Effects of dendritic interface on enantioselective catalysis by polymer-bound prolines

Dendritic effects have been observed in the past for a number of metal-based catalysts, but only rarely for organocatalysts, and particularly chiral organocatalysts. In the current study, l-proline was immobilized as an ester or amide on polyether dendritic spacers attached to polystyrene. The ester-including catalysts showed a remarkable positive dendritic effect on yield, but even more so on enantioselectivity, in the aldol reaction of acetone with aromatic aldehydes. The positive dendritic effect of the aforementioned catalytic systems on the yield, diastereo- and enantioselectivity of a three-component Mannich reaction was of an even greater magnitude. A series of experiments marked the possibility of catalysis by homogenous l-proline, detached from the resin during the reaction, highly unlikely. Model comparative studies with catalysts equipped with linear or only partially dendritic spacers emphasized the superiority of the fully dendritic spacer architecture.