Dienes singlet oxygen

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

When the valence tautomeric mixture of oxepin and benzene oxide is treated with singlet oxygen, the primary product is the 1,4-endoperoxide 3 which has proven to be too labile for isolation.219 Its formation can be rationalized by a 1,4-addition across the diene system of the benzene oxide structure 3 then rearranges to ba s-3,6,9-trioxatetracyclo[6.1.0.02 4.05 ]nonane (transbenzene trioxide, 4). 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

Photooxidation of Dienes Investigated in Micro Reactors Cas/liquid reaction 24 [CL 24] Oxidation of cyclopentadiene by singlet oxygen to 2-cyclopentene-1,4-diol... 

Singlet oxygen undergoes [4 + 2] cycloaddition with dienes. 

Since the corresponding endoperoxide precursors are all too unstable for isolation, the diimide reduction constitutes an important chemical structure confirmation of these elusive intermediates that are obtained in the singlet oxygenation of the respective 1,3-dienes. However, the aza-derivative 14 and the keto-derivative 15 could not be prepared,17> because the respective endoperoxides of the pyrroles 18) and cyclopentadienones suffered complex transformations even at —50 °C, so that the trapping by the diimide reagent was ineffective. 

Surface water t,A 320 h and 9 x 104 d for oxidation by OH and R02 radicals for olefins in aquatic system, and t,/2 = 19 h, based on rate constant of 1.0 x 107 M-1 s-1 for oxidation of dienes with singlet oxygen in aquatic system (Mill Mabey 1985). 

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

Singlet oxygen adds to cyclic 1,3-dienes to afford 1,4-endoperoxides 609). This type of reaction has first been applied successfully to the synthesis of ascaridole from a-terpinene 610) (6.4) and of the vesicatory compound cantharidine 6U) (6.5) some 25 years ago. 

Conjugated dienes and polyenes lead to unique cases of conjugated oxidations, such as the formation of endoperoxide by singlet oxygen attack on the endo-diene, e.g. u-terpinene29. 

In the literature, there are also several examples where the specific oxidation reaction is dependent on the presence of a diene. Often the substrates are conjugated dienes where there is simultaneous oxidation of both double bonds. An important prototype of this reaction is the well-known singlet oxygen oxidation of dienes to endoperoxides, for example the oxidation of a-terpinene (4) to ascaridole (5) (equation 1). 

Finally, another interesting use of singlet oxygen in the oxidation of dienes concerns the reactivity of allenes. Besides the formation of endoperoxides by addition to dienes and hydroperoxide formation via the ene reaction, singlet oxygen reacts with electron-rich... 

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

The most powerful dienophile is singlet oxygen, produced by the dye-sensitized irradiation of oxygen. Its cycloaddition to dienes to give ewrfo-peroxides (equation 86) has long been known76. 

The most powerful azo dienophile is the cisoid 4-phenyl-l,2,4-triazoline-3,5-dione 264, which is surpassed in reactivity only by singlet oxygen. The dione adds rapidly to all types of dienes and the process can be followed visually since the bright-red color of the reagent is discharged when the reaction is complete136. 

The occurrence of singlet oxygen is important for the elimination of dissociated forms of some pollutants such as phenolic, cyclic diene, and sulfur compounds. 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

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