Oxygen, singlet alkene oxidation

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. 

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). 

Udalova OV, Khaula EV, Bykhovski MY, Rufov YN, Romanov AN (2003) Thermal generation of singlet oxygen in zeohtes and its role in the alkene oxidation. Russ J Phys Chem... 

Alkene Oxidation Reactions. The reaction of triethylsilane with ozone and use of the intermediate triethylsilyl hydrotrioxide in oxidation reactions have been described. Initial oxidation reactions reported included the formation of the 9,10-endoperoxide from 9,10-dimethylanthracene (eq 1), and an allyUc hydroperoxide from 2,3-dimethyl-2-butene (eq 2). Researchers also observed a near-IR emission from triethylsilyl hydrotrioxide as it decomposed at —60°C, consistent with generation of singlet oxygen. Other workers have characterized triethylsilyl hydrotrioxide by NMR spectroscopy and and measured the kinetics of its decomposition in deuterated acetone. ... 

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 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... 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

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

The chemistry of singlet oxygen has received remarkable attention by chemists not only because of its interesting mechanistic and synthetic aspects but also because of its large environmental and biomedical significance . Among the peculiar type of reactions that O2 affords with organic substrates are [4 - - 2], ene and [2 - - 2] additions to alkenes (Scheme 1), oxidation of sulphides, phosphines etc. 

The retention of olefin geometry in the oxidation of cis and trans alkenes408 suggests a one-step concerted [2 + 2]-cycloaddition process, suprafacial in the alkene, and antarafacial in singlet oxygen.363 The strong solvent dependence, however, observed in many cases, points to a stepwise mechanism involving a perepoxide intermediate 403,409... 

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. 

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