixed orthoester that collapses (losing one equivalent of alcohol) to a vinyl ether in situ, which then undergoes the same [3,3]-shift — this route avoids strong base entirely and is often preferred industrially for its operational simplicity, though it offers less direct control over enolate geometry than the Ireland variant.

Does the Cope rearrangement ever proceed through a boat transition state in practice?

Yes, when the substrate is constrained (typically by a ring) so that a chair geometry is geometrically inaccessible. The classic example is cis,cis-1,5-cyclooctadiene-type or bicyclic systems where the tether length and existing ring fusion force the six-atom pericyclic array into a boat-like arrangement; in such cases the product stereochemistry follows the boat-TS prediction instead of the default chair prediction, which is exactly why you must draw out the actual accessible geometry for constrained or cyclic substrates rather than assuming the chair rule applies universally.

How does the aromatic Claisen rearrangement of allyl phenyl ether differ mechanistically from the aliphatic case?

The initial [3,3]-shift step is mechanistically identical — a chair-like, six-electron pericyclic transition state moving the allyl group from the ether oxygen to the ring carbon ortho to it, producing a non-aromatic cyclohexadienone intermediate. The key difference is what happens next: this dearomatized intermediate rapidly tautomerizes (a simple proton transfer, not pericyclic) back to an aromatic phenol, now bearing the allyl group at the ortho position, because rearomatization is a huge thermodynamic driving force. If both ortho positions are already blocked by substituents, the initial [3,3]-product can undergo a further formal [1,3]-shift (via a second, often stepwise or dissociative pathway) to deliver the para-allylphenol instead, but this second step is mechanistically distinct from the clean concerted [3,3]-shift.