Anisole frontier orbitals

However, the frontier orbital picture based on the free arene does not account for nearly exclusive meta selectivity in addition to [(anisole)Cr(CO)3] LUMO for anisole shows essentially the same pattern as for toluene.98-100 With a strong resonance electron donor the traditional electronic picture (deactivation of the ortho and para positions) is sufficient to account for the observed meta selectivity. In this case the balance of charge control and orbital control is pushed toward charge control by strong polarization. The same argument applies to the aniline and fluotobenzene complexes. 

Again, because of photoexcitation, the important frontier orbital of the electrophile (the LUMO) is able to interact productively with the LUMO of the benzene ring which was not productive in the ground state of any drop in energy. Certainly i/ 5 has an electron distribution ideal for explaining ortho/meta attack in anisole, and, as we saw in Chapter 4, the LUMOs of nitrobenzene do lead to reactivity at the para position. Now that photoexcitation has placed an electron in these orbitals, an electrophile can take advantage of this electron distribution, whereas, in the ground state, only a nucleophile could. 

One trend seems clear, and it is a trend readily explained with frontier orbitals. In an X-substituted benzene, like toluene or anisole, the proportion of meta attack falls as the energy of the SOMO of the attacking radical rises (Table 5-5). 

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