Asymmetric catalysis using polymer supported catalysts

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

The enantioselective Diels-Alder reaction is another main motif in chiral Lewis acid catalysis. In 1996, Itsuno and coworkers reported an asymmetric Diels-Alder reaction using polymer-supported catalysts under flow conditions. Immobilized chiral oxazoboloridune (34) was prepared from a copolymer of N-sulfonylvabne and borane having styrene moiety, affording the Diels-Alder adduct in an enantioselective manner (up to 71% yield) [126], The authors used a gravity-fed-type column for the flow reaction. Ti-TADDOL-functionalized monolithic resins (35) were developed by Altava and Luis for the asymmetric Diels-Alder reaction (Scheme 7.30). 

Chiral Catalysis. Brucine has been utilized as chiral catalyst in a variety of reactions. For example, its incorporation into a polymer support provides a chiral catalyst for performing enan-tioselective benzoin condensations. It has also been used as a chiral catalyst in the asymmetric synthesis of (I )-malic acid via the corresponding p-lactone, which results from the asymmetric cycloaddition of chloral and ketene (eq 12). Though brucine yields malic acid with 68% ee, quinidine was found to be a more selective catalyst (98% ee). 

The subject of soluble polymers as supports in catalysis has been discussed in a number of recent reviews [ 1-5]. These other reviews each focused on a particular polymer or groups of polymers or on some aspect of catalysis (e.g., organic catalysis or asymmetric synthesis) [2, 3, 6-8]. Soluble polymers use as supports in synthesis has also been reviewed, but this topic is not covered below because in synthesis the polymer is used in a stoichiometric amount and is generally not recyclable [5,9-11]. This review takes a general approach, focusing on soluble polymers used as catalyst supports. It discusses these supports within a context of the separation strategies that could be or were used to separate or recover the soluble polymer-bound catalyst from the products. This review emphasizes examples from the last few years where soluble polymers are used but includes, for completeness, earlier examples if a particular... 

In another approach, a chiral auxiliary can be fixed to solid-supported substrate and help direct the stereoselectivity of the asymmetric reactions to provide enantiopure molecules. In this chapter, we present some representative examples of resin-bound catalysts and chiral auxiliaries in asymmetric synthesis. For detailed reviews about the polymer-bound chiral ligands used in conjunction with metals and metalloids in asymmetric catalysis, chiral organic catalysts attached to polymer supports, and chiral auxiliaries on solid support, readers are referred to Refs 6 and 7. 

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