Page 1116 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1092 hydrogen peroxide or t-butyl hydroperoxide. A quite different mechanism, involving
conjugate nucleophilic addition, operates in this case. 74
CHAPTER 12
Oxidations –
O O O OH O
O
–
RCCH CHCH + OOH RC C CHCH 3 RC H + OH –
3
H
H CH 3
There have been a number of computational studies of the epoxidation reaction.
These studies have generally found that the hydrogen-bonded peroxy acid is approx-
imately perpendicular to the axis of the double bond, giving a spiro structure. 75
Figure 12.8 shows TS structures and E values based on B3LYP/6-31G computations.
∗
a
The E trend is as expected for an electrophilic process: OCH < CH ∼ CH = CH <
a
3
2
3
∗
H < CN. Similar trends were found in MP4/6-31G and QCISD/6-31G computations.
∗
The stereoselectivity of epoxidation with peroxycarboxylic acids has been well
studied. Addition of oxygen occurs preferentially from the less hindered side of the
76
molecule. Norbornene, for example, gives a 96:4 exo:endo ratio. In molecules where
two potential modes of approach are not very different, a mixture of products is formed.

Fig. 12.8. Comparison of epoxidation transition structures and activation energies for ethene and substi-
tuted ethenes. Reproduced from J. Am. Chem. Soc., 119, 10147 (1997), by permission of the American
Chemical Society.

74 C. A. Bunton and G. J. Minkoff, J. Chem. Soc., 665 (1949).
75 R. D. Bach, M. N. Glukhovtsev, and C. Gonzalez, J. Am. Chem. Soc., 120, 9902 (1998); K. N. Houk,
J. Liu, N. C. DeMello, and K. R. Condroski, J. Am. Chem. Soc., 119, 10147 (1997).
76
H. Kwart and T. Takeshita, J. Org. Chem., 28, 670 (1963).