Discussion
The major product of the following reaction is :
- A.
- B.
- C.
- D.
Answer: B
The product is (B), methylenecyclopentane.
H2O2 oxidises the tertiary amine to its N-oxide, and heating brings about the Cope elimination: a concerted, syn elimination through a five-membered cyclic transition state in which the N-oxide oxygen removes a β-hydrogen.
The β-hydrogens available are on the methyl group and on the two ring CH2 groups. Like the Hofmann elimination, the Cope elimination favours the less substituted alkene, here formed by removing a methyl hydrogen, giving the exocyclic methylene. (A), the more substituted endocyclic alkene, would be the Zaitsev product, which this elimination does not favour. (C) and (D) have the double bond away from the carbon that carried nitrogen, which an elimination cannot produce.
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