Beckmann reaction of oximes catalysed by chloral: mild and neutral procedures
Sosale Chandrasekhar* and Kovuru Gopalaiah
*Department of Organic Chemistry, Indian Institute of Science, Bangalore 560
012, India, Email: sosale
orgchem.iisc.ernet.in
S. Chandrasekhar, K. Gopalaiah, Tetrahedron Lett., 2003, 44, 755-756.
DOI: 10.1016/S0040-4039(02)02644-8
Abstract
Various ketoximes undergo the Beckmann rearrangement when heated with 0.5 molar equiv. of chloral (hydrate) in the absence of solvents and acids. Yields of the corresponding amides were excellent after a simple work-up. Aromatic aldoximes were dehydrated to the corresponding nitriles in excellent yields under similar conditions.
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The document discusses a study by Sosale Chandrasekhar and Kovuru Gopalaiah on the Beckmann rearrangement of oximes catalyzed by chloral. The researchers developed a mild and neutral procedure where various ketoximes, when heated with 0.5 molar equivalents of chloral (trichloroacetaldehyde) at around 130°C, undergo the Beckmann rearrangement to yield corresponding amides in excellent yields (80-95%). Aromatic aldoximes under similar conditions dehydrate to form nitriles. The procedure is characterized by the absence of solvents, Brønsted acids, strong Lewis acids, and by-products, making it environmentally friendly. Chloral, known for its electrophilic properties, is generated in situ by heating chloral hydrate in vacuo. The reaction mechanism likely involves the formation of a hemi-acetal intermediate, which breaks down to form the amide, regenerating chloral and enabling a catalytic process. The study highlights chloral's advantages, including its lower toxicity compared to other reagents, cost-effectiveness, and ease of storage. This method offers a simple work-up and solvent-free conditions, making it a valuable improvement for the Beckmann rearrangement.
Key Words
Beckmann Rearrangement, nitriles, catalytic, chloral, green chemistry
ID: J42-Y2003-710
