1.2- Dioxetanes photooxygenation

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Photooxygenation of bicyclic enol ether 617 at —78°C affords intermediate 1,2-dioxetane 618, which reacts with a premixed acetaldehyde without acidic additives or... 

The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen. This effect was demonstrated by the photooxygenation of / ,/ -dimethyl styrene (20). This substrate, apart from the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides as shown in Scheme 9. 

The oxazolidinone-substituted olefin Ic (Scheme 3) constitutes another fortunate substrate for the diastereoselective synthesis of a chiral dioxetane , which is of preparative value for the enantiomeric synthesis of 1,2 diols . For example, the photooxygenation of the enecarbamate Ic produces the asymmetric dioxetane 2c in >95% jt-facial diastereoselectivity. The attack of the O2 occurs from the jt face anti to the isopropyl... 

Chiral catalysts, asymmetric metal-catalyzed suHoxidations, 478-85 Chiral 1,2-dihydronaphthalenes, photooxygenation, 265-6 Chiral dioxetanes, stereoselective synthesis, 1173-8... 

The methylene-blue sensitized photooxygenation of 164 yields the isolable dioxetane 165 (70%), the first of its kind to be isolated from an imidazole, presumably via the sequence of Schemes 10 and 26. Some of its reactions are illustrated (Scheme 44). ... 

A similar photooxidation pathway was found for Mg(ll)TPP. It reacted readily with molecular oxygen to give the corresponding 15,16-dihydrobiliverdin, similar to the one shown for Mg(II)OEP in Scheme 1. Further studies have proposed that the photooxygenation of metallo-me o-tetrasubstituted porphyrins proceeds via a one-molecule mechanism involving only one oxygen molecule. Most likely, the first intermediates formed upon photooxygenation are short-lived peroxides. Such compounds are very unstable and a possible dioxetane structure is shown in formula 31. 

In 1995, the RB-sensitized photooxygenation of 6,6-diethoxyfulvene (103) in acetone-d6 at — 78 °C afforded diethoxyfulvene endoperoxide (104) as a primary product (Scheme 11.23) [117]. On the basis of 1H NMR spectroscopy at —50 °C, the initial endoperoxide 104 was assigned, but the dioxetane, 105, was not observed. Further photooxygenation resulted in the formation of bisperoxides syn-106 and the anti-106 in a 9 1 ratio. Peroxides 104 and 106 were not stable and not isolated. Above — 30 °C, endoperoxide 104 released oxygen and gave back fulvene 103, while the bisperoxides 106 decomposed to cis-butenedial, diethylcarbonate, and carbon monoxide. 

Mazur, S. and Foote, C.S. (1970) Chemistry of singlet oxygen. IX. A stable dioxetane from photooxygenation of tetramefhoxyefhylene. Journal of the American Chemical Society, 92 (10), 3225-3226. 

The first example of a chiral-auxiliary-induced [2+2] cycloaddition between 02 and oxazolidinone-functionalized enecarbamates, which proceeds with complete diastereoselectivity as a result of steric repulsions, has been reported to afford 57 <02JACS8814>. The optically active enecarbamates bearing Evans chiral auxiliary were photooxygenated at -35 °C with 5,10,15,20-tetrakis(pentafluorophenyl)porphine (TPFPP) as sensitizer and an 800 W sodium lamp as light source. The dioxetanes 57 were obtained exclusively, but they readily decomposed at room temperature to the expected carbonyl products because of their thermally labile nature. The absolute configuration of the dioxetanes 57 was established by reduction to the corresponding diols with L-methionine. 

In fact, the chemiluminescence of an authenic sample of trans- 3,4-diphenyl-1,2-dioxetane, prepared by Kopecky s procedure [118] and then added to the DCA-sensitized reaction on traws-stilbene (E° = 1.51 V vs SCE), totally disappears within a few minutes [119]. In spite of this elegant demonstration, the intermediacy of these short-lived compounds required further direct evidence. In this regard, although different mechanistic pathways can account for the reaction products [34,120,121], Schaap et al. [98] isolated in the DCA-sensitized photooxygenations of adamantylidene-adamantane (E° = 1.46 V vs SCE) 21 and/or of several substituted 2,3-diphenyl-5,6-dihydro-l,4-dioxins (Eox in the range 0.72-1.07 V vs SCE) 22a-f the correspopnding stable 1,2-dioxetanes 23, 24a-f [Eqs. (10,11)] in good yields. 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

In the TPP+BF4 -sensitized photooxygenation of 21, leading to the corresponding thermally stable 1,2-adamantylideneadamantane dioxetane, 23, direct evidence for the chain electron-transfer reaction was achieved by quenching experiments, ns laser flash photolysis, and cyclic voltammetric studies [185]. 

For example, photooxygenation of 2,3-dimethylbenzo[b]furan at —78 °C produced dioxetane 89, which isomerized at room temperature to give 2-acetoxyacetophenone (as shown in Scheme 49) <1995ACR289>. 

Photooxygenation of alkylidenecycloproparenes provided a complex mixture of cleavage products derived from an intermediate dioxetane. From 1,1-diphenylmethylenebenzocyclopropene, a spirophenanthrone 18 was isolated in 13% yield.Osmium(VIII) oxide/sodium periodate resulted in a mixture of cleavage and ring-expansion products. 

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