Oxonium alkylation
Meerwein's salt alkylation
How the Meerwein's salt alkylation 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: Et₃O⁺ BF₄⁻ (Meerwein salt). Conditions: CH₂Cl₂, 0–25 °C, anhydrous. Bonds formed: Nu–CH₂CH₃ bond; Et₂O; bonds broken: C–O⁺ (oxonium) bond. Stereochemistry: Inversion at carbon (SN2) — invisible for ethyl. Rate and selectivity: Very fast; excellent leaving group (neutral ether).
At a glance
Step 1: A stable but potent ethyl donor
Triethyloxonium tetrafluoroborate (Meerwein's salt) is a stable, isolable trialkyloxonium alkylating agent — its ethyl groups are activated toward transfer by the highly stable, neutral diethyl ether byproduct that departure will form.
Step 2: SN2 by a weak nucleophile
A weak nucleophile — even one far too poor to displace an ordinary halide, like an amide oxygen or a pyridine nitrogen — attacks one of the electrophilic ethyl carbons directly, in a straightforward SN2 displacement.
Step 3: Alkylated cation + diethyl ether
Loss of neutral diethyl ether gives the alkylated product, often as a new, more reactive cationic species (like an activated imidate from an amide) — because Meerwein's salt is such an aggressive alkylating agent, it can functionalize nucleophiles that no ordinary alkyl halide or sulfonate ester would touch.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Meerwein's salt alkylation | Delépine reaction | Stork enamine alkylation | |
|---|---|---|---|
| Reagents | Et₃O⁺ BF₄⁻ (Meerwein salt) | Hexamethylenetetramine (HMTA), then HCl/EtOH (or H₃O⁺) | Pyrrolidine, cat. TsOH, then R–X (or Michael acceptor), then H₃O⁺ |
| Conditions | CH₂Cl₂, 0–25 °C, anhydrous | CHCl₃ or EtOH, reflux; then ethanolic HCl | Toluene, Dean–Stark, then CH₃CN, reflux; aqueous acid |
| Stereochemistry | Inversion at carbon (SN2) — invisible for ethyl | Inversion at a stereogenic primary/benzylic carbon (SN2) | Less substituted enamine favoured; monoalkylation |
| Rate / selectivity | Very fast; excellent leaving group (neutral ether) | SN2 step rate-limiting; works for benzylic, allylic, α-halo carbonyls | Alkylation (SN2) is rate-limiting; reactive halides (allylic, benzylic, α-halo carbonyl) work best |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Meerwein's salt alkylation?
Et₃O⁺ BF₄⁻ (Meerwein salt). Conditions: CH₂Cl₂, 0–25 °C, anhydrous.
Which bonds are formed and which are broken?
Formed: Nu–CH₂CH₃ bond; Et₂O. Broken: C–O⁺ (oxonium) bond.
What is the stereochemical outcome?
Inversion at carbon (SN2) — invisible for ethyl
What controls the rate or the selectivity?
Very fast; excellent leaving group (neutral ether)
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go