Homogeneous epoxidation salen complexes

The oxidations catalyzed by VO(HPS)-Y, gave slow reactions, with rather low selectivities. The latter phenomenon is likely a direct result of the lower rate, giving allylic oxidation more chance to interfere. At room temperature almost no reaction was observed. The VO(salen)-Y material tested here gave a limited activity, although the selectivity towards the epoxide was higher than that obtained with the homogeneous VO(salen) complex. 

Table 6 Stirred batch reactor ARO of epoxides catalyzed by a dimeric (R,R)-Cr(III)(salen) complex 65 immobilized in a supported ionic liquid phase compared to the dimeric complex impregnated on silica 64-silica and the reported homogeneous reactions with the monomeric Cr(salen) complex 63...

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Fig. 2.1.6.7 Epoxidation of (-)-ot-pinene over various homogeneous and immobilized transition metal-salen complexes. = homogeneous complex (QH F, 4.6 mmol pivalic aldehyde, 1.85 mmol (-)-ot-pinene, 25 mg catalyst,...

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

The third investigation track demonstrated the immobilization of metal-salen complexes in mesoporous materials and their use in the hydrolytic kinetic resolution of meso and terminal epoxides. The best results were obtained over cobalt-Ja-cobsen catalysts. The catalytic activity of the (S,S)-Co(II)-Jacobsen complex immobilized on Al-MCM-41 was comparable with that of the homogeneous counterpart. Several other immobilization methods are still under investigation. 

Fig. 18 Representative examples of homogeneous catalysts for CO epoxide copolymerization Inoue s tetraphenylporphyrin aluminium chloride complex (a), zinc hw-2,6-fluorophenoxide complex (b), zinc (3-diiminate complex (c) and chromium-salen complex (d)...

Substituted alkenes as well as terminal olefins and styrene derivatives are epoxidized in high yield and enantiomeric excess under homogeneous reaction conditions. Very recently, the first chiral salen complexes which are selectively soluble in perfluorinated solvents have been synthesized and their application in asymmetric synthesis has been investigated [39,40]. 

When appHed in the asymmetric epoxidation of 2,2-dimethyl-2H-chromene, this MOF was less active than the homogeneous salen complex, but more stable over time, as the immobihzation of the Mn(salen) complexes precludes oxidation of one metal center by another. Immobilization decreases the flexibility of the hgands, explaining the slight decrease in enantioselectivity compared to the free complex. While a small loss of catalytically inactive Mn was observed after each ran, the enantioselectivity remained constant however, upon prolonged use of this catalyst, the outer layers of the MOF crystals were gradually damaged by oxidation. This... 

Asymmetric epoxidation of olefins with homogeneous chiral (salen) Mn(III) complex 13COC2936. 

Luts, T., Suprun, W., Hofmann, D., Klepel, O., Papp, H. 2007. Epoxidation of olefins catalyzed by novel Mn(III) and Mo(IV) Salen complexes immobilized on mesoporous silica gel Part I. Synthesis and characterization of homogeneous and immobilized Mn(III) and Mo(IV) Salen complexes. Journal of Molecular Catalysis A Chemical 261(l) 16-23. 

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