Mechanism and anion activation in solid–liquid phase-transfer reactions catalysed by cyclophosphazenic polypodands. Comparison with cyclic analogue crown ethers

Abstract

A kinetic study of the nucleophilic substitution of the methanesulfonic group by anions (Cl^–, Br^–, I^–, SCN^–, C₆H₅O^–, C₆H₅CH₂COO^–) catalysed by cyclophosphazenic polypodands has been performed under solid–liquid phase-transfer conditions. The results obtained show that the mechanism of PTC previously found for cyclic ligands (crown ethers, cryptands) also operates in the case of these open-chain ligands: the attack by the anionic nucleophile on the substrate occurs in the organic phase and is rate determining of the overall process. The nucleophilicity scales found (I^– > C₆H₅O^–≈ SCN^– > C₆H₅CH₂COO^–; I^– > Br^– > Cl^–) as well as the anion activation are comparable with those found for cyclic crown ethers. This indicates that the anionic reactive species involved are similar in the complexes of both ligands.