Dihydroxylation heterogeneous

Asymmetric dihydroxylation. heterogeneous dihydroxylation of c method using Pt electrodes in undiv requires much-reduced quantities of f Homoallylic alcohol derivatives 1 1.2-alkanediols are now available b better enantioselectivities have been c... 

Petri, A., Pini, D. and Salvadori, P. Heterogeneous enantioselective dihydroxylation of aliphatic olefins - A comparison between different polymeric cinchona alkaloid derivatives, Tetrahedron Lett., 1995, 36, 1549-1552. 

The research reviewed here reflects the intense activity of the preceding 30 years in the field of oxidation catalyst immobilization. Obviously, the literature contains many erroneous and unreliable results, particularly with respect to leaching of active metal components. Examples are the reactions with silica- or alumina-supported Mo, W, or Cr and organic peroxides as the oxidants. The majority of these reports simply deal with homogeneous catalysis. Nevertheless, many concepts have been proposed in which truly heterogeneous catalysis has been obtained. Examples include the Mo-polybenzimidazole epoxidation catalyst (243), the Os-tetrasubstituted dio-late catalyst for dx-dihydroxylation (391), the heteronuclear P-W epoxidation catalysts (359, 377), and the dioxirane systems (406, 407). 

The first heterogeneous osmium catalyst applicable for asymmetric dihydroxylation reactions was described by Kobayashi and coworkers (Table 9, entry 1) [38, 39]. Osmium tetroxide was enveloped in a polymer capsule by microencapsulation techniques [40,41]. The asymmetric dihydroxylation of transmethylstyrene with poly(acrylonitrile-co-butadiene-co-styrene) microencapsulated (ABS-MC) osmium tetroxide as catalyst, NMO as the cooxidant, and (DHQD)2PHAL as the chiral ligand completed in 88% yield with 94% ee [38]. The catalyst and the chiral ligand were reused in five consecutive runs without loss of activity. However, the use of NMO as cooxidant required the slow... 

Table 9 Comparison of the asymmetric dihydroxylation ((DHQD)2PHAL) with different heterogeneous osmium catalysts...

By employing polymer-bound alkaloid derivatives, heterogeneous catalytic asymmetric dihydroxylation has been achieved with good to excellent enantioselectivities in the dihydroxylation of trans-stilbene. These polymers can be recovered and reused while both the yields and the optical purities of diols were maintained. 

To explore the possibility of recycling alkaloid-Os04 complexes, several polymer-bound alkaloid derivatives have been used for heterogeneous catalytic asymmetric dihydroxylations. As chiral ligands, polymerized cinchona alkaloids or copolymers of quinine derivatives with acrylonitrile or styrene were studied [46]. In general, lower select vities and decreased rates were observed. 

Pini, D., Petri, A., Mastantuono, A., Salvador , P. Heterogeneous enantioselective hydrogenation and dihydroxylation of carbon carbon double bond mediated by transition metal asymmetric catalysts. Chiral Reactions In Heterogeneous Catalysis, [Proceedings of the European Symposium on Chiral Reactions In Heterogeneous Catalysis], 1st, Brussels, Oct. 25-26, 1993 1995, 155-176. 

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

The presence of the quinuclidine base functionality makes them effective ligands for a variety of metal-catalyzed processes (Chapters 2-4). The most representative example is the osmium-catalyzed asymmetric dihydroxylation of olefins [9]. The metal binding properties of the quinuclidine nitrogen also allow to use cinchona alkaloids as metal surface modifiers, for example, in the highly enantioselective heterogeneous asymmetric hydrogenation of a-keto esters (Chapter 2). Both... 

P. A. Jacobs, A. Severeyns, D. E. De Vos, L. Fiermans, F. Verpoort, P. J. Grobet. A heterogeneous cis-dihydroxylation catalyst with stable, site-isolated osmium-diolate reaction centres. Angew. Chem. (Int. Ed.), 2001,40 (3), 586-589. 

Asymmetric dihydroxylation. Efficient and practical polymeric catalysts for heterogeneous dihydroxylation of olefins have been developed. An electrtjchemical method using Pt electrodes in undivided cells enables a synthesis of chiral 1,2-diols that requires much-reduced quantities of potassium osmate and KjFefCN). ... 

Fig. 30. The proposed epoxidation/cis-dihydroxylation active structure for the heterogenized Mn-Me2tacn catalyst.

I 5 Aerogel Supported NartoparUcles in Catalysis Table 5.1 Different approaches to heterogenization of the dihydroxylation of olefins. 

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