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Scheme 10. Polymer-supported (salen) complexes for asymmetric epoxidation reactions.

Supported (Salen) Complexes

Asymmetric epoxidation of unfunctionalized olefins catalyzed by chiral (salen) Mn(III) complexes has proven to be a useful solution-phase reaction [47]. To simplify product isolation and to avoid degradation of the Mn-(salen) complex via formation of p-oxo-manganese (IV) dimers by spatial distribution, the polymer-supported catalyst 50 was prepared by co-polymerization of complex 51, styrene 41, and divinylbenzene 52 (Scheme 10) [48]. As stoichiometric oxidant, a combination of meta-chloroperbenzoic acid (mCPBA) and N-methyl morpholine-N-oxide (NMO) in acetonitrile is used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 41 (to give 53) and of methyl styrene, but only low enantioselectivities were obtained. However, the catalyst preserves its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by others [49].

Polymer-Supported Schiff Base Zinc Complexes

The addition of diethylzinc to aldehydes produces secondary alcohols. This process can be stereoselectively catalyzed by chiral amino alcohols which form Schiff base-zinc complexes with the aldehyde and the metal. With the aim of simplifying the work-up of these reactions and to use continuous flow processes, the polymer-supported amino alcohols 54 and 55 were synthesized (Scheme 11) [50]. The polymers were obtained by co-polymerization of the chiral monomer 56 and styrene 41 in the presence of divinylbenzene 52 or crosslinking agent 57 containing a flexible oxyethylene chain. The latter was used to ensure sufficient flexibility within the crosslinked network of the polymer and to further activate the nucleophile by coordination of the oxyethylene chain to the metal.

Treatment of an aldehyde, e.g. 58, with one of the polymeric reagents in toluene resulted in the formation of the intermediate Schiff base 59 and oxazolidine 60, as was confirmed by analytical data (Scheme 12) [50]. Subsequent addition of diethylzinc to the reaction mixture af-

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