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

Abstract

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 (chlorobenzene, THF) under solid–liquid heterogeneous conditions. While lithium and sodium salts give exclusively the ‘P-crown’ substitution product, the preferential 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 involvement of the cation in the stabilization of the transition state, following a ‘push–pull’ mechanism. The results provide strong evidence for the key role of the polyether crown substituent in determining, ia a host–guest interaction with the alkali metal p-nitrophenoxide, both the activation of the anion and the regiocontrol of the chlorine substitution in the phosphazenic ring (‘P-crown’  s. ‘P-non-crown’ substitution). As a consequence of this supramolecular control, it is possible to switch the reaction pattern toward electronically and sterically unfavored regioselective substitution at the position geminal to the macrocycle (‘P-crown’ substitution), giving easy access to species otherwise difficult to obtain following the rules of classical phosphazene chemistry.