Effect of the metal ion on the regiochemical outcome of substitution reactions promoted by crown-bearing cyclophosphazenes

Abstract

The regiochemical outcome of nucleophilic substitution reactions of PNP16C6 (1) by a series of alkaline-earth metal p-nitrophenoxides (6–8) in chlorobenzene–solid salt two-phase systems strongly depends on the identity of the metal ion M²⁺ (Ca²⁺, Sr²⁺, Ba²⁺). The regiochemistry observed on changing the salt (i.e., geminal mono- and/or disubstitution in the 'P-crown’ position), in apparent contrast with the rules of classical phosphazene chemistry, has been rationalised on the basis of a transition state in which the metal cation M²⁺ interacts with the oxygens of the polyether 1 ('host-guest’ complex) while the ion-paired anion simultaneously attacks the adjacent phosphorus atoms, following a concerted mechanism. According to the proposed mechanism the effect is not observed when the metal ion charge is shielded by a complexing agent, such as perhydrodibenzo-18-crown-6 (14), or when the bulky non-complexable tetrahexylammonium cation is used.