Catalysis of Organic..

Jones et al. 9term performance characteristics of the catalysts. This represents an area ofcontinuing study.Table 1 Catalytic data for homogeneous and poly(styrene) supported Co(III) salencomplexes in the HKR of rac-epichlorohydrin.Entry Catalyst Time [h] Conv. [%] ee [%]1 homogeneous 1.0 49 931.5 52 >992 2d 1.0 48 832.0 55 >993 2a 1.0 50 901.5 54 >994 2b 1.0 54 >995 2c 1.0 54 >99Table 2 Recycle of the poly(styrene) supported Co(III) salen complex 2b in theHKR of rac-epichlorohydrin.Cycle Time [h] Conv. [%] ee [%]1 1.0 54 >992 1.5 55 >993 2.0 55 >994 2.0 53 98Compared to existing salen systems, our new polymer-immobilizedcomplexes perform better than the homogeneous complex due to the increasedprobability for bimolecular interactions among the salen complexes (Table 1).However, the cyclic oligomeric salen complexes previously described still displaysuperior reactivity in the epoxide ring-opening reactions (36). A disadvantage of thecyclic oligomer systems is that they are more difficult to recycle via the methoddescribed here, as the low molecular weight of these catalysts makes precipitationproblematic. Normally, these low molecular weight catalysts have been recycled onthe bench scale by removing the volatile reactants followed by the addition of morestarting material to the reaction vessel and without the isolation of the catalyticspecies (47). In contrast, our polymeric Co(salen) complexes, besides the desirablecatalytic performance in the HKR, hold the advantage of more facile productseparation and catalyst recycling (Table 2). Hence, these supported catalysts areparticularly suitable for the kinetic resolution of epoxides (e.g., epichlorohydrin) thatare prone to racemization in the presence of the catalysts. For larger scale