Heterocyclic Polymers as Catalysts in Organic Synthesis: Effect of Macromolecular Design and Microenvironment on the Catalytic Activity of Polymer-Supported (Dialkylamino)pyridine Catalysts

Several new polystyrene-based resins containing (dialkylamino)pyridine pendant groups have been prepared by chemical modification of halogenated bead polymers or by suspension copolymerization of the corresponding monomers with divinylbenzene and styrene or 4-vinylpyridine. Kinetic studies on the polymeric catalysts and comparisons with low molecular weight analogues indicate that their efficiency as acylation catalysts depends on the microenvironment and the structure of the catalytic sites, the degree of functionalization, and the distance of the catalytic sites from the polymer aryl. Best results are obtained with gel-type polymers containing a three-carbon spacer group between the polystyrene rings and the catalytic site and functionalized to an extent of less than 50%, providing a hydrophobic local environment to the catalytic centers. Copolymers containing 4-vinylpyridine units in addition to the catalyst units, or high concentrations of the latter, show a strong microenvironment effect whereby activity is lowered drastically in a way that cannot be duplicated with other catalysts under homogeneous conditions.