32 Beckmann Rearrangement
R
R C
N
R'
OH
PCl5 −HCl
C
N
R'
O
PCl4
1 The stereochemical course of the Beckmann rearrangement often allows for the prediction of the reaction product to be obtained; in general the substituent R anti to either the hydroxy or the leaving group will migrate:
R C
N + O H
R' H
R
−H2O
+C
N
R'
In some cases a mixture of the two possible amides may be obtained. This has been rationalized to be a result of partial isomerization of the oxime under the reaction conditions, prior to rearrangement. With aldoximes (R D H) a migration of hydrogen is seldom found. The Beckmann rearrangement therefore does not give access to N-unsubstituted amides. The reaction with oximes of cyclic ketones leads to formation of lactams (e.g. 6 ! 7) by ring enlargement:
N
OH
H N
6
O
7
This particular reaction is performed on an industrial scale; ε-caprolactam 7 is used as monomer for polymerization to a polyamide for the production of synthetic fibers. Substituents R, R0 at the starting oxime 1 can be H, alkyl, or aryl.2,3 The reaction conditions for the Beckmann rearrangement often are quite drastic (e.g. concentrated sulfuric acid at 120 Ž C), which generally limits the scope to less sensitive substrates. The required oxime can be easily prepared from the respective aldehyde or ketone and hydroxylamine. 1. E. Beckmann, Ber. Dtsch. Chem. Ges. 1886, 19, 988–993. 2. L. G. Donaruma, W. Z. Heldt, Org. React. 1960, 11, 1–156. R. E. Gawley, Org. React. 1988, 35, 1–420.