Inorganic — [2+2+1] cycloaddition
Pauson-Khand reaction
How the Pauson-Khand reaction 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: alkyne, alkene, Co₂(CO)₈ (stoich. or cat.), CO. Conditions: Toluene/DCE, 60–110 °C (or NMO / amine N-oxide promoter, 25 °C). Bonds formed: 2 × C–C, C–C(=O); cyclopentenone ring; bonds broken: alkyne π, alkene π; Co–CO. Stereochemistry: Alkene geometry and ring-fusion set in insertion; often high de intramolecularly. Rate and selectivity: Alkene insertion into Co–C is usually rate- and selectivity-determining.
At a glance
Step 1: Alkyne–Co₂(CO)₆ complex
An alkyne coordinates to a Co₂(CO)₆ complex, forming a stable, isolable alkyne-cobalt cluster — CO ligands on cobalt hold the alkyne activated and protected until the second substrate is ready to react.
Step 2: Alkene then CO insertion
An alkene coordinates nearby and undergoes migratory insertion into a cobalt-carbon bond of the complex, followed by insertion of a coordinated CO ligand — two sequential insertion steps, the same fundamental step-type used throughout organometallic catalysis, just chained twice in a row.
Step 3: Reductive elimination
Reductive elimination releases a cyclopentenone, combining alkyne, alkene, and carbon monoxide into a single new five-membered ring — this formal [2+2+1] cycloaddition builds a densely functionalized ring system from three simple, otherwise unreactive components in one step.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- DCE
- 1,2-Dichloroethane. Higher-boiling chlorinated solvent.
- NMO
- N-Methylmorpholine N-oxide. Co-oxidant that reoxidises Os(VI) to Os(VIII).
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
How it compares with related reactions
| Pauson-Khand reaction | Associative ligand substitution at a square-planar Pt(II) complex | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | alkyne, alkene, Co₂(CO)₈ (stoich. or cat.), CO | K₂[PtCl₄] + NH₃ (aq) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | Toluene/DCE, 60–110 °C (or NMO / amine N-oxide promoter, 25 °C) | H₂O, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Alkene geometry and ring-fusion set in insertion; often high de intramolecularly | Trigonal-bipyramidal intermediate; retention of geometry | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Alkene insertion into Co–C is usually rate- and selectivity-determining | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Pauson-Khand reaction?
alkyne, alkene, Co₂(CO)₈ (stoich. or cat.), CO. Conditions: Toluene/DCE, 60–110 °C (or NMO / amine N-oxide promoter, 25 °C).
Which bonds are formed and which are broken?
Formed: 2 × C–C, C–C(=O); cyclopentenone ring. Broken: alkyne π, alkene π; Co–CO.
What is the stereochemical outcome?
Alkene geometry and ring-fusion set in insertion; often high de intramolecularly
What controls the rate or the selectivity?
Alkene insertion into Co–C is usually rate- and selectivity-determining
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam