Microenvironmental effect in polymer-supported reagents for organic chemistry

The microenvironment of a polymer-supported reagent consists of the polymer backbone and the neighboring groups surrounding the active moieties. The interactions between the groups are not negligible due to a small confined volume within the macromolecule. The focus of this dissertation is to determine whether polymer-supported reagents can be optimized for a target reaction by tailoring the microenvironment surrounding the active sites to reaction mechanism. Polymer-supported phosphine resins are synthesized and used to carry out the Mitsunobu reaction of benzyl alcohol with benzole acid. They are compared with their small molecule analogues to quantify the electronic and microenvironmental effect. It is found that the choice of groups surrounding a ligand can be as important as the choice of the ligand. Increasing the hydrophobicity surrounding the phosphine ligands results in an increase in the reaction rate (at 0.1 h: 41.7, 68.4, 83.7, and 94.5% conversion for polymers with 0, 33, 60, and 82% phenyl groups surrounding the phosphine ligands). Polymers with 60 and 82% phenyl groups give an equilibrium solution that is more pure (97.6% and 97.0% ester) than that with a comparable soluble reagent (85.3% ester). Replacing phenyl groups with carbomethoxy groups, or carboxylic acid groups, has a detrimental effect (at 0.1 h: 1.3% conversion with carbomethoxy groups and 0% conversion with carboxylic acid groups). It is proposed that decreasing the polarity of the microenvironment surrounding the active sites increases the reactivity of the intermediate in the rate-determining step, resulting in an increase in the rate of product formation. Polymer-supported sulfonic acid resins have been synthesized and used to catalyze the aldol condensation. The kinetics for the heterogeneous and homogeneous acid catalyzed reactions are studied and compared. Macroporous and lightly cross-linked gel resins outperform their small molecule analogues (at 1 h: 35.8, 50.6 and 56.5% conversion for benzenesulfonic acid, fully sulfonated 2% DVB cross-linked gel resin, and fully sulfonated 10% DVB cross-linked MR resin). Introducing phenyl rings as neighboring groups surrounding the sulfonic acid ligands results in an increase in reaction rate (at 1 h: 50.6 and 63.5% conversion for polymers with 0 and 55% phenyl groups). This is attributed to a less polar microenvironment which increases the reactivity of the intermediate in the rate determining step. Ester groups are found to compete with the reactant for the catalytic ligands. Replacing phenyl groups with carbomethoxy or carbobutoxy groups slows down the reaction (at 1 h: 63.5, 51.3, 49.2% conversions for polymers with phenyl, carbobutoxy, and carbomethoxy groups as neighboring groups). Polymer-supported sulfonic acid resins have also been used to catalyze the Prins reaction. The kinetics for the heterogeneous and homogeneous acid catalyzed reactions are studied and compared. Macroporous and lightly cross-linked gel resins significantly outperform their small molecule analogues (at 1 h: 8.3, 100, 100% conversion for benzenesulfonic acid, fully sulfonated 2% DVB cross-linked gel resin, and fully sulfonated 10% DVB cross-linked MR resin). Introducing phenyl rings as neighboring groups surrounding the sulfonic acid ligands results in an increase in the reaction rate (rate constant k: 23.1, 44.4, and 53.7 for polymers with 0, 15, and 20% phenyl rings). However, the reaction rate decreases when more than 20% phenyl rings are introduced into the catalyst (k; 53.7, 36.0, 18.6, and 6.4 for polymers with 20, 41, 55, and 80% phenyl rings). Similar behavior has been observed when carbomethoxy and carbobutoxy groups are used as neighboring groups. It is proposed that the polymeric acid catalyzed reaction rate is favored by the high ligand density and the low polarity surrounding the ligands. The above results demonstrate that polymer-supported reagents can be tailored to a reaction for maximum reaction rate, and product yield, through the microenvironmental effect. The above studies provide guidelines for designing effective polymer-supported reagents for organic reactions.