Osmium species

Reaction of the osmium(III) pentammines with zinc amalgam gives an osmium species that is stable in solution for some hours (and may be [Oslv(NH3)5(OH)(H)]2+). It is capable of forming Os(NH3)5L2+ adducts with 7r-acids like MeCN [146]. 

The [Os3(CO)io( t-H)( t-OSi)]surface catalyzes the isomerization and hydrogenation of olefins. When the hydrogenation of ethylene is carried out at 90 °C the trinuclear framework of the initial cluster remains intact in all the proposed elementary steps of the catalytic cycle [133]. However, at higher reaction temperatures the stability of the [Os3(CO)io( t-H)( t-OSi)]sujface depends on the nature of the reactant molecule. It is moderately active in the isomerization of 1-butene at 115 °C but decomposes under reaction conditions to form surface oxidized osmium species that have a higher activity [134]. 

Sodium hydrosulfite reduces the osmium tetroxide to insoluble lower-valent osmium species. 

The Criegee mechanism originally proposed involves the formation of an osmium(VI)-ester complex (106) from the [4+2] cycloaddition of the Osvul cis-dioxo moiety with an alkene, followed by the hydrolysis or reduction of (106) to cis-glycol and reduced osmium species. In support of this mechanism a variety of Osvl cyclic esters such as (107) or (108) (L = quinuclidine) have recently been synthesized from Os04 and the alkene, and characterized by an X-ray crystal structure.290,343 In solution the dimeric complex (108) dissociates to give the monomeric dioxo trigonal-bipyramidal complex (109), which is similar to (106).344... 

Osmium tetroxide is the traditional osmium species used in the dihydroxylation of olefins. For large-scale reactions, osmium tetroxide may be weighed and transferred as the solid. For many catalytic applications on a laboratory scale, the amount of osmium tetroxide required is too small to be weighed conveniently. In these cases, advantage can be taken of the solubility of osmium tetroxide in organic solvents by the preparation of a stock solution of known concentration and the use of an aliquot for the small-scale reaction. 

In 1975 Sharpless and coworkers discovered the stoichiometric aminohydrox-ylation of alkenes by alkylimido osmium compounds leading to protected vicinal aminoalcohols [1,2]. Shortly after, an improved procedure was reported employing catalytic amounts of osmium tetroxide and a nitrogen source (N-chlo-ro-N-metallosulfonamides or carbamates) to generate the active imido osmium species in situ [3-8]. Stoichiometric enantioselective aminohydroxylations were first reported in 1994 [9]. Finally, in 1996 the first report on a catalytic asymmetric aminohydroxylation (AA) was published [10]. During recent years, several reviews have covered the AA reaction [11-16]. 

The AA reaction is closely related to the asymmetric dihydroxylation (AD). Alkenes are enantioselectively converted to protected 3-aminoalcohols (Scheme 1) by syn-addition of osmium salts under the influence of the chirr 1 bis-Cinchona ligands known from the AD process (see Chap. 20.1). As for the AD reaction, a cooxidant is needed to regenerate the active osmium species. But in the AA process the cooxidant also functions as the nitrogen source. Since two different heteroatoms are transferred to the double bond, regioselectivity becomes an important selectivity issue in addition to enantioselectivity. Moreover, chemoselectivity has to be addressed due to the possible formation of the... 

Additionally, the regioselectivity is strongly solvent-dependent. The most common solvents used in the AA process are alcohols or acetonitrile with high water content (up to 50%). A large amount of water is necessary to ensure high catalytic turnover by enhancing the hydrolysis step in the catalytic cycle. However, in the Boc-AA the opposite was true. Lower water content was favorable by reducing the competitive hydrolysis of the imido osmium species [20], which... 

Arene)(e/ido-dicyclopentadiene)M(0) complexes [M = Ru (211), Os (212)] are prepared by reduction of lc or Id with 2-propanol and Na2C03 in the presence of dicyclopentadiene (Scheme 15). Protonation of derivatives 211 and 212 with HPF6 leads to cations 213 and 214 which are stabilized by an agostic C—H—M bond, as shown by the X-ray structure of the osmium species (214). The cations 213 and 214 react with CN(t-Bu) to give complexes of type 215 in which the agostic C—H—M is no longer observed (102). 

The cyclic oxo esters are very much a feature of osmium(VI) chemistry but analogous rings are rare with other elements they have been detected for ruthenium(VI) and manganese(V), but only for chromium(V) has any substantial oxo ester chemistry been uncovered. The osmium species are formed by two main routes (a) by reaction of 0s04 with alkenes, dienes or alkynes, in which case two electrons are transferred from the multiple bond to osmium(VIII) which is thus reduced to osmium(VI), or (b) by reaction of m-glycols with fra/w-[0s02(02 R)2] (R = H, Me) with elimination of water or methanol (see p. 582). 

Arrangement 6 is extremely common, and recently reported examples involve binuclear platinum species,116117 trinuclear osmium species,118 as well as some Ru2 species to which further HgX2 may be added119 to give an arrangement of type 15 ... 

Rafts, that is, large, triangulated, planar nets. Thus far this sort of structure has been found only for some osmium species. 

Many of these oxo ruthenium (and similar osmium) species can be involved in reversible redox reactions in which OH or H20 participate, for example,... 

Although all of these methods allow release of the desired diols from the corresponding osmates(VI), the oxidation state of the final osmium species differs ... 

Figure 8.6. One view of a proposed intermediate in the Sharpless AD reaction of the (DHQD)2PHAL-derived osmium species reacting with styrene [80].

In general, the mechanism for the AD reaction is depicted in Figure 1. Coordination of a ligand to osmium tetraoxide 1 generates complex 2. This species then reacts with alkene 3 producing osmium glycolate 4 that can then decompose to the desired 1,2-diols 5 and the reduced osmium species 6. Catalytically active 2 can be regenerated from 6 by an external oxidant, such as ferricyanide. 

Without added sodium periodate, olefins are converted only into ctT-diols by OSO4 upon workup, therefore, it is plausible that sodium periodate further oxidizes osmium species and triggers the cleavage of carbon-carbon bond, as shown here. 

Many researchers have focused on the preparation and catalytic properties of polymer-bound ruthenium and osmium species because of their proven ability to catalyze homogeneous reactions and the vast synthetic chemistry available for their preparation. A series of preformed polymers of [M(bpy)2(pol)nCl]Cl, where M can be a Ru(II) or Os(II) metal center coordinated to 2,2 -bipyridine ligands (bpy) and anchored to a pyridine or imidazole nitrogen of a PVP or poly(N-vinylimidazole) polymer (pol), have been prepared and characterized with respect to charge transport rates and mechanisms in drop-coated films on electrode surfaces. Electrodes coated with films of the ruthenium polymer have been shown to mediate the oxidation of nitrite, and nickel bis(2-hydroxyethyl)dithiocarbamate. ... 

The mechanism of the reactions of complexes containing more than one 0x0 group with olefins has been studied for many years in the context of the catalytic dihydroxylation and aminohydroxylation of olefins. Both the combination of a [2+2] cycloaddition followed by rearrangement to the final [3+2] addition product and direct [3+2] reactions of the olefin with the osmium species have been proposed (Scheme 13.21). Computational... 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

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