Syn-dihydroxylation of alkenes

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... 

A very effective way of carrying out syn-dihydroxylation of alkenes is by using an osmium tetroxide-tertiary amine N-oxide system. This dihydroxylation is usually carried out in aqueous acetone in either one-or two-phase systems, but other solvents may be required to overcome problems of substrate solubility.61... 

Mechanism 8-10 Acid-Catalyzed Opening of Epoxides 362 8-14 Syn Dihydroxylation of Alkenes 364 8-15 Oxidative Cleavage of Alkenes 366 8-16 Polymerization of Alkenes 369 8-17 Olefin Metathesis 373... 

Because the two carbon-oxygen bonds are formed simultaneously with the cyclic osmate ester, the oxygen atoms add to the same face of the double bond that is, they add with syn stereochemistry. The following reactions show the use of OSO4 and H202 for the syn dihydroxylation of alkenes. 

It is a chiral version of the syn dihydroxylation of alkenes by osmium tetroxide. Here is an example— though the concept is quite simple, the recipe for the reactions is quite complicated so we need to approach it step by step. 

One of the earliest known method of syn-dihydroxylation of alkenes is by treatment with dilute KMnO solution in presence of sodium hydroxide (Scheme 67). In fact the change in purple colour is the basis for the presence of double bond and this is known as Baeyer s test for unsaturation. 

There are numerous other examples of syn-dihydroxylation of alkenes in the literature. 

Osmium tetroxide in dry organic solvent was subsequently used for syn-dihydroxylations of alkenes. In fact, only a catalytic amount of osmium tetraoxide was needed the reaction was done in presence of chlorate salt as primary oxidant. The reaction is normally carried out in water-tetrahydrofuran solvent mixture (Scheme 69). Silver or barium chlorate gave better yields. 

The previous section described a method for anti dihydroxyladon of alkenes. In this section, we will explore two diflFerent sets of reagents that can accomplish syn dihydroxylation of alkenes. When an alkene is treated with osmium tetroxide (OSO4), a cyclic osmate ester is produced ... 

The stereochemical consequences of the vicinal syn dihydroxylation of alkenes are conplementary to those of vicinal anti dihydroxylation. Show the products (indicate stereochemistry) of the vicinal... 

LiBr has been reported to catalyse the dihydroxylation of alkenes (1) to afford syn-and anh -diols (2/3) with excellent diastereoselectivity, depending on the use of NaKTt (30 mol%) or PhI(OAc)2 (1 equiv.), respectively, as the oxidant.28 The authors claim that oxidation of non-benzylic halides was achieved for the first time to afford the corresponding diols in excellent yields 28 is not really justified as the development of the silver-free Prevost-Woodward dihydroxylation for non-benzylic substrates had... 

The 1 1 complex between bovine serum and an osmate ester is an enantioselective catalyst in the syn hydroxylation of certain alkenes, although synthetic applications appear to be limited. Asynunetric dihydroxylation of alkenes is considered in a review on catalytic asymmetric reactions. ... 

Epoxidation and Dihydroxylation of Alkenes There are several ways to convert alkenes to diols. Some of these methods proceed by syn addition, but others lead to anti addition. An important example of syn addition is osmium tetroxide-catalyzed dihydroxylation. This reaction is best carried out using a catalytic amount of OSO4, under conditions where it is reoxidized by a stoichiometric oxidant. Currently, the most common oxidants are f-butyl hydroperoxide, potassium ferricyanide, or an amine oxide. The two oxygens are added from the same side of the double bond. The key step in the reaction mechanism is a [3 + 2] cycloaddition that ensures the syn addition. 

Dihydroxylation of alkenes can be accomplished conveniently using catalytic OSO4 and the co-oxidant A-methylmorpholine A-oxide (NMO) in t-Bu0H/H20 (Upjohn conditions, see Scheme 5.80). This occurs by syn addition of the two hydroxy groups... 

Overall, the reaction leads to addition of two hydroxyl groups to the double bond and is referred to as dihydroxylation. Both oxygens of the diol come from osmium tetraoxide via the cyclic osmate ester. The reaction of OSO4 with the alkene is a syn addition, and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium. Thus, both hydroxyl groups of the diol become attached to the same face of the double bond syn dihydroxylation of the alkene is observed. 

The reaction is called dihydroxylation and proceeds by syn addition to the double bond. Osmium-based reagents that bear chiral ligands catalyze enantioselective dihydroxylation of alkenes. 

Once again, product stereochemistry provides key clues to the mechanism. Another important example of syn addition is osmium tetroxide-mediated cis-dihydroxylation of alkenes. Because of their limited relevance in this book, however, we won t discuss the mechanistic details of these reactions, but do consult your organic text if you are curious. 

Overall, the reaction leads to addition of two hydroxyl groups to the double bond and is referred to as dihydroxylation. Both hydroxyl groups of the diol become attached to the same face of the double bond syn dihydroxylation of the alkene is observed. 

Transformation of an alkenylsilane to a 1,2-dihydroxyalkylsilane, followed by the Peterson reaction after conversion of its a-hydroxy group to methoxy group, provides a useful method for the stereoselective synthesis of an alkene. 1-Methoxy-fraus-cyclooctene 201 is prepared by syn-elimination from )S-hydroxyalkylsilane 200, which is derived from syn-dihydroxylation of 1-trimethylsilyl-cis-cyclooctene, although the product 201 is thermally unstable and easily isomerizes to the cis-cyclooctene (Scheme 2.127, for NMO, see Scheme 2.27) [356]. 

Dihydroxylation of an alkene with 0s04 is a specifically cis reaction the two OH groups add to the same side of the alkene. So E-22 gives one diastereomer (syn as drawn) of the diol 23 while Z-22 gives, by syn addition, a diol that can be re-drawn after rotation of a bond, as anti -23. 

The AA reaction is closely related to the asymmetric dihydroxylation (AD). Alkenes are enantioselectively converted to protected 3-aminoalcohols (Scheme 1) by syn-addition of osmium salts under the influence of the chirr 1 bis-Cinchona ligands known from the AD process (see Chap. 20.1). As for the AD reaction, a cooxidant is needed to regenerate the active osmium species. But in the AA process the cooxidant also functions as the nitrogen source. Since two different heteroatoms are transferred to the double bond, regioselectivity becomes an important selectivity issue in addition to enantioselectivity. Moreover, chemoselectivity has to be addressed due to the possible formation of the... 

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