Stable Diradicals

Photolysis of spiro[fluorene-9,3 -indazole] (384) to the tribenzopentalene (385) has been rationalized in terms of the initial formation of triplet diradical (386) (76JOC2120). The spiroindazole (387) behaves differently and on irradiation in THF is converted into the dimer (388) and the stable iV-ylide (389) (76CB2596). 

A special case of fragmentation is that of 1,4-diradicals where fragmentation can lead to two stable molecules. In the case of 1,4-diradicaIs without functional-group stabilization, reclosure to cyclobutanes is normally competitive with fragmentation to two molecules of alkene. 

The reaction is ordinarily stereoselective, favoring the more stable adduct for either alkene isomer, and a long-lived triplet diradical intermediate is implicated. ... 

This result can be rationalized in terms of a cyclopropyl diradical structure by noting that the bond cleavage of the intermediate will occur to give the more stable of the two possible 1,3-diradicals. The cyclopropane ring in the final product will then incorporate this terminus ... 

Pyridine, and its monomethyl and 3,5-dimethyl derivatives " combine exothermically with dimethyl acetylenedicarboxylate in ether yielding some ether soluble materials, including trimethyl pyrrocoline-1,2,3-tricarboxylate (Section III,F,3) and deep red ether-insoluble gums. A number of crystalline compounds have been isolated from these gums by fractional crystallizations and will now be considered in detail. In the case of pyridine, Diels et al. ° isolated a red labile 1 2 molar adduct, which they formulated as (75), which isomerized rapidly on standing to a yellow stable adduct (76). These formulations are no longer accepted. Diels and Alder also suggested that the acetylenic ester first dimerized to the diradical (74) which then combined with the pyridine. 

Diradical species 4 is more stable than diradical 5, and the oxetane 6 is thus formed preferentially oxetane 7 is obtained as minor product only. Evidence for diradical intermediates came from trapping experiments, as well as spectroscopic investigations. ... 

In the case of 60, the ion has been prepared and has been shown to be a diradical in the ground state, as predicted by the discussion on page 58. Evidence that 60 is not only nonaromatic but also antiaromatic comes from studies on 64 and 66. When 64 is treated with silver perchlorate in propionic acid, the molecule is rapidly solvolyzed (a reaction in which the intermediate 65 is formed see Chapter 5). Under the same conditions, 66 undergoes no solvolysis at all that is, 60 does not form. If 60 were merely nonaromatic, it should be about as stable as 65 (which of course has no resonance stabilization at all). The fact that it is so much more reluctant to form indicates that 60 is much less stable than 65. It is noted that under certain conditions, 65 can be generated solvolytically. ... 

The cross a conjugated isomers of diradicals E H were predicted to be more stable than the linear isomers in the triplet states (Scheme 28a) [49]. 

Orbital phase discontinuity in singlet state. In contrast to the triplet state, orbital phase continuity conditions cannot be satisfied simultaneously (denoted by the dashed line in Fig. 6c) in the singlet. Thus, the singlet 1,3-diradical suffers from the orbital phase discontinuity. According to the orbital phase properties, the triplet states of TMM (1) and TM (2) were predicted to be more stable than their singlet states by the orbital phase theory [29, 31]. 

All the above-mentioned acyclic 1,3-diradicals are less stable than the a-bonded isomers. Therefore, in addition to using various substituents, other factors should be further considered in our design of persistent singlet 1,3-diradicals. In Sect. 5.2, ring structure is taken into account. Strain prevents the ring closure in the singlet state. Two linkers between the radical centers multiply the through-bond interactions. 

Stable localized singlet 1,3-diradicals are bnilt on 2,4-disilacyclobntane-l,3-diyls with electron-withdrawing o-bonds on the silicon atoms and jt-electron... 

As demonstrated above, the most stable singlet siUcon-centered monocyclic diradicals are the o-diradicals where the radicals interact with each other through the Si-Ge bonds, whereas the most stable triplet diradicals are jt-diradicals where the radicals interact with each other through the Ge-X bonds. However, for the bicyclic diradicals, 60 and 61, the conformations are fixed to exclude possibility of Jt-diradicals (Fig. 22). 

Substantial advances have been made in understanding, preparing, and detecting the carbon-centered delocalized or localized diradicals. But the silicon-centered diradicals are much less explored. Search for other stable localized singlet diradicals remains a goal of experimental and theoretical scientists. 

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