Discussion
Predict the major product of the following reaction :
- A.
- B.
- C.
- D.
Answer: B
The product is (B), with NHR meta to OMe.
Diazotisation of an anthranilic acid gives the benzenediazonium-2-carboxylate, which loses N2 and CO2 to form benzyne. Here the methoxy group ends up next to one end of the triple bond: 3-methoxybenzyne.
The amine adds to the benzyne carbon farther from OMe. That puts the developing aryl anion on the carbon next to OMe, where the electronegative oxygen stabilises it inductively; protonation then gives the 3-methoxy (meta) product. Addition to the near carbon would place the anion away from OMe and give the ortho product (A), the minor pathway. (C), the para product, cannot come from a 2,3-benzyne at all, and (D) would need two additions.
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