Orientation Rules in Nucleophilic Aromatic Photosubstitution

More than anything else it was the curious meta-activation found in the photosubstitution of nitrophenyl esters and ethers by various nucleophiles that stimulated closer investigation of this class of reactions (Section 1). In order to avoid misunderstanding, it should be stressed that the concept of activation at the meta-position vwth respect to the nitro-group does not exclude reactions taking place at other positions in the excited molecule. It merely means that, other things being equal, substitution is preferred at the meta-position over the ortho- and para-positions. 

As has been pointed out in Section 1, the simple picture in which the charge distribution of the excited aromatic compound determines the position of substitution, although containing elements of truth, cannot adequately cover the newer experimental findings and rules which we now will summarize. 

Ortho-para activation by methoxy-(alkoxy-) substituents 

However, in contradistinction to the meta-activation by the nitro-group, there is no attractive simple rationalization of the ortho-activation by the methoxy group. 

One may formulate in general terms a complementarity of rules for thermal and photochemical processes. However, such a generalization could possibly predict ortho-para orientation but not activation. First of all, careful experimental testing of the hypothesis seems required. We present a selection of very recent results.  

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