High Catalytic Activity of Chiral Amino Alcohol Ligands Anchored to Polystyrene Resins

Abstract

Enantiomerically pure (2S,3S)-2,3-epoxy-3-phenylpropanol (3) has been anchored to Merrifield resins with different degrees of cross-linking and functionalization. The resulting epoxy-functionalized resins 4 have been submitted to completely regioselective (C-3 attack) and stereospecific ring-opening with secondary amines [piperidine (a), N-methylpiperazine (b), and cis-2,6-dimethylpiperidine (c)] in the presence of lithium perchlorate to afford (2R,3R)-3-(dialkylamino)-2-hydroxy-3-phenylpropoxy resin ethers 5a-c. The progress of these two processes has been monitored by (13)C gel-phase NMR spectroscopy. Polymer-supported amino alcohols 5a-c have been evaluated as catalytic ligands in the enantioselective addition of diethylzinc to benzaldehyde, best results being obtained with the cis-2,6-dimethylpiperidine containing ligand 5c. Analogously, (2R,3R)-3-(cis-2,6-dimethylpiperidino)-3-phenyl-1,2-propanediol (11) has been anchored to a 2-chlorotrityl chloride resin (Barlos resin) in dichloromethane in the presence of diisopropylethylamine, and the anchoring process has been also monitored by (13)C gel-phase NMR spectroscopy. The resulting resin 12 has subsequently been used as a chiral ligand in the catalytic addition at 0 degrees C of diethylzinc to a family of fourteen representative aromatic and aliphatic aldehydes 8a-n, to afford the corresponding (S)-1-substituted 1-propanols 10a-n with a mean enantiomeric excess of 92%.