1,2-Dioxetanes identification

The identification of trimethylsiloxy-l,2-dioxetane and the assignment of trimethylsiloxymethyl formate as a reaction product demonstrate the feasibility of a trimethylsilyl group migration. 

Infrared (IR) spectroscopy is not characteristically definitive for the identification and structural proof of the dioxetane core. Dioxetanones 2 (a-peroxy lactones) such as 29 and 30, depicted in Section 2.16.3.1.4, show carbonyl stretching frequencies at 1835 and 1870cm-1, respectively <1997JOC1623, 1977JA5768>. 

Melting points of crystalline and stable dioxetanes have served for identification purposes. However, the melting points are frequently decomposition temperatures, which limit their use. 

Of course, the most rigorous identification is by x-ray structure analysis. The difficulty is in growing suitable crystals. More seriously, on extended exposure to x-rays, the crystals deteriorate and do not permit a definitive structure elucidation. Nevertheless, a few x-ray structures of the very stable dioxetane (9) in Eq. and the unusual cyclobutadiene-1,2-dioxetane (16) have been reported. 

As already mentioned in connection with the determination of purity, iodometric titration is useful in the identification of dioxetanes. Unfortunately this simple and convenient method is not specific since most peroxides release iodine from acidic potassium iodide. If the necessary care is taken, iodometry is quantitative and thus an excellent purity criterion. However, the sterically hindered dioxetanes (9) do not titrate well. 

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. ... 

H and nmr are the most important tools for the identification of 1,2-dioxetanes and a-peroxylactones. The dioxetanyl ring protons have characteristic... 

Except for the a-peroxylactones, which have characteristic carbonyl stretching frequencies at 1850-1875 cm (for specific values, see Table 2), infrared spectra are of no great help for the identification of 1,2-dioxetanes. The weak and controversial 0-0 stretching frequency of peroxides is not sufficiently characteristic for structural confirmation. ... 

The identification of the cw-diol (190) as a component of the mixture produced by singlet oxygen oxidation of thujopsene (187) has been cited as evidence in favour of the dioxetan intermediate (188) previously proposed to account for the formation of the dicarbonyl product (189) (Scheme 17). ... 

Reduction with lithium aluminum hydride is a useful method of identification since the respective diols are formed.12,34 Catalytic hydrogenation usually leads to decomposition of the dioxetanes into the corresponding carbonyl products.12,31 This is not surprising since transition metals promote fragmentation of these labile materials.49 In the case of dioxetane lz, the use of zinc in acetic acid was the only successful method for reducing it to its diol.31... 

There have been a few studies of the matrix photolysis of complexes between alkene molecules and species other than NOj. The charge transfer complex between O2 and 2,5-dimethylhexa-2,3,4-triene (81) has been photolyzed in Ar matrices containing various amounts of O2 and in pure Oj matrices. Oxidation products were formed even with visible light (543 nm), and these included the trisdioxetane 82, which decomposed on further irradiation (slowly at X > 500 nm, rapidly at X > 300 nm) to acetone and CO2. Identification of 82 was assisted by interpretation of its IR spectrum and the use of oxygen isotopes. At O2 concentrations between 10 and 100%, the trisdioxetane was almost the sole product in these matrix reactions. At lower Oj concentrations, a second product was observed, which was identified as dimeth-ylpropadienone (83) and was probably formed from dioxetane 84. 

It is very difficult to follow these alternatives to the very well defined dioxetan route right through to the light emitting step. A variety of fates for the intermediate peroxide, and a variety of electron transfer mechanisms are possible. Very many heterocycles and fluorescent aromatic hydrocarbons are chemiluminescent in dipolar aprotic solvents with base in the presence of oxygen. The quantum yields are rarely high, and the identification of a single well defined pathway is extremely difficult. 

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