Fragmentation of 1,2-Dioxetanes

This behavior is common to dioxetanes with small substituents [56], but as the bulk of the substituents increases, the yield of triplet acetone decreases without any significant increase in the fluorescence yield [57, 58, 59], suggesting that fragmentation to two vibrationally excited Sq molecules becomes increasingly competitive with spin-non-conservative dissociation as the density of vibrational states in the product ketone increases, but that crossing to the excited triplet surface is favored over spin-retentive crossing to the excited singlet surface, despite the spin-forbiddenness of the former. 

Recall, however, that TMD is much closer in energy to the excited surfaces corresponding to one molecule of acetone in So and one excited to either Si or Ti crossing to either of them may well be competitive with reaction on the ground-state surface to form two (5q) molecules. Consulting Fig. 9.10 once more, we see that the (J — correspondence calls for the same in-plane 

Soon after the spin-non-conservative fragmentation of dioxetanes was reported, a number of reaction path calculations were attempted diffferent assumptions about reaction path geometry were made and - not surprisingly -widely diverse transition state geometries and energies of activation were obtained [60, Table 1]. A more recent theoretical discussion of the reaction [62, pp. 183-188] cites later computational studies and comes comes down firmly for a biradical mechanism, in which the 00 bond is ruptured while the CC bond remains intact. However, in none of the computations reviewed was the reaction surface mapped out fully specifically, the reaction path via the TS suggested by Fig. 9.11 was not explored. 

As an afterthought, it might be noted that the transition structure in Fig. 9.11 appears to be on the pathway for thermal isomerization of 1,2-dioxetane to 1,3-dioxetane this reaction is not known to occur, presumably because spin-non-conservative fragmentation is a more efficient process. It is suggestive, however, that the analogous isomerization of 1,2-dimesityl-1,2-di-t-butyl-disiladioxetane to its 1,3 isomer, has been observed both in solution and in the solid state [63]. 

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Figure 9.10. Correspondence diagram (C2v) for fragmentation of 1,2-dioxetanes. (After reference [60, Fig. 7])...

Besides their thermal decompositions into carbonyl fragments, the chemistry of 1,2-dioxetanes is quite limited. Obviously one of the reasons for this is the great lability of the dioxetane ring system. However, a number of reactions with nucleophiles and electrophiles have been performed and will be briefly reviewed here. 

Figure 9.11. Skeletal symmetry coordinates of 1,2-dioxetane and (in frame) the TS (speculative) for its fragmentation...

As highlighted in Section 2.16.2, simple alkyl-substituted 1,2-dioxetanes are thermally labile compounds and decompose through a twisted diradical-like transition state to afford two carbonyl fragments one of which is predominantly a triplet-excited carbonyl (Scheme 3). Activation barriers are often in the order of 25 kcal mol. ... 

The enamine is unique in that it also has a photosensitizer (the dihydro-fullerene chromophore) in the same molecule, and brief exposure to air and room light leads to cleavage of the enamine double bond, producing the ketoamide shown below. The well-known photooxidative cleavage of enamines proceeds via an intermediate 1,2-dioxetane [119,120]. Although many 1,2-dioxetanes are relatively stable, those from enamines are not, and cleave to ketone and amide fragments, below — 40 °C in most cases. The ketoamide was characterized by FAB ms (m/e = 863), IR, and and CNMR (carbonyls at 170 ppm for the amide and 204 ppm for the ketone and overall C, symmetry for the fullerene carbons). 

The emission of transient 1,2-dioxetanes in the gas phase has recently been actively investigated and matches the ,7t fluorescence of the carbonyl fragments. Recently 7T,7r fluorescence has also been documented for the 1,2-dioxetane (18). ... 

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