Covalent asymmetric catalysis

Asymmetric catalysis using chiral ligands, including cyclic phosphine or pyra-zole fragments covalent-bonded with ferrocene system 98PAC1477. 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

A spectacular, site-isolation effect in heterogeneous asymmetric catalysis was first reported by Pugin et al. The asymmetric hydrogenation of imine 1 is important for the commercial production of fS -metolachlor, a herbicide presently produced at >10000 tons per year. In this reaction, whereas homogeneous Ir-BPPM (2) catalyst prepared with [Ir(COD)Cl]2 was deactivated after 26% conversion (turnover frequency (TOP) min = 0), the covalently immobilized Ir catalysts, Si-PPM (3)-Ir, were much more active and productive (TOP min = up to 5.1 Scheme 2.1)... 

In 2000, Breinbauer and Jacobsen reported the use of dendrimer-supported Co(salen) complexes for the asymmetric ring opening of epoxides and demonstrated the first example of a positive dendrimer effect in asymmetric catalysis [95], A series of dendrimers with up to 16 catalytic sites at the periphery were synthesized by covalently attaching Co-salen to the commercially available PAMAM dendrimers with NH2-terminals (Figure 4.30). 

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. 

In DNA-based asymmetric catalysis, DNAs role is that of a chiral scaffold, i.e., as the source of chirality for the catalyzed reaction. In this concept, it is assumed that if the catalyzed reaction takes place close to the DNA double helix, the chirality of DNA can be transferred to the reaction product, resulting in an enantiomeric excess. This can be achieved by binding of the catalyst to the DNA. Analogously to hybrid enzymes [145], two general approaches can be followed (i) the supramo-lecular approach, in which the catalyst is bound using noncovalent interactions, and (ii) the covalent approach, in which the catalyst is attached to the DNA via a covalent linkage. 

In this chapter, we have successfully developed bifunctional chiral rhodium complexes bearing chiral phebox ligands that can be used in catalytic asymmetric reactions. The N,C,N meridional geometry with the rhodium-carbon covalent bond is the key character in the phebox complexes. The metal-phebox cooperative bifunctionality significantly contributes reactivity and selectivity in the catalytic asymmetric reactions. Furthermore, the prototype of the bifunctional catalyst can be explained to a wide range of asymmetric catalytic reactions promoted by the Lewis acids, hydrides, enolates, and bory active species. Their diversity further broadens the range of opportunities for asymmetric catalysis. 

Asymmetric catalysis is an important technique for the synthesis of chiral compounds. The introduction of supported IL catalyst into the field of asymmetric catalysis might offer new approaches to improve the catalytic performance and also the reusabiUty of chiral catalysts. The first example of a supported IL asymmetric catalyst is the proUne-catalyzed aldol reaction [116]. In this work, the IL molecule covalently attached to modified silica gel was used as the support for IL-phase containing L-proUne. The modification of the silica gel surface by the IL molecule is crucial to gain high enantioselectivity. In the model reaction of acetone and benzaldehyde, the yield to 4-hydroxy-4-phenylbutan-2-one was 51% with 64% ee. Otherwise, the yield was only 38% with 12% ee without the silica gel modification. 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

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