Photosubstitution Reactions of Aromatic Compounds

In nucleophilic aromatic photosubstitution, just as in aromatic substitution in the ground state, substituents can have directing and activating effects. Four rules, which describe orientation of nucleophilic substitution in the excited state, can now be formulated. They are: (a) meta-activation by the nitro group (b) ortho/para-activation by the methoxy group (and probably also by other electron-donating groups) (c) 'x-reactivity' in biand tricyclic aromatics (i.e. position 1 in naphthalenes and azulenes, 9 in phenanthrene, 2 and 4 in biphenyls, etc.) (d) merging (resonance) stabilization during product formation. In most nucleophilic aromatic photosubstitutions the reaction proceeds via a it,ir excited triplet state, which interacts with the nucleophile. In some cases the reaction starts from a ir,it excited singlet and in some others there are indications that the aromatic molecule in its excited state undergoes dissociation, producing an ion which subsequently reacts with the nucleophile. Kinetic measurements have revealed the absolute necessity of using rate constants instead of quantum yields as a measure of reactivity. Rate constants for the process in which the triplet excited molecule undergoes deactivation by the nucleophile, leading to substitution product, have been determined for some naphthalene derivatives. From flash photolytic investigations in the nanosecond region evidence has been obtained for the occurrence of a complex (possibly a sigma-complex) formed from the nucleophile and the aromatic molecule in its reactive excited state. The complex may dissociate back into the starting materials or lead to substitution products, while a third route gives rise to the radical anion. The latter cannot be intermediate in the route to substitution product; instead, it may undergo protonation and be transformed into reduction products. INTRODUCTION In this paper a summary is given of our present knowledge on aromatic photosubstitution. The following aspects will be treated: (a) orientation rules; (b) the nature of the reactive excited state and of the primary step; kinetic results; (c) intermediates in aromatic photosubstitution. The emphasis will lie on nucleophilic aromatic photosubstitution. A 433 PAC—41----4----B