Alkenes, dihydroxylation

For alkene dihydroxylations, heavy metal oxides such as 0s04 and Ru04 can be applied. They are efficient catalysts but their toxitity makes their use less desirable and there is a dear need for non-toxic metal catalysts. Nevertheless, only a few reports have focused on the use of iron catalysts for alkene dihydroxylations. All systems described so far try to model the naturally occurring Rieske dioxygenase, an enzyme responsible for the biodegradation of arenes via cis-dihydroxylation by soil baderia [66]. 

Diols (glycols), such as those formed by dihydroxylation of alkenes, are cleaved by periodic acid (HI04). The products are the same ketones and aldehydes that would be formed by ozonolysis-reduction of the alkene. Dihydroxylation followed by periodic acid cleavage serves as a useful alternative to ozonolysis, and the periodate cleavage by itself is useful for determining the structures of sugars (Chapter 23). 

The first reaction, 8.30, is the classical alkene dihydroxylation with osmium tetroxide. In recent years the potential of this reaction has been vastly extended... 

Although a large number of asymmetric catalytic reactions with impressive catalytic activities and enantioselectivities have been reported, the mechanistic details at a molecular level have been firmly established for only a few. Asymmetric isomerization, hydrogenation, epoxidation, and alkene dihydroxylation are some of the reactions where the proposed catalytic cycles could be backed with kinetic, spectroscopic, and other evidence. In all these systems kinetic factors are responsible for the observed enantioselectivities. In other words, the rate of formation of one of the enantiomers of the organic product is much faster than that of its mirror image. 

Figure 9.10 Catalytic cycle for Os04-catalyzed asymmetric alkene dihydroxylation. The dashed line represents the phase boundary between the organic and the aqueous phase. L is the chiral ligand, e.g., 9.44.

Draw structures of ligands derived from the chiral framework of glucose, tartaric acid, binaphthol, and cinchona alkaloids that are used for efficient asymmetric hydrocyanation, epoxidation, hydroformylation, and alkene dihydroxylation reactions respectively. 

Quite recently it was reported that in addition to hydrogen peroxide, periodate or hexacyanoferrat(III), molecular oxygen21,31-34 can be used to reoxidize these metal-oxo compounds. New chiral centers in the products can be created with high enantioselectivity in the dihydroxylation reactions of prochiral alkenes. The development of the catalytic asymmetric version of the alkene dihydroxylation was recognized by Sharpless receipt of the 2001 Nobel prize in Chemistry. 

DFT-calculations show great similarities between the alkene dihydroxylation by permanganate and osmium tetroxide. The activation energy for the [3 + 2]-pathway is a little higher in energy (+ 9.2 kcal/mol) compared to osmium tetroxide, while the barrier for the [2 + 2]-pathway is more than 40 kcal/mol higher in energy (+50.5 kcal/mol)47... 

The catalytic cycle for 0s04-catalyzed alkene dihydroxylation in the presence of the cooxidant NMO is shown in Scheme 7.25. 

Suprafacial alkene dihydroxylation can also be achieved by the permanganate ion96 (see Vol. 4/1 b, pp 602-606). The lower cost and toxicity of this reagent compared to 0s04 renders this method attractive. 

Table 1 Alkenes Dihydroxylated using DHQD Ligands...

Other Applications. Other (/ ,/ )-stilbenediamine derivatives have been used to direct the stereochemical course of alkene dihydroxylation (with stoichiometric quantities of Osmium Tetroxide and epoxidation of simple alkenes with Sodium Hypochlorite and manganese(III) complexes. ... 

Some very widely used organic reactions are catalyzed or mediated by transition metals. For example, catalytic hydrogenation of alkenes, dihydroxylation of alkenes, and the Pauson-Khand reaction require Pd, Os, and Co complexes, respectively. The d orbitals of the transition metals allow the metals to undergo all sorts of reactions that have no equivalents among main-group elements. This doesn t mean that the mechanisms of transition-metal-mediated reactions are difficult to understand. In fact, in some ways they are easier to understand than standard organic reactions. A transition-metal-catalyzed or -mediated reaction is identified by the presence of a transition metal in the reaction mixture. 

As summarized in Scheme 2.8, these reactions provide access to three different overall stereochemical outcomes for alkene dihydroxylation, syn addition, anti addition, or stereorandom addition, depending on the reaction mechanism. In Section 2.5.4 we will discuss enantioselective catalyst for alkene dihydroxytation. These reactions provide further means of controlling the stereochemistry of the reaction. 

Scheme 2.8. Summary of Stereochemistry of Alkene Dihydroxylation by OSO4-Catalysis... 

Dihydroxylation.The conventional method of alkene dihydroxylation with OSO4 and Al-mcthylmorpholinc V-oxide (NMO) has been modified such that the latter reagent is replaced by substoichiometric A-methylmorpholine and 1.4 equiv of MCPBA. 

Dihydroxylation. Besides the enormously popular and effective cinchona alkaloid-based chiral auxiliaries several C2-symmetrical diamines (13), (14) and (15) have been developed to direct alkene dihydroxylation with OSO4. These efforts are probably overwhelmed by the Sharpless protocols because the approaches are not catalytic with respect to the most expensive and toxic reagent. 

In summary, it was demonstrated that the application of an ionic liquid provides a simple approach to the immobilization of the osmium catalyst for alkene dihydroxylation [56]. It is important to note that the volatility and toxicity of OSO4 are greatly suppressed when ionic liquids are used. 

Sharpless applied the same principle of using a central metal that can hold a chiral directing group proximal to an alkene substrate in an enantloselective version of the OSO4-catalyzed alkene dihydroxylation (Section 12-11). Here, the essence of the chiral auxiliary is an amine derived from the family of natural alkaloids called the cinchona (Section 25-8). One of these amines is dihydroquinine, which is added in the linked dimeric form shown on the right. Instead of H2O2 as the stoichiometric oxidant (Section 12-11), Fe ... 

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