Page 332 - Mechanism and Theory in Organic Chemistry

Rearrangements to Electron-deficient Nitrogen and Oxygen 319

Because of the high electronegativity of oxygen, an 0-X bond will cleave
heterolytically, producing a positive oxygen, only if X is an excellent leaving
group. As a result, electron-deficient oxygen is formed most frequently in re-
actions of peresters and aromatic peroxides, R-0-0-R' (R' = aryl or acyl).
In these compounds, when -OR' departs, the negative charge on the leaving
group is stabilized by resonance. Even here hcterolytic cleavages are not uni-
versal: The energy required for a heterolytic cleavage in the absence of anchi-
meric assistance is ca. 50 kcal m~le-l,l~~ whereas the energy for a homolytic
cleavage to two alkoxy radicals is only ca. 30-40 kcal m01e-l.l~~ Thus hetero-
lytic cleavage usually takes place only with anchimeric assistance.

The Beckmann Rearrangement146
The acid-catalyzed conversion of ketoximes and aldoximes to amides or amines
(the amide is often hydrolyzed to the corresponding amine under the reaction
conditions) is known as the Beckmann rearrangement after its di~c0verer.l~~
The reaction and its widely accepted mechanism are shown in Equation 6.54.
OH 0
I I I
R-C-R' ,;i-f R-NEC-R' R-N=C-R' - RNH-C-R'
I1 +

The observation that picryl ethers of oximes (106) rearrange without a
catalyst established that the role of the catalyst was to convert the hydroxyl

into a better leaving Some acids catalyze by simply protonating the
oxime as in 107. Other acids may esterify the oximes. For example, Schofield

R\
HvN - ,OH H3p;:3 C?
"\"3
H'+ CH3
\ / \ /
H3C H3C
107 108 109

144 E. Hedaya and S. Winstein, J. Amer. Chem. Soc., 89, 1661, 5314 (1967).
14= S. W. Benson and R. Shaw, in Organic Peroxides, D. Swern, Ed., Wiley-Interscience, New York,
1970, Vol. 1, p. 147.
140L. G. Donaruma and W. Z. Heldt, Org. Reactions, 11, 1 (1960).
14' E. Beckmann, Ber. Deut. Chem. Ges., 20, 1507 (1887).
148 A. W. Chapman and F. A. Fidler, J. Chem. Soc., 448 (1936).