Organic Chemistry · Orbitals and Organic Chemistry: Pericyclic Reactions

Some Examples of Sigmatropic Rearrangements

9 min read

Lab safety note: none — these reactions are discussed as chemistry concepts; any laboratory work would require proper heat control, solvent handling, and PPE per institutional rules (general principles only). The Cope and Claisen rearrangements, the oxy-Cope variant, the ortho/para selectivity of the aromatic Claisen, and the previtamin D₃ → vitamin D₃ [1,7]-H shift are standard textbook organic/biochemistry (the vitamin D pathway is described qualitatively; no clinical dosing claims are made).

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Topic 7 introduced the rules; this topic shows the reactions. The most important sigmatropic rearrangements in practice are the [3,3] shifts — the of a 1,5-diene and the of an allyl vinyl ether — plus the biological [1,7]-hydrogen shift that converts previtamin D to vitamin D in your skin. Together they illustrate everything the selection rules predict: six-electron transition states that are thermally allowed, chair-like geometry that controls stereochemistry, and rearrangements that convert O–C bonds into C–C bonds.

The Cope rearrangement is the textbook example: heating 3-methyl-1,5-hexadiene gives 1,5-heptadiene, a constitutional isomer with the same formula. Nothing is gained or lost — the σ bond between C3 and C4 migrates to become a bond between C1 and C6 while the double bonds move inward. The Claisen rearrangement starts from an allyl vinyl ether: the oxygen–carbon σ bond breaks, and a new carbon–carbon bond forms at the far end of the allyl group, producing a compound (an aldehyde or ketone with a double bond three carbons away from the carbonyl).

Why this matters

C–C bond formation without reagents. The Claisen rearrangement builds a new carbon–carbon bond using only heat — no acid, base, or catalyst — and it is a cornerstone of natural-product synthesis.
Stereochemical control. Both [3,3] rearrangements pass through chair-like transition states, so the stereochemistry of the starting material is transferred to the product predictably (the "memory" of the reaction).
Vitamin D synthesis in your body. The photochemical ring opening of 7-dehydrocholesterol gives previtamin D, which then undergoes a thermal [1,7]-hydrogen sigmatropic shift to become vitamin D₃. Your own skin runs a sigmatropic rearrangement every time you spend time in sunlight.
Exam relevance. Problems ask you to (a) identify the rearrangement type from starting and product structures, (b) predict the product of a Cope or Claisen rearrangement, and (c) explain the stereochemical outcome.

The college version

Core Concepts

The Cope rearrangement: [3,3] of a 1,5-diene

A 1,5-diene with the general carbon skeleton C=C–C–C–C=C rearranges on heating (typically 150–300 °C) to a different 1,5-diene. The migrating σ bond is the central C3–C4 bond; in the transition state, all six carbons lie in a chair-like ring with the electrons of the two double bonds and the breaking/forming σ bond delocalized. Because the product is also a 1,5-diene, the Cope rearrangement is thermodynamically driven only by substitution or strain differences — for simple, symmetric dienes it is nearly thermoneutral, and the reaction is pulled forward by relief of strain (e.g., in small rings) or by forming more substituted alkenes.

The oxy-Cope variant (a 1,5-diene with an –OH on one terminus, giving an enol that tautomerizes to a ketone or aldehyde) is irreversible because the final carbonyl is much more stable, and it runs at much lower temperatures.

The Claisen rearrangement: [3,3] of an allyl vinyl ether

In the Claisen rearrangement, an allyl vinyl ether — structure CH₂=CH–O–CH₂–CH=CH₂ — converts to a γ,δ-unsaturated carbonyl compound. Mechanistically it is identical to the Cope: a [3,3] shift through a chair transition state. The difference is that one of the double bonds is a C=C–O (vinyl ether) system; after the shift, the oxygen becomes a carbonyl (C=O) through keto–enol tautomerization, and a new C–C bond forms where the O–C bond was.

The is the classic example: heating allyl phenyl ether gives o-allylphenol (the ortho product), because the enone-like intermediate tautomerizes to regenerate the aromatic ring. If both ortho positions are blocked, the para product forms via two consecutive [3,3] shifts (the allyl group first goes to the ortho position, then a second Cope-like rearrangement delivers it to para).

The biological [1,7]-hydrogen shift: making vitamin D

Sunlight converts 7-dehydrocholesterol (a steroid with a conjugated diene in ring B) into by a photochemical electrocyclic ring opening (Topic 4's chemistry in action). Previtamin D₃ is not yet vitamin D: a thermal [1,7]-hydrogen sigmatropic shift moves a hydrogen across the seven-atom triene system, and a subsequent double-bond shift produces vitamin D₃ (cholecalciferol). This is a genuine, well-studied example of a sigmatropic rearrangement running in human biochemistry.

Chair transition states and stereochemical memory

Both [3,3] rearrangements prefer a chair-like transition state because it minimizes eclipsing and allows substituents to sit equatorial. The stereochemical consequence: a chiral allyl group transfers its configuration to the product, and (E)- vs (Z)-alkene geometry is preserved in the new double bond. This "memory effect" is why the Claisen and Cope rearrangements are used to build stereocenters predictably.

Choosing the product: counting and drawing

To predict the product of a [3,3] rearrangement: (1) identify the six atoms of the allyl/vinyl array; (2) break the bond that connects the two three-atom fragments; (3) reform it between the two terminal atoms, shifting the double bonds one position inward; (4) for Claisen, tautomerize the enol to the carbonyl. Practicing this atom-shuffling on paper is the fastest way to master the reactions.

Common Confusions

Do Not Confuse	With	Difference
Cope rearrangement	Claisen rearrangement	Both are [3,3], but Cope starts from a 1,5-diene (C–C migrating bond) and gives another 1,5-diene; Claisen starts from an allyl vinyl ether (O–C migrating bond) and gives a carbonyl compound
[3,3] shift	[1,3] shift	Count atoms on both sides of the migrating bond: [3,3] has three atoms each side (six-electron, thermally allowed); [1,3] has one and three (four-electron, thermally forbidden)
γ,δ-Unsaturated carbonyl	α,β-Unsaturated carbonyl	In the Claisen product the C=C is at the γ,δ position (three bonds from the carbonyl); α,β means adjacent to the carbonyl — different isomers with different reactivity
Previtamin D₃	Vitamin D₃	Previtamin D₃ is the photochemical ring-opened product; vitamin D₃ forms from it by the thermal [1,7]-H sigmatropic shift
Claisen product (carbonyl)	Starting enol ether	The enol ether is the starting material; the carbonyl compound is the product — after tautomerization, the O is double-bonded to carbon

Eli explains

The same idea, in plain words

Explain it like I’m 10

The Cope and Claisen rearrangements are like a line of six dancers holding hands who flip the middle pair of hands to the ends of the line — everyone keeps their partners, but the handholds move. For Claisen, one dancer's grip on oxygen changes into a grip on carbon, which makes the molecule's shape lock into a much stronger form (a carbonyl).

Worked example

Example 1: Predicting the Cope product

Problem. 3-Methyl-1,5-hexadiene (CH₂=CH–CH(CH₃)–CH₂–CH=CH₂) is heated. Predict the product.

Step 1 — Identify the [3,3] array. Number the six carbons: C1=C2–C3–C4–C5=C6. The migrating bond is C3–C4.

Step 2 — Shift the bonds. Break C3–C4; the C1=C2 double bond moves to C2–C3, and the C5=C6 double bond moves to C4–C5; the new σ bond forms between C1 and C6.

Step 3 — Re-draw with the substituent. The methyl on C3 moves with that carbon; C3 becomes the terminal vinyl carbon of the product chain, so the methyl now sits on the terminal alkene carbon.

Answer. 1,5-Heptadiene (CH₂=CH–CH₂–CH₂–CH₂–CH=CH₂ with the methyl relocated): the product is the isomeric 1,5-diene CH₂=CH–CH₂–CH₂–CH=CH–CH₃, i.e., 1,5-heptadiene (methyl now on the far alkene carbon).

Example 2: Predicting the Claisen product

Problem. Allyl vinyl ether (CH₂=CH–O–CH₂–CH=CH₂) is heated. Predict the organic product.

Step 1 — Identify the array. The six-atom chain is O–C(=C)–C... counting from the vinyl ether: C=C–O–CH₂–CH=CH₂. The migrating bond is the O–CH₂ bond.

Step 2 — Do the [3,3] shift. The O–C bond breaks; a new C–C bond forms between the vinyl ether's terminal carbon and the allyl group's terminal carbon; the double bonds shift inward.

Step 3 — Tautomerize. The initial product is an enol, which rapidly tautomerizes to the more stable carbonyl.

Answer. Pent-4-enal: CH₂=CH–CH₂–CH₂–CHO (a γ,δ-unsaturated aldehyde). The oxygen ends up as the aldehyde carbonyl, and the new C–C bond is between the former vinyl-ether terminal carbon and the former allyl terminal carbon.

Example 3: Explaining the ortho selectivity of the aromatic Claisen

Problem. Allyl phenyl ether is heated. Why is the major product o-allylphenol rather than an open-chain ketone?

Step 1 — Recognize the substrate. The aromatic ring makes the system a vinyl ether whose "double bond" is part of the aromatic ring.

Step 2 — Do the [3,3] shift. The allyl group migrates to the ortho carbon of the ring, giving a cyclohexadienone-like intermediate (an enone).

Step 3 — Tautomerize to regain aromaticity. The intermediate enol tautomerizes, restoring the aromatic ring and placing an –OH on the ring carbon adjacent to the allyl group.

Answer. The product is o-allylphenol; aromaticity is recovered by keto–enol tautomerization, which is strongly favorable and drives the reaction. If both ortho positions are substituted, a second [3,3] shift delivers the allyl group to the para position.

Key takeaways

Cope: [3,3] shift of a 1,5-diene → isomeric 1,5-diene; thermally allowed; often reversible and near-thermoneutral; driven by strain relief or alkene substitution.
Oxy-Cope: 1,5-diene with –OH at a terminus → enol → carbonyl; irreversible, lower temperature.
Claisen: allyl vinyl ether → γ,δ-unsaturated carbonyl (aldehyde or ketone); O–C bond replaced by a new C–C bond; proceeds via enol tautomerization.
Aromatic Claisen: allyl phenyl ether → o-allylphenol (para if ortho blocked, via two [3,3] shifts).
Both [3,3] shifts go through chair-like six-membered transition states, transferring alkene geometry and chiral information to the product.
Biological example: previtamin D₃ → vitamin D₃ by a thermal [1,7]-hydrogen sigmatropic shift, following the photochemical ring opening of 7-dehydrocholesterol in skin.
Exam trap: a [3,3] rearrangement is NOT a [1,3] shift — count the atoms on both sides of the migrating bond.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

What is the migrating bond in a Cope rearrangement, and what functional group does the product retain?
Show answer
The central C3–C4 σ bond of the 1,5-diene; the product retains the 1,5-diene functional group (an isomerization).
What functional group transformation does a Claisen rearrangement accomplish overall?
Show answer
An allyl vinyl ether (O–C bond) becomes a γ,δ-unsaturated carbonyl compound (new C–C bond) — overall, an O–C to C–C bond conversion with a carbonyl formed by enol tautomerization.
Why is the irreversible while a simple Cope is often reversible?
Show answer
The oxy-Cope gives an enol that tautomerizes to a much more stable carbonyl, making the overall process strongly exergonic; a simple Cope interconverts two similar 1,5-dienes and is nearly thermoneutral.
When allyl phenyl ether is heated, why is the ortho product favored, and what happens if both ortho positions are blocked?
Show answer
The first [3,3] shift delivers the allyl group to the ortho carbon; aromatization by tautomerization drives it. If both ortho positions are occupied, a second [3,3] shift moves the group to the para position.
Which biological molecule is made from previtamin D₃ by a sigmatropic rearrangement, and what shift is it?
Show answer
Vitamin D₃ (cholecalciferol), formed by a thermal [1,7]-hydrogen sigmatropic shift.
Draw (in words) the atom-shuffling steps that convert allyl vinyl ether to pent-4-enal.
Show answer
Break the O–CH₂ bond; form a new C–C bond between the vinyl-ether terminal carbon and the allyl terminal carbon; shift the double bonds inward; tautomerize the enol to the aldehyde (pent-4-enal, CH₂=CH–CH₂–CH₂–CHO).

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Key vocabulary

Cope rearrangement: [3,3] sigmatropic shift of a 1,5-diene to an isomeric 1,5-diene
Claisen rearrangement: [3,3] shift of an allyl vinyl ether to a γ,δ-unsaturated carbonyl
Oxy-Cope rearrangement: Cope variant with –OH on a terminus; gives an enol that tautomerizes
Aromatic Claisen rearrangement: Claisen shift of allyl phenyl ether, giving o- or p-allylphenol
γ,δ-Unsaturated carbonyl: Carbonyl with a C=C three carbons away (counting the carbonyl carbon as 1)
Previtamin D₃: Photochemical product of 7-dehydrocholesterol in skin

Sources & references

openstax.org — Organic Chemistry

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