Heterocycle synthesis — indole via alpha-haloketone
Bischler indole synthesis
How the Bischler indole synthesis 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: Aniline (excess), α-haloketone (PhCOCH₂Br). Conditions: Step 1: 25–80 °C; step 2: acid (or aniline·HBr), 150–200 °C. Bonds formed: C–N (SN2); C–C (ring closure); aromatic pyrrole ring; bonds broken: C–Br; C=O; ortho C–H. Stereochemistry: None. Rate and selectivity: Cyclisation (high T) is slow; SN2 alkylation is fast.
At a glance
Step 1: Aniline N displaces bromide (SN2)
An aniline's nitrogen performs an ordinary SN2 displacement on an α-haloketone, forming an α-amino ketone — nucleophilic substitution exactly like the Williamson ether synthesis, just with an amine nucleophile and a carbon electrophile bearing a ketone.
Step 2: Ortho carbon attacks the activated carbonyl
Tautomerization to the enol form, followed by intramolecular electrophilic attack of the aniline ring onto that same carbon (a Friedel-Crafts-type cyclization, the same logic used throughout the Pictet-Spengler and quinoline-forming reactions), closes a new five-membered ring.
Step 3: Dehydration gives the indole
Dehydration and aromatization give a substituted indole — this provides a distinct disconnection into the indole scaffold from the Fischer indole synthesis covered earlier, building the ring through direct N-alkylation rather than a [3,3]-sigmatropic rearrangement.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Bischler indole synthesis | Leimgruber-Batcho indole synthesis | Madelung indole synthesis | |
|---|---|---|---|
| Reagents | Aniline (excess), α-haloketone (PhCOCH₂Br) | DMF-DMA (Me₂NCH(OMe)₂), pyrrolidine; then H₂, Pd/C or Raney Ni | N-acyl-o-toluidide, NaNH₂ or n-BuLi (≥2 equiv.) or t-BuOK |
| Conditions | Step 1: 25–80 °C; step 2: acid (or aniline·HBr), 150–200 °C | DMF, 110 °C; then H₂ (1 atm), EtOAc or MeOH, 25 °C | Neat or high-boiling solvent, 200–400 °C (milder with BuLi, THF, 25 °C) |
| Stereochemistry | None | None at the product; enamine is the E isomer | None (aromatic product) |
| Rate / selectivity | Cyclisation (high T) is slow; SN2 alkylation is fast | Condensation is slow but drives off MeOH; cyclization is fast | Benzylic deprotonation and ring closure need very strong base |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Bischler indole synthesis?
Aniline (excess), α-haloketone (PhCOCH₂Br). Conditions: Step 1: 25–80 °C; step 2: acid (or aniline·HBr), 150–200 °C.
Which bonds are formed and which are broken?
Formed: C–N (SN2); C–C (ring closure); aromatic pyrrole ring. Broken: C–Br; C=O; ortho C–H.
What is the stereochemical outcome?
None
What controls the rate or the selectivity?
Cyclisation (high T) is slow; SN2 alkylation is fast
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam