Oxidation of alkenes with singlet oxygen

Certain compounds react with singlet oxygen in a different manner, giving cyclic 

This reaction is not usually a major factor with alkenes bearing only alkyl substituents, but becomes important with vinyl ethers. Enamino ketones undergo a clean oxidation to an a-diketone  

Singlet oxygen undergoes 4 + 2 cycloaddition reactions with dienes, generating peroxides  

SECTION 10.3. CLEAVAGE OF CARBON-CARBON DOUBLE BONDS 

Osmium tetroxide used in combination with sodium periodate can also effect alkene cleavage cleanly.Successful oxidative cleavage of double bonds using ruthenium 

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Scheme 12.16. Oxidation of Alkenes with Singlet Oxygen... 

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. ... 

Heterogeneous palladium catalysts proved to be active in the conversion of simple alkenes to the corresponding allylic acetates, carbonyl compounds, and carboxylic acids.694 704 Allyl acetate or acrylic acid from propylene was selectively produced on a palladium on charcoal catalyst depending on catalyst pretreatment and reaction conditions.694 Allylic oxidation with singlet oxygen to yield allylic hydroperoxides is discussed in Section 9.2.2. 

In fact, the oxidation with singlet oxygen of alkenes not possessing allylic hydrogens is known to involve a 1,2-dioxetane intermediate15 16 (see Section 9.2.2). 

During the conversion of alkenes into epoxy alcohols by allylic oxidation with singlet oxygen, an oxygen atom is usually removed from the allylic hydroperoxide by reduction, only to be subsequently reintroduced in the resulting epoxy alcohol by epoxidation with an external oxygen donor. 

Other oxidations with singlet oxygen are conversions of alkenes into epoxides [43, of secondary alcohols into ketones via alcohol hydroperoxides [44, 45] (equation 9) and the oxidative degradation of tertiary amines to secondary amines [46] (equation 10). 

Epoxldation. Dye-sensitized photooxidation of aliphatic olefins results in formation of altylic hydroperoxides from reaction with singlet oxygen. Photo-IMiidation with -diketone sensitizers follows a different course epoxides are the foain products. Of several a-diketones examined, epoxides are obtained in highest yields with biacetyl, which is oxidized at the same time mainly to acetic acid. In some cases yields of epoxides in biacetyl photooxidation are 90% or more. Both cis- and trans-alkenes yield mainly tran.r-epoxides. Both epoxidatioh and ketone destruction are much slower with benzil. The nature of this epoxidation reaction is discussed, although many points of ambiguity remain. This reaction is Of practical importance. ... 

Often, the hydroperoxides generated in the oxidation of alkenes are not the ultimately desired product. They can be reduced without extensive prior purification to the corresponding allylic alcohol. The sequence of singlet-oxygen oxidation followed by reduction constitutes a reasonably general method of synthesis of allylic alcohols from olefins with allylic migration of the double bond. Scheme 9.14 records some specific examples. 

Photochemical oxidations of various substrates such as amines [763], alkenes [769,770,772,775,778-780], fiiran [764,765], sulfide [766,773], phytol [767], bi-adamantylidene [768], thiophenolate [771], catechol [774], allylsilane [776], and imines [777], etc., through reactions with singlet oxygen sensitized by porphyrins have been extensively studied. Photochemical atrop isomerization of picket-fence-type porphyrin should be noted here as another interesting type of photoreaction [793]. 

Also among the oxidative reactions that add oxygen at a carbon-carbon double bond is the reaction with singlet oxygen. For most alkenes this reaction proceeds with the specific removal of an allylic hydrogen so that the initial product is an allylic hydroperoxide. This result, along with other detailed mechanistic studies,... 

Alkenes with different aUyhc hydrogens may yield various regioisomeric products on reaction with singlet oxygen. For controUing the regioselectivity of such aUylic oxidations by Oj, several general effects are... 

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... 

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