Beckmann rearrangement of acetophenone oximes to the corresponding amides organo-catalyzed by trifluoroacetic acid for sustainable NSAIDs synthesis

• High yield of acetaminophen and acetanilide is obtained under mild conditions.
• Easy Beckmann rearrangement, easy recovery of the reagents, no work-up operation.
• A sustainable process can be implemented since TFA is recycled in high extent.
• TFA acts as an organocatalyst, Brønsted acid catalysis is not verified.
• The key step of the mechanism is the exchange of the trifluoroacetyl group.

The Beckmann rearrangement of acetophenone oximes to the corresponding amides (4-hydroxyacetophenone oxime to N-acetyl-4-hydroxyacetanilide and acetophenone oxime to N-phenylacetamide) is investigated by using trifluoroacetic acid (TFA) as catalyst. The reaction occurs either in the presence or in the absence of a suitable solvent. High selectivity and practically quantitative yield to amide is achieved in both cases at TFA/substrate > 3. Both TFA and the solvent (whenever present) could be easily reused by distillation since no protonation of amides occurs. The reaction proceeds via a multistep reaction path and the role of TFA is related not only to its acidity but also mainly to its ability on forming reactive trifluoroacetylated intermediates. In particular, the highly reactive trifluoroacetylated amide is actually the effective catalyst. Finally, a likely reaction path is proposed.

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