Nitramines rearrangement

Depending on the amine substrate and the nitrating conditions used, it is not uncommon for an intermediate nitramine to undergo direct rearrangement to the ring-nitrated product without prior isolation, in which case, the formation of the nitramine as an intermediate can only be postulated. Due to the high o-selectivity often observed with this type of reaction the o/p-ratio can be very different to that where the aromatic ring is directly nitrated. 

Amine substrates whose rings are strongly deactivated with nitro groups are Al-nitrated with relative ease 2,4-dinitro-A-methylaniline undergoes Al-nitration on treatment with 70 % nitric acid at room temperature. It is known that the Al-nitration of anilines is favoured by 

Chemists at the Naval Air Warfare Center (NAWC), Weapons Division, China Lake, have reported many examples of polynitroarylamine synthesis via Bamberger rearrangements of arylnitramines. The nitration of 4-amino-2,5-dinitrotoluene (36) with a mixture of nitric acid and acetic anhydride in glacial acetic acid at room temperature yields the nitramine (37) which on treatment with neat sulfuric acid, provides 4-amino-2,3,5-trinitrotoluene (38) as the sole product. Nitration of 3,4-dinitroaniline (39) with a solution of nitric acid in acetic anhydride yields A,3,4-trinitroaniline (40) acid-catalyzed rearrangement of the latter in neat sulfuric acid furnishes a 74 % yield of isomeric 2,3,4- (41) and 2,4,5- (42) trinitroanilines in a 4 6 ratio.Accordingly, a mixture of products can be expected when an unsymmetrical arylnitramine has two unsubstituted ortho positions available. 

Reaction mixtures containing an arylnitramine with an o-nitro group require careful work-up because the formation of diazophenols is often a facile and sometimes spontaneous process. The 

Diazophenol formation is most competitive when a nitramine substrate contains an electron-withdrawing nitro group ortho to the nitro group being displaced and hence meta to the nitramine functionality, assumedly because that site is then activated towards nucleophilic aromatic substitution. Heating nitramines in inert chlorinated solvents also favours diazophenol formation but this is suppressed by using urea or sulfamic acid as additives. 

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VI. REARRANGEMENT INVOLVING NITRO GROUPS A. The Nitramine Rearrangement... 

Confirmation of the radical mechanism was provided by Ridd and Sandall35, who detected radical cations by nitrogen-15 NMR of the reaction mixture for the nitramine rearrangement of 2.6-dibromo-jV-nitroaniline and of A -methyl/V-nitroaniline labelled with nitrogen-15 in the nitro group. The results did not allow the authors to indicate whether the products were formed within the solvent cage or from separated radicals. 

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