Abstract
Orientation and mechanism in photoinduced aromatic substitutions were critically reviewed from the theoretical and experimental points of view. Four empirical rules proposed by Havinga et al. for photonucleophilic substitutions were re-examined and it was found that three of them can be rationalized by the frontier electron method. One of the empirical rules, merging resonance stabilization, was shown to lack sound theoretical basis, and it should therefore be replaced by an orientation rule for reactions involving one-electron transfer prior to σ-complex formation. The reaction mechanism of this type can be reasonably described by charge-transfer theory developed by Nagakura and Tanaka and referred as SN (ETANR) Ar* (Electron Transfer and Addition of the Nucleophile Radical). The mechanism of photocyanation of methoxy-substituted aromatics (e. g. substituted anisoles) was re-examined by referring to data on photoionization of the aromatic substrates. It may safely be concluded that the photocyanation of p-halogenoanisoles is not of the SRN+1 Ar* mechanism, but of the SN 2 Ar* type.
