Catalytic activity and anion activation in S(N)2 reactions promoted by complexes of silicon polypodands. Comparison with traditional polyethers

The catalytic activity of silicon polypodands was evaluated in anion- promoted reactions under solid - liquid phase- transfer catalysis ( SL- PTC) conditions and compared with that exhibited by common PTC agents. Results showed that these many- armed ligands are particularly efficient catalysts, even comparable with the more sophisticated crown ethers. Kinetics under homogeneous conditions indicated that the anion activation realized by silicon polypodands does not appreciably change with the number of chains and is comparable or slightly lower than that of traditional polyethers. In addition, the sequence of reactivity is determined by the cation ( NH4+ < K+ < Na+ < Li+), thus revealing an involvement of the metal ion in the transition state of the reactions (" metal ion electrophilic catalysis'').