Cu-mediated alkynylation
Castro-Stephens coupling
How the Castro-Stephens 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: Aryl iodide, cuprous acetylide (or CuI, terminal alkyne). Conditions: Pyridine or DMF, reflux ≈ 115 °C, inert atmosphere. Bonds formed: C(sp²)–C(sp) bond; CuI; bonds broken: alkyne C–H; C–I. Stereochemistry: None; linear alkyne. Rate and selectivity: Aryl iodides react best; ArBr is sluggish without Pd.
At a glance
Step 1: Cu(I) and base form the copper acetylide
Cu(I) deprotonates a terminal alkyne, forming a copper acetylide — the same first step shared by Sonogashira, Glaser, and Cadiot-Chodkiewicz coupling.
Step 2: Acetylide attacks the aryl iodide
This copper acetylide's nucleophilic carbon performs a direct oxidative-addition-like displacement of halide from an aryl halide — without any palladium involved at all, predating the later Pd/Cu Sonogashira coupling by decades.
Step 3: Reductive elimination releases the alkyne
Reductive elimination-type collapse releases the coupled aryl alkyne and regenerates Cu(I) — this stoichiometric or catalytic copper-only process was the direct historical predecessor that Sonogashira later improved by adding a palladium co-catalyst for milder conditions.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- Ph
- Phenyl. –C₆H₅.
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
- CuI
- Copper(I) iodide. Co-catalyst for Sonogashira, Ullmann and click chemistry.
How it compares with related reactions
| Castro-Stephens coupling | CBS reduction (Corey-Bakshi-Shibata) | Ullmann coupling | |
|---|---|---|---|
| Reagents | Aryl iodide, cuprous acetylide (or CuI, terminal alkyne) | (S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂ | ArX (I > Br), Cu powder or Cu(I) salt |
| Conditions | Pyridine or DMF, reflux ≈ 115 °C, inert atmosphere | THF, –20 to 25 °C, anhydrous | Neat or DMF/nitrobenzene, 150–250 °C (milder with ligands) |
| Stereochemistry | None; linear alkyne | Predictable absolute configuration: hydride to the face set by the chiral boron-N framework | Hindered biaryls can show axial chirality |
| Rate / selectivity | Aryl iodides react best; ArBr is sluggish without Pd | Hydride transfer is rate-limiting; catalyst turnover fast | Oxidative addition to Cu is rate-limiting; needs high T |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Castro-Stephens coupling?
Aryl iodide, cuprous acetylide (or CuI, terminal alkyne). Conditions: Pyridine or DMF, reflux ≈ 115 °C, inert atmosphere.
Which bonds are formed and which are broken?
Formed: C(sp²)–C(sp) bond; CuI. Broken: alkyne C–H; C–I.
What is the stereochemical outcome?
None; linear alkyne
What controls the rate or the selectivity?
Aryl iodides react best; ArBr is sluggish without Pd
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam