Beckmann Rearrangement of Oximes Catalyzed by Cyanuric Chloride in Ionic Liquids

Abstract

The Beckmann rearrangement of a variety of ketoximes has been carried out in imidazolium-based ionic liquids in the presence of catalytic amounts of 2,4,6-trichloro[1,3,5]triazine. This mild and ‘green’ procedure is highly regioselective affording the corresponding N-substituted amides in very good to quantitative yields. The ionic liquids were easily recovered and recycled several times.

oximes - amides - Beckmann rearrangement - ionic liquids - cyanuric chloride catalysis

References and Notes

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In a typical procedure, the oxime (0.3-1 mmol) was dissolved in the IL (about 1 mL) and added, under stirring, with the appropriate quantity of cyanuric chloride (TCT, 2 or 20 mol%). The mixture was left at the desired temperature (60 °C or 130 °C) and monitored by TLC (eluant EtOAc-light PE) until completion. The amide was isolated by direct chromatography (silica gel) of the reaction mixture (eluant EtOAc-light PE). Alternatively, the mixture was extracted with MTBE (4 × 3 mL) and concentrated under vacuum. The crude was purified on silica gel column to afford the pure product. The amide was identified (¹H NMR) by comparison with the authentic sample. The residue of the IL was dissolved in CH₂Cl₂, filtered on Celite, and reused after removal of the organic solvent under vacuum.

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Note added in proofs: The use of the IL previously treated with a solid base (K₂CO₃) in order to remove residual acid impurities, as suggested by one referee, gave the same results. It was expected because AnalaR grade IL [13b] [15] were used in our experiments.

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