Chemiluminescence concerted mechanism

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

One is the concerted decomposition of a dioxetanone structure that is proposed for the chemiluminescence and bioluminescence of both firefly luciferin (Hopkins et al., 1967 McCapra et al., 1968 Shimomura et al., 1977) and Cypridina luciferin (McCapra and Chang, 1967 Shimomura and Johnson, 1971). The other is the linear decomposition mechanism that has been proposed for the bioluminescence reaction of fireflies by DeLuca and Dempsey (1970), but not substantiated. In the case of the Oplopborus bioluminescence, investigation of the reaction pathway by 180-labeling experiments has shown that one O atom of the product CO2 derives from molecular oxygen, indicating that the dioxetanone pathway takes place in this bioluminescence system as well (Shimomura et al., 1978). It appears that the involvement of a dioxetane intermediate is quite widespread in bioluminescence. 

Certain Schiff bases, i.e. 122, were synthesized as model compounds for Latia luciferin. This compound exhibits strong blue chemiluminescence ( max 385 nm) on oxidation with oxygen in DMSO/potassium t.-butylate, the main products being acetone and 2-formamido pyridine 124. The mechanism suggested by Me Capra and Wrigglesworth includes the concerted bond cleavage of a dioxetane derivative 123. 

Figure 8 Chemiluminescent mechanism for 1,2-dioxetanes (A) a concerted decomposition process (B) a two-step biradical process.

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. 

The 1,2-dioxetanes are another important group of chemiluminescent compounds. These compounds are oxidatively cleaved thermally in a concerted fashion to yield two carbonyl moieties, one of which is excited. The mechanism of this cleavage has been described as a chemically initiated electron-exchange... 

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.

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