Chiral catalysis/catalysts solid-supported

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. 

Asymmetric catalysis provides access to several synthetically important compounds, and immobilized catalysts together with solid-supported chiral ligands have been equally instrumental. Chiral ligands immobilized on a solid support provide the advantage of being rapidly removable post-reaction while retaining their activity for further applications [139]. 

Glos and co-workers introduced the aza-bis(oxazolines) 258 and 259 (Fig. 9.78) as a new class of chiral C2-symmetric bis(oxazoline) ligands.These catalysts were used in various reactions such as enantioselective allylic substitution and cyclopropanation it was also shown that these new catalysts could easily be tethered to a polymeric support, as shown in structure 259, allowing for facile recovery of the catalyst. There have been other examples of bis(oxazoline) ligands immobilized on solid supports and their use in catalysis.These methods have shown mixed results. 

Keywords Asymmetric synthesis, Chiral catalysis, Diversity-based approaches, Supported chiral catalysts, Solid-phase chemistry... 

There are several different approaches to fixing a molecular catalyst into a host material, some of these methods have been reviewed recently by On et al.[881 in 2001 and by De Vos et af.[89] Reviews from the perspective of chiral catalysis appeared in 2002 by Song and Lee,[90] and in 2004 by Li,[91] and noncovalently bound catalytic species on solid supports have been reviewed in 2004 by Horn et al.[92] This section is intended to complement these recent reviews and highlight as well as define the approaches encountered and to update some of the latest developments in this field. 

Simple L-alanine, L-valine, L-norvaline, L-isolecucine, L-serine and other linear amino acids [ 121 ] or chiral amino acids with a binaphthyl backbone [ 122] and peptides have also been used as asymmetric catalysts [123,124,125,126]. Solid-supported proline-terminated peptides have been used for heterogeneous catalysis of the asymmetric aldol reaction [ 127]. Apart from proline and derivatives, other cyclic compounds such as 5,5-dimethyl thiazolidinium-4-car-boxylate (DMTC) [128], 2-fert-butyl-4-benzyl imidazolidinones [129], (l/ ,25)-2-aminocy-clopentanecarboxylic acid [130], (5 -5-(pyrrolidin-2-yl)tetrazole, (5)-l,3-thiazolidine-4-car-boxylic acid, (5)-5,5-dimethyl-l,3-thiazolidine-4-carboxylic acid, and (5)-hydroxyproline are effective catalysts in asymmetric aldol reactions [126,131,132,133,134,135]. 

Solid-Supported Chiral Dendrimer Catalysts for Asymmetric Catalysis... 

Soluble dendrimers bearing catalytic centers located at the periphery can be covalently attached onto the surface of conventional solid supports (such as polymer beads or silica gels), leading to another type of solid-supported dendrimer catalyst. It is expected that this type of immobihzed catalysts would combine the advantages of both the traditional supported catalysts and the dendrimer catalysts. First, the catalytically active species at the dendrimer surface are more easily solvated, which makes the catalytic sites more available in the reaction solutions (relative to cross-hnked polymers). Second, the insoluble supported dendrimers are easily removed from the reaction mixtures as precipitates or via filtration (relative to soluble dendrimers). These solid-supported peripheraUy functionalized chiral dendrimer catalysts have attracted much attention over the past few years [12, 113], but their number of applications in asymmetric catalysis is very limited. 

The use of chiral esters as 2tx-substrates permit the production of diastereomer-ically enriched cycloadducts. For example, the menthyl ester 48 was obtained as a 4 ldiastereomeric mixture. The most intriguing feature of polystyrene-bound catalyst 46 is the fact that it can be removed from the reaction mixture by simple filtration and repeatedly reused. This means that the two basic problems of homogeneous catalysis, separation and recycling of the catalyst, can be solved by using the solid-supported complex 46. Additionally, the environmental problems associated with chromium can also be effectively eliminated. 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

In a newsletter in 2000, Dow CMS announced it had joined forces with the Center for Applied Catalysis to speed the commercialization of solid-supported homogeneous chiral catalysts [104]. However, in the information given on the homepage of the Center for Applied Catalysis dated 2002, Johnson Matthey is marketing these catalysts [105]. Johnson Matthey, on the other hand, an-... 

Chiral separation or sorption is another important technique in chirotechnology. In fact, due to the high cost of chiral catalysts, industries generally prefer chiral separation over asymmetric catalysis to obtain optically pure compounds. As in asymmetric heterogeneous catalysis, a chiral selector (a chiral molecule in optically pure form) can be immobilized on a solid support to make a chiral stationary phase (CSP) of use in direct chiral separation. The basic principle of chiral separation is that the chiral selector interacts differently with the enantiomers of a racemic or enantioenriched mixture to form transient diastereoisomeric species of different stability, and this fine distinction leads to the separation of enantiomers during elution. This topic has also produced a huge number of papers and the readers are referred to the previous reviews for more knowledge on this field [70-73]. 

Chiral solid catalysts usually have two functions, activation and control. The activating function ensures that the solid actually catalyzes a reaction (chemical catalysis), and the control function provides the stereochemical direction that yields the required enantiomer. Early studies were carried out with metallic catalysts supported on inherently chiral solids such as quartz, cellulose (Harada and Yoshida, 1970), and polypeptides (Akabori et al., 1956 Beamer et al., 1967), in which the metal provided the activating function and the support provided the control function. More recent emphasis has been on binding chiral molecules to nonchiral supports. 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

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