Salen complex, immobilised

Covalent attachment chiral Co(salen) complexes to polystyrene and silica gave efficient and highly enantioselective catalysts for the hydrolytic kinetic resolution (HKR) of terminal epoxides, including epichlorohydrin. These systems provide practical solutions to difficulties with the isolation of reaction products from the HKR. Removal of the supported catalyst by filtration and repeated recycling was demonstrated with no loss of reactivity or enantioselectivity. The immobilised catalysts have been adapted to a... 

The "Ship In A Bottle" Approach Immobilised Salen Complexes for the Stereoselective Epoxidation of Olefins... 

Stereoselective Epoxidation with Immobilised Salen Complexes... 

Scheme 9.21 Immobilisation of a Mnlll(salen) complex in zeolite Y.

Fig ure 11.8 Procedures followed for the immobilisation of cationic chiral [Mn salen)] complexes into Al-MCM-41 through electrostatic interactions. 

Langanke J, Leitner W (2008) Regulated Systems for Catalyst Immobilisation Based on Supercritical Carbon Dioxide. 23 91-108 Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

Figure 5 shows that the (salen-2) complexes of V and the Co(salen-5) complex retained their catalytic properties upon entrapment in the host materials. In contrast to the Mn(salen-2) complex which loses only some of its epoxide selectivity upon immobilisation, the corresponding Co and Cr complexes show an additional decrease in stereoselectivity as well. Strikingly, the immobilised Co(salen-5) complex achieved with 100 % conversion, 96 % selectivity and 91 % de even better results in the epoxidation of (-)-a-pinene than its homogeneous counterpart. However, it is worth notifying that among the (salen-2) complexes neither the homogeneous nor the occluded Jacobsen complex catalysed the epoxidation of (-)-a-pinene best. 

We can conclude that a comparison of the respective catalytic results of these new heterogeneous catalysts and their homogeneous counterparts showed that the entrapment of the organometallic complex was achieved without considerable loss of activity and selectivity. The immobilised catalysts are reusable and do not leach. The oxidation system applies only O2 at RT instead of sodium hypochloride at 0°C. A disadvantage is the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid which has to be separated from the reaction mixture. The best results so far - 100 % conversion, 96 % selectivity and 91 % de - were achieved with the immobilised Cobalt(salen-5) complex in the epoxidation of (-)-a-pinene. 

To get a greener process, Jackson and coworkers reported a study dedicated to the use of immobilised Schiff base ligands for asymmetric thioether oxidation (Scheme 7.9). The salen ligand derivative 13, linked to a Wang resin, formed an extremely robust titanium complex, which allows the system to be reused several times without erosion of conversion and enantioselectivity. No leaching of titanium was detected in the reaction solvent. 

Another strategy to immobilise salen-titanium complexes was reported with silica MCM-41 inorganic support. The obtained catalysts gave high enantiomeric excesses in the cyanation of benzaldehyde, but were poorly... 

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