Catalyst supports chiral membranes

Methods for (bio)catalyst retention include (i) heterogenization on supports (ii) recovery through phase change, such as precipitation or extraction of the catalyst and (iii) membrane filtration of a homogeneous catalyst. All methods, in principle, enable repeated use of a chiral catalyst without much loss of activity or selectivity. In a recent review (Kragl, 2001), examples are given from laboratory and... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Polymeric membranes also show potential for application in the area of chiral catalysis. Here metallocomplexes find use as homogeneous catalysts, since they show high activity and enantioselectivity. They are expensive, however, and their presence in the final product is undesirable they must be, therefore, separated after the reaction ends. Attempts have been made to immobilize these catalysts on various supports. Immobilization is a laborious process, however, and often the catalyst activity decreases upon immobilization. An alternative would be a hybrid process, which combines the homogeneous catalytic reactor with a nanofiltration membrane system. Smet et al. [2.98] have presented an example of such an application. They studied the hydrogenation of dimethyl itaconate with Ru-BINAP as a homogeneous chiral catalyst. The nanofiltration membrane helps separate the reaction products from the catalyst. Two different configurations can be utilized, one in which the membrane is inserted in the reactor itself, and another in which the membrane is extraneous to the reactor. Ru-BINAP is known to be an excellent hydrogenation catalyst... 

ImmobiUzation is and will be an important issue in the future. Much research has been focused on the use of simple insoluble polymeric systems as supports, but there have also been attempts to immobUize chiral catalysts via encapsulation. The chiral membrane reported by Vankelecom [122], and the microencapsidated chiral catalysts prepared by Kobayashi et al. [ 123] represent excellent examples of such variations, hi addition, the immobiUzation of catalysts using thin films was reported [ 14]. 

The use of soluble polymers or dendrimers as chiral catalyst supports is another interesting way for catalyst separation [13]. Behaving like a homogeneous catalyst during the reaction, the catalyst can easily be separated by precipitation at the end of the reaction. High catalytic activities were reported using this approach. In addition, even use in membrane reactors may be possible using the ball-shaped dendrimers. 

The characteristic functionalities of naturally occurring polymers are, in most cases, related to their specific chiral structure. In nature, proteins, nucleic acids, and polysaccharides are constructed of readily available chiral monomers such as sugars and amino acids. Both natural and synthetic chiral polymers are finding application as chromatographic supports, polymeric reagents and catalysts, chiral membranes, and materials for preparation of cholesteric liquid crystal polymers (471,472). 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

Besides the use of homogeneously soluble polymethacrylates or poylstyrene, as for the examples described above, other soluble supports may be used in order to yield a catalyst which can be retained by ultra- or nanofiltration membranes. Several groups have introduced catalysts (chiral and nonchiral) coupled to dendrimers and dendrimer-like structures [54, 59-76]. Compared with catalysts coupled to polymers, such complexes offer the advantage of a more defined structure. Thus, the number of active sites can be controlled more accurately. As these will be present at the surface of a globular structure they will be easily accessible. 

Wandrey and co-workers pioneered the use of homogeneous catalysts bound to soluble supports in continuously operating membrane reactors (CFMRs). In 1996 Kragl and Dreisbach reported on a chiral polymer-enlarged homogeneous catalyst, which was used for the enantioselective addition of diethylzinc to benzaldehyde [Eq. (3)] [9]. The catalyst consisted of a soluble polymeric support, a copolymer of 2-hydroxyethylmethacrylate and octadecyl methacrylate, combined with a,a-diphe-nyl-L-prolinol as the active organocatalytic site (2). 

Chiral manganese salen catalysts have been widely used for the asymmetric oxidation of unactivated olefins. The dendritic polyglycerol-supported Mn-salen catalyst (44) was developed for the asymmetric epoxidation of the chromene derivative in a continuous membrane fiow reactor. This fiow system involves the continuous removal of the product (and unreacted substrate) from the high-molecular-weight dendritic catalyst (44) by filtration through a nanomembrane (Scheme 7.33). Under optimal conditions, 70% conversion with up to 92% ee was achieved [133]. In this system, however, the dendritic catalyst (44) worked as a homogeneous catalyst rather than a heterogeneous one. 

Chitosan has already been used as a support in several catalytic applications, including some which exploit its chirality 15-19), Finally, and a major benefit, is the ability of chitosan to form films and fibres readily, 13) allowing the coating of reactor walls and the forming of catalyst systems with controlled architecture, e.g. films, membranes, porous beads. Such behaviour relies on the ability of chitosan to dissolve in dilute aqueous acid (via protonation of the amino function) to give solutions from which films can be cast. Chitosan is completely insoluble in virtually all other solvent systems. 

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