Protection of 4‐Fluoroanisole from Aromatic Nucleophilic Photosubstitution by Cyclodextrins [1]

Aromatic nucleophilic photosubstitution reactions of 4‐fluoroanisole with cyanide and water have been investigated in aqueous α‐ β ‐, and γ‐cyclodextrin solutions. Although photoreactivity of the α‐ or β‐cyclodextrin‐complexed 4‐fluoroanisole toward nucleophilic substitution is suppressed almost completely, none of the cyclodextrins had a measurable influence on the rate of 4‐fluoroanisole fluorescence quenching by I − . In aqueous γ‐cyclodextrin, the efficiency of photohydroxylation was increased slightly but the efficiency of photocyanation was unaffected. From Benesi‐Hildebrand plots, the dissociation constants of the complexes of 4‐fluoroanisole with α‐ and β ‐cyclodextrins were obtained. A good correlation between the relative quantum yields and the degree of complexation was found. Explanation for the lack of a marked effect on photosubstitution by γ‐cyclodextrin is presented.