Osmium ligands

Various other chiral diamines have also been explored for use with 0s04, some of which are illustrated in Scheme 12.8. They presumably function by forming hexaco-ordinate chelates with 0s04. The reactant in Entry 3 also raises the issue of diastereo-selectivity with respect to the allylic substituent. Normally, the dihydroxylation is anti toward such substituents.52 There are thus matched and mismatched combinations with the chiral osmium ligand. The R, 7 -diamine shown gives the matched combination and leads to high diastereoselectivity, as well as high enantioselectivity. 

The reason for this must, of course, lie in the way in which the substrate interacts with the osmium-ligand complex. However, even as we write this book, the detailed mechanism of the asymmetric dihydroxyiation is still under discussion. What is known is that the ligand forms some sort of... 

When the osmium/ligand complex was recycled, both activity and chemoselectivy dropped significantly while chemoselectivity was remained. The enantioselectivity decreased slightly even when an initial amount of K2OSO4 was added to the mixture. 

Fig. 4.48. Effect of the osmium ligands on the stoichiometry of osmilation of fullerenes. n = osmilation degree osmium atoms per mol Ceo- [Hawkins JM (1992) Acc. Chem. Res. 25 150]...

The usefulness of osmium derivatives in nucleic acid chemistry has prompted an investigation of the synthesis and reactions of osmium ligand complexes [e.g. (334)—(336)] of common nucleosides. Such dipyridylosmium esters underwent relatively rapid transesterification reactions with glycols. Other groups have reported n.m.r. studies on the complexes formed between nucleosides and transition-metal carbonyls e.g, [Rh(COa)Cl]2 and W(CO)e and on the ionic binding of Li+ and Cl ions to nucleosides (based on Li and C1 nuclear magnetic relaxation times). ... 

A -alkyl derivatives of tubercidin and of 9-(6-deoxy-6-nitro-j8-D-glucopyrano-syl)adenine have been reported. Guanosine reacted with tetrakis(hydroxy-methyl)phosphonium chloride to give the JV -substituted phosphine (499). The preparation and properties of osmium-ligand complexes of nucleosides are mentioned in Chapter 18. ... 

Coordination Compounds. Osmium in oxidation states from +2 to +8 forms a wide range of complexes with nitrogen ligands. Amine... 

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

This is the second of the common oxidation states for iron and is familiar for ruthenium, particularly with Group 15-donor ligands (Ru probably forms more nitrosyl complexes than any other metal). Osmium(II) also produces a considerable number of complexes but is usually more strongly reducing than Ru". 

Osmium(II) forms no hexaaquo complex and [Os(NH3)g] +, which may possibly be present in potassium/liquid NH3 solutions, is also unstable. [Os(NH3)5N2] and other dinitrogen complexes are known but only ligands with good 7r-acceptor properties, such as CN, bipy, phen, phosphines and arsines, really stabilize Os , and these form complexes similar to their Ru analogues. 

Vectorial transfer of electronic energy in rod-like ruthenium-osmium complexes with bis-2,2, 2"-terpyridine ligands 97CC333. 

In the iron, ruthenium, and osmium derivatives, there are eases of r] re-switeh on thermolysis followed by the elimination of small ligands. Organo-ruthenium speeies eontaining pyrazol-l-ylborate or -methane ligands with bulky substituents often have uneoordinated pyrazol-l-yl moieties and agostie R—B(C) - - - M interaetion. The latter sometimes influenees the properties of the jj -eoordinated speeies as well. 

Another important reaction associated with the name of Sharpless is the so-called Sharpless dihydroxylation i.e. the asymmetric dihydroxylation of alkenes upon treatment with osmium tetroxide in the presence of a cinchona alkaloid, such as dihydroquinine, dihydroquinidine or derivatives thereof, as the chiral ligand. This reaction is of wide applicability for the enantioselective dihydroxylation of alkenes, since it does not require additional functional groups in the substrate molecule ... 

The actual catalyst is a complex formed from osmium tetroxide and a chiral ligand, e.g. dihydroquinine (DHQ) 9, dihydroquinidine (DHQD), Zj -dihydroqui-nine-phthalazine 10 or the respective dihydroquinidine derivative. The expensive and toxic osmium tetroxide is employed in small amounts only, together with a less expensive co-oxidant, e.g. potassium hexacyanoferrate(lll), which is used in stoichiometric quantities. The chiral ligand is also required in small amounts only. For the bench chemist, the procedure for the asymmetric fihydroxylation has been simplified with commercially available mixtures of reagents, e.g. AD-mix-a or AD-mix-/3, ° containing the appropriate cinchona alkaloid derivative ... 

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