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Lossen rearrangement

Chemical rearrangement reaction

Lossen rearrangement

Chemical rearrangement reaction

The Lossen rearrangement is the conversion of an acyl hydroxamate to an isocyanate and carboxylic acid side product. The isocyanate can be used further to generate ureas in the presence of amines or generate amines in the presence of H2O.

Reactant is an acylated hydroxamate. Reaction is heat- or base-mediated. Products are an isocyanate and carboxylic acid.
General reaction scheme for the Lossen rearrangement.

Reaction mechanism

The acyl hydroxamate is first converted to its conjugate base. A concerted rearrangement event then occurs in which the amide substituent migrates to the amide nitrogen, the nitrogen donates a pair of electrons to form the isocyanate C=N double bond, and the N–O σ-bond breaks.

Nitrogen of the general acyl hydroxomate is deprotonated and shown to be in equilibrium with its conjugate base. Three curved arrows are used to show the concerted rearrangement of the deprotonated acyl hydroxomate to isocyanate and carboxylic acid side product. One arrow begins at the sigma bond between the amide carbon and the migrating substituent and ends at the nitrogen atom. Another arrow begins from the nitrogen lone pair and ends at the C-N sigma bond. And another arrow begins at the N-O sigma bond and ends at oxygen atom.
Arrow pushing mechanism for the Lossen rearrangement.

Historical references

References

References

  1. (2010). "Comprehensive organic name reactions and reagents". John Wiley & Sons, Inc..
  2. Yale, H. L.. (1943). "The Hydroxamic Acids". [[Chem. Rev.]].
  3. (1974). "The Chemistry of Hydroxamic Acids and N-Hydroxyimides". [[Angew. Chem. Int. Ed. Engl.]].
  4. (1991). "Degradation Reactions". Comprehensive Organic Synthesis.
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