The yield and enantioselectivity of an asymmetric aldol reaction, catalyzed by a proline derivative immobilized on polystyrene via dipolar cycloaddition, are remarkably improved by the dendronization of the support.

Bidentate phosphine ligands have been prepared on polystyrene beads modified with polyether dendron spacers. When complexed to Pd(0), these systems exhibited a negative dendritic effect on Heck catalysis (contrary to the analogous monodentate phosphine systems), but mostly a positive influence on carbonylation. This opposite influence of the dendronization falls into line with other differences in the optimal ligand structure for the two reactions. The negative effect on the Heck catalysis with bidentate phosphines may indicate that dendrimer-induced reduction in the cross-linking upon Pd complexation is responsible for the positive effect in the corresponding monodentate phosphine systems.

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In order to delineate the properties of the spacer architecture responsible for the strong positive dendritic effect exhibited by polymer-supported proline-based catalysts, we prepared two series of polystyrene-bound model catalysts. The first series was based on a linear and partially dendritic spacers (of reduced branching and valency) imitating the length of the second generation spacer, while the second series was based on the first generation dendron spacer with one functional (proline-terminated) and one non-functional arm. Comparative studies of the model and original (fully dendritic) catalysts in the asymmetric aldol reaction of aromatic aldehydes with acetone disclose the features characteristic to the dendritic architecture, such as proximity between the terminal catalytic units and enhanced branching, as crucial for inducing higher yield and enantioselectivity in catalysis.Keywords: aldol reaction; asymmetric organocatalysis; dendrimers; solid-phase synthesis; supported catalysts Over the past years a growing number of studies in the field of asymmetric catalysis were devoted to organic catalysts, metal-free small molecules, capable of promoting chemical transformation with high efficiency and enantioselectivity.[1] Although heterogenized catalysts benefit from a number of economic, environmental and technical advantages as compared to their homogeneous analogues, reports of successful immobilization of enantioselective organocatalysts on solid supports remain relatively scarce.[2] l-Proline provides an excellent model for organocatalyst immobilization studies due to its simplicity and the ready availability of various derivates, [3] in spite of a question mark on the economic profitability of developing a successful immobilized proline catalyst because of the low price of this amino acid and the possibility of its easy recovery from homogeneous reaction mixtures. Moreover, immobilization frequently provides catalysts with an altered reactivity profile and can indirectly afford valuable mechanistic information. In earlier studies, unfortunately, the immobilization of proline led to reduced selectivity, [4] and only recently have a limited number of covalently heterogenized proline-based catalysts, exhibiting enantioselectivity approaching that of their soluble analogues, been reported. [5] While these catalysts demonstrated excellent results in the aldol reactions with cyclic ketones, only a few supported prolinecontaining peptides could match the performance of proline in the reaction with acetone. [5a-c] Recently, we observed that the introduction of a short dendritic spacer between the polymer core and the 4-hydroxyproline-derived catalytic units immobilized via azide-alkyne "click" chemistry provides a more active and remarkably more enantioselective catalyst, as compared to the spacer-less non-dendritic analogue. (Scheme 1). [6] Although this was an extraordinary manifestation of a positive dendritic effect, the first of its kind in supported organocatalysis, [7] the question of the...