Page 1142 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1118 hydrogen and shift of the double bond to provide an allylic hydroperoxide as
the initial product.
CHAPTER 12
Oxidations O H O OH
O

The allylic hydroperoxides generated by singlet oxygen oxidation are normally reduced
to the corresponding allylic alcohol. The net synthetic transformation is then formation
of an allylic alcohol with transposition of the double bond.
A number of methods of generating singlet oxygen are summarized in
Scheme 12.16. Singlet oxygen is usually generated from oxygen by dye-sensitized
photoexcitation. Porphyrins are also often used as sensitizers. An alternative chemical
1
means of generating O involves the reaction of hydrogen peroxide with sodium
2
hypochlorite (Entry 2). The method in Entry 3 involves formation of unstable
trioxaphosphetane intermediates from O and phosphine or phosphate esters. The
3

adducts are formed at low temperature (−70 C) and decomposition with generation of

singlet oxygen occurs at about −35 C. The peroxide intermediate in Entry 4 is formed

by photolytic addition of oxygen to diphenylanthracene and reacts at around 80 C
1
to generate O . The method in Entry 5 involves formation of an unstable precursor
2
1
of O , a trialkylsilyl hydrotrioxide. The half-life of the adduct is roughly 2.5 min
2
at −60 C.

H ) SiOH
(C H ) SiH + O 3 (C H ) SiOOOH (C 2 5 3 + O O
2 5 3
2 5 3
Scheme 12.16. Generation of Singlet Oxygen
1 a
Photosensitizer + h ν 1 [Photosensitizer] ∗
1 [Photosensitizer] ∗ 3 [Photosensitizer] ∗
3 3 1 O + Photosensitizer
[Photosensitizer]* + O 2 2
2 b
H O + – OCl 1 O H O + Cl –
2 2 2 + 2
3 c O
(RO) P + O 3 (RO) P O (RO) P O+ O 2
1
3
3
3
O
4 d Ph Ph
O 1
O + O 2
Ph Ph
5 e (C H ) SiH + O 3 (C 2 5 3 (C H ) SiOH + 1 O 2
H ) SiOOOH
2 5 3
2 5 3
a. C. S. Foote and S. Wexler, J. Am. Chem. Soc., 86, 3880 (1964).
b. C. S. Foote and S. Wexler, J. Am. Chem. Soc., 86, 3879 (1964).
c. R. W. Murray and M. L. Kaplan, J. Am. Chem. Soc., 90, 537 (1968).
d. H. H. Wasserman, J. R. Sheffler, and J. L. Cooper, J. Am. Chem. Soc., 94, 4991 (1972).
e. E. J. Corey, M. M. Mehotra, and A. U. Khan, J. Am. Chem. Soc., 108, 2472 (1986).