Immobilization and Recycling

The resulting derivatives were applied with success in the standard asymmetric allylic alkylation (up to 97 % ee) [134, 136] or in transformations involving either specific allylic substrates (2-cycloalkenyl derivatives, up to 99% ee) [135, 137], unsymmetrical substrates (monosubstituted allyl acetate, up to 83% ee) [140], or especial nucleophiles (nitroalkanes [141], iminoesters [138 a], or diketones [139, 140, 142]). Such ligands were also effective in the formation of quaternary chiral carbon through allylic substitution (eq. (6)) [138, 143], deracemiza-tion of vinyl epoxides (up to 99% ee) [144], or alkylation of ketone enolates [138 b], and deracemization of allylic derivatives [145]. 

The recycling of enantioselective catalysts has attracted less attention than achiral catalytic processes. Nevertheless, several approaches have been investigated with success in that direction. As such, chiral diphosphanes have been modified in order to obtain congeners able to operate efficiently in water (Structure 34) 

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]. 

Polymer-supported chiral catalysts have likewise been prepared in order to obtain access to reusable systems (cf. Section 3.1.1, especially Section 3.1.1.3). For example, copolymerized functionalized BINAP [160, 161] could be applied in the enantioselective hydrogenation of olefinic substrates (up to 94 % ee). Similarly, the copolymerization of vinyl-BINAPHOS with styrene derivatives led to a heterogenized auxiliary which made it possible to hydroformylate styrene and vinyl acetate (Rh catalysis) with selectivities and enantioselectivities close to those provided by the parent homogeneous catalytic system [162]. 

Chiral auxiliaries have also been included in dendrimeric structures (cf. Section 3.2.2). Two general strategies are possible for that purpose. Thus, multiple auxi- 

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De Vos DE, Vankelecom IFJ, Jacobs PA (eds) (2000) Chiral catalysts immobilization and recycling. Wiley-VCH, Weinheim... 

H. U. Blaser, B. Pugin, M. Studer, Enantioselective heterogeneous catalysis academic and industrial challenges, in D. E. de Vos, I. F. J. Vankelecom, P. A. Jacobs (Eds.), Chiral Catalyst Immobilization and Recycling, Wiley-VCFI, Weinheim, 2000, p. 1. 

Chiral Catalyst Immobilization and Recycling, WiJey-VCH, Weinheim, 2 000, p. 43. 

Alkali-catalyzed transesterifications have several drawbacks in addition to the problem of free fatty acids and water in the feedstock. They are energy intensive, recovery of the glycerol is difficult, the basic catalyst has to be removed from the product and the alkaline waste water requires treatment. These disadvantages could be circumvented by employing a lipase catalyst. But, in order to be economically viable, the enzyme costs have to be minimized through effective immobilization and recycling. 

Salvador , P., Pini, D., Petri, A., Mandoli, A. Catalytic heterogeneous enantioselective dihydroxylation and epoxidation. Chiral Catalyst Immobilization and Recycling 2000, 235-259. 

De Vos, D.E., Van-kelecom, I.F.J. and Jacobs, P.A. (2000) Chiral Catalyst Immobilization and Recycling, Wiley-VCH Verlag GmbH, Weinheim. 

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