Osmium dihydroxylation catalysts, olefins

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]). 

Enantioselective c -dihydroxylation of olefins using osmium catalyst in the presence of cinchona alkaloid ligands. 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

The computational study of the osmium dihydroxylation of aliphatic al-kenes is much more complicated than the case of aromatic alkenes due to the large number of conformations that the former could adopt. To overcome this issue, we considered the system to be composed of two different parts the catalyst and the olefin. For the catalyst, the conformation considered is that from the X-ray structure. As already shown in the study of styrene [95], and in some experimental works [98], the catalyst is a fairly rigid molecule. For the aliphatic alkenes under study, there is a large number of possible conformations in addition, the stability of an olefin conformation is also affected by the interactions between the olefin substituent and the catalyst. Therefore, the catalyst must be included in the conformational search. The conformational analysis was done using a scheme based on the systematic search approach [99]. The strategy consisted of two parts first we developed a method to identify all of the possible conformations afterwards, we screened all of the possible conformations at MM level to select the most stable. Finally, we only carried out the relatively expensive QM/MM calculations on these selected conformations. 

Osmium is unrivalled as catalyst for the asymmetric cis-dihydroxylation of olefins. However, Sato and coworkers reported that the perfluorosulfonic acid resin, Nafion (see Chapter 2) is an effective catalyst for the trans-dihydroxyla-tion of olefins with H202 [86]. The method is organic solvent-free and the catalyst can be easily recycled (see Fig. 4.33). The first step of this reaction is epoxi-dation which is probably carried out by resin-supported peroxysulfonic acid formed in situ. This is followed by acid-catalyzed epoxide-ring opening. 

After their leading publication on the osmium-catalyzed dihydroxylation of olefins in the presence of dioxygen [208], Beller et al. [209] recently reported that alcohol oxidations could also be performed using the same conditions. The reactions were carried out in a buffered two-phase system with a constant pH of 10.4. Under these conditions a remarkable catalyst productivity (TON up to 16 600 for acetophenone) was observed. The pH value is critical in order to ensure the reoxidation of Os(VI) to Os(VIII). The scope of this system seems to be limited to benzylic and secondary alcohols. 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

Kobayashi S, Sugiura M. Immobilization of osmium catalysts for asymmetric dihydroxylation of olefins. Adv. Synth. Catal. 2006 348 1496-1504. 

The discovery of iron complexes that can catalyze olefin czs-dihydroxylation led Que and coworkers to explore the possibility of developing asymmetric dihydroxylation catalysts. Toward this end, the optically active variants of complexes 11 [(1R,2R)-BPMCN] and 14 [(1S,2S)- and (lP-2P)-6-Me2BPMCN] were synthesized [35]. In the oxidation of frans-2-heptene under conditions of limiting oxidant, 1R,2R-11 was foimd to catalyze the formation of only a minimal amount of diol with a slight enantiomeric excess (ee) of 29%. However, 1P-2P-14 and 1S,2S-14 favored the formation of diol (epoxide/diol = 1 3.5) with ees of 80%. These first examples of iron-catalyzed asymmetric ds-dihydroxylation demonstrate the possibility of developing iron-based asymmetric catalysts that may be used as alternatives to currently used osmium-based chemistry [45]. 

An alternative way to use the polymer support is to microcapsulate the catalyst Kobayashi developed a microcapsulated osmium tetroxide using phenoxyethoxy-methyl-polystyrene, and applied this to the asymmetric dihydroxylation of olefins in water (Scheme 3.34) [64]. The reaction proceeded smoothly in water with cata-... 

Song CE, Jung D, Roh EJ, Lee SG, Chi DY (2002) Osmium tetro3dde-(QN)2PHAL in an ionic liquid a highly efficient and recyclable catalyst system for asymmetric dihydroxylation of olefins. Chem Commun 3038-3039... 

Oxidative rearrangements, via oxythallation, have been improved in yield and selectivity by the use of thallium(iii) nitrate supported on clay rather than in methanolic solution. Thus, cyclohexene gave an 85% yield of dimethoxymethyl-cyclopentane while 1-tetralone, which normally gives a complex mixture of products, gave a 1 1 mixture of methyl indane-l-carboxylate and 2-methoxytetralone. An efficient, large-scale procedure for the direct cis-dihydroxylation of olefins has been reported. The oxidant is t-butyl hydroperoxide and the catalyst osmium tetroxide, with the reaction conducted under alkaline conditions (E%N OH ), so facilitating a rapid turnover of catalyst via enhanced hydrolysis of the osmate esters. The method appears to be more advantageous for the more substituted olefins than the Hofmann and Miles procedure. 

Corey, E. J. Noe, M. C. Sarshar, S. The Origin of High Enantioselectivity in the Dihydroxylation of Olefins Using Osmium Tetraoxide and Cinchona Alkaloid Catalysts. J. Am. Chem. Soc. 1993,115, 3828-3829. 

Recently Kobayashi and coworkers reported on a new type of microencapsulated osmium tetroxide using phenoxyethoxymethyl-polystyrene as the support [39]. With this catalyst, asymmetric dihydroxylation of olefins has been successfiiUy performed using (DHQD)2PHAL as a chiral ligand and K3[Fe(CN)6] as a cooxidant in H20/acet-one (Scheme 1.11). 

The dihydroxylation of olefins with an osmium catalyst also utilizes chiral tertiary amine ligands to achieve high yields and enantioselectivity. Soon after Krief and coworkers reported on the coupled 02/PhSeCH2Ph oxidation [34], Bel-ler and coworkers discovered a direct 02-coupled catalytic aerobic oxidation of olefins was possible using a phosphate-buffered pH 10.4 solution (Scheme 5.21) [69]. Under increased pressure with air rather than O2, the catalyst remains active and a TOP of 40 h is possible. This system does not quite achieve as high an enantioselectivity as the AD-Mix methods [70]. 

Mono-, di-, and trisubstituted olefins undergo osmium-catalyzed enantioselective dihydroxylation in the presence of the (R)-proline-substituted hydroquinidine 3.9 to give diols in 67-95% yields and in 78-99% ee.75 Using potassium osmate(VI) as the catalyst and potassium carbonate as the base in a tm-butanol/water mixture as the solvent, olefins are dihydroxylated stereo- and enantioselectively in the presence of 3.9 and potassium ferricyanide with sodium chlorite as the stoichiometric oxidant the yields and enantiomeric excesses of the... 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

The matching and mismatching of chiral olefin 54 and catalyst was examined briefly by using stoichiometric quantities of osmium tetroxide with achiral and chiral ligands [60], The monothio acetal derived from camphor (54) was dihydroxylated with osmium tetroxide in the presence of quinuclidine, DHQD-OAc, or DHQ-OAc, With the achiral quinuclidine as ligand, the ratio of (2S,3R) to (2R,3S) diasteriomers 55 and 56 was 2.5 1. With DHQD-OAc as the chiral ligand, catalyst and substrate are matched and the ratio is enhanced to 40 1 while with DHQ-OAc catalyst and substrate are mismatched and a reversed selectivity of 1 16 is observed. 

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