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40.1.1.4.1.5.2 Variation 2: Hofmann–Martius Rearrangement

DOI: 10.1055/sos-SD-040-00241

Purchase, R.; Sainsbury, M.Science of Synthesis, (200940389.

In another classical rearrangement an anilinium chloride bearing a single N-alkyl group is transformed into a mixture of 2- and 4-alkylanilinium chlorides on heating at a temperature greater than 200°C (Scheme 36).[‌134‌] The reaction is known as the HofmannMartius rearrangement {see also Science of Synthesis, Vol. 31b [AreneX (X=N, P) (Section 31.28.1.33.3)]. The migration is intermolecular in nature and there is evidence that an alkyl chloride 126 and an aniline 127 are initially formed (Scheme 36).[‌135‌] This electrophilic alkyl chloride subsequently enters into a FriedelCrafts alkylation of the aniline, or its equivalent, to afford a mixture of 2- and 4-alkylated anilines in a ratio that depends upon the alkyl chloride that is generated; polyalkylated products may also form. In at least one case the drastic conditions traditionally employed to bring about such rearrangements can be reduced by the use of an acid catalyst; thus, 3-(N-phenylamino)indol-2-one 128 is converted into a mixture of 3-(2-aminophenyl)indol-2-one 129A and 3-(4-aminophenyl)indol-2-one 129B simply by treatment with a catalytic amount of trifluoroacetic acid in benzene at 25°C. Its analogue 130 gives the related indol-2-ones 131A and 131B under the same conditions or, alternatively, by heating it in toluene (without trifluoroacetic acid) at 80°C (Scheme 36).[‌136‌]

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