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 ident ified by the critical neighboring group content. The Prins reaction between formaldehyde and styrene is probed in the present study with immobilized s ulfonic acid ligands as the catalyst. Cross-linked polystyrene beads were s ulfonated to varying degrees of substitution. Styrene was also copolymerize d with butyl methacrylate and methyl methacrylate followed by complete sulf onation of the phenyl rings. The reaction kinetics were correlated with the neighboring group content, defined as the mole percent of neighboring grou ps (i.e., phenyl, carbobutoxy, or carbomethoxy) relative to the total sites (neighboring and sulfonic acid) in the polymer. Increasing the phenyl grou p content from 0 to 25% (i.e., decreasing the degree of substitution from 1 00 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 co ntent, CNGC) shifts to 15 and 10%, respectively. The rate constants are low er at the CNGC when phenyl groups are replaced by ester groups. When the ne ighboring group content increases from 20 to 55%, the rate constant decreas es 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 hi gher concentration of styrene within the polymer and lead to an immediate r eaction with protonated formaldehyde. When sulfonation drops below a given level, product formation decreases due to slower formation of protonated fo rmaldehyde. The critical neighboring group content can thus be an important variable in tuning the performance of a catalyst for a given reaction thro ugh an optimum microenvironmental effect.