Singlet oxygen cycloaddition

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... 

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. 

Another interesting cycloaddition, the detailed mechanism of which is still under investigation, is the addition of singlet oxygen to alkenes producing 1,2-dioxetanes (Section 5.15.3.3.2). 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

Cas/liquid reaction 25 [CL 25) [4 + 2] Cycloaddition of singlet oxygen to a-terpinene... 

Using a single-channel chip micro reactor, singlet oxygen is generated by photochemical means in presence of catalytic amounts of Rose Bengal [21]. By [4 -i- 2] cycloaddition of this oxygen species to a-terpinene, the product ascaridole is obtained. 

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

Vassilikogiannakis and coworkers described a simple sequential process for the biomimetic synthesis of litseaverticillol B (4-159) which includes a cycloaddition of 4-158 and singlet oxygen to give 4-160, followed by ring opening to afford the hydro-genperoxide 4-161 (Scheme 4.34) [55]. Reduction of 4-161 led to the hemiacetal 4-162, which underwent an aldol reaction to afford 4-159. 

In contrast to some analogous structures, pyrido[2,l-f][l,2,4]triazine 108 is quite stable toward atmospheric oxygen. It reacts, however, with singlet oxygen in a cycloaddition reaction, and the intermediate 109 thus formed loses nitrogen resulting in the pyridone 110 (Scheme 8) <2001JHC205>. 

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). 

Chiral oxazolidine compounds have also been used as chiral auxiliaries for asymmetric Diels-Alder reactions. Adam et al.8 demonstrated the cycloaddition of optically active 2,3-dimethyl oxazolidine derivatives with singlet oxygen. As shown in Scheme 5-9, the reaction of chiral substrate 39 with singlet oxygen provides product 40 in high diastereomeric ratio. 

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 reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

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. 

For the formation of 15 (Figure 6.9) a different mechanism has been proposed [62]. Reaction of singlet oxygen with the corresponding amine (iminodiacetic methylester) leads - after two H-abstractions - to an azomethine ylide as a key intermediate. This ylide can add to the double bond via a [3-i-2]-cycloaddition related to the Prato reaction (Section 4.3.4). 

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... 

Singlet oxygen reacts through the [4- -2]-cycloaddition mode with the sterically hindered and non-planar 1,3-diene system in l,l -dimenthene (493) to afford the cis-endoperoxide 494, albeit in moderate yield (Scheme 135) ". Cycloaddition of O2 to more planar bis(dialine) (495) proceeds smoothly leading predominantly to the trans-endoperoxide trans-496 (70%), while cis-496, which would be expected to derive from a concerted process, constitutes the minor (15%) product (Scheme 136) . 

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