1,2-Dioxetanes thermal decomposition mechanism

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Although the activation parameters obtained from the thermal decomposition of a great number of diverse dioxetane derivatives have been interpreted on the basis of the biradical mechanism, no general interpretation of the excitation efficiencies has been given on fhe basis of fhis mechanism Furfhermore, most theoretical... 

Lucigenin (10,10 -dimethyl-9,9 -biacridinium or bis-Af-methylacridinium (38)), in the presence of hydrogen peroxide in alkaline media, exhibits chemiluminescence with a maximum emission wavelength at 445 nm. Lucigenin chemiluminescence was first reported in 1935 by Glen and Petsch, and the 1,2-dioxetane 39 was postulated as a key intermediate. Nevertheless, the mechanism of lucigenin chemiluminescence was only elucidated by McCapra and Richardson, who also proposed the thermal decomposition... 

An alternative thermal decomposition of 1,2-dioxetanes bearing an aromatic electron donor involves a CIEEL mechanism and is described in Section 2.16.6.1. 

Probably the strongest support in favor of the diradical mechanism is the lack of a deuterium isotope effect in the thermal decomposition of franx-3,4-diphenyl-1,2-dioxetane. In the concerted mechanism, the ring carbon of the dioxetane changes its hybridization state from sp to sp in the activated complex (23) and an inverse secondary isotope effect k lkp) would be expected. Consequently, a diradical mechanism was argued to accommodate these results. Similarly, in the thermal decarboxylation of the dimethyl a-peroxylactone, a negligible (A ///A ) = 1.06 0.04) secondary isotope effect was observed. Presumably, in the a-peroxylactone decomposition, a diradical mechanism similar to that of dioxetanes (Eq. 66) upholds. 

Dioxetanes such as tetramethyl-1,2-dioxetane (525) are known to undergo thermal decomposition to form two carbonyl compounds via a concerted or stepwise (radical) mechanism, accompanied by chemiluminescence (Section 5.6) (Scheme 6.254).135,511,1440 The degradation of 525 results principally in acetone phosphorescence (2max = 430 nm) and the reaction is very sensitive to quenching by oxygen. 

Fig. 32. Possible mechanisms for the thermal decomposition of dioxetanes [adapted from Hummelen et al. (H21), with permission). In (a), a concerted bond cleavage leads directly to two carbonyl products, one of which is in the excited state and emits light (M15, M16). The substituents R1-R4 can be simple alkyl or alicyclic groups. In (b), homolytic bond cleavage leads to a biradical that exists as an equilibrium mixture of singlet-state (f ) and triplet-state (t t) forms. As before, chemiluminescence emission probably occurs via the excited singlet-state carbonyl product arising from the homolytic bond cleavage of the intermediate biradical (R5, R6). The substituents R and Ar can include uncharged alkyl, alicyclic, and aromatic groups.

The activation parameters are collected in Table III for the 1,2-dioxetanes (1) and a-peroxylactones (2). Clearly, variation of substituent structure has a minor effect on the activation parameters. A significant exception is the diadamantylidene system (lz), which is unusually thermally stable.38 Assuming a diradical mechanism for the thermal decomposition of 1,2-dioxetanes, O Neal and Richardson97 were able to reproduce the experimental activation parameters with good precision, employing thermokinetic calculations developed by Benson.9741... 

The photolytic decomposition of 1,2-dioxetanes has been studied much less extensively in comparison to their thermal decompositions. Bartlett and Schaap34 observed that prolonged irradiation times in the preparation of 1,2-dioxetanes In by photooxygenation decreased the yields substantially. Presumably the dioxetane was photosensitive. This was confirmed by Wilson and Schaap" in their classical paper on the mechanism of chemiluminescence of dioxetane In, showing that the photoenergized 9,10-diphenyl- and 9,10-dibromoanthracene singlets efficiently induced the decomposition of In. 

D Mechanism and energy diagram of the thermal decomposition of dioxetanes... 

Weak emission of light has been observed in the thermal decomposition of cw-1,2-cyclobutylene dinitrite. Since this reaction possibly proceeds through the same type of biradical as would be involved in stepwise decomposition of dioxetans, the difference between this case and the decomposition of the analogous dioxetan has been tentatively suggested as support for a concerted mechanism for the latter. For simple unsymmetrical dioxetans, undergoing... 

Kinetic data for the thermal decomposition of 1,2-dioxetans (209) and (210) indicate a first-order reaction, and the activation parameters suggest a two-step decomposition mechanism via the biradical (211). The analogous... 

Tetramethyldioxetane is the prototype for all chemiluminescent processes. It will generally be the case that a dioxetane or similar structure will be formed. Thermal decomposition of this high energy structure then produces an excited state product. Details vary, but many of the basics of Figure 16.19 will be involved. Thus, species containing a strained 0-0 bond play a special role in chemiluminescent mechanisms. For that reason, we now discuss some aspects of O2 chemistry that are relevant to the formation of dioxetanes and related species. 

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