Bamberger rearrangement, nitrenium ions

It is now apparent that nitrenium ions occur in a variety of contexts including the so-called Bamberger rearrangement (i.e., acid-catalyzed isomerization of N-hydroxyaniline 14 to 4-aminophenol 15, Fig. 13.9), and the aforementioned catabolism of arylamines. The earlier workers, however, for the most part did not explicitly consider the possibility of such a species. Heller et al. were the first to present kinetic evidence for such an intermediate. Interestingly, they chose to represent it as the iminocyclohexadienyl cation (16), With the customary ellipsis of allowing a single valency structure to represent mesomeric systems. ... 

Although 68e could not be prepared, the less reactive pivalic acid ester analogues 71a-d (Scheme 29) were synthesized. Hydrolysis rate constants at 40°C were pH independent from pH 1.0 to 7.0 and k i,s correlated with to give a of —6.0. The products derived from 71a-c are summarized in Scheme 29. These products are consistent with a nitrenium ion mechanism. In particular, the products and product ratios derived from the meta-hxomo ester 71b were consistent with those previously reported for 68e and the Bamberger rearrangement of N-(3-bromophenyl)hydroxylamine, with the exception of the rearrangement products. The rearrangement products 72 appear to be derived from intramolecular aminolysis of 73. These... 

The application of the azide clock methodology to nitrenium ions was made by Fishbein and McClelland who showed that NJ trapped a reactive intermediate identified as the nitrenium ion 75m, during the Bamberger rearrangement of N-(2,6-dimethylphenyl)hydroxylamine (Scheme 33)7 Kinetic studies showed that the NJ-solvent partitioning occurred after the rate-limiting step of the reaction so an Sn2 process could be eliminated. The selectivity ratio, was determined to be 7.5 M . Assuming that k is... 

The acid-catalyzed conversion of A-hydroxyanilines 177 intopara-hydroxyanilines 179 is well-known as the Bamberger rearrangement (see reviews in References 23,265-267). It is the opinion now that this intermolecular reaction follows a mechanism of monomolec-ular Svl nucleophilic substitution via intermediate nitrenium ion 178 (see review in Reference 268) (equation 70). 

Fishbein, J. C., McClelland, R. A., Halide Ion Trapping of Nitrenium Ions Formed in the Bamberger Rearrangement of N Arylhydroxylamines. Lifetime of the Parent Phenylnitrenium Ion in Water, Can. J. Chem. 1996, 74, 1321 1328. 

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