Osmium catalysts heterogeneous

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...

Incidentally, this is the first reported heterogeneous osmium-catalyst-mediated AD reaction of alkenes using molecular oxygen as the cooxidant. Under identical conditions, the turnover numbers of the heterogeneous catalyst are similar to those of the homogeneous system. 

Jacobs and coworkers published a completely different type of heterogeneous osmium catalyst. Their approach is based on two details from the mechanism of the ds-dihydroxylation (1) tetrasubstituted olefins are smoothly osmylated to an osmate(VI) ester, but these esters are not hydrolyzed under mild conditions, and (2) an Os monodiolate complex can be reoxidized to ds-dioxo Os without release of the diol subsequent addition of a second olefin results in an Os bisdiolate complex. These two properties make it possible to immobihze a catalytically active osmium compound by the addition of OSO4 to a tetrasubstituted olefin that is covalently linked to a silica support The tetrasubstituted diolate ester which is formed at one side of the Os atom is stable, and keeps the catalyst fixed on the support material. The catalytic reaction can take place at the free coordination sites of Os (Scheme 1.12) [40]. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

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. 

This reaction has been improved by addition of 2,6-lutidine to suppress the side reactions and increase the yields of aldehydes. In addition, this oxidation has been modified by using osmium tetraoxide and oxone as oxidants, which directly convert 1,2-disubstituted olefins into two individual carboxylic acids. On the basis of this modification, osmium tetraoxide has been made to be a three-dimensional networked nanomaterial that, in combination with oxone, forms a superior heterogeneous catalyst, which even oxidizes alkynes into carboxylic acids7... 

Haber has already demonstrated that Os is also a promising heterogeneous catalyst for NH3 synthesis, but the number of molecular Os—N2 or Os— N2H4 complexes is rather limited. This is caused by the fact that osmium forms relatively labile N2 compounds, which are usually not a result of direct N2 activation, but formed (a) by intermolecular N-"N coupling between two osmium nitridos, (b) by (thermal/photochemical) activation of... 

Early in 2003, Choudary et al. studied the catalytic activity of a unique tri-functional heterogeneous catalyst system consisting of palladium, osmium, and tungsten species for tandem Heck olefination followed by asymmetric dihydro>ylation reaction induced by cinchona alkaloid (DHQD)2PHAL in the presence of a tertiary amine such as N-methylmorpholine (NMM) in one pot. The trimetal catalyst system of Pd-Os-W was embedded into hexagonal layered double-hydroxides (LDHs). As shown in Scheme 7.57, the corresponding almost enantiopure diol was achieved in high yield. This remarkable result was not clearly understood by the authors. 

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