Using Microenvironmental Effects: Loading, Activity, and Stereoselectivity Relative to Polymer Bound Nabh4 Reagents
Year: 2007
Type: article
Abstract: The microenvironment of a polymer support plays an active role during the course of a reaction.This study investigated how reduction reactions of a prochiral ketone were affected by concentrating stereogenic centers within a polymer matrix.A 1% divinylbenzene crosslinked polystyrene polymer functionalized with isocyanato groups as the linkers was used as the original polymer support.A variety of chiral polyols were synthesized from δ-gluconolactone and attached to the polymer support by means of a carbamate bond.The chiral reducing agents were then prepared by allowing the alcohol groups to react with a solution of sodium borohydride.The results of the reduction reactions indicated that as the number of stereogenic centers increased at the linking point on the polymer, the % enantiomeric excess of the alcohols produced during the reaction also increased from 6.8 % enantiomeric excess to 34.2 % enantiomeric excess.The rate constants also experienced an effective two-fold increase in magnitude when the polyol with the largest number of stereocenters was used.However, it was interesting to note that the effective loading of the polymers remained effectively constant at 4.0-5.0meg of hydride per gram of polymer for all the polymers that used polyols as the linking points.Overall, the results of this study suggest that polar, asymmetric pockets were formed within the polymer matrix when polar functional groups containing chiral appendages were concentrated within a non-polar polymer.While this change in microenvironment had a positive effect on the activity and chiral induction of the reaction reaction, it also appeared to increase the interaction between the linking sites on the polymer and the polyol sites of the chiral groups as the polymers were being prepared.
Author: James R. Blanton
Institution: University of South Carolina
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
Funder: Citadel Foundation
OpenAlex ID: W138793363