Of 1.2-dioxetanes

The recently discovered preparative methods for the synthesis of 1.2-dioxetane derivatives (see Section V.) have made these compounds and their chemiluminescent decomposition the subject of especially intensive study. 

As mentioned in Section II. C., the concerted bond cleavage of 1.2-dioxetane derivatives has been proposed to be of general importance in respect of the excitation step of a large number of chemiluminescence reactions. The first experimental results concerning simple dioxetanes were obtained by M. M. Rauhut and coworkers in their work on activated oxalic ester chemiluminescence 24>. From experimental data on the reaction of e.g. bis (2.4-dinitrophenyl)oxalate with hydrogen peroxide in the presence of rubrene, they concluded that 1.2-dioxetanedione... 

Diels-Alder reaction of 3-vinylindole 131 with aryne in the presence of air gives, besides primary Diels-Alder product 132, the methyl 12-methyl-12H-[3]-benzoxepino[l,2-l7]indole-5-carboxylate 135. This can be explained by the formation of 1,2-dioxetane 133, its cyclo reversion and final intramolecular cyclization of dienol 134 or its tautomers (Scheme 26 (1996JCS(P1)1767)). 

The cleavage of 1,2-dioxetanes constitntes the model chemiluminescent process, which may be initiated thermally, by electron transfer or in catalytic reactions (e.g. in complexes formed between dioxetanes and transition metals). In the subsequent subsections, we review the most recent significant developments in this area. 

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 . 

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. 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

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... 

The n-7T state selectivity observed in the decomposition of 1,2-dioxetanes, even in cases in which the it-it excited state of the carbonyl product possesses lower... 

In view of the above exposed facts, to date there is no direct experimental evidence of the intermediary 1,4-dioxy biradical in the decomposition of 1,2-dioxetanes. Therefore, it appears that the asynchronous (biradicaloid or biradical-like) concerted mechanism (merged mechanism) is the one consistent with aU the experimental and theoretical data currently available. 

The mechanism of 1,2-dioxetane formation in the reaction of lucigenin with hydrogen peroxide suggests a nucleophilic attack of peroxide anion on position 9 of the acri-dinium ring, followed by deprotonation and subsequent formation of 1,2-dioxetane ring... 

Several other examples of 1,2-dioxetane derivatives containing easily oxidizable groups have been reported and the high singlet quantum yield observed in their decomposition was attributed to the occurrence of the intramolecular CIEEL sequence Based on this concept, Schaap and coworkers have introduced the concept of induced chemiluminescence, which is very relevant for investigations into firefly luciferin bioluminescence and has led to the development of chemiluminescent probes widely used in immunoassays (Section N. 

The n-jr state selectivity observed in the decomposition of 1,2-dioxetanes, even in cases in which the it-it excited state of the carbonyl product possesses lower energy105-108, appears to be best explained by the concerted biradicaloid decomposition of the dioxetanes, or by the intermediacy of an extremely short-lived biradical60. These studies provide strong evidence for the asynchronous, concerted mechanism because a... 

For reviews of 1,2-dioxetanes see Adam, in Patai, Ref. 743, pp. 829-920 Bartlett Landis, in Wasserman Murray Ref. 781, pp. 243-286 Adam Adv. Heterocycl. Chem. 1977, 21, 437-481. See also Inouc Hakushi Turro Kokagaku Toronkai Koen Yoshishu 1979, 150 [C.A. 92, 214798q] Adam Encarnacidn Chem. Ber. 1982, 115, 2592 Adam Baader Angew. Chem. Int. Ed. Engl. 1984, 23, 166 [Angew. Chem 96, 156]. 

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