Inorganic — π-allyl palladium chemistry
Tsuji-Trost allylic substitution
How the Tsuji-Trost allylic substitution 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: allylic acetate/carbonate, soft Nu–H (e.g. dimethyl malonate), base, cat. Pd(PPh₃)₄. Conditions: THF, 25–65 °C. Bonds formed: allylic C–Nu bond; bonds broken: allylic C–O (leaving group). Stereochemistry: Soft Nu: net retention (inversion at ionisation + inversion at attack). Rate and selectivity: Ionisation to the π-allyl complex is usually rate-limiting.
At a glance
Step 1: Ionisation: π-allyl Pd(II)
Pd(0) oxidatively adds an allylic substrate (an allylic acetate, halide, or carbonate) in an SN2-like, backside-attacking process, ionizing the leaving group and forming a symmetric, resonance-stabilized π-allyl palladium complex.
Step 2: Soft Nu attacks the allyl terminus
Because the allyl group is bound symmetrically across both its termini, a nucleophile can attack either end — for a 'soft' nucleophile (like a stabilized malonate anion), attack happens directly at the allyl carbon, opposite the metal, exactly like backside SN2 on the palladium-activated system.
Step 3: Product and Pd(0)
This releases the substituted allylic product with net overall retention of configuration (two inversions cancel: one at ionization, one at nucleophilic attack) and regenerates Pd(0) — a uniquely versatile way to install nucleophiles at an allylic position with predictable, controllable stereochemistry.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- LG
- Leaving group. The group that departs with the bonding electrons (good ones are weak bases: I⁻, Br⁻, TsO⁻…).
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- Pd(PPh3)4
- Tetrakis(triphenylphosphine)palladium(0). Classic Pd(0) pre-catalyst for cross-coupling.
- L2
- Ligand set. Spectator ligands on the metal that tune its reactivity.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Tsuji-Trost allylic substitution | Trost asymmetric allylic alkylation | Associative ligand substitution at a square-planar Pt(II) complex | |
|---|---|---|---|
| Reagents | allylic acetate/carbonate, soft Nu–H (e.g. dimethyl malonate), base, cat. Pd(PPh₃)₄ | Allylic acetate, soft nucleophile (dimethyl malonate, BSA/KOAc), Pd₂(dba)₃ or [Pd(allyl)Cl]₂, chiral bisphosphine ligand L* (Trost ligand) | K₂[PtCl₄] + NH₃ (aq) |
| Conditions | THF, 25–65 °C | CH₂Cl₂ or THF, 25 °C | H₂O, 25 °C |
| Stereochemistry | Soft Nu: net retention (inversion at ionisation + inversion at attack) | Ligand pocket selects the face and terminus attacked: high ee | Trigonal-bipyramidal intermediate; retention of geometry |
| Rate / selectivity | Ionisation to the π-allyl complex is usually rate-limiting | Oxidative ionisation or nucleophilic attack can be rate-limiting; fast enantiodetermining attack | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Tsuji-Trost allylic substitution?
allylic acetate/carbonate, soft Nu–H (e.g. dimethyl malonate), base, cat. Pd(PPh₃)₄. Conditions: THF, 25–65 °C.
Which bonds are formed and which are broken?
Formed: allylic C–Nu bond. Broken: allylic C–O (leaving group).
What is the stereochemical outcome?
Soft Nu: net retention (inversion at ionisation + inversion at attack)
What controls the rate or the selectivity?
Ionisation to the π-allyl complex 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