Advantages of polymer-supported multivalent organocatalysts for the Baylis-Hillman reaction over their soluble analogues

Immobilization of well-defined catalytic units onto insoluble support promises significant benefits, but frequently results in a reduced activity and selectivity of the heterogenized catalysts. Recently, we showed that introduction of a dendritic spacer between the support and the units could remedy the compromised activity and/or selectivity of heterogenized catalysts and, in particular, of the systems based on N-alkylated imidazoles. These catalysts exhibit an outstanding multivalency effect on the activity in the Baylis-Hillman reaction, while preserving very high chemoselectivity. In order to better understand this remarkable effect, we decided to synthesize and examine soluble analogues of the supported systems. These soluble catalysts display poor chemoselectivity, although it improves with the increase of the dendritic generation. Though the consumption of the limiting aldehyde reactant (conversion) displays the opposite trend, experiments demonstrated that the chemoselectivity is generation-dependent rather than conversion-dependent. A hydrophobic "pocket" effect was implicated as responsible for the differences between the polystyrene-bound and the soluble catalysts. An MS analysis of the crude reaction mixture revealed that the formation of multiple adducts, which incorporate several enone and several nitrobenzaldehyde fragments into a single molecular structure (as opposed to one-to-one stoichiometry of the Baylis-Hillman reaction), is responsible for the decline in the chemoselectivity.