Alkene epoxidation chiral catalyst recycling

Song et al. [62] reported poly-salen Co(III) complexes 18, 19 as catalyst for HKR (Figure 5) of terminal alkene epoxides. The polymeric catalysts provided product epoxides with excellent conversion (>49%) and high chiral purity (ee s, 98%) and the catalytic system could be recycled once with retention of activity and enantioselectivity. 

Many attempts were undertaken to produce chiral epoxides for chemical syntheses. This can be achieved by the use of chiral catalysts. The first applicable and relatively simple procedure of chemical chiral epoxidations was described by Katsuki and Sharpless [2], later called the Katsuki-Sharpless epoxidation. In this reaction, allyl alcohols are epoxi-dized in the presence of tartrate esters, e.g., (—)-diethyl tartrate. This allows the production of either (/ )- or (S)-epoxides depending on the selection of (R)- or (5)-tartrate ester as chir additive. However, the reaction is limited to ally lie alcohols and is somewhat sensitive to steric hindrances. In the meantime, a number of different catalysts have been developed for the epoxidation of cw-alkenes. The Jacobsen-Katsuki reaction allows the epoxidation of fran5-alkenes and terminal olefins [3]. All of these approaches, however, are limited to the epoxidation of activated double bonds like allylic alcohols or require expensive catalysts, and usually the regiospecificity of these reactions is not sufficient for practical applications. Furthermore, the chiral catalysts, although usually they can be recycled, are often very exj nsive. 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

Asymmetric epoxidation of terminal alkenes with hydrogen peroxide was optimized with electron-poor chiral Pt(II) complexes bearing a pentafluorophenyl residue, as described in Section 23.3.1.6. The same catal3rtic system was made more sustainable by the employment of water as the solvent under micellar conditions. Surfactant optimization revealed the preferential use of neutral species like Triton-XIOO to solubihze both the catalyst and substrates. In several cases an increase of the asymmetric induction was observed (Scheme 23.43). The use of an aqueous phase and the strong affinity of the catalyst for the micelle allowed the recycling of the catalytic system by means of phase separation and extraction of the reaction products using an apolar solvent (hexane). The aqueous phase containing the catalyst was reused for up to three cycles with no loss of activity or selectivity. 

Kureshy and coauthors [82] also reported that chiral Mn(Salen) catalyst immobilized in the nanopores of MCM-41 and SBA-15 (Scheme 10.14) showed higher enantioselectivity (70% ee) than its homogeneous counterpart (45% ee) for the enantioselective epoxidation of styrene with aqueous NaOCl as the oxidant. In addition, the immobilized chiral Mn(Salen) could smoothly catalyze the epoxidation of bulkier alkenes such as 6-cyano-2,2-dimethylchromene into their epoxides with enanatioselectivity (up to 92% ee) comparable to those of the homogeneous counterparts. The heterogeneous catalyst could be recycled four times without notable loss of activity and enantioselectivity. 

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