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

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The Lossen rearrangement is the conversion of a hydroxamic acid (1) to an isocyanate (3) via the formation of an O-acyl, sulfonyl, or phosphoryl intermediate hydroxamic acid O-derivative (2) and then conversion to its conjugate base. Here, 4-Toluenesulfonyl chloride is used to form a sulfonyl O-derivative of hydroxamic acid. ^{[1] [2]}

The isocyanate can be used further to generate ureas in the presence of amines (4) or generate amines in the presence of H₂O (5).

The mechanism below begins with an O-acylated hydroxamic acid derivative that is treated with base to form an isocyanate that generates an amine and CO₂ gas in the presence of H₂O. The hydroxamic acid acid derivative is first converted to its conjugate base by abstraction of a hydrogen by a base. Spontaneous rearrangement kicks off a carboxylate anion to produce the isocyanate intermediate. The isocyanate in the presence H₂O hydrolyzes and then decarboxylation via abastraction of a hydrogen by a base generates an amine and CO₂ gas.

Several reviews have been published. ^{[3] [4] [5]}

Hydroxamic acids are commonly synthesized from their corresponding esters.^[6]