Discussion
The major product formed in the following reaction sequences is
- A.
- B.
- C.
- D.
Answer: B
The major product is (B), ethyl indole-3-acetate.
(i) K2CO3 lets the secondary amine displace the allylic bromide, giving the N-benzyl-N-(4-ethoxycarbonylbut-2-enyl) aryl iodide. (ii) Pd(0) adds oxidatively to the C-I bond, and an intramolecular Heck reaction closes a five-membered ring (5-exo); beta-hydride elimination and isomerisation give the aromatic indole with a CH2CO2Et group at C3. (iii) H2 over Pd/C in acetic acid removes the N-benzyl group by hydrogenolysis, giving the free N-H indole.
The quinoline-type products (A) and (C) would need a six-membered (6-endo) cyclisation, which the intramolecular Heck reaction does not favour here. (D), the indoline, would need the aromatic indole C2=C3 bond to be hydrogenated, which these mild conditions do not do.
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