Asymmetric Dihydroxylation of Olefins

Alternatively, the process can be performed with K3Fe(CN)6 as the stoichiometric oxidant in tert-butanol/water mixtures [298]. In this case the olefin osmy-lation and osmium reoxidation steps are uncoupled, since they occur in different phases, resulting in improved enantioselectivities. However, from a practical point of view, the improved enantioselectivities (about 5-10% ee) are probably offset by the use of a less attractive oxidant. 

Beller et al. [85] recently described the aerobic dihydroxylation of olefins catalyzed by osmium at basic pH, as mentioned above. When using the hydroquini-dine and hydroquinine bases, they were able to obtain reasonable enantioselectivities (54% ee to 96% ee) for a range of substrates. An alternative route towards enantiopure diols, is the kinetic resolution of racemic epoxides via enantioselec-tive hydrolysis catalyzed by a Co(III)salen acetate complex, developed by Jacob- 

---

The history of asymmetric dihydroxylation51 dates back 1912 when Hoffmann showed, for the first time, that osmium tetroxide could be used catalytically in the presence of a secondary oxygen donor such as sodium or potassium chlorate for the cA-dihydroxylation of olefins.52 About 30 years later, Criegee et al.53 discovered a dramatic rate enhancement in the osmylation of alkene induced by tertiary amines, and this finding paved the way for asymmetric dihydroxylation of olefins. 

Corey et al.66 have developed a bidentate chiral ligand 93 for asymmetric dihydroxylation of olefins. As shown in Table 4-13, asymmetric dihydroxylation of a series of olefins using 93 as a chiral catalyst and OsCU as the oxidant gives good to excellent yield as well as good enantioselectivity in most cases. 

Hirama and co-workers71 developed another chiral bidentate ligand 92 for OsCU-mediated dihydroxylation of /m .v-disubstituted and monosubstituted olefins. As shown in Table 4-14, asymmetric dihydroxylation of olefins using (S,S)-(—)-92b as the chiral ligand provides excellent yield and enantioselectivity. 

Chiral compounds 91a and 91b, as shown in Table 4-15, were first reported by Jacobsen et al.55 for the asymmetric dihydroxylation of olefins. These catalysts can be used for asymmetric dihydroxlation of a variety of substrates. 

TABLE 4-15. Asymmetric Dihydroxylation of Olefins with 0s04 Induced by 91a or 91b... 

Excitingly, the electrochemical Os-catalyzed asymmetric dihydroxylation of olefins with Sharpless s ligands yielding the chiral diol (138) via the chiral adduct (137) has been reported [184]. The asymmetric dihydroxylation of olefins (136) is performed by recycling a catalytic amount of potassium ferricyanide [K3Fe(CN)6] in the presence... 

SCHEME 178. Osmium-catalyzed catalytic asymmetric dihydroxylation of olefins by H2O2 as terminal oxidant... 

A polymeric cinchona alkaloid-derived ligand 44 was prepared and used to catalyze the asymmetric dihydroxylation of olefins (see the diagram below).66 Both aliphatic and aromatic olefins afforded diols with good enantioselectivities. 

Success in the use of Ti tartrate catalyzed asymmetric epoxidation depends on the presence of the hydroxyl group of the allylic alcohol. The hydroxyl group enhances the rate of the reaction, thereby providing selective epoxidation of the allylic olefin in the presence of other olefins it also is essential for the achievement of asymmetric induction. The role played by the hydroxyl group in this reaction is described in a later section of this chapter. The need for a hydroxyl group necessarily limits the scope of this asymmetric epoxidation to a fraction of all olefins. Fortunately, allylic alcohols are easily introduced into synthetic intermediates and are very versatile in organic synthesis. The Ti tartrate catalyzed asymmetric epoxidation of allylic alcohols has been applied extensively as documented in the literature and in this review. The development of methods aimed at catalytic asymmetric epoxidation of unfunctionalized olefins is described in Chapter 6B, whereas the catalytic asymmetric dihydroxylation of olefins, which provides an alternate method for olefin functionalization, is described in Chapter 6D. 

The cis dihydroxylation of olefins mediated by osmium tetroxide represents an important general method for olefin functionalization [1,2]. For the purpose of introducing the subject of this chapter, it is useful to divide osmium tetroxide mediated cis dihydroxylations into four categories (1) the stoichiometric dihydroxylation of olefins, in which a stoichiometric equivalent of osmium tetroxide is used for an equivalent of olefin (2) the catalytic dihydroxylation of olefins, in which only a catalytic amount of osmium tetroxide is used relative to the amount of olefin in the reaction (3) the stoichiometric, asymmetric dihydroxylation of olefins, in which osmium tetroxide, an olefinic compound, and a chiral auxiliary are all used in equivalent or stoichiometric amounts and (4) the catalytic, asymmetric dihydroxylation of olefins. The last category is the focus of this chapter. Many features of the reaction are common to all four categories, and are outlined briefly in this introductory section. 

Figure 6D.3. Schematic representation of electrocatalytic asymmetric dihydroxylation of olefins.

Osmium-catalysed dihydroxylation of olefins is a powerful route towards enantioselective introduction of chiral centers into organic substrates [82]. Its importance is remarkable because of its common use in organic and natural product synthesis, due to its ability to introduce two vicinal functional groups into hydrocarbons with no functional groups [83]. Prof. Sharpless received the 2001 Nobel Prize in chemistry for his development of asymmetric catalytic oxidation reactions of alkenes, including his outstanding achievements in the osmium asymmetric dihydroxylation of olefins. 

Fig. 11.5. Os-catalyzed asymmetric dihydroxylation of olefins by enantiomeric differentiation (chiral induction). (Reprinted from S. Torii, Electrochemical Asymmetric Syntheses of Chiral Diols and Epoxides Interface, Winter 1997, p. 46, Scheme 1. Reproduced by permission of the Electrochemical Society.)...

In 1994, Hanessian and co-workers [50] reported the first examples of metal-free three-dimensional triple-stranded helicates through spontaneous self-assembly of chiral C2-symmetrical diols and chiral C2-symmetrical diamines. The initial observation resulted from the utilization of enantiopure C2-symmetrical vicinal trans-1,2-diaminocyclohexane [51,52] as ligands in the asymmetric dihydroxylations of olefins [53] and as reagents for asymmetric synthesis [54], When equimolar amounts of (5,5)-frfl x-l,2-diaminocyclohexane (28) and its (i ,i )-enantiomer (29) were individually mixed with (5,5)-frfl x-l,2-cyclohexanediol and heated in refluxing benzene, crystals of the respective supraminol complexes 28 30 and 29 30 were formed quantitatively (Scheme 12). This was the physical basis for the separation of racemic diols with tr[Pg.104]

Figure 20 Fundamental steps in the commercial catalytic asymmetric dihydroxylation of olefins catalyzed by osmium tetroxide.

Example 8.10. Osmium-tetroxide catalyzed asymmetric dihydroxylation of olefins. Substituted olefins such as styrene can be dihydroxylated to give vicinal diols ... 

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. 

Ionic solvents have been used to facilitate the Heck reaction (13-9), the oxidation of alcohols with hypervalent iodine reagents (19-3)," and the catalytic asymmetric dihydroxylation of olefins (15-48) using a recoverable and reusable osmium/... 

Recently, electrochemical Os-catalyzed asymmetric dihydroxylation of olefins with Sharpless s ligand has been developed by Amundsen and Balko [450] and by Torii and coworkers [451,452]. 

Pini, D., Petri, A., Salvador , P. Heterogeneous catalytic asymmetric dihydroxylation of olefins a new polymeric support and a process improvement. Tetrahedron 99A, 50,11321-11328. 

Jiang, R., Kuang, Y., Sun, X., Zhang, S. An improved catalytic system for recycling Os04 and chiral ligands in the asymmetric dihydroxylation of olefins. Tetrahedron Asymmetry 2004,15, 743-746. 

Ogino, Y., Chen, H., Kwong, H. L., Sharpless, K. B. The timing of hydrolysis-reoxidation in the osmium-catalyzed asymmetric dihydroxylation of olefins using potassium ferricyanide as the reoxidant. Tetrahedron Lett. 1991, 32, 3965-3968. 

Corey, E. J., Noe, M. C. Kinetic Investigations Provide Additional Evidence That an Enzyme-like Binding Pocket Is Crucial for High Enantioselectivity in the Bis-Cinchona Alkaloid Catalyzed Asymmetric Dihydroxylation of Olefins. J. Am. Chem. Soc. 1996, 118, 319-329. 

The presence of the quinuclidine base functionality makes them effective ligands for a variety of metal-catalyzed processes (Chapters 2-4). The most representative example is the osmium-catalyzed asymmetric dihydroxylation of olefins [9]. The metal binding properties of the quinuclidine nitrogen also allow to use cinchona alkaloids as metal surface modifiers, for example, in the highly enantioselective heterogeneous asymmetric hydrogenation of a-keto esters (Chapter 2). Both... 

Not surprisingly, fhe classical Os-catalyzed asymmetric dihydroxylation of olefins (cf. Section 2.2) continues to be of interest. The basic principles, such as type of catalyst, stay relatively fhe same and instead efforts are concentrated on making the reaction more suitable for operation under process-like conditions. A modification fhat could improve fhe operabihty is to replace the conventional t-BuOH/ H2O solvent mixture by ionic liquid containing mixtures, either as a monophasic... 

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... 

The growing interest manifested by synthetic chemists towards the use of the asymmetric dihydroxylation of olefins has resulted in so many varied apphca-tions that it is beyond the scope of this limited review to enumerate them all. Throughout this concise review, the specificities of the catalytic AD of alkenes have been discussed and the salient features of the almost limitless transformations of the resulting chiral diols have been illustrated through the use of selected examples. Whilst the AD of olefins is now considered as one of the most important tools in asymmetric synthesis, let us not forget that the enantioselectiv-ity displayed by the asymmetric dihydroxylation process can depend upon subtle steric, electronic or conformational effects that are far less pronounced in other asymmetric catalytic reactions. Such is the case in our last example [141], featuring an elegant approach towards vitamin E by Tietze and coworkers... 

Asymmetric dihydroxylation of olefins bearing chiral substituents can take place with double diastereodifferentiation ( 1.6), so that matched and mismatched pairs will be observed [498, 753, 762],... 

Ligands have also been attached to polyethylene glycol so that they can be recovered for recycle by the foregoing methods. A phosphine isocyanate has been reacted with polyethylene glycol for use in the Staudinger reaction with alkyl azides to form a phosphine imine.173 An alkaloid has been attached to the monomethyl ether of polyethylene glycol for use in the Sharpless asymmetrical dihydroxylation of olefins. The reaction was complete in the same time, with no decrease in yield or enantioselectivity, as when the alkaloid was used by itself.174 (Asymmetrical reactions are covered in Chap. 10.)... 

The Sharpless group is best known for development of a method for the asymmetrical dihydroxylation of olefins.104 A typical oxidation uses 1 mol% of potassium osmate dihydrate, 1 mol% ligand, potassium carbonate, potassium ferricyanide, and methylsulfonamide in aqueous tert-butyl alcohol. Other oxidants such as A-melhylmor pholine N oxide can be used in place of the potassium ferricyanide. One of the preferred ligands is a bis(dihydro-quinidine ether) of phthalazine (10.46). 

---