Inorganic — C–H functionalization
Iridium-catalyzed C–H borylation
How the Iridium-catalyzed C–H borylation 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)OMe]₂, dtbpy, B₂pin₂ (or HBpin). Conditions: THF or hexane, 25–80 °C. Bonds formed: C–B (aryl boronate); H–Bpin; bonds broken: aryl C–H; B–B. Stereochemistry: Sterically controlled regioselectivity (meta to substituents). Rate and selectivity: C–H oxidative addition is turnover-limiting.
At a glance
Step 1: C–H oxidative addition
An electron-rich Ir(III) catalyst, bearing boryl ligands (from B₂pin₂), oxidatively adds a C–H bond of the arene — a genuine oxidative addition at carbon-hydrogen, distinct from the CMD mechanism covered earlier, and selective for the least sterically hindered C–H bond rather than any electronically activated position.
Step 2: Ir(V) hydride aryl boryl
This gives an Ir(V) hydride-aryl-boryl intermediate — a higher oxidation state than most Pd chemistry in this library reaches, reflecting iridium's tolerance for higher formal oxidation states.
Step 3: Two reductive eliminations
Reductive elimination forges the new C–B bond and releases the borylated arene, while a second reductive elimination releases H–Bpin, regenerating the active catalyst — because this reaction is controlled by sterics rather than electronics, it borylates positions that Friedel-Crafts and EAS chemistry can't easily reach.
Abbreviations and terms
- Me
- Methyl. –CH₃.
- OMe
- Methoxy. –OCH₃.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
- CMD
- Concerted metalation–deprotonation. C–H activation where a carboxylate shuttles the proton as the C–M bond forms.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
How it compares with related reactions
| Iridium-catalyzed C–H borylation | C–H activation via concerted metalation-deprotonation (CMD) | Directed C–H activation via cyclometalation | |
|---|---|---|---|
| Reagents | [Ir(cod)OMe]₂, dtbpy, B₂pin₂ (or HBpin) | Pd(OAc)₂ (or PdCl₂ + carboxylate), arene, carboxylate/carbonate base | Pd(OAc)₂ (5–10 mol%), directing-group substrate, coupling partner (Ar–I), oxidant/base |
| Conditions | THF or hexane, 25–80 °C | DMA or toluene, 100–140 °C; PivOH additive | AcOH or toluene, 80–120 °C |
| Stereochemistry | Sterically controlled regioselectivity (meta to substituents) | Site selectivity set by acidity of C–H and directing groups | Site-selective ortho functionalisation |
| Rate / selectivity | C–H oxidative addition is turnover-limiting | C–H cleavage is often rate-determining (KIE kH/kD ≈ 2–5) | C–H cleavage (CMD) is usually rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Iridium-catalyzed C–H borylation?
[Ir(cod)OMe]₂, dtbpy, B₂pin₂ (or HBpin). Conditions: THF or hexane, 25–80 °C.
Which bonds are formed and which are broken?
Formed: C–B (aryl boronate); H–Bpin. Broken: aryl C–H; B–B.
What is the stereochemical outcome?
Sterically controlled regioselectivity (meta to substituents)
What controls the rate or the selectivity?
C–H oxidative addition is turnover-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
- C–H activation via concerted metalation-deprotonation (CMD)
- Directed C–H activation via cyclometalation
- 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
- Contact process