The Hofmann and Curtius rearrangement reactions can be applied to synthesize primary amines from carboxylic acid derivatives such as amides and acyl azides. In the Hofmann rearrangement, a primary amide undergoes deprotonation in the presence of a base, followed by halogenation to generate an N-haloamide. A second proton abstraction produces a stabilized anionic species, which rearranges to an isocyanate intermediate via an alkyl group migration from the carbonyl carbon to the neighboring nitrogen. The addition of water to the isocyanate yields a carbamic acid that undergoes spontaneous decarboxylation to produce a primary amine.
The Curtius rearrangement also involves an isocyanate intermediate. Here, however, acyl azides undergo a concerted rearrangement under thermal conditions to generate the isocyanate with the expulsion of a nitrogen molecule. Next, hydration, followed by the loss of CO2, yields the desired amine. The Curtius rearrangement is useful for the synthesis of carbamate esters, primary amines, and urea derivatives by the reaction of carbamic acid with alcohols, water, and amines, respectively.
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