Pd-catalyzed cross-coupling

Hiyama coupling: full catalytic cycle

How the Hiyama coupling works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.

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In short. A 3-step mechanism. Typical reagents: ArX, Ar′Si(OMe)₃ (or ArSiF₃), Pd(0) cat., TBAF (F⁻ activator). Conditions: THF or DMF, 60–100 °C. Bonds formed: Ar–Ar′ (C–C σ); Si–F; bonds broken: C–X; C–Si. Stereochemistry: Retention at sp² carbons; E/Z alkenes preserved. Rate and selectivity: Transmetalation (needs F⁻ activation) is usually rate-limiting.

At a glance

ReagentsArX, Ar′Si(OMe)₃ (or ArSiF₃), Pd(0) cat., TBAF (F⁻ activator)

Solvent / conditionsTHF or DMF, 60–100 °C

Bonds formedAr–Ar′ (C–C σ); Si–F

Bonds brokenC–X; C–Si

StereochemistryRetention at sp² carbons; E/Z alkenes preserved

Rate / selectivityTransmetalation (needs F⁻ activation) is usually rate-limiting

Step 1: Oxidative addition

Pd(0) oxidatively adds an aryl halide, forming Ar–Pd(II)–X.

Step 2: Fluoride activation, then transmetalation

An organosilane, normally too unreactive to transmetalate on its own, is first activated by fluoride (or hydroxide), which coordinates silicon and makes the organic group far more willing to transfer to palladium in the transmetalation step.

Step 3: Reductive elimination

Reductive elimination forms the new C–C bond and regenerates Pd(0) — because organosilanes are exceptionally low in toxicity and easy to handle compared to boron, tin, or zinc partners, Hiyama coupling is often favored for large-scale, environmentally-conscious synthesis.

Abbreviations and terms

Ar: Aryl group. Any aromatic ring substituent such as phenyl.
X: Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
L: Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
Ph: Phenyl. –C₆H₅.
OMe: Methoxy. –OCH₃.
THF: Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
DMF: N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
TBAF: Tetrabutylammonium fluoride. Soluble fluoride source; strong Si–F bond cleaves silyl ethers.
L2: Ligand set. Spectator ligands on the metal that tune its reactivity.

How it compares with related reactions

	Hiyama coupling	Buchwald-Hartwig amination	Heck reaction
Reagents	ArX, Ar′Si(OMe)₃ (or ArSiF₃), Pd(0) cat., TBAF (F⁻ activator)	ArBr, R₂NH, Pd₂(dba)₃ / bulky phosphine (XPhos, BINAP), NaOtBu	ArBr, alkene (e.g. methyl acrylate), Pd(OAc)₂/PPh₃, Et₃N
Conditions	THF or DMF, 60–100 °C	Toluene or dioxane, 80–110 °C	DMF or MeCN, 80–120 °C
Stereochemistry	Retention at sp² carbons; E/Z alkenes preserved	No new stereocentre; retention of aryl geometry	Syn insertion, syn β-H elimination; E-alkene favoured
Rate / selectivity	Transmetalation (needs F⁻ activation) is usually rate-limiting	Bulky, electron-rich ligand speeds oxidative addition and C–N reductive elimination	Aryl adds to the less hindered alkene carbon

Test yourself

Try to answer each question before opening it.

Which reagents and conditions are typical for the Hiyama coupling?

ArX, Ar′Si(OMe)₃ (or ArSiF₃), Pd(0) cat., TBAF (F⁻ activator). Conditions: THF or DMF, 60–100 °C.

Which bonds are formed and which are broken?

Formed: Ar–Ar′ (C–C σ); Si–F. Broken: C–X; C–Si.

What is the stereochemical outcome?

Retention at sp² carbons; E/Z alkenes preserved

What controls the rate or the selectivity?

Transmetalation (needs F⁻ activation) is usually rate-limiting

Study guides that help with this reaction

Related mechanisms

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