Alkenes vicinal dihydroxylation

Ruthenium complexes alcohol oxidation, 788-9 alkene/alkyne vicinal dihydroxylation, 556 dioxetane decomposition, 1189-90... 

When asymmetric epoxidation of a diene is not feasible, an indirect route based on asymmetric dihydroxylation can be employed. The alkene is converted into the corresponding syn-diol with high enantioselectivity, and the diol is subsequently transformed into the corresponding trans-epoxide in a high-yielding one-pot procedure (Scheme 9.5) [20]. No cpirricrizalion occurs, and the procedure has successfully been applied to natural product syntheses when direct epoxidation strategies have failed [21]. Alternative methods for conversion of vicinal diols into epoxides have also been reported [22, 23]. 

Syn-Dihydroxylation. When the reaction was first discovered, the syn-dihydroxylation of alkenes was carried out by using a stoichiometric amount of osmium tetroxide in dry organic solvent.56 Hoffman made the observation that alkenes could react with chlorate salts as the primary oxidants together with a catalytic quantity of osmium tetroxide, yielding syn-vicinal diols (Eq. 3.11). This catalytic reaction is usually carried out in an aqueous and tetrahydrofuran solvent mixture, and silver or barium chlorate generally give better yields.57... 

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]. 

Dihydroxylation, the addition of two hydroxy groups across a C = C bond, converts fluorinated alkenes into different products depending on the presence or absence of a fluorine atom at the hydroxylated carbon. Partially fluorinated alkenes with vicinal hydrogen atoms attached to the C = C bond can be hydroxylated to vicinal diols. When the reaction is performed with a sufficiently strong oxidizing agent, the initially formed diols are oxidized to vicinal diketones as the end products. 

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. 

The compound is a vicinal diol (glycol) containing seven carbon atoms. Glycols are commonly made by dihydroxylation of alkenes, and this glycol would be made by dihydroxylation of 3-ethylpent-2-ene, which effectively becomes the target compound. 

Z)-l,2-disubstituted alkenes proved to be the most difficult class. In fact, they are not osmylated efficiently with the all purpose ligands 1F/2 F. Further studies, however, led to the discovery of the indolinyl ligands 11/21 that allowed cis dihydroxylation of these alkenes in up to 80% eel0. It should be kept in mind, however, that in the case of 1,1-disubstituted alkenes and of (Z)-l,2-disubstituted alkenes, a lowering of difference in steric requirement between the two vicinal substituents inevitably means a drop in the 7t-face discrimination since the two enantiotopic alkene 7t-faces lend to become quasi-homotopic . 

Sharpless asymmetric dihydroxylation One-pot enantioselective synthesis of vicinal diols from simple alkenes. 406... 

The presence of the anfi-vicinal diol unit in the phytosphingosine 67 strongly suggests the application of the double diastereocontrolled AD to the Z-alkene 64 (Scheme 25). However, in order to access the desired product 64, the mismatched diol should be obtained. Horikawa and coworkers studied both the achiral and the chiral dihydroxylation of the optically active (E)- and (Z)-allylic... 

The Sharpless asymmetric hydroxylation can take one of two forms, the initially developed asymmetric dihydroxylation (AD)1 or the more recent variation, asymmetric aminohydroxylation (AA).2 In the case of AD, the product is a 1,2-diol, whereas in the AA reaction, a 1,2-amino alcohol is the desired product. These reactions involve the asymmetric transformation of an alkene to a vicinally functionalized alcohol mediated by osmium tetraoxide in the presence of chiral ligands (e.g., (DHQD)2-PHAL or (DHQ)2-PHAL). A mixture of these reagents (ligand, osmium, base, and oxidant) is commercially available and is sold under the name of AD-mix p or AD-mix a (vide infra). 

Dihydroxylation Reaction or sequence of reactions in which an alkene is converted to a vicinal diol. 

A common oxidation is the conversion of an alcohol such as 2-propanol (1) to a ketone (acetone, 2), and it involves loss of hydrogen atoms from oxygen and from carbon. Similarly, the conversion of an alkene such as cyclopentene (3) to a vicinal diol (4) (see dihydroxylation in Chapter 10, Section 10.7.1) involves the... 

Section 17.3 reviewed the oxidation of alkenes to give dihydroxylation. Ozonolysis is a good method for the oxidative cleavage of alkenes, but vicinal diols are also subject to oxidative cleavage and the products are aldehydes or ketones. Two common reagents used for this purpose are periodic acid (HIO4) and lead tetraacetate [Pb(OAc)J. 

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