SOMO-HOMO interactions

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

The dominant factor that gives rise to the observed high reactivities of per-fluoro-n-alkyl radicals, particularly in their additions to electron-rich alkenes, would appear to be the high electrophilicities of these very electron-deficient radicals [114]. A perfluoro-n-alkyl radical, which one can assume to have a low-lying SOMO, should exhibit a dominant SOMO-HOMO interaction in its additions to alkenes, and polarization of the type shown in Fig. 1 will stabilize the early transition state in which little radical character has been transferred to the substrate alkene. Therefore, if steric hindrance is equivalent for a series of alkenes, the rates of addition of RF should correlate with the alkene IPs (which should reflect HOMO energies). As Fig. 2 indicates, there is indeed a respectable correlation between log kadd for typical perfluoro-n-alkyl radicals and terminal... 

Considering the very large number of interactions to be taken into account, the FO approach loses much of its charm. Even when a radical can be described by a single configuration, scheme (a) above shows that it may be necessary to consider four orbital interactions including the three-electron SOMO-HOMO interaction, which may pose a problem as it can be either attractive or repulsive (p. 12). 

This interpretation is consistent with the nucleophilic properties which are generally associated with the methyl radical. In passing, note that Canadell s rule states that any radical, nucleophilic or electrophilic, reacts with an alkene at the site having the largest HOMO coefficient.62 Canadell and co-workers argue that the three-electron SOMO-HOMO interaction is stabilizing, due to the energetic proximity of these orbitals. See, however, p. 12. 

In the previous sections, the reactions of nucleophilic alkyl and acyl radicals with electron-deficient aromatics via SOMO-LUMO interaction have been described. At this point, we introduce the reactions of electrophilic alkyl radicals and electron-rich aromatics via SOMO-HOMO interaction, though the study is quite limited. Treatment of ethyl iodoacetate with triethylborane in the presence of electron-rich aromatics (36) such as pyrrole, thiophene, furan, etc. produces the corresponding ethyl arylacetates (37) [50-54]. 

This reaction comprises firstly of SH2 reaction on the iodine atom of ethyl iodoacetate by an ethyl radical, formed from triethylborane and molecular oxygen, to form a more stable Chester radical and ethyl iodide. Electrophilic addition of the a-ester radical to electron-rich aromatics (36) forms an adduct radical, and finally abstraction of a hydrogen atom from the adduct by the ethyl radical or oxidation by molecular oxygen generates ethyl arylacetate (37), as shown in eq. 5.20. Here, a nucleophilic ethyl radical does not react with electron-rich aromatics (36), while only an electrophilic a-ester radical reacts with electron-rich aromatics via SOMO-HOMO interaction. 

In the previous chapters, Bu3SnH has been used as a typical and useful radical reagent in a benzene solvent. Generally, radical reactions with Bu3SnH initiated by AIBN, proceed effectively in benzene, which bears a conjugated Tr-system. Probably, the formed radicals are somewhat stabilized through the SOMO-LUMO or SOMO-HOMO interaction between the radicals and benzene. 

With anisole, the SOMO/HOMO interaction (B) is strong, and with nitrobenzene the SOMO/LUMO interaction (A) is strong, but with benzene neither is stronger than the other. Product development control can also explain this, since the radicals produced by attack on nitrobenzene and anisole will be more stabilised than that produced by attack on benzene. However, this cannot be the explanation for another trend which can be seen in Table 7.1, namely that a p-nitrophenyl radical reacts faster with anisole and benzene than it does with nitrobenzene. This is readily explained if the SOMO of the p-nitrophenyl radical is lower in energy than that of the phenyl radical, making the SOMO/HOMO interactions (C and D) strong with the former pair. 

When the SOMO/HOMO interaction is the more important one, and... 

A word of warning. In this chapter, we have not tried any quantitative correlations. No doubt they could be made, but they will be complicated. The reason is that the interactions we have been looking at, especially the SOMO/ HOMO interactions, are often between orbitals quite close in energy. Such interactions lead to first-order perturbations and the third term of equation 2-7 is not appropriate. 

Carbon-centered radicals are nucleophilic or electrophilic species, depending upon the substituents at the radical center. Electron-donating substituents like alkyl or alkoxy groups increase the nucleophilicity - of the radicals whereas electron-withdrawing groups (EWG) like ester or nitrile groups increase their electrophilic nature . In nucleophilic radical reactions SOMO LUMO interaction dominates whereas electrophilic radical reactions are controlled by SOMO-HOMO interactions. 

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. 

Figure 13.5. The SOMO-HOMO interactions for methyl and acetate radicals with methyl acrylate and methyl vinyl ether.

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