Synthesis of chiral quaternary ammonium polymers for asymmetric organocatalysis application

Main-chain chiral quaternary ammonium polymers have been synthesized using cinchonidine or 10,11-dihydrocinchonidine as a chiral source. Since the quinuclidine moiety of cinchonidine is easily quaternized with halide to form cinchonidinium salt, the quaternized reaction (Menshutkin reaction) was applied to the dimeric compound of cinchonidine and dihalide. The quaternization polymerization between cinchonidine dimer and dihalide smoothly occurred to afford the chiral polymers containing cinchonidinium salt structure in its main-chain. This atom economical addition polymerization was also applied to dimer of 10,11-dihydrocinchonidine. The corresponding chiral quaternary ammonium polymers were found to perform as polymeric organocatalysts for enantioselective benzylation of N-diphenylmethylene glycine tert-butyl ester to afford the desired (S)-phenylalanine derivative in high yields and high enantioselectivities (up to 95% ee).