Olefins reaction + singlet

A simple way to rationalize the site selectivity is the formation of a perepoxide between the olefin and singlet oxygen as the rate-determining step of this reaction. The formation of this intermediate is irreversible, at least within the time scale of the ene reaction. The low activation energy ca 10 kcal mol ) provides support for the assumption of irreversibility. This complex is formulated in such a way that Oa is over the monosubstimted side and Ob over the disubstituted side of the olefin (Scheme 4). 

Oxetane formation is not only a triplet reaction. Singlet n,n states of aliphatic ketones add stereospecifically to electron-deficient olefins, whereas their triplet states transfer energy to these olefins 134>. With olefins such as 1-methoxy-l-butene, oxetane formation takes place from both singlet and triplet states 135>. 

Spectra from the chemiluminescent gas phase reactions at 0,5 torr, of ozone with ethylene, tetramethylethylene, trans-2-hutene, and methyl mercaptan at room temperature are presented, and a summary of the general features of the emissions obtained from reaction in the gas phase of ozone with fourteen different olefins is given. The emitting species in the ozone-olefin reactions have been tentatively identified as electronically excited aldehydes, ketones, and a-dicarbonyl compounds. The reaction of ozone with hydrogen sulfide, methyl mercaptan, and dimethylsulfide produces sulfur dioxide in its singlet excited state. 

Relative rates of reaction of a number of olefins with singlet and triplet methylene have been determined in the gas phase Singlet methylene was generated by direct photolysis of ketene at 2600 A and the triplet by mercury-photosensitized decomposition of ketene. The evaluation of relative... 

Other cyclisation reactions lead to dioxetans and dioxetanones [2, 7]. The most convenient method is reaction of olefins with singlet oxygen [8, 9, 10]. Limitations of this method include olefins with insufficient reactivity and those having allylic H-atoms capable of taking part in the ene reaction which leads to allylic hydroperoxides. 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Singlet oxygen reacts with olefins presumably by the "ene" reaction to form allyflc hydroperoxides (45,57), eg, l-methyl-2-propenyl hydroperoxide [20733-08-8] is produced from 2-butene (eq. 19). The regioselectivity of this reaction has been investigated (58). 

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

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