Palladium cross-coupling: reading a catalytic cycle
Palladium-catalysed cross-coupling joins two fragments that would not react on their own, and it won a Nobel Prize in 2010. Behind dozens of named reactions sits a short list of elementary steps. Learn those and every cycle becomes readable.
The three core steps
Most couplings alternate between palladium(0) and palladium(II) through three steps.
- Oxidative addition. A Pd(0) species inserts into a carbon–halogen bond (Ar–X), giving Ar–Pd(II)–X. Palladium is oxidised from 0 to +2, and its coordination number rises by two. Reactivity follows the C–X bond strength: Ar–I > Ar–Br > Ar–OTf >> Ar–Cl. See Oxidative addition of methyl iodide to Pt(0).
- Transmetalation. A second organometallic partner (R–M, where M is B, Sn, Zn, Cu, Si or Mg) passes its organic group to palladium, and the halide goes to M. The palladium now carries both organic groups: Ar–Pd–R. See Transmetalation at a Pd(II) center.
- Reductive elimination. The two organic groups on palladium join to form the new C–C bond, and palladium returns to Pd(0), ready for another cycle. See Reductive elimination.
Because the catalyst is regenerated, a small amount (typically 0.1–5 mol %) is enough. Phosphine or N-heterocyclic carbene ligands tune the rate of each step: electron-rich ligands speed up oxidative addition, and bulky ligands speed up reductive elimination.
One cycle, many reactions: what changes is the partner
| Reaction | Partner (R–M) | New bond | Notes |
|---|---|---|---|
| Suzuki–Miyaura | Boronic acid or ester | C(sp²)–C(sp²) | Needs a base to activate boron; tolerates water; non-toxic by-products |
| Stille | Organostannane | C–C | Very tolerant of functional groups; tin by-products are toxic |
| Negishi | Organozinc | C–C | Highly reactive; moisture-sensitive |
| Kumada | Grignard reagent | C–C | Cheap; limited functional-group tolerance |
| Hiyama | Organosilane | C–C | Needs fluoride or base activation |
| Sonogashira | Terminal alkyne (with Cu(I)) | C(sp²)–C(sp) | Copper acetylide does the transmetalation |
Variants that change a step
Heck reaction. No organometallic partner. After oxidative addition, the alkene inserts into the Pd–C bond (migratory insertion), and a β-hydride elimination releases the substituted alkene and a Pd–H species. A base then regenerates Pd(0). See Heck reaction and Migratory insertion.
Buchwald–Hartwig amination. The partner is an amine. After oxidative addition, the amine coordinates and is deprotonated by a strong base to give a palladium amido complex, and reductive elimination forms the C–N bond. See Buchwald-Hartwig amination.
Copper variants. The older Ullmann reaction uses copper to make C–C, C–O and C–N bonds (Ullmann coupling), and the Chan–Lam coupling (Chan-Lam coupling) forms C–N and C–O bonds with copper under air.
How to read any catalytic cycle
- Find the catalyst resting state. The cycle begins and ends with the same species, usually L₂Pd(0).
- Label each arrow. Is a bond added (oxidative addition, insertion), exchanged (transmetalation, ligand substitution) or released (reductive elimination, β-hydride elimination)?
- Track the oxidation state and electron count of the metal. Oxidative addition raises the oxidation state by 2, and reductive elimination lowers it by 2. Transmetalation and insertion leave it unchanged.
- Identify what is consumed: the two coupling partners and the base. The catalyst is not consumed.
- Ask where the base acts. In Suzuki it activates boron, in Heck it removes HX, and in Buchwald–Hartwig it deprotonates the amine.
Common reasons a coupling fails
- Oxygen oxidises Pd(0) and phosphine ligands, so reactions are normally run under nitrogen or argon.
- β-Hydride elimination from alkyl–palladium intermediates leads to alkenes instead of the desired product, which is why coupling of sp³ alkyl partners is harder than of aryl partners.
- Aryl chlorides are cheap but unreactive towards oxidative addition and need specially designed electron-rich ligands.
- Homocoupling (two identical fragments joining) happens when the partner reacts with itself, for example copper-catalysed alkyne dimerisation in the Glaser coupling or Eglinton coupling.
Where this connects
The oxidative addition and reductive elimination steps also appear in hydrogenation with the Wilkinson catalyst, in the Monsanto acetic acid process and in many other metal-catalysed reactions. For the aromatic partners, see the aromatic substitution guide. Cross-coupling is often the step that builds the carbon skeleton in drug synthesis, which is why it appears so often in pharmaceutical chemistry courses.
Practise in the interactive app Browse all 345 reactions
More study guides
- How to learn reaction mechanisms (without memorising everything)A practical method for learning organic reaction mechanisms: understand electron flow, group reactions into a few patterns, and practise by drawing.
- Curved-arrow notation: rules and the mistakes students makeHow to read and draw curved arrows in organic mechanisms: where they start and end, full-headed versus fish-hook arrows, and the most common errors.
- SN1, SN2, E1 or E2? How to decide which mechanism winsA clear decision guide for substitution versus elimination: substrate, nucleophile or base strength, solvent and temperature, with examples.
- Nucleophiles, electrophiles and leaving groups: what makes a reaction goWhat decides how reactive a nucleophile, an electrophile or a leaving group is, and how solvent, charge, size and resonance change reactivity.