Inorganic — organometallic (coupling step)
Transmetalation at a Pd(II) center
How the Transmetalation at a Pd(II) center 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: Ar–Pd(II)–X + Ar′–B(OH)₂ (base-activated) or Ar′–ZnX / Ar′–SnR₃. Conditions: Aqueous base (K₂CO₃, K₃PO₄) / THF or dioxane, 60–100 °C. Bonds formed: Pd–C(Ar′); B–O / B–X; bonds broken: C–B (Ar′–B); Pd–Br. Stereochemistry: Gives cis-diaryl Pd(II). Rate and selectivity: Often rate-limiting; base converts ArB(OH)₂ to the more nucleophilic boronate.
At a glance
Step 1: Pd(II) halide meets the activated boronate
After oxidative addition has already put an aryl (or vinyl) group and a halide on Pd(II), an organometallic reagent (like an arylboronic acid, activated by base, or an organozinc) approaches.
Step 2: Aryl group hops from B to Pd
The organic group transfers directly from the incoming metal (boron, zinc, tin, or magnesium) to palladium, displacing the halide — both metal centers exchange one organic ligand for the other.
Step 3: cis-Diaryl Pd(II)
Pd now holds two organic groups, cis to each other — exactly the setup needed for the reductive elimination step that follows in every cross-coupling reaction.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- K2CO3
- Potassium carbonate. Mild inorganic base.
How it compares with related reactions
| Transmetalation at a Pd(II) center | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | Ar–Pd(II)–X + Ar′–B(OH)₂ (base-activated) or Ar′–ZnX / Ar′–SnR₃ | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | Aqueous base (K₂CO₃, K₃PO₄) / THF or dioxane, 60–100 °C | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Gives cis-diaryl Pd(II) | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Often rate-limiting; base converts ArB(OH)₂ to the more nucleophilic boronate | Initiation/phosphine-or-ether release often limits; [2+2] steps fast | Alkyl must be cis to CO; ligand then fills the vacant site |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Transmetalation at a Pd(II) center?
Ar–Pd(II)–X + Ar′–B(OH)₂ (base-activated) or Ar′–ZnX / Ar′–SnR₃. Conditions: Aqueous base (K₂CO₃, K₃PO₄) / THF or dioxane, 60–100 °C.
Which bonds are formed and which are broken?
Formed: Pd–C(Ar′); B–O / B–X. Broken: C–B (Ar′–B); Pd–Br.
What is the stereochemical outcome?
Gives cis-diaryl Pd(II)
What controls the rate or the selectivity?
Often rate-limiting; base converts ArB(OH)₂ to the more nucleophilic boronate
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam