Inorganic — directed asymmetric-adjacent catalysis
Crabtree hydrogenation: directed by a nearby group
How the Crabtree hydrogenation 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: [Ir(cod)(PCy₃)(py)]⁺ PF₆⁻, H₂ (1 atm). Conditions: CH₂Cl₂, 25 °C. Bonds formed: two C–H bonds (syn addition); bonds broken: C=C π; H–H. Stereochemistry: Face selectivity set by the directing OH. Rate and selectivity: Alkene insertion/oxidative addition; works for tetrasubstituted alkenes.
At a glance
Step 1: Directing group binds Ir
Crabtree's iridium catalyst first coordinates to a polar directing group on the substrate (commonly a free hydroxyl or ether oxygen near the alkene), anchoring the metal on one specific face of the molecule before any hydrogenation chemistry happens.
Step 2: H₂ adds, alkene inserts
With the metal held in place by that directing group, H₂ undergoes oxidative addition and the alkene inserts into the Ir–H bond — the same fundamental steps as Wilkinson's hydrogenation, but the substrate's own functional group, not just steric bulk, controls which face reacts.
Step 3: Reductive elimination
Reductive elimination releases the saturated product with the new stereocenters set by whichever face the directing group held the catalyst against — this directing-group strategy lets even highly hindered, tetrasubstituted alkenes hydrogenate cleanly and predictably.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- py
- Pyridine. Weak amine base / nucleophilic catalyst and solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Wilkinson
- Wilkinson’s catalyst, RhCl(PPh₃)₃. Homogeneous hydrogenation catalyst.
How it compares with related reactions
| Crabtree hydrogenation | Asymmetric phase-transfer catalysis | Jacobsen-Katsuki epoxidation | |
|---|---|---|---|
| Reagents | [Ir(cod)(PCy₃)(py)]⁺ PF₆⁻, H₂ (1 atm) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr | Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene |
| Conditions | CH₂Cl₂, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring | CH₂Cl₂, 0 °C, optional pyridine N-oxide |
| Stereochemistry | Face selectivity set by the directing OH | High ee: chiral ion pair shields one enolate face | High ee for cis-alkenes; salen pocket selects the face |
| Rate / selectivity | Alkene insertion/oxidative addition; works for tetrasubstituted alkenes | Alkylation of the ion pair is rate-limiting | Oxygen transfer to alkene is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Crabtree hydrogenation?
[Ir(cod)(PCy₃)(py)]⁺ PF₆⁻, H₂ (1 atm). Conditions: CH₂Cl₂, 25 °C.
Which bonds are formed and which are broken?
Formed: two C–H bonds (syn addition). Broken: C=C π; H–H.
What is the stereochemical outcome?
Face selectivity set by the directing OH
What controls the rate or the selectivity?
Alkene insertion/oxidative addition; works for tetrasubstituted alkenes
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam