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USING MICROENVIRONMENTAL EFFECTS TO INCREASE LOADrNG AND STEREOSELECTIVITY FOR POLYMER BOUND

Past studies have illustrated that the microenvironment of the functional groups attached to polymer-bound reagents and catalysts plays a major role in their reactivity. This study investigated the effect that concentrating stereogenic centers within a polymer matrix had on the reduction of acetophenone. Both crosslinked polystyrenes and crosslinked polyurethanes were functionalized using chiral appendages derived from 5gluconolactone and used as supports for 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 13 % enantiomeric excess to 54 % enantiomeric excess. Overall, the results of this study suggest that asymmetric pockets were formed within the polymer matrix when polar functional groups containing chiral appendages were concentrated within a non-polar polymer. INTRODUCTION Previously, it had been established that the microenvironment surrounding the functional group in a crosslinked polymer could affect that group's reactivity. The microenvironment includes the nature of the groups on the polymer near the functional group in question; and, to some extent, the type of solvent that is used to swell the polymer. (Alexandratos and Miller, 1996; Pillai and Mathew, 1993) More recently, it has also been shown that the microenvironment within the polymer matrix in the vicinity of the functional group may be tailored to optimize the effect that group has on the reaction. (Alexandrotos and Miller, 2000) It has been demonstrated that the chemical environment of the polymer matrix can be modified to be more compatible with the reaction. For instance, a more polar polymer can be used to interact more favorably with a growing polypeptide by increased solvation of all the reactive sites. Conversely, by this approach, more nonpolar polymers have been prepared to increase the concentrations of the nonpolar reactants in the polymer matrix. (Krishnakumar and Mathew, 2002) While these previous studies focused on optimizing the kinetics and product yields for the reaction, it may also be possible to introduce more stereochemical control as well. In particular, examples using polymer or dendrimer supported chiral auxiliaries have begun to appear in the literature that yield high degrees of chiral induction in reduction and alkylation reactions. (Van Heerbeek, et al., 2002; Sato, et al., 2002; Hutchison, et al., 2004; Sashiwa and Shigemasa, 2002; Schmitzer, et al., 2001) In both instances, the high degree of chiral induction appears to be related to the microenvironment of the reactive groups on the polymer or dendrimer. While it is not as apparent in the alkylations reported by Hutchison using polymer supported ephedrines, the reduction reactions reported by Schmitzer using a dendrimer support illustrates that the microenvironment does play an important role. Because it was shown that a critical concentration of chiral groups was required before significant chiral induction occurred, the high degree of chiral induction appears to be related to the microenvironment of the reactive groups on the polymer or dendrimer. In this report, we extend this idea that the microenvironment of a polymer or dendrimer may be manipulated to prepare reagents capable of reducing prochiral ketones to secondary alcohols with moderate to high stereoselectivities. In order to accomplish this goal, polymers with pockets of polar, asymmetric environments within a relatively nonpolar polymer matrix were prepared. Furthermore, these polymers were prepared from readily available starting materials to circumvent the time and expense associated with obtaining dendrimers. Two different polymer matrices were utilized in this study: polystyrene that was either chloromethylated (Merrifield resin) or aminomethylated; and polyurethanes that utilized differing degrees of crosslinking or crosslinks with different chemical characteristics. In all cases, the asymmetric region of the polymer was prepared by using 8-gluconolactone or derivatives using 8-gluconolactone as a starting material. METHODS AND MATERIALS All starting materials and solvents were obtained from the Aldrich Chemical Company and used without further purification. The FT-IR spectra were obtained using a Nicolet 550 Magna IR spectrometer. The polymer loadings and %ee data for the products were obtained from gas chromatographic data using a Hewlett-Packard 5890 gas chromatograph equipped with a 3390A integrator and a Supelco Beta Dex 120 column (30 m x 0.25 mm x 0.25 um film). The % enantiomeric excess was determined by comparing the areas of the R and S isomers relative to the area of a hydrocarbon reference. The signals of the R and S isomers were identified using samples of the actual compounds. All NMR spectra were obtained using an Anasazi Eft, 60 MHz spectrometer. Preparation of (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanamidomethylatedpolystyrene, 1. A dry 250 mL round-bottom flask equipped with reflux condenser, magnetic stirrer bar, and septa was purged with nitrogen. To the round-bottom flask were added 15 mL of dry methanol and 2.9 g of aminomethylated polystyrene (0.9 meq of N/g of polymer) and the polymer was allowed to swell. The round-bottom flask was then heated to 60-62EC and allowed to equilibrate at this temperature. In a separate flask that was equipped with a septum and purged with nitrogen, 1.0 g (0.0056 mols) of 8gluconolactone was dissolved in a mixture of 25 mL of dry methanol and 5 mL of dry DMSO with gentle heating. The resulting solution was transferred via cannula to the reaction vessel. The mixture was allowed to stir for 72 h at 60-62°C. The reaction was terminated by filtering the polymer and washing it successively with 3x15 mL portions of the following solvents: THF, 1:1 (v:v) water: THF, water, 1:1 (v:v) watenTHF, and THF. The polymer was dried to yield 3.23 g of an off-white colored solid (70% of theoretical weight). FT-IR analysis: 3500-3300, 1660, 1300-1000 cm".