Triplet oxygen reactions

Selenophenes, dioxirane oxidation, 1157 Selenyl radicals, triplet oxygen domino reactions, 223, 227... 

Thiophene, dioxirane oxidation, 1156 Thiourea, dialkyl peroxide synthesis, 706 1,4-Thioxane, oxidation, 805-6 Thiyl radicals, triplet oxygen domino reactions, 221-3, 224, 225, 226 THPO (1,2,3,4-tetrahydronaphthyl hydroperoxide), 331-2 Thromboxanes, from arachidonic hydroperoxides, 612... 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen. These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. 

Attempts to trap carbenes 3b and 3d with molecular oxygen, a reaction frequently used to characterize triplet carbenes, were successful at high (>5%) concentrations of O2 in the argon matrix22. In both cases, the corresponding silyl formate was detected, and it was assumed that a carbonyl oxide and a dioxirane are intermediates in this carbene trapping... 

Another interest in the use of triplet oxygen lies in the oxidation of dienes with photochemical activation (Type 1, above) with formation of endoperoxides as products. The first example of this reaction was observed in the early 1970 s. Thus, reaction of ergosteryl acetate (107) in the presence of trityl tetrafiuoroborate and Lewis acids in the presence of light yielded the endoperoxide 108 (equation 28), With certain Lewis acids this reaction could be thermally, rather than photochemically, activated. Cation radicals were shown to be the intermediate active species, as was borne out by a comparative oxidation of the isomeric lumisteryl acetate which was inactive under these conditions but reacted easily with singlet oxygen . This reaction was later extended to other substrates. Thus, the intermediacy of cation radicals was also indirectly observed by the fact that the r-butyl substituted 1,3-cyclodiene 109 gave a dimeric product 110 (equation 29) via the cation radical intermediate in addition to the usual endoperoxide llOa ,... 

A study of Livingston and Wei (102) showed that the dimerization reaction of anthracene is occurring via the singlet excited state although it may be partially due to triplet reaction based on the effect of oxygen on the quantum yield of cyclomerl-zatlon. Recent results (105), however, indicate that dimerization of 9-methylanthracene occurs via excited state complex formation from the singlet excited state. 

Fig. 1 (B) illustrates the second major function of carotenoids, namely, photoprotection of chlorophyll, either by direct triplet-triplet energy transfer [labeled T-T j from Chl to the carotenoid or by indirect transfer via the formation of singlet oxygen. The reaction steps for each of the two mechanisms are shown in Fig. 1, bottom, right. Further details on both the light harvesting by carotenoids and photoprotective role of carotenoids will be described in the following sections.

Although the multiplicity of excited states of heteroaromatic N-oxides is disputed, it is established that for pyridazine N-oxides the triplet state is involved in oxygen abstraction reactions, whereas the singlet state is involved in rearrangements and isomerizations. ... 

One of the modes of deactivation of an excited triplet is by energy transfer to the molecular oxygen. The reaction may be written as ... 

The activation energy of the cyclo-elimination has been estimated at 3 kcal mole-1 below Tg [88]. Since the isobutene yield decreases in the presence of naphthalene, benzophenone and oxygen, the reaction is thought to proceed through the excited triplet state of the ester side group [88]. 

Cvetanovic (3) has concluded that the reaction of triplet oxygen with olefins is electrophilic, and that the reaction of hydrogen atoms is free radical in nature, the oxygen atom reaction rate constants showing larger variations with structural changes in the olefins. In view of the... 

To address the possibility that both the singlet and triplet diradicals could be participants, we examined the chemistry of 72c in the presence of molecular oxygen. The reaction of TMM diyls with oxygen serves as a diagnostic for the intervention of triplet chemistry [28]. While in principle both the singlet and triplet diyl can react, a combination of the low oxygen concentration available under the reaction conditions, combined with the... 

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