Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Named Reactions/Curtius Rearrangement

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The Curtius rearrangement (or Curtius reaction or Curtius degradation), as first defined by Theodor Curtius, is a chemical reaction that involves the rearrangement of an acyl azide to an isocyanate.[1][2] Several reviews have been published.[3][4]

The Curtius rearrangement

The isocyanate can be trapped by a variety of nucleophiles. Often water is added to hydrolyze the isocyanate to an amine.[5] When done in the presence of tert-butanol, the reaction generates Boc-protected amines, useful intermediates in organic synthesis.[6][7]

Carboxylic acids 1 can be easily converted to acyl azides 3 using diphenylphosphoryl azide 2.[8][9][10]

Using DPPA to convert an acid to a BOC-protected amine

Likewise, when the Curtius reaction is performed in the presence of benzyl alcohol, Cbz-protected amines are formed.[11]

Reaction mechanism[edit]

The first step of the Curtius rearrangement is the loss of nitrogen gas forming an acyl nitrene (2). Once formed, acyl nitrenes very quickly rearrange by migration of R-group forming the desired isocyanate (3).

The mechanism of the Curtius rearrangement

Scope[edit]

In one variation called the Darapsky degradation (A. Darapsky, 1936) a Curtius rearrangement takes place as one of the steps from an α-cyanoester to an amino acid.[12]

References[edit]