Tetramethyl-l,2-dioxetane

Excitation appears to be general for this reaction but yields of excited products vary substantially with the substituent R. The highest yield reported is from tetramethyl-l,2-dioxetane [35856-82-7] (TMD) where the yield of triplet acetone is 50% of total acetone formed (18,19). Probably only one carbonyl of the two produced can be excited by the thermal decomposition, and TMD provides 100% of the possible yield of triplet acetone. Singlet excited acetone is also formed, but at the low yield of 0.1—0.3% (17—21). Other tetraaLkyldioxetanes behave similarly to TMD (22). 

Tetrakis(bromomethyl)-9,9-dimethyl-l,2,4,5,7,8-hexoxonane, 3173 Tetramethyl-l,2-dioxetane, 2508... 

About 30 years ago, an enthalpy of formation was reported for 3,3,4,4-tetramethyl-l,2-dioxetane . Both by direct microcalorimetric combustion measurements of the neat solid and by reaction calorimetry (of the solid itself, and in acetone solution to form acetone), a consensus value was derived. Now, is the enthalpy of formation plausible , notwithstanding the very large error bars Consider reaction 6 for the dioxetane that produces 2,3-dimethyl-2,3-butanediol . The liquid phase enthalpy of reaction is —329 kJmoU. It is remarkable that this value is compatible with that for the dialkyl peroxides, ca —335 kJmoU, despite the ring strain that might be expected. 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Tetramethylammonium ozonide, 736 Tetramethyl-l,2-dioxetane (TMD) chemical titration, 1224 chemiluminescence, 1221, 1234 quantum yield standard, 1224, 1226 N,N, N, A -Tetramethyl-p-phenylenediamine hydrogen peroxide determination, 735, 631, 633... 

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. 

Emission of phosphorescence by 1,2-dioxetanes and a-peroxylactones has also been observed, but is quite rare. Thus, in degassed acetonitrile the 430 nm emission exhibited by the tetramethyl-l,2-dioxetane (7) has been assigned to acetone phosphorescence. Similarly, this acetone phosphorescence has been detected for the dimethyl-a-peroxylactone. ° For the acetyl derivative (19), both the n,TT fluorescence and phosphorescence of 2,3-butanedione have been reported. Thus, if the photoexcited luminescence spectrum of the carbonyl product is known or can be readily measured, the chemiluminescence spectrum can be used as corroborative structure confirmation of the 1,2-dioxetane or a-peroxylactone. 

For the specific case of tetramethyl-l,2-dioxetane, the heat of reaction was determined experimentally by differential scanning calorimetry (DSC) to be A/fg = — 61 kcal/mole. Its activation enthalpy is about 25 kcal/moleThus, a total of... 

The specificity of this reaction has been used to chemically titrate both the excited-singlet acetone and the triplet acetone produced through thermal decomposition of tetramethyl-l,2-dioxetane. (Cf. Section 7.6.4.) For this purpose the thermolysis was carried out in the presence of /ra i-l,2-dicy-anoethylene, and the quantities of singlet and triplet acetone formed were obtained from the yields of dioxetane and m-I,2-dicyanoethylene, respectively (TUrro and Lechtken, 1972). 

Oxidation of luminol (see Example 7.22) and thermolysis of endoperox-ides or 1,2-dioxetanes provide important examples of chemiluminescent reactions. Tetramethyl-l,2-dioxetane (181) has been studied in great detail the thermolysis is clearly first order and the activation enthalpy in butyl phthal-ate is AH == 21 kcal/mol. The enthalpy difference between the reactants and ground-state products is A// = -63 kcal/mol. 

T24. Turro, N. J., and Lechtken, P., Thermal decomposition of tetramethyl-l,2-dioxetane. Selective and efficient chemelectronic generation of triplet acetone. J. Am. Chem. Soc. 94, 2886-2888 (1972). 

Singlet sensitization of tetramethyl-l,2-dioxetan by pyrene results in the formation of triplet acetone.296 In a theoretical study related to the one reported in ref. 293, it was concluded that the triplet surface intersects the ground-state surface between (15) or (16) and cyclo-octatetraene. Evidence for triplet product... 

The dark initiation of the photosensitized degradation of a styrene-methyl isopropenyl ketone copolymer by thermally generated 7i(3iwr ) acetone using the tetramethyl-l,2-dioxetan dissociation reaction has been studied.217 The reaction... 

Treatment of 3-halo hydroperoxides of tetrasubstituted olefins with base re-iiKs exclusively in formation of allylic hydroperoxides. In this case treatment of he ji-lialo hydroperoxide with silver acetate in CH Clj proved effective. For niiiiiple, tetramethyl-l,2-dioxetane (2) was obtained in this way in an optimum deldof 35% ... 

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

Tetramethyl-l,2-dioxetane, prepared by treating the corresponding bromohydroperoxide with base, was a generous gift from Dr. K.R. Kopecky, University of Alberta, Department of Chemistry, Edmonton, Alberta, Canada. 

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