Hinweise auf den Radikalischen ablauf der meisenheimer-Umlagerung

Thermal Rearrangement of Benzyldimethylamine Oxide

“…The positive entropy of activation (+7.9 cal deg-I) is not consistent with an anionic mechanism, since reactions of neutral inolecules to form ions generally show negative entropies of activation (6). This evidence, combined with that of Schollkopf et al (2), thus clearly points to a homolytic mechanism for this reaction.…”

“…Schollltopf et al attempted to distinguish between carbonium ion, carbanion, and free radical pathways on the basis of a substituent effect (2). Clearly, enhancement of the rate by electron-withdrawing substituents on the benzyl group rules out a carboniuin ion intermediate.…”

“…Den~onstration of a radical intermediate in the one case therefore does not require that a radical intermediate be involved in the other. ICinetic evidence presented by Schollkopf et al (2) excludes a carbonium ion intermediate, but does not clearly distinguish between radical and anionic mechanisms.…”

“…Me?N-0-+CH?CsH:, Recently, a radical inechanisin has been suggested (2). The presence of a methylphenylainine oxide radical during the rearrangement of n~ethylphenylbenzylainine oxide was demonstrated by electron spin resonance.…”

Short-chain aliphatic amines in human urine: a mathematical examination of metabolic interrelationships

“…For tertiary amine oxides with no b-hydrogen, migration of an alkyl group from the nitrogen to the oxygen may occur on heating to yield O,N,N-trisubstituted hydroxylamines (Meisenheimer rearrangement) [23]. This rearrangement has been shown to be predominantly intramolecular [24,25], although free radical or ion pair mechanisms cannot be discounted [26,27]. During the production of aldehydes and secondary amines from aliphatic amine oxides (via acylation with acetic anhydride or acetyl chloride) transfer of oxygen from the initial nitrogen to the adjacent carbon occurs (Polonovski reaction) [28].…”