Nucleophilic catalysis — Michael/aldol tandem
Baylis-Hillman reaction
How the Baylis-Hillman reaction 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: Activated alkene (methyl acrylate), aldehyde, DABCO (10–100 mol %). Conditions: Neat or MeOH/CH₃CN, 25 °C, days (slow). Bonds formed: C–C (α-carbon to carbinol); C=C (regenerated); bonds broken: C=O π; α C–H; C–N. Stereochemistry: Racemic unless chiral amine/Brønsted acid used. Rate and selectivity: Proton transfer/elimination is rate-limiting; accelerated by protic additives.
At a glance
Step 1: Michael addition of the amine
A tertiary amine (like DABCO) adds in a Michael fashion to an activated alkene (like an acrylate), generating a stabilized enolate with a positively charged ammonium tail.
Step 2: Aldol addition to the aldehyde
That enolate then attacks an aldehyde's carbonyl carbon in an ordinary aldol addition, forming a new C–C bond and a new alkoxide.
Step 3: Proton transfer, then elimination of DABCO
An intramolecular proton transfer, followed by elimination of the amine catalyst (an E1cb-type step), regenerates the double bond and releases the catalyst — the product combines both a new C–C bond and a retained alkene, all from one catalytic cycle.
Abbreviations and terms
- MeCN
- Acetonitrile. Polar aprotic solvent.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- DABCO
- 1,4-Diazabicyclo[2.2.2]octane. Unhindered tertiary amine, a good nucleophilic catalyst (Baylis–Hillman).
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- E1cB
- Elimination, unimolecular conjugate base. Deprotonation first to a carbanion, then loss of the leaving group.
How it compares with related reactions
| Baylis-Hillman reaction | Robinson annulation | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | Activated alkene (methyl acrylate), aldehyde, DABCO (10–100 mol %) | Ketone + methyl vinyl ketone (MVK); NaOEt or KOH | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | Neat or MeOH/CH₃CN, 25 °C, days (slow) | EtOH, 25–80 °C; heat for dehydration | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Racemic unless chiral amine/Brønsted acid used | Quaternary stereocentre; often racemic (proline gives ee) | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Proton transfer/elimination is rate-limiting; accelerated by protic additives | Michael first, then intramolecular aldol; E1cB dehydration last | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Baylis-Hillman reaction?
Activated alkene (methyl acrylate), aldehyde, DABCO (10–100 mol %). Conditions: Neat or MeOH/CH₃CN, 25 °C, days (slow).
Which bonds are formed and which are broken?
Formed: C–C (α-carbon to carbinol); C=C (regenerated). Broken: C=O π; α C–H; C–N.
What is the stereochemical outcome?
Racemic unless chiral amine/Brønsted acid used
What controls the rate or the selectivity?
Proton transfer/elimination is rate-limiting; accelerated by protic additives
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam