The Glaser Coupling

What the Glaser coupling does

Take a terminal alkyne — a molecule ending in a C≡C–H group — and the Glaser coupling stitches two of them together, tip to tip, into a single conjugated 1,3-diyne. The two acetylenic hydrogens are lost, a brand-new carbon–carbon single bond appears between the two sp carbons, and you are left with a rigid, rod-straight R–C≡C–C≡C–R skeleton. Because both ends come from the same kind of alkyne, the product is symmetric.

The magic ingredient is copper. A copper(I) salt (typically CuCl) plus a base (ammonia, pyridine, or a diamine) turns each terminal alkyne into a copper acetylide, R–C≡C–Cu. Two of these copper acetylides then couple, and the copper is oxidized back to its working state by molecular oxygen from the air. Net, the reaction is a two-electron oxidation of the two alkyne carbons — which is why it is called an oxidative coupling and why you cannot run it in the absence of an oxidant.

    2 R-C≡C-H   +   ½ O₂   ──CuCl, amine──→   R-C≡C-C≡C-R   +   H₂O

    the two terminal alkynes fuse tip-to-tip into a symmetric 1,3-diyne

Unlike palladium cross-couplings (Suzuki, Sonogashira, Heck) that join different fragments, Glaser is fundamentally a homocoupling: it joins two copies of the same partner. That constraint is also its superpower — for building symmetric struts, hooping a molecule back onto itself in a macrocycle, or spinning terminal-diyne monomers into rigid carbon networks, homocoupling is exactly what you want.

The step-by-step mechanism

The Glaser coupling is a copper-mediated redox process. The accepted mechanism runs through copper acetylides and a bimetallic C–C bond-forming step:

1. Deprotonation → copper acetylide. The amine base removes the terminal alkyne's acidic proton (pK_a ≈ 25 for a typical R–C≡C–H — acidic because the resulting carbanion sits in an sp orbital, 50% s-character, hugging the nucleus). Copper(I) coordinates to the alkyne π-system and then binds the carbon, giving a σ-bonded copper(I) acetylide R–C≡C–Cu^I. The amine also keeps the copper soluble and tunes its redox potential.
2. Oxidation of copper. Molecular oxygen (or Cu(II) in the Eglinton variant) oxidizes copper(I) to copper(II). The acetylide is now bound to an electron-poor, oxidized metal — R–C≡C–Cu^II — with meaningful radical / acetylide-anion character on the terminal carbon. This is the key activating event: oxidation drains electron density and primes the two carbon ends to bond.
3. C–C bond formation. Two of these oxidized acetylide units come together on a bimetallic copper cluster and couple. The two sp carbons form the new C(sp)–C(sp) σ bond in what is best described as a two-electron oxidative reductive-elimination from a di-copper acetylide — the two electrons from the forming bond flow onto the two coppers, reducing them.
4. Turnover. The freshly reduced copper(I) is released, ready to grab two more alkynes, and molecular oxygen closes the catalytic cycle by re-oxidizing copper — being reduced itself to water. In the Hay version this whole loop turns over many times per copper; in Eglinton, stoichiometric Cu(II) is consumed as the oxidant.

  step 1:  R-C≡C-H  +  Cu⁺  +  base  →  R-C≡C-Cu(I)  +  base-H⁺
  step 2:  R-C≡C-Cu(I)         ──O₂──→  [R-C≡C-Cu(II)]     (Cu oxidized)
  step 3:  2 [R-C≡C-Cu(II)]    →  R-C≡C-C≡C-R  +  2 Cu(I)   (new C-C bond)
  step 4:  2 Cu(I) + ½O₂ + 2H⁺ →  2 Cu(II) + H₂O            (O₂ closes the cycle)

The electron bookkeeping is the whole story: the alkyne carbons start in a C–H bond and end in a C–C bond, formally losing two electrons and two protons. Oxygen is the electron sink. Remove the oxidant and the cycle jams — you get a single stoichiometric turnover at most, then nothing.

Reagents, catalyst, and conditions

The Glaser coupling has three practical dialects. All three share the same mechanism; they differ in copper source, base, and oxidant.

• Classic Glaser (1869). CuCl and aqueous ammonia in ethanol, open to air. Copper(I) chloride is dissolved in ammonia; the terminal alkyne is added and air (O₂) is bubbled or simply allowed in. Simple and cheap, but copper acetylides can precipitate and the reaction can be messy.
• Eglinton (1956). Stoichiometric copper(II) acetate in pyridine (often methanol/pyridine), typically warmed to 40–60 °C. Here Cu(II) is itself the oxidant, so no external O₂ is strictly required. Because it works well under high dilution, Eglinton conditions are the classic choice for macrocyclization — the intramolecular coupling of a di-terminal-alkyne.
• Glaser–Hay (1962). Catalytic CuCl with the bidentate diamine TMEDA (N,N,N′,N′-tetramethylethylenediamine) and molecular oxygen, in acetone, dichloromethane, or o-DCB at room temperature. TMEDA chelates copper, keeps it soluble in organic solvents, and stabilizes the redox couple. This is the modern default: cleaner, milder, higher-yielding, and often just stir-under-a-balloon-of-O₂ simple.

Typical conditions to keep in your head: 5–20 mol% CuCl, ~10–20 mol% TMEDA, O₂ balloon or air, acetone or CH₂Cl₂, 20–40 °C, 1–12 h. Yields for well-behaved substrates are commonly 70–95%. Substrate concentration matters: run dilute (10⁻³–10⁻² M) for macrocyclizations to favor the intramolecular ring over intermolecular oligomers.

Scope and selectivity

Because the reaction goes through a copper acetylide, its scope is essentially "anything with a terminal alkyne." Aryl-, alkyl-, silyl-, and heteroaryl-acetylenes all couple. There is no stereochemistry to worry about — the new bond is between two sp carbons, which are linear, so no cis/trans or R/S question arises. Regiochemistry is likewise fixed: coupling always happens at the terminal carbon, because that is the only place the copper acetylide can form.

• Requires a terminal alkyne. Internal alkynes (R–C≡C–R′) have no acidic C–H and simply do not react.
• Symmetric products only. The unavoidable limitation. Two different alkynes give a statistical mixture (1 : 2 : 1 of the two homocouplings and the cross-product). For unsymmetrical diynes, switch to the Cadiot–Chodkiewicz coupling (terminal alkyne + 1-haloalkyne).
• Functional-group tolerance. Copper is mild — esters, ethers, free alcohols, halides, and many heterocycles survive. Free amines can bind copper and slow things down; strongly reducing or copper-chelating groups can interfere.
• Silyl-protected alkynes. TMS- or TIPS-acetylenes can be desilylated in situ (with a fluoride or mild base) to unmask the terminal alkyne, giving a one-pot deprotection/coupling — handy for controlling which alkyne couples.

Glaser vs related alkyne couplings

	Glaser / Hay / Eglinton	Cadiot–Chodkiewicz	Sonogashira
Bond formed	C(sp)–C(sp): alkyne–alkyne	C(sp)–C(sp): alkyne–alkyne	C(sp)–C(sp²): alkyne–aryl/vinyl
Partners	Two terminal alkynes (same)	Terminal alkyne + 1-haloalkyne	Terminal alkyne + aryl/vinyl halide
Product	Symmetric 1,3-diyne	Unsymmetrical 1,3-diyne	Aryl- or vinyl-alkyne (not a diyne)
Metal(s)	Cu only	Cu only	Pd + Cu (co-catalyst)
Oxidant	O₂ or Cu(II) — required	Amine oxidant / no net oxidation	None — Pd(0)/Pd(II) redox is internal
Nature	Oxidative homocoupling	Oxidative-free cross-coupling	Reductive cross-coupling
Selectivity issue	Only symmetric products cleanly	Glaser homocoupling is the side reaction	Glaser homocoupling is the side reaction
Year / who	1869 Glaser; 1956 Eglinton; 1962 Hay	1957 Cadiot & Chodkiewicz	1975 Sonogashira, Tohda, Hagihara

The through-line: Glaser homocoupling is both a tool and a nuisance. When you want a symmetric diyne, you run it deliberately. When you are running a Sonogashira or Cadiot–Chodkiewicz coupling, the same chemistry lurks as the dominant side reaction — which is why those protocols so carefully exclude oxygen.

Worked example: two phenylacetylenes → 1,4-diphenylbutadiyne

The textbook Glaser–Hay: homocouple phenylacetylene (ethynylbenzene) to make 1,4-diphenyl-1,3-butadiyne, a classic conjugated diyne and a common building block for optoelectronic materials.

    2 Ph-C≡C-H  ──CuCl (10 mol%), TMEDA (12 mol%), O₂, acetone, rt, 3 h──→  Ph-C≡C-C≡C-Ph

• Reagents. Phenylacetylene (the sole substrate — two molecules combine per diyne), CuCl 0.10 equiv, TMEDA 0.12 equiv, dry acetone, O₂ balloon.
• Conditions. Stir at room temperature under an oxygen atmosphere for 2–4 h; the solution takes on the blue-green tint of the copper–TMEDA complex.
• Workup. Dilute with ether, wash the copper out with dilute aqueous ammonia and then water, dry, and concentrate. The product often crystallizes directly.
• Yield. Typically 85–95% of 1,4-diphenylbutadiyne — pale solid, mp ≈ 87 °C — a symmetric conjugated diyne ready for further elaboration.

Note the atom economy: the only stoichiometric co-products are water (from O₂) and the two protons the base carried off. Copper and TMEDA are catalytic. That efficiency is why the Hay variant, not the original 1869 recipe, is what you will actually run in a modern lab.

Real-world applications

• Shape-persistent macrocycles. Intramolecular Glaser (Eglinton, high dilution) closes rings of aryl–diyne units into rigid molecular hoops and boxes — the foundation of much of supramolecular and dynamic covalent chemistry.
• Carbon-rich materials and graphdiyne. Oxidative alkyne coupling of hexaethynylbenzene monomers builds graphdiyne, a two-dimensional carbon allotrope woven from benzene rings linked by diyne struts — grown as films on copper foil, which doubles as the catalyst.
• Polydiacetylenes and sensors. Diacetylene (1,3-diyne) monomers self-assemble and topochemically polymerize into conjugated polydiacetylenes that change color under heat, strain, or binding — the basis of colorimetric biosensors and time–temperature indicators.
• Natural-product and drug diyne cores. Many bioactive polyacetylene natural products carry conjugated diyne units; Glaser and Cadiot–Chodkiewicz couplings are the standard disconnection for assembling them.
• Bioconjugation and materials ligation. The diyne linkage is short, rigid, linear, and metabolically stable, making Glaser-type couplings a useful stitching reaction in polymer and peptide conjugation.

Limitations and side reactions

• Only symmetric diynes. The defining constraint. Cross-coupling two different terminal alkynes gives statistical mixtures; use Cadiot–Chodkiewicz for unsymmetrical diynes.
• Over-oxidation and polyyne/polymer formation. Unhindered or concentrated substrates can run past the diyne to higher polyynes and dark, intractable copper-acetylide polymers. Dilution, ligand tuning, and controlled O₂ keep this in check.
• Copper-acetylide hazard. Dry copper(I) acetylides are shock-, friction-, and heat-sensitive primary explosives (copper acetylide, Cu₂C₂, is the same class of compound behind acetylene-line incidents). Never isolate or dry them; always destroy copper residues with dilute acid or ammonia during workup.
• Amine and chelator interference. Substrates bearing free amines, thiols, or strong copper chelators can sequester the catalyst and stall the reaction.
• It sabotages Sonogashira. Any Sonogashira reaction — terminal alkyne + Pd + Cu co-catalyst — will suffer Glaser homocoupling of the alkyne if oxygen is present, wasting the precious alkyne. Rigorous degassing is the standard fix.

Historical discovery

The German chemist Carl Andreas Glaser reported the reaction in 1869, observing that copper(I) phenylacetylide, on exposure to air, coupled to give diphenylbutadiyne. That makes Glaser coupling the oldest known transition-metal-mediated carbon–carbon coupling — it predates the Ullmann reaction (1901), and it long predates the palladium cross-couplings (Heck, Suzuki, Negishi, Sonogashira) of the 1970s that won the 2010 Nobel Prize.

For almost a century it stayed a laboratory curiosity, hampered by messy copper-acetylide precipitation. Two modernizations rescued it. In 1956, Geoffrey Eglinton and A. R. Galbraith introduced copper(II) acetate in pyridine, enabling the clean macrocyclizations that opened up cyclic-alkyne and annulene chemistry. In 1962, Allan S. Hay at General Electric added the chelating diamine TMEDA with catalytic copper and O₂, giving the soluble, high-yielding, catalytic conditions that made the reaction genuinely practical. Today "Glaser–Hay" is the name most chemists reach for.

Safety and practical notes

• Never dry-isolate copper acetylides. They are impact-sensitive explosives. Keep them wetted; quench copper with dilute HNO₃ or ammonia at the end of the reaction.
• Control the oxygen. You need O₂ to turn over, but a large headspace of pure O₂ over volatile organic solvent is a flammability concern — use a balloon, keep it away from ignition sources, and prefer air or dilute O₂ where possible.
• Watch the exotherm on scale. Oxidative couplings can run away as they self-heat; add substrate slowly and keep cooling available.
• Copper waste. Streams contain dissolved copper — collect and dispose of them as heavy-metal waste rather than down the drain.
• Dilution is your friend for rings. For macrocyclizations, work at high dilution and add substrate by syringe pump to beat the intermolecular oligomerization.