Osmium complexes catalysts

Diols are applied on a multimilhon ton scale as antifreezing agents and polyester monomers (ethylene and propylene glycol) [58]. In addition, they are starting materials for various fine chemicals. Intimately coimected with the epoxidation-hydrolysis process, dihydroxylation of C=C double bonds constitutes a shorter and more atom-efficient route to 1,2-diols. Although considerable advancements in the field of biomimetic nonheme complexes have been achieved in recent years, still osmium complexes remain the most efficient and reliable catalysts for dihydroxylation of olefins (reviews [59]). 

Doyle MP (2004) Metal Carbene Reactions from Dirhodium(II) Catalysts. 13 203-222 Drudis-Sole G, Ujaque G, Maseras F, Lledds A (2005) Enantioselectivity in the Dihydroxyla-tion of Alkenes by Osmium Complexes. 12 79-107... 

The potential of osmium complexes as homogeneous catalysts has been highlighted in two recent reviews.100,101... 

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. 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

There are only very few applications of osmium complexes in radical chemistry. Gasanov and coworkers studied the efficiency of M3(CO)12 (M=Fe, Ru, Os) in Kharasch addition reactions in the presence and absence of DMF as a ligand and found that the efficiency of the catalyst decreases in the order Fe>Ru>Os [93]. DMF activates all three systems. Its role was attributed to the generation of... 

Only one example of an NHC-containing olefin metathesis catalyst containing a transition metal other than ruthenium has been reported in the literature. The NHC-osmium complexes 53a and 53b (Scheme 2) are synthesized from the dichloro(i]6-p-cymene)osmium dimer by addition of the NHC prepared in situ and abstraction of the chloride, followed by introduction of the ben-zylidene moiety with phenyl diazomethane. 

Unfortunately, the immobilization of the alkaloid ligands did not result in the simultaneous immobilization of the osmium catalyst due to the weak binding of cinchonidine to the osmium complexes. In most studies, leaching of the osmium catalyst was reported and supplementation of the osmium catalyst was necessary after recovery of the immobilized chiral ligand. In other studies, the recycled ligands were used without further addition of the osmium catalyst resulting in reduced yields or longer reaction times. 

Because it forms stronger bonds with ligands, osmium complexes are generally less active catalysts than analogous ruthenium complexes. The exception is in oxidation chemistry where certain osmium complexes are better and more selective catalysts. Because of this, osmium tetroxide and other osmium 0X0 complexes are widely used in the synthesis of complex organic molecules. 

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