Bromine compounds, Hofmann rearrangement

Scheme 9.103. The Hofmann rearrangement, which results in the loss of carbon dioxide (CO2) and the production of an amine from oxidation of an amide with bromine (Br ) or chlorine (CI2).The oxidation produces the A -halo-compound, which, on treatment with base, undergoes rearrangement to generate the N-alkylisocyanate (cyclohexylisocyanate in this case). Treatment of the Af-alkyhsocyanate with water, produces the transient carbamic acid. The carbamic acid undergoes spontaneous decarboxylation in water to carbon dioxide (CO2) and cyclohexylamine (Chapter 10). Details of the Hofinann rearrangement will be discussed subsequently. But see Hofmann, A. W. Chem. Ber., 1881,7 /, 2725.

The same isocyanate derivative is also seen in the Hofmann rearrangement (Scheme 10.36). TTiat is, if the carboxylic acid chloride from the 2-methylbutanoic acid is first converted to the corresponding amide by reaction with ammonia (NH3) and the latter treated with bromine (Br2) and sodium hydroxide (i.e.,NaOBr formed from reaction of the two reagents), the corresponding A -bromo compound forms. 

A number of methods involve the rearrangement of carboxylic acid derivatives via nitrenes. The best known of these is the Hofmann degradation of amides. This involves treating an amide with bromine and alkali. The A-bromo compound undergoes an a-elimination in the presence... 

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