Catalyst enantioselective 75 - recycling

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. 

The next step in the use of transfer hydrogenation catalysts for recycling of the unwanted enantiomer is the dynamic kinetic resolution. This is a combination of two reaction systems (i) the continuous racemization of the alcohol via hydrogen transfer and (ii) the enantioselective protection of the alcohol using a... 

The polymer-supported chiral catalyst is recyclable. The catalytic AROM/CM example shown in Scheme 11.12 is representative. Excellent enantioselection and ef... 

An investigation of various catalysts, the solvent, and the additive used in the meso-stilbene oxide enantioselective ring opening with anilines at room temperature has been carried out.29 The best catalyst, solvent, and additive proved to be (17), toluene and triphenylphosphane, respectively. The reaction gave 95% of the syn-fi-wm m alcohol with 78% ee. A single recrystallization increased the enantiomeric excess to 98%. The catalyst is recyclable. 

Hoveyda and Schrock attached (97a) to polymer via attached styrene groups yielding the first reported supported chiral molybdenum olefin metathesis catalyst, (290) (Scheme 27). Supported complex (290) is less active than (97a), but it gives similar ranges of ees for enantioselective transformations like desymmetrization. The catalyst is recyclable and, even though the conversions have eroded, the enantioselectivity is still relatively high. Table 14. 

Liquid or supercritical carbon dioxide has also been used as a friendly medium providing easy recycling of the catalyst. Enantioselective catalyses (hydro-formylations, hydrogenations) have been described with different catalysts [157-159]. 

Water-soluble polymer-bound Pd(0)-phosphine catalyst has also been efficiently used in aqueous or mixed aqueous/organic media, the catalyst being recycled by solvent or thermal preparation methods [17]. Amphiphilic resin-supported palladium-phosphine complexes show high catalytic activity in allylic substitution reactions of various allylic acetates with different nucleophiles in aqueous media [18, 19]. Enantiomeric excess up to 98% is obtained using amphiphilic resin-supported MOP ligand or resin-supported P,N-chelating palladium complexes, the catalyst being recyclable [20,21]. The catalyst could be recovered by simple filtration and re-used without any loss of activity and enantioselectivity. 

Impressive enantioselectivities (up to >99.9% enantiomeric excess) were observed with a large range of thioethers. However, moderate yields were obtained [ca. 30-40%), which was attributed to a kinetic resolution in the oxidation of sulfoxide to sulfone, thus reducing the yield in sulfoxide. The heterogeneous nature of the catalyst was confirmed by inductively coupled plasma (ICP) spectroscopic analysis of the liquid phase (<1 ppm of titanium). The catalyst was recycled by simple filtration, and was reused at least 8 times in oxidation of thioanisole without any loss of enantioselectivity. 

Schulz and coworkers have developed an attractive approach to recycling Cu(II) bisoxazoline complexes (Scheme 17.57) [78]. Formation of charge transfer complexes such as (252) allow for efficient and selective reaction along with facile precipitation and reuse the following completion of the reaction. Application of (252) in Diels-Alder reaction between 3-acryloyl-oxazolidin-2-one (244) and (238) gives cycloadduct (mt)-(246) in moderate to excellent yield with high diastereo- and enantioselectivity. The catalyst was recycled 11 times with essentially no loss of reactivity or selectivity. 

A number of enantioselective hydrogenation reactions in ionic liquids have also been described. In all cases reported so far, the role of the ionic liquid was mainly to open up a new, facile way to recycle the expensive chiral metal complex used as the hydrogenation catalyst. 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

In spite of these limitations, three examples of (salen)-metal complex adsorption have been described. In the first one, Jacobsen s complex (la-MnCl) was adsorbed on Al-MCM-41 [27] by impregnation with a solution of the complex in dichloromethane, an approach that prevents the possible cationic exchange. The results in the epoxidation of 1,2-dihydronaphthalene with aqueous NaOCl were comparable to those obtained in solution, with only a slight reduction in enantioselectivity (55% ee instead of 60% ee). However, recycling of this catalyst was not described. 

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