Dioxetans, bridged

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). 

In contrast, the radical cation of the tetracychc system is significantly distorted The parent system has D2d symmetry and a b2 HOMO, whereas the radical cation is distorted toward 2 equiv structures of Cav symmetry ( E), with a two-center three-electron N-N bond (3 +). The ESR data (an = 0.709 mT, 4N ah = 0.768 mT, 8H, N—C—N ah = 0.414 mT, 8H, N—C—support the rapid interconversion of the two structures. The structure of 3 " is one of many doubly or multiply bridged diaza compounds forming three-electron N—N bonds (e.g., 4 " ). Many additional examples involving three-electron S—S or I I bonds are also known. Dioxetane radical cations (e.g., 5 ), characterized by ESR spectroscopy as intermediates in oxygenations (cf.. Section 5), contain analogous three-electron 0—0 bonds. 

While the formation of 1,3-dione (22) by singlet oxygenation of the cyclopropene 21a via dioxetane lqj appears to be quite straightforward, the transformation of cyclopropene 21b into dione 23 has been postulated to involve rearrangement of the initial 1,2-dioxetane laic to lak, as shown in Eq. (16).93 Even the bridged annulene 24 affords the... 

Several accounts of syntheses of the sulphur-containing cyclophanes (489)—(494) have been produced, the n.m.r. spectra of (493) and (494) indicating that unhindered rotational movement is possible in the bridges of both. Consideration of other related conformational points is promised for the future. Tetrathio-1,2-dioxetans (495) are postulated as intermediates... 

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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