Inorganic — intramolecular aminopalladation
Aza-Wacker cyclization
How the Aza-Wacker cyclization 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: Pd(OAc)₂ (cat.), base, O₂ or benzoquinone (BQ). Conditions: DMSO or toluene, 25–80 °C. Bonds formed: C–N, Pd–C then C=C; Pd–H; bonds broken: N–H, alkene π, β C–H. Stereochemistry: Syn-aminopalladation (inner-sphere); syn β-H elimination. Rate and selectivity: Aminopalladation usually rate-limiting; Pd(0) reoxidized by O₂ or BQ.
At a glance
Step 1: Pd(II) activates the alkene
Pd(II) coordinates an alkene tethered to a pendant nitrogen nucleophile (like a sulfonamide), activating the alkene toward nucleophilic attack — the same electrophilic-alkene-activation strategy as the ordinary Wacker oxidation covered much earlier.
Step 2: Amido-Pd, then syn-aminopalladation
The anionic nitrogen first coordinates to palladium as an amido ligand, then delivers onto the alkene via migratory insertion — syn-aminopalladation, an inner-sphere pathway distinct from the anti-nucleopalladation used by external water in ordinary Wacker oxidation.
Step 3: β-Hydride elimination; Pd(II) regenerated
β-hydride elimination releases the cyclized enamine/imine product and a Pd–H species, and reoxidation regenerates Pd(II) — this builds nitrogen heterocycles (pyrrolidines, indolines) directly from simple aminoalkenes, using the identical catalytic logic as Wacker oxidation just redirected toward ring formation.
Abbreviations and terms
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- Ts
- Tosyl. p-Toluenesulfonyl (p-CH₃C₆H₄SO₂–).
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- Pd(OAc)2
- Palladium(II) acetate. Pd(II) pre-catalyst, reduced in situ to Pd(0).
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Aza-Wacker cyclization | Associative ligand substitution at a square-planar Pt(II) complex | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | Pd(OAc)₂ (cat.), base, O₂ or benzoquinone (BQ) | K₂[PtCl₄] + NH₃ (aq) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | DMSO or toluene, 25–80 °C | H₂O, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Syn-aminopalladation (inner-sphere); syn β-H elimination | Trigonal-bipyramidal intermediate; retention of geometry | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Aminopalladation usually rate-limiting; Pd(0) reoxidized by O₂ or BQ | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Aza-Wacker cyclization?
Pd(OAc)₂ (cat.), base, O₂ or benzoquinone (BQ). Conditions: DMSO or toluene, 25–80 °C.
Which bonds are formed and which are broken?
Formed: C–N, Pd–C then C=C; Pd–H. Broken: N–H, alkene π, β C–H.
What is the stereochemical outcome?
Syn-aminopalladation (inner-sphere); syn β-H elimination
What controls the rate or the selectivity?
Aminopalladation usually rate-limiting; Pd(0) reoxidized by O₂ or BQ
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam