Chain-walking thioamide synthesis
Willgerodt-Kindler reaction
How the Willgerodt-Kindler reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Aryl alkyl ketone, S₈, secondary amine (morpholine). Conditions: Neat or DMF, 120–160 °C. Bonds formed: C=S (thioamide); C–N; H₂S; bonds broken: C=O; chain C–H bonds. Stereochemistry: Achiral product; carbonyl carbon ends as CH₂. Rate and selectivity: Slow; high T, excess amine; microwave accelerates.
At a glance
Step 1: Ketone + morpholine → enamine
An aryl alkyl ketone condenses with a secondary amine (like morpholine) and elemental sulfur; the amine first forms an enamine with the ketone — ordinary enamine-forming condensation.
Step 2: Sulfur adds, C=N⁺ migrates down the chain
Through a cascade of sulfur insertions and imine/enamine tautomerizations (a full step-by-step mechanism that remains only partly settled even today), the carbonyl's original position 'walks' down the alkyl chain to the far, terminal carbon.
Step 3: Thioamide at the chain terminus
The result is a thioamide with the carbonyl-derived functionality relocated to the chain's terminus — a strikingly unusual transformation where the net constitutional change (moving a functional group down a chain) is far easier to see than the many small mechanistic steps that actually accomplish it.
Abbreviations and terms
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
How it compares with related reactions
| Willgerodt-Kindler reaction | Aza-Wittig reaction | Baeyer-Drewson indigo synthesis | |
|---|---|---|---|
| Reagents | Aryl alkyl ketone, S₈, secondary amine (morpholine) | R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone | o-Nitrobenzaldehyde, acetone, aq. NaOH |
| Conditions | Neat or DMF, 120–160 °C | THF or toluene, 25–110 °C, anhydrous | H₂O/acetone, 20–25 °C, then air |
| Stereochemistry | Achiral product; carbonyl carbon ends as CH₂ | Imine E/Z set at ring collapse; no stereocentre needed | Product is the E-indigo, planar and strongly H-bonded |
| Rate / selectivity | Slow; high T, excess amine; microwave accelerates | Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol) | Aldol step fast; indigo precipitates as deep blue solid |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Willgerodt-Kindler reaction?
Aryl alkyl ketone, S₈, secondary amine (morpholine). Conditions: Neat or DMF, 120–160 °C.
Which bonds are formed and which are broken?
Formed: C=S (thioamide); C–N; H₂S. Broken: C=O; chain C–H bonds.
What is the stereochemical outcome?
Achiral product; carbonyl carbon ends as CH₂
What controls the rate or the selectivity?
Slow; high T, excess amine; microwave accelerates
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go