Functional crown ethers with chlorocyclophosphazene sub-units as anion activators and promoters of highly regioselective reactions

The effect of the metal ion on the regiochemical outcome has been revealed in the nucleophilic substitution reactions of diphosphaza[16]crown-6 with a series of alkali metal p-nitrophenoxides in low polarity solvents (chlorob enzene, THF) under solid-liquid heterogeneous conditions. While lithium and sodium salts give exclusively the 'P-crown' substitution product, the pref erential formation of mono- and di-'P-non-crown' derivatives was found with the corresponding potassium and caesium salts. The regiochemistry observed on changing the salt has been rationalized in terms of the differing invol vement of the cation in the stabilization of the transition state, followin g a 'push-pull' mechanism. The results provide strong evidence for the key role of the polyether crown substituent in determining, via a host-guest in teraction with the alkali metal p-nitrophenoxide, both the activation of th e anion and the regiocontrol of the chlorine substitution in the phosphazen ic ring ('P-crown' vs. 'P-non-crown' substitution). As a consequence of thi s supramolecular control, it is possible to switch the reaction pattern tow ard electronically and sterically unfavored regioselective substitution at the position geminal to the macrocycle ('P-crown' substitution), giving eas y access to species otherwise difficult to obtain following the rules of cl assical phosphazene chemistry.