Charged Ambident Radicals

This stereochemistry is similar to that in the reduction of a,/3-unsaturated ketones. Stork found that octalones like 7.88 were reduced exclusively to the frans-decalone 7.90, even in cases when the trans- was less stable than the corresponding c/.v-decalin. In this reaction, an electron is fed into the LUMO 

In contrast, C-substituted benzenes like biphenyl 7.96 are reduced to 3-substituted cyclohexa-1,4-dienes 7.99, and this too fits the analysis. The Hiickel coefficients for the SOMO of the radical anion 7.97 also reflect the total 7r-electron distribution, since the other three filled orbitals lead to a more or less even distribution of 7i-electron population. So, regardless of whether it is the Coulombic or the frontier orbital term that is more important, both contributions lead to protonation at C-4 to give the radical 7.98. Reduction and protonation of this intermediate (or possibly a mixture with the 1-protonated isomer) leads to the observed product 7.99. Further reduction of this molecule then takes place, but now the benzene ring is an X-substituted one. The major final product, accordingly, is the hydrocarbon 7.100, which has been reduced 1,4 in one ring and 2,5 in the other. 

The Birch reduction of benzoic acid is the same type as that of biphenyl, and the product, with protonation successively at C-4 and C-l is the acid 7.102. In the reaction medium, it will be benzoate ion 7.101 that is being reduced. As a result of the delocalisation of the negative charge in the benzoate ion, we should probably regard the carboxylate ion more as a C- than as a Z-substituent. 

For example the radical cation 7.131 is generated by oxidation of 2-methylnaphthalene. The odd electron is in the HOMO of naphthalene, the highest coefficient of which is at C-l. The methyl group, as an X-substituent, will further enhance the coefficient at this site relative to the other Q-positions thus, the total electron population at this site will be higher than at the other positions, and yet the nucleophile, an acetate ion, attacks at this site. That an anion should attack a site of relatively high electron population is easily accounted for by the SOMO/HOMO interaction. The intermediate radical 7.132 eventually gives l-acetoxy-2-methylnaphthalene when a radical abstracts the hydrogen atom. 

The radical cations of conjugated systems can also take part in pericyclic reactions. Examples are known of cycloadditions, electrocyclic reactions and sigmatropic rearrangements. One noticeable feature of some of 

The experimental observation is that hydride reduction (Felkin-Anh control) of the ketone 7.153 gives the anti-alcohol anti-7.154 as the major product, but electron-transfer reduction gives its syn diastereoisomer syn-7.154.1067 This is in contrast to the reduction on p. 423, which differs in having a silyl group on the oxygen atom in a neutral radical. 

In cyclohexanones, the pyramidalisation is similarly greater in the radical anion 7.155 than it was in the ketone 5.138 (see p. 230), thus explaining the high degree of synthetically useful selectivity for the formation of the equatorial alcohol trans-1.157 from the ketone 7.156, as a result of axial protonation.1068 

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The absorption of quinoxaline 1,4-dioxide at long wavelength has been interpreted in terms of intramolecular charge transfer involving ambident Af-oxide groups.- ESR and optical spectra of quinoxaline cation radical have also been measured. ... 

Radicals are soft most of them do not have a charge, and in most chemical reactions they react with uncharged molecules. Thus the Coulombic forces are usually small while the orbital interactions remain large. This is borne out by such well-known reactions as the attack of radicals at the conjugate position of o,/l-unsaturated carbonyl compounds like methyl methacrylate 7.1, rather than at the carbonyl group, and the attack by the ambident -carbonyl methyl radical 7.2 from the carbon atom, not from the oxygen atom. The clean and industrially important polymerisation giving poly(methyl methacrylate) (PMMA) demonstrates both of these typically soft patterns of behaviour. 

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