Alkenes reactions with singlet oxygen

If the alkene or alkadiene has at least one hydrogen on the carbon adjacent to the double bond, reaction with singlet oxygen may give hydroperoxides. The mechanism of this reaction is related to [4 + 2] cycloadditions and is presumed to occur through a HLickel pericyclic transition state (see Section 21-10D) ... 

Gerdil et al. reported two examples involving the solid-gas reactions of inclusion complexes of tri-o-thymonide with alkene or epoxycyclopentanone. The complex 5 crystallized in /T3 2, and the reaction with singlet oxygen gave endoperoxide 6 however, the [other hand, the reaction of chiral crystals of the complex 7 with hydrogen chloride gave two products 8 and 9, ees of which were 9% and 22%, respectively. [13]... 

Some substituted alkenes react with singlet oxygen to form a dioxetane in a -f- cycloaddition reaction. Most dioxetanes readily decompose to carbonyl compounds in an exothermic reaction that is accompanied by a bluish luminescence. The chemiluminescence will be dealt with in more detail in Section 7.6.4. 

A further interesting extension of the allyl alcohol epoxidation reaction is the metal-catalyzed direct hydroxy-epoxidation of alkenes. In such a reaction an ene reaction with singlet oxygen, 02, or Schenk reaction forming an ally lie hydroperoxide is combined with metal-catalyzed intramolecular epoxidation to form epoxy alcohols For example, with 2,5-dimethyl-2,4-hexadiene (83) two different isomers, 84 and 85, were obtained with Ti(OPr-/)4 and VO(acac)2, respectively (equation 16). Similar reactions were also car-... 

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. ... 

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]. 

Gerdil et al. reported two examples involving the solid-gas reactions of inclusion complexes of tri-o-thymonide with alkene or epoxycyclopentanone. The complex crystallized in a chiral fashion, and the reaction with singlet oxygen or hydrogen chloride gave products up to... 

The enol tautomers of 1-(2, 4, 6 -trialkylphenyl)-2-methyl-1,3-diketones form a range of alkene, epoxide, ether, and hydroperoxide products on reaction with singlet oxygen. The product distribution is substantially affected by the solvent, apparently owing to the disruption of intramolecular hydrogen bonding of the enols in polar solvents. 

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,... 

Apart from high stereo selectivity, the reaction also demonstrates a high and unusual regioselectivity. The reaction of alkene (I) with singlet oxygen yield equal amount of (II) and (III), resulting from hydrogen abstraction from the cis-methyl functionalities. But trans-compound (IV) behaves differently. 

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... 

Autoxidation reaction of alkenes with singlet oxygen... 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

The reaction of cis- and frans-stilbene oxides with phenylphosphonothioic dichloride in the presence of magnesium gives cis- and fra/ts-stilbene and (7).13 Phenylphosphinidene sulphide is postulated as being an intermediate. The zwitterion (8) bears a remarkable similarity to the controversial perepoxides which are thought to be intermediates in the reaction of singlet oxygen with alkenes. 

The reaction of singlet oxygen with cis and trans alkenes shows an unexpected regioselectivity for hydrogen abstraction on the large alkyl group of the double bond . 

Highly regioselective ene reactions of singlet oxygen with alkyl-substituted alkenes can also be used for synthetic purposes. For example, in the photooxygenation ofl-t-butylcycloheptene only one allylic hydroperoxide, by the exclusive geminal hydrogen... 

Pyrano[2,3- ]-l,2,4-trioxines 260a-c arise in low yield from the reaction of the appropriate alkene, 3,4-dihydro-277-pyran 258, with singlet oxygen in the presence of methylene blue (MB), followed by condensation of the intermediate oxetane 259 with acetaldehyde or acetone as appropriate (Scheme 43) <1997H(46)451>. 

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