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47.1.4.5.5 Variation 5: Thermolytic β-Elimination from Ammonium Hydroxides

DOI: 10.1055/sos-SD-047-00322

Kostikov, R. R.; Khlebnikov, A. F.; Sokolov, V. V.Science of Synthesis, (201047824.

The classical Hofmann alkene synthesis from alkylamines (or from alkyl halides) includes the preparation of a quaternary ammonium hydroxide 88 and its pyrolysis at 105150°C. Frequently, the ammonium hydroxides are obtained by reaction of the respective ammonium salts with silver(I) oxide, barium oxide, or ion-exchange resins in situ without isolation of the hydroxide as a pure compound. The reaction proceeds by proton abstraction from an α-carbon position by the hydroxide anion to give an intermediate ammonium ylide. The latter then undergoes an intramolecular fragmentation to an alkene molecule and a tertiary amine. The transition structure 89 of the elimination is a five-membered ring, and the process is a syn elimination (Scheme 34) (see also HoubenWeyl, Vol. 5/1b, pp 219235).[‌192‌]

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