Reactions of aldehydes with diethylzinc catalysed by polymer-supported ephedrine and camphor derivatives: comparisons of enantiomeric excesses achieved with various supports: optimisation of support parameters to enable high enantiomeric excesses to be obtained†
The reactions of benzaldehyde with diethylzinc catalysed by PS ephedrine or camphor derivatives have been investigated in some depth in order to identify the crucial factors necessary to successfully prepare PS chiral catalysts for such reactions. The most important factor is found to be a favourable interaction of the polymer matrix with the reaction solvent so that the polymer will dissolve or swell to allow the other reactants easy access to the catalytic sites. Accordingly toluene is a better reaction solvent than hexane. The ephedrine-derived catalytic groups reduce the solubility of the linear polymers in toluene and, almost certainly, the swelling properties of the crosslinked polymers. Thus, of the polymers investigated the better linear ones had <ca. 1.5 mmol per g of catalyst sites and the better insoluble ones were 1% crosslinked gels with <ca. 1.0 mmol per g of catalyst sites. Site–site interactions and microenvironmental effects do not appear to play a major role in these PS reaction systems. For the reaction of benzaldehyde with diethylzinc, using the best linear PS ephedrine derivatives 10f, 10g or 11e affords 1-phenylpropanol (1) with 83–88% enantiomeric excesses (ee)s of the (R)-enantiomer; using the best linear PS camphor derivative 21 affords the alcohol 1 with a 98% ee of the (S)-enantiomer; using the best crosslinked PS ephedrine derivative 12a affords the alcohol 1 with a 78–81% ee and using the best crosslinked PS camphor derivative 22 affords alcohol 1 with a 97% ee of the (S)-enantiomer. These values are close to those obtained using analogues of non-polymeric catalysts under similar reaction conditions.