Dioxetans substituents

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). 

In 1982, the Schaap group demonstrated that chemiluminescence can be induced by the addition of a base to dioxetanes bearing a phenolic substituent [11]. Herein, the same group presents a method utilizing aryl esterase to catalyze the cleavage of a naphthyl acetate-substituted dioxetane in aqueous buffer at ambient... 

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. 

This qualitative interpretation of structural and electronic similarity has also been employed to rationalize the fact that the quantum yield for the dioxetane derivative 6, in which the phenoxy substituent is directly linked to the peroxidic ring, is two orders of magnitude higher than for the dioxetane 7, in which the trigger function is separated by a methylene bridge. Furthermore, the different quantum yields were rationalized in terms of a competition between the intramolecular (pathway A) and intermolecular back-electron transfer (pathway B) in the decomposition of 7, whereas the intramolecular back-electron transfer was believed to occur exclusively in the decomposition of 6, due to the higher stability of the radical anion of the benzaldehyde derivative, as compared with the radical anion of acetone (Scheme 14). 

A recent theoretical study on the effect of substituents on the strain energies of small ring compounds has provided some valuable insight into the differences between 1,2-dioxetanes and 1,3-dioxetanes <2002JOC2588>. The C-H bonds within 1,2-dioxetane have been calculated to be stronger than those within 1,3-dioxetane by some 8 kcal mol-1 at the G2 level of theory. Calculations at the same level of theory indicate that 1,2-dioxetane is more strained than 1,3-dioxetane by some 6 kcal mol-1. Somewhat surprising is that this study has also shown that 1,2-dioxetanes are more strained than dioxiranes by some 7-12 kcal mol-1, which is in stark contrast to the case for the parent hydrocarbons and our expectations. The vibrational frequencies and the moments of inertia have also been calculated for the parent 1,2- and 1,3-dioxetanes <1997PGA2471>. 

The increasing of conjugation within the pendant substituents is also another factor that increases the stability of dioxetanes. For example, phenylethynyl dioxetane 35 has been shown to be more thermally stable than the styryl analogue 36, which in turn is more stable that the simple olefinic dioxetane 37 <1999TL4571>. 

Numerous dioxetanes with varying atom (X) and protecting group (PG) have been synthesized over the last decade in order to study the CIEEL mechanism. The following serve as prototypical examples. The most prevalent trigger is that of a siloxyphenyl substituent such as that incorporated into dioxetane 54. Tetrabutylammonium fluoride (TBAF) is used in an aprotic solvent, such as dimethyl sulfoxide (DMSO) or acetonitrile, to desilylate to afford the unstable phenolate 55 (Scheme 11) <2002MI305>. 

Actually, steric hindrance between geminal substituents of a dioxetane (3,3-steric interaction) appears to especially stabilize the peroxidic ring [6d], Conversely, the presence of an easily oxidized substituent, that has low... 

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