Oxygen, singlet with alkenes

Autoxidation reaction of alkenes with singlet oxygen... 

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

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. 

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,... 

Reaction of Alkenes With Singlet Oxygen in Microreactors... 

Alberti, M.N. and Orfanopoulos, M. (2006) Stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen. Tetrahedron, 62 (46), 10660-10675. 

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). 

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. 

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. 

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... 

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... 

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. ... 

---