Polymer-supported N-alkyl-α,α,-diphenyl-L-prolinols as catalysts for the alkylation of aldehydes using diethylzinc

Polystyrene beads containing N-alkyl-alpha,alpha-diphenyl-L-prolinol moieties (7) linked to the support via a long N-alkyl group, which also serves as a spacer chain, have been prepared. When used to catalyse the reactions of aldehydes with diethylzinc at 10 mole % in toluene at 20 degrees C they afford the expected alcohols in 59-83% e.e. Where comparisons can be made with the results obtained with N-methyl-alpha,alpha-diphenyl-L-prolinol (3), the PS (polymer-supported) catalysts afford lower percentage e.e. values. Evidence is provided that this is due in part to the presence of the spacer chain, and in part because with the PS catalysts not all the catalyst sites are readily accessible. The latter means that the PS catalysts cannot compete satisfactorily with the uncatalysed reactions between aldehydes and diethylzinc which produce racemates. Thus, if higher percentage e.e. values are to be obtained in future with PS alpha,alpha-diphenyl-L-prolinol derivatives, the catalyst sites will most likely need to be bound to the polymer via the phenyl groups or substituents on the five-membered ring.