Formylation meta complexes

The Z-substituted benzene (benzaldehyde, Figure 11.2) is not activated toward electrophilic attack since the HOMO of benzene is scarcely affected. No preferred site for attack of the electrophile can be deduced from inspection of the HOMOs. The interaction diagram for a Z-substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with a formyl substituent are shown in Figure 11.4. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 9.204 / , 6a — 9.2031/ , and 6a -9.1291/ , respectively. Thus, by SHMO, the ortho and meta channels are favored over the para channel, with no distinction between the ortho and meta pathways. Experimentally, meta substitution products are usually the major ones, contrary to the SHMO predictions. Either the SHMO method fails in this case or the predominance of meta products may be attributed to steric effects. 

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