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DOI: 10.1055/sos-SD-040-00241

Purchase, R.; Sainsbury, M.Science of Synthesis, (200940388.

There are a number of rearrangement reactions in which a group bonded to a nitrogen atom migrates to an adjacent carbon atom sometimes when that carbon is part of an aromatic ring.[‌125‌] These processes all have a part to play in the synthesis of amines or their derivatives but are too numerous to provide more than an overview in this section. One of the best known is the FischerHepp reaction {see also Science of Synthesis, Vol. 31b [AreneX (X=N, P) (Sections and]}, which is used to convert N-nitroso-N-alkylarylamines 123 into N-alkyl-4-nitrosoanilines when they are treated with hydrochloric acid (Scheme 35). The rearrangement dates back to 1886[‌126‌] and was once thought to be a dissociative process; however, nowadays the key steps are considered to be intramolecular in nature so that homolytic cleavage of the protonated substrate affords a radical and a radical cation that are held within a solvent cage, before recombination to give the product takes place.[‌127‌,‌128‌] Nevertheless, when the FischerHepp rearrangement of N-methyl-N-nitrosoaniline is conducted in the presence of montmorillonite K 10 clay, N-methyl-4-nitrosoaniline, N-methylaniline, and aniline are formed. In this process the rearrangement product predominates when a polar protic solvent is employed, but if a nitrite trap (hydrazine, sulfamic acid or sodium nitrite) is introduced the rearrangement is completely inhibited.[‌129‌] The FischerHepp change also occurs with N-nitrosodiphenylamines and, when they are heated in hydrochloric acid and acetic acid, they afford N-aryl-4-nitrosoarylamines.[‌130‌,‌131‌] The reaction is not confined to arenes and also works with nitrogen heterocycles, ultimately giving the appropriate amines through the reduction of the initial products. In one example where a heterocycle is involved 9-nitroso-9H-carbazole (124) is treated with acetic acid and nitrous acid at 10°C to provide 3-nitro-9H-carbazole (125), presumably by the oxidation, in situ, of the corresponding 3-nitroso derivative (Scheme 35).[‌132‌,‌133‌]

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