Pd-catalyzed cross-coupling
Negishi coupling: full catalytic cycle
How the Negishi 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, R–ZnX, Pd(PPh₃)₄ or Pd/phosphine (cat.). Conditions: THF, 25–65 °C, no added base. Bonds formed: Ar–R (C–C σ); ZnXBr; bonds broken: C–Br; C–Zn. Stereochemistry: Retention at sp² carbons; sp³ organozincs also work. Rate and selectivity: Oxidative addition (I > Br > Cl) often rate-limiting.
At a glance
Step 1: Oxidative addition
Pd(0) oxidatively adds an aryl or vinyl halide, forming Ar–Pd(II)–X — the same first step as every Pd-catalyzed coupling in this library.
Step 2: Transmetalation with organozinc
An organozinc reagent (R–ZnX) performs transmetalation, transferring its organic group to palladium and displacing halide — zinc's oxophilicity drives this ligand exchange forward efficiently, even without added base.
Step 3: Reductive elimination
Reductive elimination forges the new C–C bond and regenerates Pd(0) — Negishi coupling tolerates a broader range of functional groups than Suzuki, since organozincs are milder nucleophiles than boronic acids under basic conditions.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- 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.
- Et
- Ethyl. –CH₂CH₃.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- 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
| Negishi coupling | Buchwald-Hartwig amination | Heck reaction | |
|---|---|---|---|
| Reagents | ArBr, R–ZnX, Pd(PPh₃)₄ or Pd/phosphine (cat.) | ArBr, R₂NH, Pd₂(dba)₃ / bulky phosphine (XPhos, BINAP), NaOtBu | ArBr, alkene (e.g. methyl acrylate), Pd(OAc)₂/PPh₃, Et₃N |
| Conditions | THF, 25–65 °C, no added base | Toluene or dioxane, 80–110 °C | DMF or MeCN, 80–120 °C |
| Stereochemistry | Retention at sp² carbons; sp³ organozincs also work | No new stereocentre; retention of aryl geometry | Syn insertion, syn β-H elimination; E-alkene favoured |
| Rate / selectivity | Oxidative addition (I > Br > Cl) often 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 Negishi coupling?
ArBr, R–ZnX, Pd(PPh₃)₄ or Pd/phosphine (cat.). Conditions: THF, 25–65 °C, no added base.
Which bonds are formed and which are broken?
Formed: Ar–R (C–C σ); ZnXBr. Broken: C–Br; C–Zn.
What is the stereochemical outcome?
Retention at sp² carbons; sp³ organozincs also work
What controls the rate or the selectivity?
Oxidative addition (I > Br > Cl) often 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