Inorganic — directed C–H functionalization
Directed C–H activation via cyclometalation
How the Directed C–H activation via cyclometalation 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: Pd(OAc)₂ (5–10 mol%), directing-group substrate, coupling partner (Ar–I), oxidant/base. Conditions: AcOH or toluene, 80–120 °C. Bonds formed: Pd–C (palladacycle); then C–Ar; bonds broken: ortho C–H. Stereochemistry: Site-selective ortho functionalisation. Rate and selectivity: C–H cleavage (CMD) is usually rate-limiting.
At a glance
Step 1: Directing group binds Pd(II)
A substrate bearing a coordinating directing group (like a pyridine or amide) first binds a Pd(II) center through that group, positioning the metal in close, fixed proximity to a specific nearby C–H bond — the directing group's whole job is geometric, not electronic.
Step 2: CMD gives a five-membered palladacycle
The same concerted metalation-deprotonation (CMD) mechanism covered earlier then activates that specific, nearby C–H bond, forming a stable five- or six-membered palladacycle — the directing group is what makes this activation happen at one particular C–H bond instead of any of the many similar ones elsewhere on the molecule.
Step 3: Coupling installs the new group
This palladacycle then reacts with an external coupling partner (via oxidative addition/reductive elimination, exactly as in any Pd-catalyzed coupling) to install a new substituent at that one specific, directed position — this strategy is how modern C–H functionalization achieves site-selectivity without needing a pre-installed halide.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
- Pd(OAc)2
- Palladium(II) acetate. Pd(II) pre-catalyst, reduced in situ to Pd(0).
- CMD
- Concerted metalation–deprotonation. C–H activation where a carboxylate shuttles the proton as the C–M bond forms.
How it compares with related reactions
| Directed C–H activation via cyclometalation | Crabtree hydrogenation | C–H activation via concerted metalation-deprotonation (CMD) | |
|---|---|---|---|
| Reagents | Pd(OAc)₂ (5–10 mol%), directing-group substrate, coupling partner (Ar–I), oxidant/base | [Ir(cod)(PCy₃)(py)]⁺ PF₆⁻, H₂ (1 atm) | Pd(OAc)₂ (or PdCl₂ + carboxylate), arene, carboxylate/carbonate base |
| Conditions | AcOH or toluene, 80–120 °C | CH₂Cl₂, 25 °C | DMA or toluene, 100–140 °C; PivOH additive |
| Stereochemistry | Site-selective ortho functionalisation | Face selectivity set by the directing OH | Site selectivity set by acidity of C–H and directing groups |
| Rate / selectivity | C–H cleavage (CMD) is usually rate-limiting | Alkene insertion/oxidative addition; works for tetrasubstituted alkenes | C–H cleavage is often rate-determining (KIE kH/kD ≈ 2–5) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Directed C–H activation via cyclometalation?
Pd(OAc)₂ (5–10 mol%), directing-group substrate, coupling partner (Ar–I), oxidant/base. Conditions: AcOH or toluene, 80–120 °C.
Which bonds are formed and which are broken?
Formed: Pd–C (palladacycle); then C–Ar. Broken: ortho C–H.
What is the stereochemical outcome?
Site-selective ortho functionalisation
What controls the rate or the selectivity?
C–H cleavage (CMD) is usually rate-limiting
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam
Related mechanisms
- Crabtree hydrogenation
- C–H activation via concerted metalation-deprotonation (CMD)
- Iridium-catalyzed C–H borylation
- Associative ligand substitution at a square-planar Pt(II) complex
- Asymmetric phase-transfer catalysis
- Aza-Wacker cyclization
- Catalytic hydrogenation of an alkene (Pd/C, H₂)
- Catellani reaction