Formation of Dioxetanes

Biadamantylene dissolved in methylene chloride at -78 °C and treated with a solution of triphenyl phosphite ozonide in the same solvent and subsequently with a mixture of methanol and pyridine gives biadamantylene dioxetane in 91% yield (equation 64). The dioxetane decomposes at 150 °C [1101]. 

In the oxidation of indene [39] and of 1,2-diphenylcyclobutene [25] with singlet oxygen generated by irradiation of the solutions of the compounds in the presence of sensitizers, dioxetanes are the probable intermediates in the conversion of the unsaturated hydrocarbons into dialdehydes and diketones, respectively. Different products may be formed depending on the solvents used (equation 65) [25]. 

The stereospecific addition of singlet oxygen to cis- and trans-1,2-diethoxyethylene under irradiation with rose bengal as a sensitizer in fluo-rotrichloroethane at -78 °C gives cis- and rra/w-l,2-diethoxydioxetane [40], Similarly, cis- and rra/i5-a,a -dimethoxystilbene give cis- and trans-, 2-d -methoxy-l,2-diphenyloxetane (equation 66) [35]. 

Tetramethoxyethylene in ether at -70 °C treated with oxygen under irradiation in the presence of zinc tetraphenylporphyrin furnishes a 94% yield of 1,1,2,2-tetramethoxydioxetane [37], Chemically generated singlet oxygen converts substituted ketenes into a-peroxylactones (equation 67) 20], 

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Other speculative mechanisms [26] may be proposed based on the presence of singlet oxygen and C = C in oxidized polymer. The reaction of the latter may lead to the transient formation of dioxetanes, the decomposition of which has an even higher quantum yield of luminescence than CIEEL mechanism [27],... 

Initially we chose to test elements of this approach to artemisinin in an abbreviated version of 4 that lacked the 2P-(3-oxobutyl) and 3a-methyl groups. Hence, on low-temperature ozonolysis of the vinyl silane 5 in methanol, transient and stereoexclusive formation of dioxetane 6 was observed upon immediate analysis by NMR (Eq. 3). On standing, the dioxetane 6 underwent rearrangement and cyclization to furnish hydroperoxy-lactone 7 in 54% isolated yield on a scale sufficient for X-ray structural study.44... 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

A less frequent reaction is the formation of dioxetanes from compounds containing double bonds [20, 36, 37, 38, 39, 40, 41, 42], Such compounds are not stable and disintegrate to dicarbonyl compounds (equation 8). 

Another type of chain reaction is sometimes observed when electrochemical oxidation of unsaturated hydrocarbons is carried out in the presence of atmospheric oxygen, which may result in the formation of dioxetanes exemplified by the oxidation of biada-mantylidene [Eq. (53)] and related compounds [121-123]. Similar behavior has been observed for other alkenes [124]. 

N.B. Dioxygen can be implicated in reactions induced by anodic electron transfers. It is particularly the case in the oxidation of electron-rich double bonds in the presence of molecular oxygen. The reaction below exemplifies such a catalytic process (here in the case of adamantylideneadamantane) with the formation of dioxetane [149, 150]. Note that in this case the reaction is catalytic in electron (i.e. catalytic amount of charge extracted by the anode). 

The stereoselectivity of the allylic hydroperoxidation also depends on several factors. With chiral allylic alcohols or allylic amines 200, a hydrogen bond is developed between and the vicinal hydroxyl or amino group. The facial differentiation results from an approach of 02 in the transition state that minimizes 1,3-allylic strain. 201 and 202 can be obtained with a diastereoisomeric excess higher than 90% in CCI4. As previously indicated for the formation of dioxetanes and 1,4-endoperoxides, the selectivity decreases considerably in the presence of hydroxylic solvents [123]. When hydrogen bonding is no more possible in 203, the stereofacial differentiation is steered by steric and electronic repulsion effects at the level of the possible diastereoisomeric transition states and 204 is formed selectively (Scheme 54). 

In the second mechanism proposed (scheme 2), there is a formation of dioxetane which is well known to be stabilized in strong acidic medium [19], so. the overall oxidation rate should be decreased as we observed. 

Tetramethyldioxetane is the prototype for all chemiluminescent processes. It will generally be the case that a dioxetane or similar structure will be formed. Thermal decomposition of this high energy structure then produces an excited state product. Details vary, but many of the basics of Figure 16.19 will be involved. Thus, species containing a strained 0-0 bond play a special role in chemiluminescent mechanisms. For that reason, we now discuss some aspects of O2 chemistry that are relevant to the formation of dioxetanes and related species. 

The near-inseparability of conditions for the formation of dioxetanes and of epoxides was first observed when 7,7 -binorbornylidene was oxidized with photosensitizers in a series of solvents (21). 

It is useful to include some examples of the formation of dioxetans in this section since very few of these unstable compounds are known. Hydroperoxy-oxirans (15) have been converted into 1,2-dioxetans (16) under phase-transfer conditions without any of the five-membered isomer (17) being formed. The... 

The activated alkenes not have acidic allylic hydrogen atom, to undergo addition with singlet oxygen on photolysis, with the formation of dioxetanes. The addition is stereospecific. 

Clennan, E.L. and Nagraba, K., Reactions of singlet oxygen with alkoxy-substituted dienes. Formation of dioxetanes in the singlet oxygenations of s-cis fixed dienes (Z,Z)- and ( ,Z)-4,5-diethyl-idene-2,2-dimethyl-l,3-dioxolanes,/. Org. Chem., 52, 294, 1987. 

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