Q5(b). Cope elimination of an amine oxide

Introduction. A tertiary amine oxide with a β-hydrogen decomposes on heating (100–150°C) into an alkene and N,N-dimethylhydroxylamine. This is the Cope elimination: an intramolecular syn elimination (Ei) through a five-membered cyclic transition state, with no base or ion intermediates.

Scheme: The O⁻ removes a β-H on the same side as the C–N bond.

Figure: Sketch of the Cope transition state: O⁻ takes the syn β-H, the C–H pair becomes the C=C π bond, and the C–N pair stays on N.

1. The substrate is (1-methylcyclopentyl)dimethylamine oxide. The β-hydrogens are on the ring CH₂ groups (C-2, C-5; only the one H on each that is cis to the C–N bond can be used) and on the methyl group (three H, any of which can rotate into place). Counted alone, the H atoms favour the methyl side: the Cope elimination, like the Hofmann elimination, usually removes H from the β-carbon with the most H.
2. In the transition state the O⁻ pulls a β-H while the C–N bond breaks; the five atoms (C_α, C_β, H, O, N) must lie nearly in one plane, so the H and the N-O group must be syn-periplanar.
3. In a cyclopentane ring a ring C–H next to C–N can be eclipsed with it (the ring is nearly planar and flexible), so a ring H can reach the planar transition state as easily as a methyl H. When more than one β-H can reach it, the product ratio follows the stability of the competing transition states; the one leading to the trisubstituted endocyclic alkene is lower: X = 1-methylcyclopent-1-ene is expected as the major product (the selectivity is not high).
4. Removing a methyl H gives methylenecyclopentane (disubstituted, exocyclic), the minor product. (In the six-membered analogue the ring H cannot be syn-periplanar to an equatorial or axial C–N in the chair, and methylenecyclohexane becomes the major product.)
5. By-product: (CH₃)₂N-OH. The elimination is concerted; no rearrangement of the carbon skeleton occurs.

Conclusion. X = 1-methylcyclopentene, formed by Cope (Ei) syn elimination through a planar five-membered transition state; methylenecyclopentane is the minor alkene and Me₂NOH the by-product.

Application. The mild Cope elimination, which gives no skeletal rearrangement, is used to make alkenes from amines (via H₂O₂ oxidation) and to make trans-cyclooctene from N,N-dimethylcyclooctylamine oxide.