Mechanism:
Reaction starts with protonation of hydroxyl group of the oxime by acid. Removal of a water molecule promotes the transfer of alkyl group on the nitrogen atom generating a carbocation. The carbocation then rearranges into quaternary amine ion which act as an excellent electrophilic site. Water molecule attacks on the sp hybridized carbon atom as a nucleophile and then deprotonation regenerates the acidic proton (catalyst) and gives amide as the final product. [1][2][3]
Salient Features:
1. Oxime that serves as a starting material for Beckmann Rearrangement is prepared by the reaction of ketone with hydroxylamine.
2. Rearrangement of amide-oximes to urea derivatives is known as Tiemann Reaction.
3. Reaction is stereospecific in nature. Alkyl group that is anti to the hydroxyl group always migrates.
Examples:
Ex-1 [5]
Ex-2 [6]
Ex-3 [7]
Ex-4 [8]
Ex-5 [9]
References:
1. T. Law, A. Plagens, Named Organic Reactions, John Wiley and Sons, Inc., New York, 1998, pp. 22-24. DOI Link
2. M. B. Smith, J. March in March's Advanced Organic Chemistry, 5th ed., John Wiley and Sons, Inc., New York, 2001, 1349, 1381, 1384, 1415-1416. DOI Link
3. L. G. Donaruma, W. Z. Heldt, Organic Reactions 11, 1. DOI Link
4. R. E. Gawky, Organic Reactions 35, 1. DOI Link
5. J. A. Robl. E. Dieber-McMaster. R. Sulskv. Tetrahedron Letters 1996, 37, 8985. DOI Link
6. M. Han, D. F. Covey, Journal of Organic Chemistry 1996, 61, 7614. DOI Link
7. O. Muraoka, B.Z. Zheng, K. Okumura, G. Tanabe, T. Momose, C. H. Eugster, Journal of the Chemical Society, Perkin Transactions 1, 1996, 13, 1567 DOI Link
8. A. N. Komdtsu, S. Simizu, T. Sugita, Synthetic Communications 1992, 22, 277 DOI Link
9. P. W. Jeffs, G. Molina, N. A. Cortex, P. R. Hauck, J. Wolfram, Journal of Organic Chemistry 1982, 47, 3876. DOI Link