Pd-catalyzed cross-coupling
Suzuki-Miyaura coupling: full catalytic cycle
How the Suzuki-Miyaura coupling 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: ArBr, Ar′B(OH)₂, Pd(PPh₃)₄ (cat.), base (K₂CO₃). Conditions: Toluene/EtOH/H₂O or dioxane/H₂O, 80–100 °C. Bonds formed: Ar–Ar′ (C–C σ); bonds broken: C–Br; C–B. Stereochemistry: Retention at sp² carbons; biaryl axis possible. Rate and selectivity: Oxidative addition (I > Br > Cl) or transmetalation is rate-limiting.
At a glance
Step 1: Oxidative addition
Pd(0) oxidatively adds an aryl halide, forming Ar–Pd(II)–X — the same oxidative addition mechanism from earlier, now the first step of a coupling cycle.
Step 2: Transmetalation
Base activates an arylboronic acid, and transmetalation transfers the second aryl group onto palladium, displacing halide and putting two aryl groups cis to each other on Pd.
Step 3: Reductive elimination
Reductive elimination forges the new C–C bond between the two aryl groups and regenerates Pd(0) — three mechanistic steps you already know individually, chained into the most widely used cross-coupling reaction in industry.
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.
- EtOH
- Ethanol. Polar protic solvent.
- K2CO3
- Potassium carbonate. Mild inorganic base.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- Pd(PPh3)4
- Tetrakis(triphenylphosphine)palladium(0). Classic Pd(0) pre-catalyst for cross-coupling.
- L2
- Ligand set. Spectator ligands on the metal that tune its reactivity.
How it compares with related reactions
| Suzuki-Miyaura coupling | Buchwald-Hartwig amination | Heck reaction | |
|---|---|---|---|
| Reagents | ArBr, Ar′B(OH)₂, Pd(PPh₃)₄ (cat.), base (K₂CO₃) | ArBr, R₂NH, Pd₂(dba)₃ / bulky phosphine (XPhos, BINAP), NaOtBu | ArBr, alkene (e.g. methyl acrylate), Pd(OAc)₂/PPh₃, Et₃N |
| Conditions | Toluene/EtOH/H₂O or dioxane/H₂O, 80–100 °C | Toluene or dioxane, 80–110 °C | DMF or MeCN, 80–120 °C |
| Stereochemistry | Retention at sp² carbons; biaryl axis possible | No new stereocentre; retention of aryl geometry | Syn insertion, syn β-H elimination; E-alkene favoured |
| Rate / selectivity | Oxidative addition (I > Br > Cl) or transmetalation is rate-limiting | Bulky, electron-rich ligand speeds oxidative addition and C–N reductive elimination | Aryl adds to the less hindered alkene carbon |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Suzuki-Miyaura coupling?
ArBr, Ar′B(OH)₂, Pd(PPh₃)₄ (cat.), base (K₂CO₃). Conditions: Toluene/EtOH/H₂O or dioxane/H₂O, 80–100 °C.
Which bonds are formed and which are broken?
Formed: Ar–Ar′ (C–C σ). Broken: C–Br; C–B.
What is the stereochemical outcome?
Retention at sp² carbons; biaryl axis possible
What controls the rate or the selectivity?
Oxidative addition (I > Br > Cl) or transmetalation is rate-limiting
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam