Catalysis dioxetanes

A peculiar effect was observed in the decomposition of 19 a with anthracene as fluorescer when oxygen was carefully removed from the solutions an increase of the chemiluminescence decay rate and of the dioxetane cleavage resulted. It was suggested that this was due to a catalytic effect of triplet anthracene (formed by energy transfer from triplet formate) on the decomposition of the dioxetane. When oxygen is present, triplet anthracene is quenched. Whether such a catalytic effect of triplet anthracene or similar compounds on dioxetane cleavage actually exists has not yet been fully established positive effects were observed by M. M. Rauhut and coworkers 24> in oxalate chemiluminescence and by S. Mazur and C. S. Foote 80> in the chemiluminescent decomposition of tetramethoxy-dioxetane, where zinc tetraphenylporphy-rin seems to exert a catalytic effect. However, the decomposition of trimethyl dioxetane exhibits no fluorescer catalysis 78h... 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

Further evidence for ground-state complexation as the cause of the special catalysis was obtained by a spectroscopic study in a model system. Such complexes are typically characterized by a shift of the maximum of the porphyrin Soret absorption band relative to that of the non-complexed porphyrin. In the presence of a high concentration of tetramethy 1-1,2-dioxetane, used as a model for the co-ordinating ability of [21], the absorption maximum of ZnTPP was determined to be shifted by 1.2 nm. 

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. 

For the 1,2-dioxetanes the first route can be executed by catalysis with bases (HO , MeO ) or by metal cations (Ag", for the more labile a-peroxy-... 

For primary and secondary bromides base-catalysis is required, while for tertiary bromides silver acetate or silver oxide are more effective cyclization catalysts. For tertiary substrates dehydrobromination leading to allylic hydroperoxides is a serious side reaction when base-catalysis is employed and, thus, silver ion catalysis is essential. Furthermore, the silver salts must be freshly prepared because metallic silver that might be present due to exposure to light causes decomposition of the dioxetane. The tetramethyl-l,2-dioxetane (7) was the first example prepared in this way (Eq. 13). For primary substrates, abstraction of the base-sensitive dioxetanyl hydrogens are probably responsible for the low yields. For secondary substrates, both side reactions might operate. 

Catalytic reductions over platinum or palladium, which are usually quantitative methods for the identification of organic peroxides, are problematic. Little of the expected 1,2-diol is obtained because the dioxetane fragments into its carbonyl products due to metal catalysis." However, lithium aluminium hydride reduction under subambient conditions affords the expected 1,2-diol quantitatively. Again, the sterically hindered dioxetane (9) is an exception. Here zinc in acetic acid proved successful. ... 

A dramatic solvent effect in the thermolysis of tetramethyldioxetane, which followed the isokinetic relationship A/7 = /3A5 for a variety of solvents, formed the basis for the postulation of the concerted mechanism. However, it was shortly thereafter reported that the dramatic solvent effect in methanol was the result of catalysis by transition-metal ion impurities. In the presence of metal-ion complexing agents such as EDTA or Chelex 100, the menacing catalysis could be suppressed. That utmost care must be taken in measuring reliable kinetic parameters in 1,2-dioxetane decomposition cannot be overemphasized. 

Endoperoxides are easily formed by Diels-Alder addition of singlet oxygen to 1,3-dienes. On acid catalysis these endoperoxides behave like 1,2-dioxetanes combining efficiently with aldehydes and... 

The catalytic mechanism for IDO and TDO is believed to proceed via the formation of a hydroperoxide at C-3 of the indole ring, followed either by dioxetane formation or Criegee rearrangement, as shown in Figure 26. Formation of the hydroperoxide could either take place via nucleophilic attack upon heme-bound dioxygen, or via the formation of an indole radical, followed by recombination with iron(III)-superoxide. The structure of human IDO was published in 2006. Site-directed mutagenesis of active site residues has established that Phe-226, Phe-227, and Arg-231 contribute toward catalysis. ... 

The most cmsory glance at the literatiu-e would quickly lead to one to conclude that the mechanism of catalysis was cast in stone. The reaction had been proposed as occiuring through either Criegee (49) or dioxetane (57) pathways (Scheme 3) and has been... 

Keywords Catalysis Chemiluminescence Chromism Cycloreversion Dioxetane Flex activation Mechanochemistiy Polymers Self-healing... 

It might be thought that these four-membered cyclic peresters would most readily be compared with dioxetans. However there is a marked difference in their behaviour. The dioxetanones are considerably less stable than the dioxetans, and their reactions are much more susceptible to catalysis by electron donating compounds and substituents. They are key intermediates in firefly and coelenter-ate bioluminescence, and being a part of an electron-rich molecule, suffer decomposition as soon as they form. 

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