1.2- Dioxetanes methyl-substituted

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 biradical-like concerted mechanism, in which the kinetic features reflect the biradical character and the formation of excited-state products can best be rationalized by the concerted nature of the complex reaction coordinate, was proposed to optimally reconcile the experimentally determined activation and excitation parameters of most 1,2-dioxetanes studied and has been called the merged mechanism51-87. Specifically, both thermal stability and singlet and triplet quantum yields in the series of methyl-substituted 1,2-dioxetanes, including the parent 1,2-dioxetane11 50 51, could be readily rationalized on the basis of the merged mechanism and qualitative quantum mechanics considerations86. 

An interesting approach to 1,4,22 -dioxaphosphinanes has been reported. Methyl-substituted 1,2-dioxetans (357) react with triphenylalkylidenephosphoranes (358) by nucleophilic attack of the ylide carbon at the peroxide bond of (357) the 2,2,2-triphenyl-1,4,22 -dioxaphosphinanes (360) are... 

However the view that the 0-0 bond simply stretches is altered by the fact [33] that 3,3-(i.e. geminal) substitution seems more important than 3,4-substitu-tion. A group of dioxetans of this sort has been synthesised with methyl and ethyl substituents so disposed as to make this point. Some workers hold the view that the transition state involves twisting about the C-C bond. Large groups by virtue of their inertia would resist this [34]. Very many bridged dioxetans have also been synthesised [35], such as (4) to (7). 

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