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Sulfoxide activation rearrangement

Pummerer rearrangement

How the Pummerer rearrangement works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.

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In short. A 3-step mechanism. Typical reagents: Ac₂O (or TFAA); AcO⁻ as base/nucleophile. Conditions: Heat or 0–25 °C in CH₂Cl₂. Bonds formed: C–O (α-acyloxy); C=S⁺ (thionium); bonds broken: S=O (as AcO leaving); α C–H. Stereochemistry: α-Carbon often racemic. Rate and selectivity: Deprotonation/elimination to the thionium is rate-limiting.

At a glance

ReagentsAc₂O (or TFAA); AcO⁻ as base/nucleophile
Solvent / conditionsHeat or 0–25 °C in CH₂Cl₂
Bonds formedC–O (α-acyloxy); C=S⁺ (thionium)
Bonds brokenS=O (as AcO leaving); α C–H
Stereochemistryα-Carbon often racemic
Rate / selectivityDeprotonation/elimination to the thionium is rate-limiting

Step 1: Sulfoxide O is acylated

++SOOOOδ⁻SOOOO1 · SULFOXIDE O IS ACYLATEDmethyl phenyl sulfoxideC₇H₈OSNUCLEOPHILEacetic anhydrideC₄H₆O₃ELECTROPHILEacyloxysulfoniumC₉H₁₁O₂S⁺INTERMEDIATEacetateC₂H₃O₂⁻BYPRODUCTS=O oxygen is the nucleophile; the acyloxy group is now an excellent leaving group.

An electrophile (commonly acetic anhydride) activates a sulfoxide's oxygen, converting it into a good leaving group — the sulfur-oxygen analog of protonating a carbonyl.

Step 2: α-Deprotonation, loss of AcO⁻

+++OOHSOOSOOHOO2 · Α-DEPROTONATION, LOSS OF ACO⁻acetate (base)C₂H₃O₂⁻BASEacyloxysulfoniumC₉H₁₁O₂S⁺INTERMEDIATEthionium ionC₇H₇S⁺CATIONacetic acidBYPRODUCTacetateLEAVING GROUPSulfur lone pair stabilizes the carbocation: C=S⁺ is the thionium resonance form.

Base removes a proton from the carbon alpha to sulfur as the activated oxygen leaves, forming a resonance-stabilized thionium ion (a sulfur-stabilized carbocation) in a single concerted-feeling elimination.

Step 3: Nucleophile traps the thionium

+OOSOOS3 · NUCLEOPHILE TRAPS THE THIONIUMacetateC₂H₃O₂⁻NUCLEOPHILEthionium ionC₇H₇S⁺CATIONα-acetoxy sulfideC₉H₁₀O₂SPRODUCTNet: sulfur is reduced (S=O → S), the α-carbon is oxidized (C–H → C–OAc).

The departed acetate (or another nucleophile present) attacks this thionium ion's carbon, giving an α-acyloxy (or α-substituted) sulfide — net effect: the oxidation state has moved from sulfur onto the adjacent carbon, a useful way to functionalize a position alpha to sulfur.

Abbreviations and terms

OAc
Acetoxy. –OC(=O)CH₃, an acetate group.
DCM
Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
Ac2O
Acetic anhydride. Acetylating agent.

How it compares with related reactions

Pummerer rearrangementBoekelheide rearrangementBamberger rearrangement
ReagentsAc₂O (or TFAA); AcO⁻ as base/nucleophile2-Methylpyridine N-oxide, Ac₂O (or TFAA); then aqueous baseN-Phenylhydroxylamine, dilute H₂SO₄ (aq)
ConditionsHeat or 0–25 °C in CH₂Cl₂Ac₂O, reflux (TFAA works at 25 °C)H₂O, 25–60 °C
Stereochemistryα-Carbon often racemicNone (achiral product)None; para-selective
Rate / selectivityDeprotonation/elimination to the thionium is rate-limitingN-acylation fast; [3,3] shift rate-limitingLoss of water to give the nitrenium ion is rate-limiting

Test yourself

Try to answer each question before opening it.

Which reagents and conditions are typical for the Pummerer rearrangement?

Ac₂O (or TFAA); AcO⁻ as base/nucleophile. Conditions: Heat or 0–25 °C in CH₂Cl₂.

Which bonds are formed and which are broken?

Formed: C–O (α-acyloxy); C=S⁺ (thionium). Broken: S=O (as AcO leaving); α C–H.

What is the stereochemical outcome?

α-Carbon often racemic

What controls the rate or the selectivity?

Deprotonation/elimination to the thionium is rate-limiting

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