Microenvironmental effect in polymer-supported reagents. 2. The Prins reaction and the influence of neighboring group content on catalytic efficiency

The microenvironmental effect on catalysis is quantified by the neighboring group content. Opposing variables in the reaction mechanism are then identified by the critical neighboring group content. The Prins reaction between formaldehyde and styrene is probed in the present study with immobilized sulfonic acid ligands as the catalyst. Cross-linked polystyrene beads were sulfonated to varying degrees of substitution. Styrene was also copolymerized with butyl methacrylate and methyl methacrylate followed by complete sulfonation of the phenyl rings. The reaction kinetics were correlated with the neighboring group content, defined as the mole percent of neighboring groups (i.e., phenyl, carbobutoxy, or carbomethoxy) relative to the total sites (neighboring and sulfonic acid) in the polymer. Increasing the phenyl group content from 0 to 25% (i.e., decreasing the degree of substitution from 100 to 75%) increases the rate constant from 23.1 to 56.2 M-1 s(-1) while a further increase lowers the rate constant. The carbobutoxy and carbomethoxy groups show the same trend, but the neighboring group content at which the maximum rate constant is observed (i.e., the critical neighboring group content, CNGC) shifts to 15 and 10%, respectively. The rate constants are lower at the CNGC when phenyl groups are replaced by ester groups. When the neighboring group content increases from 20 to 55%, the rate constant decreases 3-fold with phenyl groups, 5.5-fold with carbobutoxy groups, and la-fold with carbomethoxy groups. A less ionic microenvironment may allow for a higher concentration of styrene within the polymer and lead to an immediate reaction with protonated formaldehyde. When sulfonation drops below a given level, product formation decreases due to slower formation of protonated formaldehyde. The critical neighboring group content can thus be an important variable in tuning the performance of a catalyst for a given reaction through an optimum microenvironmental effect.