Synthesis of novel well-defined chain-end-functionalized polystyrenes with dendritic chiral ephedrine moieties and their applications as highly enantioselective diethylzinc addition to N-diphenylphosphinoyl arylimines

A series of novel well-defined chain-end-functionalized polystyrenes having 2, 4, 8, and 16 chiral ephedrine moieties dendritically distributed at their hyperbranched chain-ends were quantitatively synthesized. Their well-defined architectures were fully confirmed by elemental analysis, FTIR, SEC as well as by 1H and 13C NMR spectroscopies. These polymers were precisely controlled in the molecular weight and molecular weight distribution as well as well-defined in chain-end-functionalities. These dendritic chiral polymers serve as highly enantioselective chiral ligands in the enantioselective addition of diethylzinc to a series of N-diphenylphosphinoyl arylimines. Among them, chiral dendrimer having eight ephedrine moieties at the chain-ends afforded the corresponding enantiomerically enriched phosphinoylamides in good to high yields with enantioselectivities up to 93% ee. The obtained enantioselectivities are comparable with those obtained by using N-benzylephedrine and its corresponding copolymer as chiral ligands.