Curtius Rearrangement of Acyl Azides to Isocyanates

by James

Description: The Curtius rearrangement is a rearrangement of acyl azides to give isocyanates, expelling nitrogen gas. The isocyanates can then be used in other reactions to give products such as ureas and carbamates.
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Notes: Although acid halides are generally used, acid bromides or anhydrides are also good substrates here.

Examples:

Notes: Example 1 shows a Curtius rearrangement which gives the isocyanate. In example 2, treatment of the isocyanate (formed after step 2) with methanol gives a carbamate (shown). If the isocyanate is treated with water, the product loses CO2, forming an amine.

Mechanism: In step 1, addition of azide anion to the acid chloride provides a tetrahedral intermediate (Step 1, arrows A and B) which then eliminates chloride ion (Step 2, arrows C and D). Upon heating, rearrangement occurs, forming a new C-N bond and breaking the C-C bond, leading to loss of nitrogen (Step 3, arrows E and F). A resonance form of the carbocation formed can be drawn (arrow G) that represents the isocyanate.

Notes: Different nucleophiles can be added to isocyanates. When alcohols are added to isocyanates, the products are called carbamates. When amines are added to isocyanates, ureas are formed. Finally, when water is added to isocyanates, the product is unstable and loses carbon dioxide (similar to the Hoffman rearrangement); the final product is an amine.

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