Dendrimers as chiral catalysts?; A critical note

The possibility of using dendrimers as carriers for chiral ligands in the diethylzinc addn. to benzaldehyde is investigated with poly(propylene imine) based dendrimers. Optically active b-amino alcs. at the periphery are prepd. by the addn. of R-styrene oxide to the primary amine end groups. High chem. yields of 1-Ph propanol are obtained for all generations. However, the enantiomeric excess of the product decreases from 11% to 0% by increasing the generations and hence the no. of chiral end groups from 1 to 64. The decrease in enantiomeric excess is explained by an increase of d. of chiral end groups, which results in several frozen-in conformations. The close packing in the shell of a dendrimer thus results in a multi-site catalyst.