Osmium tetroxide, alkene additions

Another important reaction associated with the name of Sharpless is the so-called Sharpless dihydroxylation i.e. the asymmetric dihydroxylation of alkenes upon treatment with osmium tetroxide in the presence of a cinchona alkaloid, such as dihydroquinine, dihydroquinidine or derivatives thereof, as the chiral ligand. This reaction is of wide applicability for the enantioselective dihydroxylation of alkenes, since it does not require additional functional groups in the substrate molecule ... 

There are many reagents that add two OH groups to a double bond. Osmium tetroxide (0s04) and alkaline KMn04 give syn addition from the less-hindered side of the double bond. Less substituted double bonds are oxidized more rapidly than more substimted alkenes. Osmium tetroxide adds rather slowly but almost quantitatively. The cyclic ester (78) is an intermediate and can be isolated, but usually decomposed... 

Problems of acyl anion equivalents met above in the synthesis of similar TMs disappear if (25) is made from the alkene (26), A Wittig is the obvious method to make (26) and reaction between (27) and PhgCO will probably give (26), An alternative is the dehydration of (28), made by Grignard addition to ester (20), Osmium tetroxide was used for the hydroxylation. 

The most widely used reagent for oxidation of alkenes to glycols is osmium tetroxide. Osmium tetroxide is a highly selective oxidant that gives glycols by a stereospecific syn addition.39 The reaction occurs through a cyclic osmate ester that is formed by a [3 + 2] cycloaddition.40... 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

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. 

Hydroxylation of alkenes is the most important method for the synthesis of 1,2-diols (also called glycol). Alkenes react with cold, dilute and basic KMn04 or osmium tetroxide (OSO4) and hydrogen peroxide to give cis-1,2-diols. The products are always syn-diols, since the reaction occurs with syn addition. 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents Bis(tributyltin) oxide, 41 Hydrogen hexachloroplatinate(IV)-Copper(II) chloride, 145 4-Methoxy-2,2,6,6-tetramethyl-1 -oxopiperidinium chloride, 183 Osmium tetroxide, 222 Potassium nitrosodisulfonate, 258 Samarium(II) iodide, 270 From alkenes by addition or cleavage reactions... 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide as HO—OH. Of particular importance are alkaline permanganate (MnO40) and osmium tetroxide (0s04), both of which react in an initial step by a suprafacial cycloaddition mechanism like that postulated for ozone. 

The conversion of alkenes to 1,2-diols by osmium tetroxide is also an olefin addition reaction. In this case a hydroxy group is added to each carbon of the olefin group, and the addition is termed an oxidative addition since the diol product is at a higher oxidation level than the alkene reactant. Oxidation of the carbon atoms of the alkene takes place in the first step, which is the reaction with 0s04 to produce the intermediate osmate ester. 

Osmium tetroxide is commonly used to add two OH groups to a double bond.15 The mechanism gives syn addition from the less hindered side of the alkene. Since 0s04 is expensive and highly toxic it is therefore mostly used in a catalytic fashion using stoichiometric cooxidants, like H202 or /V-methylmorpholine-/V-oxide (NMO). 

Osmium tetroxide and permanganate are the textbook examples for the direct addition of the hydroxyl function to double bonds as shown in Scheme 3. They have been rationalized to be feasible because of their large thermodynamic exothermi-cities,30 and the existence of a low-energy pathway discussed in Section 2.1 for the transfer of two oxygen atoms from the metal to the adjacent alkene carbons. 

Vinyl and ethynyl groups attached to an imidazole ring can be catalytically reduced to the saturated (or less unsaturated) species and cleaved by oxidation. The corresponding 4-carbaldehyde is formed in 71% yield when l-methyl-2,5-diphenyl-4-styrylimidazole is oxidized with osmium tetroxide. However, they may not react like aliphatic alkenes and alkynes not all addition reactions occur normally, Michael additions are known, and the compounds can act as dienophiles in DielsAlder reactions (e.g., Scheme 132). 

We now turn to the stereochemistry governed by a ring system, and we shall look at both nucleophilic and electrophilic attack, since usually they have similar stereochemical preferences rather than contrasting preferences. In addition to several reactions that are straightforwardly electrophilic attack, we shall see several which can be described as electrophilic in nature, like the reactions of alkenes with osmium tetroxide, with peracids, with some 1,3-dipoles, and with boranes, and of dienes with dienophiles in Diels-Alder reactions. Some of these reactions are pericyclic, the pericyclic nature of which we shall meet in Chapter 6. For now, it is only their diastereoselectivity that will concern us. 

Whereas hydrolysis of epoxides leads to the trans diaxial addition of water and the formation of trans glycols (1,2-diols), cis glycol formation involves the addition of osmium(VIII) oxide (osmium tetroxide, OsO ) or cold dilute aqueous potassium manganate(Vll) (potassium permanganate) to an alkene. 

Addition has also been shown to occur when the pyridinlum iodide (139) is irradiated. The product Is assigned the structure (140) and the relevance of this reaction to the photoreactivity of Kosower solvent polarity probes has been discussed. The use of osmium tetroxide to hydroxylate alkenes is a well known procedure which is often carried out in aromatic solvents. These arene solvents form charge transfer complexes with the osmium tetroxide and the photochemistry of these has now been examined. It is shown that with benzene and alkyl benzenes isolable adducts are formed that from benzene Is assigned structure (141). 

The addition of two hydroxylic groups to the double bonds of unsaturated ketones is carried out by the same methods used for hydroxylations of alkenes (equations 71-83). As an example, hydroxylation of the double bond in 3p-hydroxyandrost-5-en-17-one is accomplished by treatment with one equivalent of osmium tetroxide in pyridine and subsequent reductive cleavage of the osmate ester with sodium bisulfite in aqueous pyridine (equation 442) [950. ... 

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