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.