Stable Biradicals

An anchor, as defined above, contains stable molecules, conformers, all pairs of radicals and biradicals formed by a simple bond fission in which no spin re-pairing took place, ionic species, and so on. Figure 1 shows some examples of species belonging to the same anchor. Thus, an anchor is a more general and convenient temi used in the discussion of spin re-pairing. 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

Photoelectron spectroscopy has routinely been used to determine the ionization energies of stable molecules. It has also been adapted for the investigation of reactive species, including radicals, biradicals, and carbenes, usually generated chemically or by using pyrolysis. ... 

The above biradicals, with the exception of the oxygen molecule, are all highly unstable there are, however, a number of much more stable species that show evidence of biradical character. Thus the hydrocarbon (135) exists, in part, in solution as a biradical ... 

Azidofurans lose nitrogen giving products reminiscent of those formed from furyl carbenes and biradicals.278 External nitrenes apparently add to furan double bonds just as carbenes do, and again the initial products collapse very readily to give nonheterocyclic products. The nitrene produced oxidation of N-aminophthalimide reacts as in Scheme 54. The products are relatively stable derivatives of but-2-endial and are therefore of potential synthetic value, initially the butene link is Z but easily isomerizes to E on silica columns.279... 

The use of MO theory to find deep minima in the So surface, or geometries of stable molecules, is well known. A simplified rule would be to choose the geometry so as to allow efficient overlap of valence orbitals of the constituent atoms in a way giving bonding orbitals for all available electrons from pairs or larger sets of suitably hybridized atomic orbitals. No atomic orbitals occupied by one electron should be left over dangling free and unable to interact with others, since that would give radicals, biradicals, etc. Chemical intuition allows one to proceed almost automatically in cases of molecules of familiar types. 

An important factor that stabilizes the complex or transition state leading to biradical intermediate is the dipole-dipole interaction between the addends. The orientation given in 13 would be more stable than 14 from this factor, but the orientation in 14 is preferred by the orbital... 

Whichever mechanism operates, it appears to be generally true that singlet aromatic carbenes react with the lower alcohols to form ethers at rates approaching the diffusion limit. On the other hand, aromatic carbenes that are clearly triplets do not give any ether at all from reaction with alcohols. Instead, these triplets behave as is expected of biradicals and abstract a hydrogen atom from the oxygen bearing carbon of the alcohol. The stable products of this reaction are those due to the combination and disproportionation (10) of the pair of radicals (Lapin et al., 1984). The more com-... 

Five- or six-membered saturated cyclic ketones can also react by another pathway that does not involve decarbonylation. In these reactions, the biradical initially formed by a-cleavage undergoes internal disproportionation without loss of carbon monoxide, resulting in the formation of either an unsaturated aldehyde or a ketene. Methanol is usually added to convert the reactive ketene to a stable carboxylic-acid derivative (Scheme 9.2). 

The reaction between an asymmetrical alkene and an aromatic ketone gives two different orientations of cycloaddition through two different 1,4-biradicals. The route through the more stable biradical produces the major product ... 

The author believes it is not wise to take advantage of only geometrical symmetry synthetic efforts should be directed towards triplet and quartet D or A molecules due to topological symmetry, e.g. [47] and [48] (Azuma et ai, 1974 Kuhn et al., 1966). Of special interest is a stable perchloro-derivative of the Schlenck hydrocarbon [11]. Due to restricted rotation of the aryl groups, the biradical exists in two stereoisomeric forms meso and dl. They... 

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