Polyglycerol-supported Co- and Mn-salen complexes as efficient and recyclable homogeneous catalysts for the hydrolytic kinetic resolution of terminal epoxides and asymmetric olefin epoxidation

In this report we demonstrate the suitability of hyperbranched polyglycerol as high-loading support for asymmetric catalysis. A polyglycerol-supported unsymmetrical salen ligand was prepared and purified by means of ultrafiltration. The polymeric ligand was metalated with cobalt acetate to afford the corresponding supported Co-salen complex which was further utilized in the hydrolytic kinetic resolution (HKR) of terminal epoxides. Kinetic studies for HKR of 1,2-epoxyhexane using polyglycerol-supported Co-salen showed improved catalytic performance compared to the respective non-immobilized catalyst. This observation points to the presence of a positive dendrimeric effect assuming a cooperative bimetallic mechanism. Furthermore, a -polymer-supported Mn-salen catalyst was prepared and successfully applied in the asymmetric epoxidation of olefins. The respective dendritic catalyst was recycled by precipitation up to three times in the epoxidation of 6-cyano-2,2-dimethylchromene (ee up to 95%). Experiments with repetitive batches revealed enhanced stability of the catalyst as a result of immobilization whereby the total turnover number increased from 23 in a single batch to around 80 for four repetitive batches. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).