Heterogeneous immobilization, chiral catalysts

Despite the well-known advantages of insoluble supports, there are several drawbacks in using these solids as supports for chiral catalysts due to the heterogeneous nature of the reaction conditions. The catalyst resides in the solid phase while reactants are in solution, which can often result in a decreased reaction rate owing to diffusion problems. Furthermore, the matrix effect of the solid support, though sometimes favorable, is difficult to predict and can often lead to lower enantioselectivities for the immobilized chiral catalysts than those for their... 

It is the view of the present authors that the most important problem here is the high complexity of many heterogeneous systems, because this leads to poor predictability of the catalytic properties. Additional problems with immobilized chiral catalysts include not only their lack of commercially availability but also the very high costs involved in their preparation. 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

The production of enantiomerically pure products is of great importance in chemical industry. The most desirable way to obtain these products is by chiral catalysis. Homogeneous complexes can often be used as chiral catalysts however, because of their difficult regenerability, the development of heterogeneous chiral catalysts by immobilization of these complexes is difficult but highly desired. 

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. 

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

Baiker, A. (2000) Design of new chiral modifiers for heterogeneous enantioselective hydrogenation a combined experimental and theoretical approach, in Chiral Catalyst Immobilization and Recycling (eds D.E. De Vos, I.F.J. Vankelecom and P.A. Jacobs), Wiley-VCH Verlag GmbH, Weinheim, pp. 155-71. 

Soai et al. [62a] first reported the use of sihca gel or alumina as a heterogeneous support for chiral catalysts in the enantioselective addition of dialkylzincs to aldehydes. Chiral N-alkyhrorephedrines (R = Me, Et, n-Pr) were immobilized covalently on (3-chloropropyl)silyl-functionalized alumina or silica gel via a nucleophilic substitution. However, the catalytic activities and enantioselectivities were only moderate (24—59% ee) in comparison with those of homogeneous and polymer-... 

The most common polymer supports used for chiral catalyst immobilization are polystyrene-based crosslinked polymers, although poly(ethylene glycol) (PEG) represents an alternative choice of support. In fact, soluble PEG-supported catalysts show relatively high reactivities (in certain asymmetric reactions) [le] which can on occasion be used in aqueous media [le]. Methacrylates, polyethylene fibers, polymeric monoliths and polynorbornenes have been also utilized as efficient polymer supports for the heterogenization of a variety of homogeneous asymmetric catalysts. 

Such a reaction can constitute an ideal model for the search of solid chiral catalysts. Heterogeneous catalysts have advantages over homogeneous catalysts in their easy separation and recovery from the reaction mixture. A few examples of chiral auxiliaries immobilized on some solid supports, such as polymers [6] and alumina or silica gel [7] have been recently reported for the above mentionned reaction [5]. In the case of alumina and silica as heterogeneous supports, the enantiomeric excesses obtained are moderate and the role of the solid is not clearly understood. 

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