Oxygen with alkenes

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. 

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. 

Although 1,2-dioxetanes are excluded as intermediates in the formation of allylic hydroperoxides, they may be formed as primary products in the reaction of singlet oxygen with alkenes not possessing allylic hydrogen 360 363 366 368-370 374... 

Ab initio molecular orbital calculations, coupled with activation energies and entropies from experimental data, have been employed to determine the nature of the intermediates in the reaction of singlet oxygen with alkenes, enol ethers, and enamines.214 Allylic alkenes probably react via a perepoxide-like conformation, whereas the more likely pathway for enamines involves a zwitterionic cycloaddition mechanism. The reactions of enol ethers are more complex, since the relative stabilities of the possible intermediates (biradical, perepoxide, and zwitterionic) here depend sensitively on the substituents and solvent polarity. 

The ene reaction of singlet oxygen with alkenes was originally discovered [22,23] by Schenck in 1943. It is of synthetic [21,24], biochemical [25-27], and environmental significance [25,28] (Scheme 1). Over the years, the mechanism of this re-... 

Figure 23 Scheme for reactions of singlet oxygen with alkenes. 

Stratakis, M. and Orfanopoulos, M. (2000) Regioselectivity in the ene reaction of singlet oxygen with alkenes. Tetrahedron, 56 (12), 1595-1615. 

The intermolecular and intramolecular 13C isotope effects were used in studies of the mechanism of the ene reaction of singlet (1Ag) oxygen with alkenes.89 The intermolecular 13C isotope effect was determined for reaction of 3-isopropyl-2,4-dimethylpent-2-ene 40 with 102 in methanol/2-propanol (1 1) at 10°C using a sunlamp and Rose Bengal as sensitizer. 

The reaction of singlet oxygen with alkenes constitutes an important chapter in organic chemistry. The discovery that certain enol ethers from zwitterionic peroxides as the primary event has led to the development of new methods for preparing a variety of 1,2,4-trioxanes . 

Reviews on the following aspects of four-membered heterocycles have appeared 1,2-dioxetans and the reaction of singlet oxygen with alkenes/ /8-lactam antibiotics, small ring compounds of sulphur and selenium," recent developments in the synthesis of azetidines, the chemistry of benzazetes and of thiacyclobutenes (thietes) and their valence tautomers, and conformational barriers and interconversion pathways in some small ring systems. ... 

In this chapter, we begin with the structure and physical properties of haloalkanes. We then study radical halogenation of alkanes as a vehicle to introduce an important type of reaction mechanism, namely the mechanism of radical chain reactions. Reactions of oxygen with alkenes and a radical mechanism for HBr addition to alkenes complete the chapter. 

---