Reactions of Alkenes with Singlet Oxygen

Among the oxidants that add oxygen at carbon-carbon double bonds is singlet oxygen.161 For most alkenes this reaction proceeds with the removal of an allylic 

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. 

5e (C2H5)3SiH + 03 — (C2H5)3SiOOOH — (C2H5)3SiOH + 1Oz 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165 

Many alkenes present several different allylic hydrogens, and in this type of situation it is important to be able to predict the degree of selectivity.166 A useful generalization is that there is a preference for removal of a hydrogen from the more congested side of the double bond.167 

Singlet oxygen decays to the ground-state triplet oxygen at a rate which is strongly dependent on the solvent. Measured half-lives range fi om about 700 ps in carbon 

Wasserman and R. W. Murray, eds.. Singlet Oxygen, Academic Press, New York, 1979 A. A. Primer, Chem. Rev. 79 359 (1979) A. Primer, ed., Singlet Oxygen, CRC Press, Boca Raton, Florida, 1985 C. S. Foote and E. L. Clennan, m Active Oxygen in Chemistry, C. S. Foote, J. S. Valentine, A. Greenberg, and J. F Liebman, eds., Blackie Academic Professional, London, 1995, pp. 105-140. 

SECTION 12.2. ADDITION OF OXYGEN AT CARBON-CARBON DOUBLE BONDS 

Many alkenes present several different allylic hydrogens, and in this type of situation 

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Autoxidation reaction of alkenes with singlet oxygen... 

Reaction of Alkenes With Singlet Oxygen in Microreactors... 

OLy Epoxy alcohols In the presence of Ti(0-/-Pr)4, allylic hydroperoxides are converted into a,p-epoxy alcohols. The precursors are readily available by the ene reaction of alkenes with singlet oxygen. The oxygen transfer involves the corresponding allylic alcohol. 

In the case of ene reactions between cycloalkene or alkenes and diethyl azodi-carboxylate (DEAD) (e.g. Table 2.9, entry (3)), the ratio (AV AV) was foimd to be smaller than unity (0 < 1). This result can be considered as an indication of a stepwise process in which a pericyclic transition state is not involved. Stephenson and Mattern, however, observed that the ratios S/R and kH/kp in the ene reaction with DEAD as enophile shown in Table 2.9 entry (4), were roughly equal, which was explained in terms of a concerted ene reaction. To explain this discrepancy fenner and coworkers proposed a mechanism comparable to the formal ene reactions of alkenes with singlet oxygen or triazolindiones as enophiles forming in the first step three-membered rings between the alkenes and one center of the enophile prior to hydrogen transfer. To clarify the mechanism of the ene reaction with DEAD, it... 

A. A. Frimer. Chemical Reviews, volume 79, American Chemical Society (1979), p. 3S9. The reactions and their mechanisms of alkenes with singlet oxygen are the subjects of this review. 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

Dioxetanes. These unstable compounds have been isolated from the reactions of alkenes and singlet oxygen (5,487-488). Kopecky et al have reported a nonphotochemical route to 1,2-dioxetanes from (S-halo hydroperoxides, prepared by reaction in ether of an alkene, 98% HjOj, and 1,3-dibromo- or I, .l-diiodo-5,5-dimethylhydantoin (1, 208). Treatment of jS-halo hydroperoxides derived from trisubsLiluted olefins with sodium hydroxide in methanol at low lemperatures gives 1,2-dioxetanes, as formulated fortrimethyl-l,2-dioxetane (1) ... 

AUylic hydroperoxides. The product ratios of the allylic hydroperoxides obtained on oxidation of alkenes with singlet oxygen differ significantly from those obtained by base-catalyzed isomerization of /3-halo hydroperoxides, which involves perepoxide intermediates. A third mechanism must be operating in the reaction of triphenyl phosphite ozonide (3, 323-324 4, 559). This last reaction presumably proceeds by an ionic mechanism, since singlet oxygen is not formed at — 70° from the ozonide. ... 

Ene reactions of simple alkenes with singlet oxygen have been studied by both computational and experimental methods.56,57,59 The reactions may proceed via a concerted or a stepwise mechanism [Equation (9)]. For a stepwise mechanism, four types of intermediates, biradical, zwitterion, perepoxide,... 

Diastereomeric ratios of 19 81 to 16 84 (R, R )/(R, S )] are reported for the reactions of chiral trisubstituted alkenes with singlet oxygen. With 2-methyl-4-phenyl-2-pentene, two regioiso-meric products, secondary and tertiary allylic hydroperoxides, are formed in a 93 7 ratio40. 

The kinetic hydrogen isotope effects which have been observed in the hydrogen-ene reactions of deuterium-labelled alkenes with singlet oxygen, tV-pheny I triazolincdione, nitrosopentafluorobenzene, formaldehyde, methyl chloroacrylate, and ethyl propiolate as the enophiles, and the stereoselectivity of the reactions, imply the initial formation of a complex between the alkene group and the enophile. The various products which have been identified in the metalloene reactions can be rationalised on die basis of a similar mechanism (Scheme 9-2).67... 

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