1,2-Dioxetans thermolysis

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 PM3 calculations of the So and the vertical (Franck-Condon) Ti energies as a function of the 0-0 bond length [<7(0-0)] have successfully reproduced the experimental activation energy for the dioxetane thermolysis . However, an unusual shape has been found for the energy profile A flat plateau, in which the ground-state energy... 

In summary, although the computed structural details of the reaction profile depend on the method used for calculations, the general salient mechanistic conclusion is that the dioxetane thermolysis starts with the 0—0 bond rupture to generate the 0C(H2)—C(H2)0 triplet diradical, which is followed by C—C bond cleavage to afford the final ketone products one of them is formed preferentially in its triplet excited state. Since even simple 1,2-dioxetanes still present a computational challenge to resolve the controversial thermolysis mechanism, the theoretical elucidation of complex dioxetanes constitutes to date a formidable task. 

Adam W, Arnold MA, Grune M, Nau WM, Pischel U, Saha-Moller CR (2002b) Spiroimiodihydantoin is the major product in the photooxidation of 2 -deoxyguanosine by triplet states and oxyl radicals generated from hydroxyacetophenone photolysis and dioxetane thermolysis. Org Lett 4 537-540... 

As to the nature of the electronically excited state, the investigation of the thermolysis of tetramethyl-1.2-dioxetane revealed a high yield (about 50%) of excited triplet acetone 34> ... 

Alkoxy-dioxetanes undergo chemiluminescent thermolysis in the pres-... 

Lipid hydroperoxides are also generated in singlet molecular oxygen mediated oxidations and by the action of enzymes such as lipoxygenases and cyclooxygenases. Chemiluminescence (CL) arising from lipid peroxidation has been used as a sensitive detector of oxidative stress both in vitro and in vivo . Several authors have attributed ultra-weak CL associated with lipid peroxidation to the radiative deactivation of O2 and to triplet-excited carbonyls (63, 72) (equations 35 and 36) " . It has been proposed that the latter emitters arise from the thermolysis of dioxetane intermediates (61, 62) (equation 35), endoperoxide (73) (equation 37) and annihilation of aUtoxyl, as well as peroxyl radicals ... 

In parallel with the ab initio calculations, also semiempirical smdies on the thermolysis of 1,2-dioxetane were performed. Most computations have been conducted by the PM3 method because it is the best semiempirical method for describing lone electron pairs on adjacent atoms . As an illustration, only the PM3 method reveals that in the dioxetane molecule the 0-0 bond is longer and weaker compared with the C—C one, as manifested by the computed values of bond lengths [rf(0—O) = 1.600 > d(C—C) = 1.522 A] and bond orders [n(0—O) = 0.973 < w(C—C) = 0.989] . In contrast, the AMI and MNDO semiempirical methods exhibit the opposite trends, i.e. AMI gives d 0—0) = 1.334 A, d(C-C) = 1.539 A, n(O-O) = 0.995 and n(C-C) = 0.976, whereas MNDO furnishes d(0-0) = 1.316 A, d(C-C) = 1.558 A, n(O-O) = 0.996 and n(C-C) = 0.9622 f-8. Nevertheless, despite the quantitative differences in the computed bond lengths, bond orders and bond angles, both the AMI and PM3 methods disclosed qualitatively similar reaction trajectories . 

Acetyl-protected 1,2,3,4-tetrahydropyrazines 105, which are prepared by treatment of 2,3-dihydropyrazine with acetic anhydride and zinc (Scheme 27), undergo photooxidation to produce new dioxetanes 106 <1995JA9690>. Upon thermolysis, the dioxetanes 106 decompose quantitatively to tetraacyl ethylenediamines 107. Dimethyldioxirane oxidation of tetrahydropyrazine 105 affords novel epoxide 108, which is also generated by deoxygenation of dioxetane 106 with dimethyl sulfide. In 2,3,4,5-tetrahydropyrazine 1-oxide 109, which is prepared... 

Another type of photochemistry without light involves the actual formation of the triplet reactant excited state by thermolysis of appropriately selected dioxetanes as depicted in equation 210 ". 

The thermolysis of dioxetanes in the presence of 02 also yields peroxyl radicals (alkylperoxyl and acetylperoxyl), and these generate upon their reaction with dGuo mainly Z and 4-HO-8-oxo-G with only small amounts of 8-oxo-G... 

Adam W, Arnold MA, Saha-Moller CR (2001) Photooxidative damage of guanine in DG and DNA by the radicals derived from the a cleavage of the electronically excited carbonyl products generated in the thermolysis of alkoxymethyl-substituted dioxetanes and the photolysis of alkoxyac-etones. J Org Chem 66 597-604... 

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. 

For example, 3-pentyl- and 3-neopentyl-l,2-dioxetane undergo thermolysis in xylene at 60 °C with first-order rate constants (kx) of 4.6 and 9.2 x 10 4 s, respectively (Scheme 4) <2001HAC459>. Chemiexcitation yields were in the order of 0.02 (0T) and <0.0005 (0s) for both derivatives. 

The thermal stability of dioxetanes can be increased in several ways. As expected, the most useful way to increase the thermal stability of dioxetanes has been to increase the steric bulk around the core. Heavily substituted dioxetane 33 is an example of a stable dioxetane at room temperature and requires elevated temperatures for decomposition. Thermolysis at 90 °C in toluene affords light (Amax = 411nm) whose spectrum is in good agreement with the fluorescence spectrum of dicarbonyl 34 (Scheme 5) <2000CC821>. 

Thermolysis of 2,1,3-silaphosphaoxetane 38 furnishes the transient silanone, which immediately dimerizes to the l,3-disila-2,4-dioxetane 65 (Scheme 4) <19960M1845>. 

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