The thermal decomposition of an acyl azide to an isocyanate and nitrogen gas is known as the Curtius rearrangement. The R group of the acyl azide migrates to its nearest nitrogen, releasing nitrogen gas to form the isocyanate product, which can further react with nucleophiles in solution, i.e. exposure to water produces a 1° amine.

• Both protic and Lewis acids can be used to catalyze the reaction, which lowers the reaction temperature. Moreover, photochemical conditions are also possible, but several side-products are to be expected.
• In the absence of nucleophilic solvents, the isocyanate product can be isolated. Exposure to amines and alcohols yield urea derivatives and carbamates respectively.
• Acyl azides can be prepared by (i) reacting acid chlorides or mixed anhydrides with alkali azide or trimethylsilyl azide, (ii) reacting carboxylic acids with diphenyl phosphoryl azide, etc.

• Reactant: Acyl Azide
• Product: Isocyanate
• Type of Reaction: Rearrangement

Mechanism

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