Cope elimination
/ kohp /
The Cope elimination is a gentle way to turn an amine into an alkene without using any strong base or acid — the molecule eliminates itself, internally, when warmed. It is the close relative of the Hofmann elimination, but it goes through an amine oxide and a tidy cyclic transition state rather than a charged quaternary salt and an external base.
You run it in two steps. First, oxidize a tertiary amine with hydrogen peroxide (or a peracid) to make an amine oxide, R3N+-O-, which carries a negatively charged oxygen sitting right next to the nitrogen. Second, simply heat it. The oxygen swings around and plucks a hydrogen from the carbon two bonds away (the beta carbon) while the C-N bond breaks, all in one concerted motion through a five-membered ring transition state. Because the hydrogen and the leaving group must reach each other within that ring, the geometry is syn (the H and the nitrogen leave from the same face) — the opposite arrangement to the anti-periplanar E2.
Two features make it worth knowing. Like the Hofmann elimination, it tends to give the less-substituted (Hofmann) alkene, because the bulky amine oxide prefers to grab a hydrogen from the least hindered beta carbon. And because it needs no added base and no acidic conditions, it is prized for making alkenes in delicate molecules that would not survive harsher elimination — the only requirements are the syn-coplanar hydrogen and a bit of heat. It is the intramolecular, base-free cousin of Hofmann elimination.
Oxidize N,N-dimethyl-2-phenyl-1-aminopropane to its amine oxide and heat: the oxygen pulls a syn beta-hydrogen and the C-N bond breaks, giving the alkene plus N,N-dimethylhydroxylamine, all with no external base.
A self-contained, base-free elimination through a neat five-membered ring.
The Cope elimination is syn (H and N leave from the same face through a cyclic transition state) — exactly opposite to the anti-periplanar E2 — and like Hofmann elimination it favours the less-substituted alkene.