By Dennis G Morrell; Organic Reactions Catalysis Society.; North American Catalysis Society
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These polymer-supported ligands were used as auxiliaries in the catalytic asymmetric addition of diethylzinc to various aromatic aldehydes. In the heterogeneous reactions, the products with highest ee of 97% were obtained using 4A, while low ee was seen when 3A was used. Considering the enantioselectivity and reusability, the best results were obtained with 4A. After recycling of the polymer-supported tartaric acid, the catalytic reaction resulted in an ee of 96%. Thus, a new polymer-bound catalytic system has been developed for C-C bond forming reaction.
An easy preparation procedure, a simple separation and good retention of the active metal in the polymeric material are regarded as the significant features of this new heterogenization method. Different polyanions have been found to be suitable for entrapment. By entrapment of the Pt-colloid in alginate, a constant enantioselectivity and a good activity during 25 hydrogenation cycles were observed using the catalyst in cyclohexane as solvent. The destructive hydrogenation of the chiral modifier which takes place in acetic acid is avoided in this solvent.
When using cyclohexane or toluene as solvent, no Pt-leaching was detected. In these solvents no difference in activity and enantioselectivity of the entrapped catalyst was observed. The decrease in enantioselectivity of chirally modified platinum catalysts on reuse is caused by the partial hydrogenation of the aromatic ring system of the modifier. Recent investigations have shown the chemical changes of CIN vary with the ethyl pyruvate conversion using acetic acid as solvent . Under reaction conditions CIN is hydrogenated sequentially first at the quinuclidine group and then at the quinoline ring (Figure 5).