Osmium catalysts coordination complexes

Species such as XXV, XXVI, or XXVII readily form coordination complexes when treated with AuCl, H20So(C0)j q, Idn(CO)3(r -C5Hj), Fe(C0)3(PhCH=CHC(0)CH3>, or [RhCl(CO)2]2 ( ) Tw results are of special interest. First, the skeletal nitrogen atoms in XXV-XXVII do not participate in the coordination process. Presumably, they are effectively shielded by the aryloxy units and are of low basicity. Second, coordinatlve crosslinking can occur when two phosphine residues bind to one metal atom. Ligand-exchange reactions were detected for the rhodium-bound species. The tri-osmium cluster adducts of XXV, XXVI, and XXVII are catalysts for the isomerization of 1-hexane to 2-hexene. 

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]. 

The kinetic investigation of this reaction reveals the reaction is first-order in substrate, catalyst and hydrogen concentration, and thus yields the rate law r=kCat[Os][alkyne][H2]. The proposed mechanism as given in Scheme 14.6 is based on the rate law and the coordination chemistry observed with these osmium complexes. 

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

A noted earlier, coordination of transition-metal ions to water-soluble polymers can allow for facile catalyst recovery, by ultrafiltration, from water-soluble substrates and/or products. For example, Han and Janda [22] used an osmium complex of the water-soluble polymeric chiral ligand 8 as a catalyst for the asymmetric dihydroxylation of alkenes in aqueous acetone (Eq. 5). However, they suggested that the catalyst should be recovered by precipitation with methylene chloride. Obviously the use of an ultrafiltration membrane for catalyst separation would be far more attractive. nu... 

The calculations were done using the MM2 force field, including new parameters developed for this type of complexes. A new set of parameters were developed for the metal catalyst, concretely for an osmium tetroxide with a tertiary amine coordinated. They developed these parameters based on X-ray structures and DFT calculations on related systems. As they pointed out in their paper, the scarcity of data makes quantitative predictions from these models unreliable. The purpose of this paper is to qualitatively identify the factors responsible for the observed face selectivities and rates in the AD reaction . 

Other chiral metal catalysts such as iridium, osmium, and rhenium catalysts have also been utilized for asymmetric cyclopropanation. In particular, iridium catalyst gave exceptional enantio- and diastereoselectivities. Chiral Ir(III) salen complexes, which contain a-coordinated aryl ligands, were found to catalyze the cyclopropanation of st3rrene derivatives with TDA. Higher yields (99%), higher diastereoselectivity t/c = 1 99), and enantioselectivity (99% ee) were observed when the reactions were conducted at lower temperature (—78°C) (98). For osmium, an Schiff-based complex of [Os(fBu-salch)(OH)Cl] (H2tBu-salch... 

In contrast with the Osborn rhodium catalysts, in which the exclusive formation of the product arises from a cis-addition to the alkyne due to the difference of coordination power of the substrates, in these osmium complexes, the strong tendency of the alkyne to undergo insertion to yield the observable styryl intermediate is the step that determines the selectivity (83-85). Complex Os-HCl(CO)(P Prs)2 is also active in tandem ROMP-hydrogenation of norbomene and 2,5-norbornadiene (86). 

Some iV-alkylation reactions catalyzed by other noble metal catalysts have also been reported. In 2011, Gusev and co-workers reported an Osmium complex-catalyzed N-alkylation of amines at 200 °C with a low catalyst loading (0.1 mol%) [122]. In 2014, Zhu and co-workers disclosed a [ReH7(PCy3)2]-catalyzed amination of alcohols with anilines under CO atmosphere (Eq. 29) [123]. The authors proposed that coordination of CO with Re might lead to decomposition of ReH7(PCy3)2 to a rhenium carbonyl complex, which was believed to be the active... 

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