1.2- Dioxetanes, review

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. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

As mentioned in the introductory section, other bioluminescent systems will not be discussed in detail in this review. However, the dioxetane approach has been validated in the bioluminescence mechanism of Cypridina hilgendorfii, Latia, and bacteria 90,178,179,180). As in Cypridina luciferin 119, a Schiff s base grouping is apparently involved in the bioluminescent oxidation the same may be true of Latia nerit-oides 120. 

Section II covers the synthesis of the cyclic peroxides with medium ring size from 5 to 7. Section HI covers the synthesis of 1,2,4-trioxanes. Classification in sub-sections and sub-sub-sections is done according to the type of reaction by which the cyclic peroxide system is formed. Syntheses of dioxirans, 1,2-dioxetanes, trioxolanes (ozonides), tetrox-anes, and macrocyclic peroxides are not discussed in this review. 

The cleavage of 1,2-dioxetanes constitntes the model chemiluminescent process, which may be initiated thermally, by electron transfer or in catalytic reactions (e.g. in complexes formed between dioxetanes and transition metals). In the subsequent subsections, we review the most recent significant developments in this area. 

A wealth of experimental data on the thermal dioxetane decomposition and the excited-state generation in the thermolysis process has been comprehensively surveyed in previous reviews". During the last decade, computational elucidation of the thermal cleavage received major attention and in the present subsection we consider the relevant smd-ies. Computations on the dioxetane thermolysis were conducted by both ab initio and semiempiricaP methods at different levels of sophistication. 

The addition of singlet oxygen to alkenes also gives dioxetanes. A number of mechanisms have been proposed and the literature abounds with theoretical and experimental results supporting one or more possible intermediates (a) 1,4-diradicals, (b) 1,4-dipolar, (c) perepoxides, or (d) concerted (Scheme 95). Both ab initio and semi-empirical calculations have been done and to date the controversy is still not resolved. These mechanisms have been reviewed extensively (77AHC(21)437, 80JA439, 81MI51500 and references therein) and will not be discussed here, except to point out that any one mechanism does not satisfactorily account for the stereospecificity, solvent effects, isotope effects and trapped intermediates observed. The reaction is undoubtedly substrate-dependent and what holds for one system does not always hold for another. 

The major mechanistic interest in dioxetanes and dioxetanones comes from their chemiluminescent behavior. This process has been reviewed in the reactions section. 

For reviews of 1,2-dioxetanes see Adam, in Patai, Ref. 743, pp. 829-920 Bartlett Landis, in Wasserman Murray Ref. 781, pp. 243-286 Adam Adv. Heterocycl. Chem. 1977, 21, 437-481. See also Inouc Hakushi Turro Kokagaku Toronkai Koen Yoshishu 1979, 150 [C.A. 92, 214798q] Adam Encarnacidn Chem. Ber. 1982, 115, 2592 Adam Baader Angew. Chem. Int. Ed. Engl. 1984, 23, 166 [Angew. Chem 96, 156]. 

The chemistry of some ring systems having two heteroatoms, i.e. dioxetanes, dithietanes, oxathietanes and thiazetidines are described. Next, the review considers compounds having either silicon or boron in a four-membered ring. Some thermolysis processes are interesting in the silicon series and the first thermally stable 1,2-dihydro-1,2-diborete is described. 

The body of evidence summarized within the aforementioned reviews clearly indicates that simple 1,2-dioxetanes decompose thermally through a twisted diradical-like transition state to afford predominantly a triplet-excited carbonyl with no direct emission of light (Scheme 1), while dioxetanes bearing an aromatic electron-donor moiety display intramolecular charge-transfer-induced decomposition with accompanying effective emission of light (Scheme 2). 

Dioxetanes have been the sole subject of several specialized reviews in recent years (Bartlett and Landis, 1979 Horn et al., 1978-79 Adam, 1977 T. Wilson, 1976 Turro et al., 1974a Mumford, 1975). These articles cover with depth which is not possible here such topics as (1) preparation, (2) physical and spectroscopic characterization, (3) experimental techniques, especially for the study of chemiluminescence, (4) mechanisms of decomposition and chemiexcitation, (5) ground state transformations, and (6) reactions involving dioxetanes as postulated intermediates. The interested reader is referred to these articles for details on these specialized topics, and for some interesting historical perspectives. 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

Besides their thermal decompositions into carbonyl fragments, the chemistry of 1,2-dioxetanes is quite limited. Obviously one of the reasons for this is the great lability of the dioxetane ring system. However, a number of reactions with nucleophiles and electrophiles have been performed and will be briefly reviewed here. 

Reviews have appeared of data available on singlet oxygen quwtum yields, chemiluminescent reactions with participation of singlet oxygen and dioxetanes, photooxidation of unsaturated fatty acids, and the photooxidation of furans. ... 

Little has been reported in the period covered by this review on the photoreactions of oxetans. 2,2,4,4-Tetraphenyloxetan-3-one undergoes decarbony-lation on irradiation in benzene to give tetraphenyloxiran and reduction on irradiation in propan-2-ol to give the corresponding oxetan-3-ol. The photodecomposition of 1,2-dioxetans is of particular interest tetramethyl-... 

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