Cu(II)-mediated stoichiometric homocoupling
Eglinton coupling
How the Eglinton 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: terminal alkyne, Cu(OAc)₂ (stoichiometric). Conditions: Pyridine (or pyridine/MeOH), 25–60 °C, no O₂ needed. Bonds formed: C(sp)–C(sp) bond (1,3-diyne); bonds broken: 2 × C–H (alkyne). Stereochemistry: None (linear sp carbons). Rate and selectivity: Cu(II) supplies the oxidising equivalents; good for macrocyclic diynes.
At a glance
Step 1: Cu(II) acetylide forms
A stoichiometric Cu(II) salt (typically copper acetate, with pyridine) directly deprotonates and coordinates a terminal alkyne, forming a copper acetylide without needing added O₂ as a separate oxidant.
Step 2: Two acetylides come together
Because copper is already in the +2 oxidation state, a second equivalent of copper acetylide can couple directly, with copper itself supplying the oxidizing equivalents rather than relying on atmospheric oxygen.
Step 3: Coupling, Cu(II) → Cu(I)
Reductive coupling forms the 1,3-diyne, with Cu(II) reduced to Cu(I) in the process — this stoichiometric variant of Glaser coupling is often preferred for sensitive or highly functionalized alkynes where the milder, well-defined Cu(II)/pyridine conditions give cleaner reactions than aerobic Cu(I)/O₂.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
How it compares with related reactions
| Eglinton coupling | McMurry coupling | Reformatsky reaction | |
|---|---|---|---|
| Reagents | terminal alkyne, Cu(OAc)₂ (stoichiometric) | TiCl₃ or TiCl₄, Zn (or Zn–Cu, LiAlH₄) | α-Bromo ester, Zn (activated), aldehyde or ketone; then H₃O⁺ |
| Conditions | Pyridine (or pyridine/MeOH), 25–60 °C, no O₂ needed | THF, reflux, inert atmosphere | THF or benzene/Et₂O, 40–80 °C |
| Stereochemistry | None (linear sp carbons) | Mixture of E/Z for unsymmetrical alkenes | Chair TS; modest syn/anti selectivity |
| Rate / selectivity | Cu(II) supplies the oxidising equivalents; good for macrocyclic diynes | Pinacol coupling at low T; deoxygenation needs heat | Zn insertion (needs activation) is the slow step |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Eglinton coupling?
terminal alkyne, Cu(OAc)₂ (stoichiometric). Conditions: Pyridine (or pyridine/MeOH), 25–60 °C, no O₂ needed.
Which bonds are formed and which are broken?
Formed: C(sp)–C(sp) bond (1,3-diyne). Broken: 2 × C–H (alkyne).
What is the stereochemical outcome?
None (linear sp carbons)
What controls the rate or the selectivity?
Cu(II) supplies the oxidising equivalents; good for macrocyclic diynes
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go