Olefins osmium-catalyzed asymmetric

Because most olefins are prochiral starting materials, the dihydroxylation reaction creates one or two new stereogenic centers in the products. Since the discovery of the first stoichiometric asymmetric dihydroxylations [7], catalytic versions with considerable improvements in both scope and enantioselectivity have been developed [8]. From the standpoint of general applicability, scope, and limitations, the osmium-catalyzed asymmetric dihydroxylation (AD) of alkenes has reached a level of effectiveness which is unique among asymmetric catalytic methods. As there are recent reviews in this field [9], this section is primarily oriented toward a summary of aspects of fundamental understanding and interesting practical application of catalytic dihydroxylations. 

Norrby, P. O., Kolb, H. C., Sharpless, K. B. Calculations on the reaction of ruthenium tetroxide with olefins using density functional theory (DPT). Implications forthe possibility of intermediates in osmium-catalyzed asymmetric dihydroxylation. Organometallics 994,13, 344-347. 

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. 

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... 

From the standpoint of general applicability, and scope the osmium-catalyzed asymmetric dihydroxylation of alkenes (Sharpless dihydroxylation) has reached a level of effectiveness which is unique among asymmetric catalytic methods . In the presence of an optimized catalyst ligand system nearly every class of olefin can be dihydroxylated with high enantioselectivities. 

Figure 10.7 Postulated transition-state structure for asymmetric dihydroxylation of olefins for (a) osmium-catalyzed asymmetric dihydroxylation of prochiral olefins and for (b) an artificial cis-dihydroxylation by anchoring of OSO4 to a host protein.

Hoi-Lun, K. Sorato, C. Ogino, Y Hou, C. Sharpless, K. B. Preclusion of the "Second Cycle" in the Osmium-catalyzed Asymmetric Dihydroxylation of Olefins Leads to a Superior Process. Tetrahedron Lett. 1990, 31, 2999-3002. 

More recently Backvall and coworkers developed a novel and robust system for osmium-catalyzed asymmetric dihydroxylation of olefins by H2O2 with methyltrioxo-rhenium (MTO) as the electron transfer mediator [19]. Interestingly, here MTO catalyzes oxidation of the chiral ligand to its mono-N-oxide, which in turn reoxidizes Os to Os ". This system gives vicinal diols in good yields and high enantiomeric excess up to 99%. 

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

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. 

Arrington, M. P., Bennani, Y. L., Gobel, T., Walsh, P., Zhao, S. H., Sharpless, K. B. Modified cinchona alkaloid ligands improved selectivities in the osmium tetroxide catalyzed asymmetric dihydroxylation (AD) of terminal olefins. Tetrahedron Lett. 1993, 34, 7375-7378. 

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]. 

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. 

Since excellent results were obtained in the asymmetric aminohydroxylation in homogeneous phase by Sharpless [169], heterogeneous systems appeared to be of great interest. Nandanan has reported the first heterogeneous osmium tetroxide-catalyzed asymmetric aminohydroxylation of various olefins using polymer-supported bisdehydroquinine ligand 273 (Scheme 111) [170]. When chloramine T was used as nitogen source, yields and ee were moderate with all olefins. 

Sharpless asymmetric dihydroxylahon (AD) reaction or osmium-catalyzed dihydroxylation. Scheme 2.9, was initially developed in 1980s (1980 stoichiometric version, 1988 catalyhc version ) for the preparation of chiral diols from olefins. " The most popular standard set of reactants called as AD-mix-p or AD-mix-a has been developed for this reaction and is still intensively used for producing the chiral products with up to 99% ee.33/34 Commercially available AD-mix is composed of potassium osmate K20s02(0H)4 and powdered K3Fe(CN)g and K2CO3, respectively. P and... 

The osmium-catalyzed vicinal dihydroxylation of olefins with single oxygen donors, typically tert-butyl hydroperoxide or N -methylmorpholine-JV-oxide (NMO), has been known for three decades and forms the basis of the Sharpless asymmetric dihydroxylation of olefins. Recently, Sharpless and coworkers reported that particularly electron-deficient olefins are dihydroxylated more efficiently with NMO (Eq. 2) when the pH of... 

The focus of this chapter is to acquaint the reader with details of catalytic asymmetric dihydroxylation with osmium tetroxide and the scope of results that one can expect to achieve with current optimum conditions. The literature through mid-1992 has been reviewed in compiling this chapter. Osmium tetroxide catalyzed hydroxy]ations of olefins and acetylenes are the subject of an extensive review by Schroder published in 1980 [2a]. A comprehensive review of research and industrial applications of asymmetric dihydroxylations is in preparation [2b]. 

The most impressive methodology utilizing CT, which has been developed by the group of Sharpless, is the vicinal aminohydroxylation of olefins catalyzed by osmium tetroxide [15]. The method has been elegantly extended to a practical asymmetric synthesis [16]. The reaction system was employed to the achiral aminohydroxylation of a,P-unsaturated amides to afford two hydroxysulfonamide regio-isomers. The crude mixtures were cyclized to the aziridines in a one-pot procedure, without the need for purification of the intermediates [17] (Scheme 10). 

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. 

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

One of the foremost methods for oxidation of olefins is stereospecific syn-di-hydroxylation by treatment with OSO4. Because of the toxicity and cost of osmium, however, the need for methods that would employ substoichiometric quantities of this reagent was identified early on [180], A number of stoichiometric oxidants for catalytic dihydroxylations were examined, including metal chlorates, HjOa, NMO [181], f-BuOOH [182], and KjFeiCN) [40, 183], The mechanism of 0s04-catalyzed dihydroxylation reactions has been the subject of much debate details of these mechanistic considerations are beyond the focus of this book and are amply discussed elsewhere [40-42,47,49,184-186]. A number of interesting stereoselectivity trends of general importance in substrate-controlled diastereoselective dihydroxylations [40-43] and catalytic asymmetric dihydroxylations are discussed below (Section 9.8) [42, 44-49]. 

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