Osmium-mediated

Gauvin, R. M. Rozenberg, H. Shimon, L. J. W. Ben-David, Y. Milstein, D. Osmium-mediated C—H and C—C bond cleavage of a phenolic substrate p-quinone methide and methylene arenium pincer complexes. Chem. Eur. J. 2007, 13, 1382-1393. 

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... 

Osmium-mediated c/5-addition of nitrogen and oxygen to olefins. Regioselectiv-ity may be controlled by ligand. Nitrogen sources (X-NClNa) include ... 

A vincinal amino alcohol grouping is present in a fair number of natural products which possess useful biological activity, such as antibiotics122. Such a functionality has been produced from alkenes via osmium-mediated aminohydroxylation (equation 22)123. The reaction proceeds in 40-97% yield and is enantioselective if chiral osmium-Cinchona alkaloid complexes are used to mediate the reaction. 

A stoichiometric procedure for the osmium-mediated, enantioselective aminohydrox-ylation of traws-alkenes RCH=CHR (R = Ph, Et, Pr1) has been developed employing chiral complexes between tert-butylirnidoosmium (BufN=0s03) and derivatives of cinchona alkaloids. The success of the reaction is dependent on a ligand acceleration effect corresponding diols are the by-products. The e.e. varies between 40 and 90%486,487. 

Side Note 17.2. Alternatives to the Osmium-mediated cis-vt c-Dihydroxylation... 

An extended addition increases the effective mole ratio of reaction components, which can have a beneficial impact on reaction selectivity. This is particularly true for reactions using catalysts. In the dihydroxylations in Figure 8.7, olefin 13 was added over 6 hours in order to ensure high enantiomeric purity of the product [6], Excess olefin, which accumulates if the addition rate is faster than the reaction rate, is thought to coordinate with the catalyst in a fashion that leads to non-enantioselective dihydroxylation [6]. In the osmium-mediated dihydroxylation of... 

Fig. 10. Hammett plots of osmium mediated dihydroxylations of substituted stUbenes using pyridine left), quinuclidine (middle) and DHQD-CLB (right) as ligands... 

Abstract The oxidative functionalization of olefins is an important reaction for organic synthesis as well as for the industrial production of bulk chemicals. Various processes have been explored, among them also metal-catalyzed methods using strong oxidants like osmium tetroxide. Especially, the asymmetric dihydroxylation of olefins by osmium(Vlll) complexes has proven to be a valuable reaction for the synthetic chemist. A large number of experimental studies had been conducted, but the mechanisms of the various osmium-catalyzed reactions remained a controversial issue. This changed when density functional theory calculations became available and computational studies helped to unravel the open mechanistic questions. This mini review will focus on recent mechanistic studies on osmium-mediated oxidation reactions of alkenes. 

Osmium-mediated hydroxylation of carbon-carbon double bonds has been known about for many years. However, using OSO4, two alternative mechanisms which lead to the first stable product — an osmium complex with two carbon-oxygen single bonds — are still under scrutiny. As shown in Scheme 45, in one possible mechanism a concerted cycloaddition forms the two new bonds simultaneously, while in another the two new bonds are formed... 

Scheme 45. Two possible mechanisms of osmium-mediated hydroxylation of carbon-carbon double bonds with osmium tetraoxide. [(Note L—ligand e.g., pyridine.)]...

Osmium-mediated cw-addition of nitrogen and oxygen moieties to mono-, di- and tri-substituted olefins to yield vicinal amino or amido alcohols ... 

In Nature, laccases are found in many higher plants, fungi, and microorganisms, where they catalyze the oxidation of phenolic compounds to o- and p-quinones, coupled to the four-electron reduction of atmospheric oxygen to water [4]. Here, the osmium mediator takes the place of quinines as the natural reducing substrate. 

A detailed electrokinetic analysis of osmium mediator/laccase biocathodes was published in 2008 by GaUaway and Calabrese Barton [31]. In their work, the authors described the biomolecular rate constants for mediation to be between 250 and 9.4 X10" s when the redox potentials of mediator and laccase are close or far... 

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