Dioxetane

2-Dioxetanes are highly endothermic compounds. This is partly due to ring strain, but above aU to the low bond energy of the peroxide bond. 

The typical reaction of 1,2-dioxetanes is thermal decomposition. On warming tetramethyl-l,2-dioxetane in benzene or other solvents, blue light is emitted. Such a phenomenon is known as chemiluminescence [19]. It has been demonstrated that, according to the principle of conservation of orbital symmetry, one mole of acetone in an electronically excited state is formed. In this way, an electronically excited molecule (indicated by ) is created by a thermal process. With emission of light, the ground state is restored  

Two syntheses are available for the preparation of 1,2-dioxetanes starting from I q I P-halohydro-peroxides or alkenes. ——  

The electrophilic bromination of alkenes, for example, with l,3-dibromo-5,5-dimethyl-hydantoin in the presence ofconcentrated hydrogen peroxide, leads to P-bromo hydroperoxides. These are cyclized with bases or with silver acetate to give 1,2-dioxetanes, for 

Q 1 Tetramethyl-l,2-dioxetane,yellow crystals, mp 76-77 °C, emits light a few degrees above ---- its melting point. 

Azetidin-2-ones are more reactive than y- and 5-lactams because of ring strain. This is true for the alkaline fission to give salts of / amino carboxylic acids, as well as for the acid-catalysed hydrolysis to fi-carboxyethylammonium salts. Starting from alkenes and chlorosulfonyl isocyanate, a stereocontrolled synthesis of / -amino carboxylic acids can be realized. Ammonia and amines react with azetidin-2-ones, also with ring-opening, to produce / -amino carboxylic amides. Hence they are acylated by azetidin-2-ones  

Azetidin-2-ones are reduced chemoselectively by diisobutylaluminium hydride or by chloroaluminium and dichloroaluminium hydrides in THF to form azetidines [10]. 

The azetidin-2-one system is present in penicillins (see p 159) and cephalosporines (see p 389). These natural products are known as / -lactam antibiotics. They block the biosynthesis of compounds which form the bacterial cell walls. The / -lactam antibiotics are the most prescribed antibiotics today. 

Ci5 -Azetidine-2-carboxylic acid is a cyclic amino acid, not present in proteins, found in agaves and liliaceous plants. It was first isolated from lilies of the valley  

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Yields of excited states from 1,2-dioxetane decomposition have been determined by two methods. Using a photochemical method (17,18) excited acetone from TMD is trapped with /n j -l,2-dicyanoethylene (DCE). Triplet acetone gives i7j -l,2-dicyanoethylene with DCE, whereas singlet acetone gives 2,2-dimethyl-3,4-dicyanooxetane. By measuring the yields of these two products the yields of the two acetone excited states could be determined. The yields of triplet ketone (6) from dioxetanes are determined with a similar technique. 

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

In addition to ready thermal decomposition, 1,2-dioxetanes are also rapidly decomposed by transition metals (39), amines, and electron-donor olefins (10). However, these catalytic reactions are not chemiluminescent as determined by the temperature drop kinetic method. 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Chemiluminescence and bioluminescence are also used in immunoassays to detect conventional enzyme labels (eg, alkaline phosphatase, P-galactosidase, glucose oxidase, glucose 6-phosphate dehydrogenase, horseradish peroxidase, microperoxidase, xanthine oxidase). The enhanced chemiluminescence assay for horseradish peroxidase (luminol-peroxide-4-iodophenol detection reagent) and various chemiluminescence adamantyl 1,2-dioxetane aryl phosphate substrates, eg, (11) and (15) for alkaline phosphatase labels are in routine use in immunoassay analyzers and in Western blotting kits (261—266). 

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). 

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

The conversion of small rings to smaller ones, without loss, is not common. 3-Chloroazetidine isomerizes reversibly to 2-chloromethylaziridine (Section 5.09.2.2.5). Flash vacuum pyrolysis can convert isoxazoles to azirines (Section 5.04.4.3). More common is the isomerization of medium-sized, i.e. five- or six-membered rings, e.g. certain succinimides (Scheme 23) (81JOC27) to azetidinediones, or bicyclic 1,2-dioxetanes to bis-oxiranes (Section 5.05.4.3.2). 

Dioxetanes applications, 7, 484—485 electrophilic reactions, 7, 461 nucleophilic reactions, 7, 463-464 photochemical reactions, 7, 459 spectroscopy, 7, 455... 

Dioxetan-3-ones spectroscopy, 7, 455 synthesis, 7, 469, 476 thermal reactions, 7, 459... 

The early stages in the oxidation of disilene have been treated theoretically for the parent molecule H2Si=SiH2.95 The first intermediate along the reaction coordinate is the open-chain trans diradical 64 (Scheme 16), which is in equilibrium with a gauche form, 65. From the latter, closure to the 1,2-dioxetane 66 would probably be rapid. The open-chain form can react with a second molecule of disilene to give the diradical 67, which could collapse into two molecules of the disilaoxirane 68. If similar steps are followed in the oxidation of 3, they must be quite rapid, since the relative configuration at the silicon atoms is maintained in both products, 59a and 61a.93... 

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