Organocatalysis — enamine activation
Proline-catalyzed aldol reaction
How the Proline-catalyzed aldol 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: L-Proline (10–30 mol%), acetone (donor), aldehyde (acceptor). Conditions: DMSO or DMF, 25 °C. Bonds formed: C–C (aldol); bonds broken: C=O π (aldehyde). Stereochemistry: High ee; Zimmerman–Traxler-like TS with COOH H-bond. Rate and selectivity: C–C bond formation from enamine is rate-limiting.
At a glance
Step 1: Proline + ketone → enamine
Proline's secondary amine condenses with a ketone donor to form an enamine — the same Schiff-base-derived enamine chemistry used by the aldolase enzyme covered much earlier, just with a small, chiral organic molecule standing in for the enzyme's active-site lysine.
Step 2: COOH directs the aldehyde; face-selective attack
Proline's own pendant carboxylic acid hydrogen-bonds to and activates the incoming aldehyde electrophile while simultaneously directing which face of the enamine attacks it — a single small molecule performing both the nucleophile-activation and the stereocontrol roles that a whole enzyme active site usually provides.
Step 3: Hydrolysis releases the aldol
Hydrolysis releases the aldol product with high enantioselectivity and regenerates proline — this discovery, that a simple amino acid could replicate enzyme-like aldolase stereocontrol, helped launch the field of asymmetric organocatalysis.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- Ph
- Phenyl. –C₆H₅.
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- ee
- Enantiomeric excess. How much more of one enantiomer than the other is present.
How it compares with related reactions
| Proline-catalyzed aldol reaction | MacMillan iminium organocatalysis | CBS reduction (Corey-Bakshi-Shibata) | |
|---|---|---|---|
| Reagents | L-Proline (10–30 mol%), acetone (donor), aldehyde (acceptor) | Chiral imidazolidinone·HCl (5–20 mol%), enal, nucleophile or diene | (S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂ |
| Conditions | DMSO or DMF, 25 °C | MeOH/H₂O or CH₂Cl₂, 25 °C | THF, –20 to 25 °C, anhydrous |
| Stereochemistry | High ee; Zimmerman–Traxler-like TS with COOH H-bond | High ee: bulky group shields one iminium face | Predictable absolute configuration: hydride to the face set by the chiral boron-N framework |
| Rate / selectivity | C–C bond formation from enamine is rate-limiting | Nucleophilic addition to the iminium is rate-limiting | Hydride transfer is rate-limiting; catalyst turnover fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Proline-catalyzed aldol reaction?
L-Proline (10–30 mol%), acetone (donor), aldehyde (acceptor). Conditions: DMSO or DMF, 25 °C.
Which bonds are formed and which are broken?
Formed: C–C (aldol). Broken: C=O π (aldehyde).
What is the stereochemical outcome?
High ee; Zimmerman–Traxler-like TS with COOH H-bond
What controls the rate or the selectivity?
C–C bond formation from enamine is rate-limiting
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