MICROENVIRONMENTAL EFFECT IN POLYMER-SUPPORTED REAGENTS .1. INFLUENCEOF COPOLYMER ARCHITECTURE ON THE MITSUNOBU REACTION

The microenvironment surrounding the active sites in polymer-supported reagents can be tailored for maximized kinetics and yields in organic reactions. Results with the Mitsunobu reaction are presented. Cross-l inked copolymers of poly(vinylbenzyl chloride) are substituted with di phenylphosphine ligands at 18, 40, 67, and 100% substitution and used under Mitsunobu conditions to probe benzyl benzoate formation. It is f ound that the choice of groups surrounding a ligand can be as importan t as the choice of ligand. Decreasing the percent substitution while i ncreasing the number of unsubstituted phenyl rings directly bonded to the polymer backbone increases the percent alcohol conversion at a 0.1 h contact time (41.7, 68.4, 83.7, and 94.5% conversion for polymers a t 100, 67, 40, and 18% substitution). Polymers at 18 and 40% substitut ion give an equilibrium solution that is purer (97.6 and 97.0% ester) than that with a comparable soluble reagent (85.3% eater). The rapid c onversion and high yield obtained as the percent substitution decrease s is not due to a dilution effect: replacing the phenyl rings in the p olymer at 18% substitution with carbomethoxy groups yields a polymeric reagent which allows for only 1.3% alcohol conversion at 0.1 h and a maximum product yield of 29.8%. Replacing the ester groups with the mo re strongly hydrogen-bonding carboxylic acid groups results in no conv ersion of alcohol. Thus, increasing reactant conversion with decreasin g degree of substitution on a polystyrene support is a microenvironmen tal effect of the less polar aromtic rings superimposed on the inheren t electronic effect of the CH(2)PPh(2) ligand. It is proposed that dec reasing the polarity of the microenvironment surrounding the active si tes increases the reactivity of the benzoate/phosphonium ion pair and lowers the energy of the S(N)2 transition state (due to the accompanyi ng charge dispersal, as described by the Hughes-Ingold theory), result ing in an increase in the rate of product formation.