Discussion
The starting material Y in the following reaction is
- A.
- B.
- C.
- D.
Answer: B
Y is (B), 1-bromo-2-nitrobenzene.
This is the Bartoli indole synthesis: an ortho-substituted nitroarene reacts with three equivalents of a vinyl Grignard reagent at low temperature to give a 7-substituted indole. The first equivalent attacks the nitro oxygen and converts the nitro group to a nitroso group, the second adds to the nitroso compound, a [3,3]-sigmatropic rearrangement forms the new C-C bond, and the third acts as a base. The ortho substituent, here Br, is needed for the rearrangement and ends up at C7 of the indole.
The aniline (A) and the azide (C) do not undergo this reaction with vinyl Grignard reagents, and the dibromopyrrole (D) already contains the five-membered ring, so it cannot supply the benzene ring of the product.
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