Olefins, cis-dihydroxylation

L. Jr (2002) Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts Two faces of an Fe boOH coin. /. Am. Chem. Soc., 124, 3026-3035. 

K. Suzuki, P. D. Oldenburg, L. Que, Jr., Iron-catalyzed asymmetric olefin cis-dihydroxylation with 97% enantiomeric excess, Angew. Chem. Int. Ed. 47 (2008) 1887. 

The cis dihydroxylation of olefins (10—>11, Scheme 2), one of organic chemistry s most venerable reactions, was first reported by Makowaka in 1908.9 It is also one of the most useful reactions, since it converts an olefin, itself a pivotal functional group, to a vicinal diol, another pivotal functional group present in many natural products and unnatural molecules. The original dihydroxyl-... 

The reaction of an olefin with osmium tetroxide is the most reliable method for cis-dihydroxylation of a double bond, particu-... 

The cis dihydroxylation of olefins mediated by osmium tetroxide represents an important general method for olefin functionalization [1,2]. For the purpose of introducing the subject of this chapter, it is useful to divide osmium tetroxide mediated cis dihydroxylations into four categories (1) the stoichiometric dihydroxylation of olefins, in which a stoichiometric equivalent of osmium tetroxide is used for an equivalent of olefin (2) the catalytic dihydroxylation of olefins, in which only a catalytic amount of osmium tetroxide is used relative to the amount of olefin in the reaction (3) the stoichiometric, asymmetric dihydroxylation of olefins, in which osmium tetroxide, an olefinic compound, and a chiral auxiliary are all used in equivalent or stoichiometric amounts and (4) the catalytic, asymmetric dihydroxylation of olefins. The last category is the focus of this chapter. Many features of the reaction are common to all four categories, and are outlined briefly in this introductory section. 

Inclusion in the reaction of a cooxidant serves to return the osmium to the osmium tetroxide level of oxidation and allows for the use of osmium in catalytic amounts. Various cooxidants have been used for this purpose historically, the application of sodium or potassium chlorate in this regard was first reported by Hofmann [7]. Milas and co-workers [8,9] introduced the use of hydrogen peroxide in f-butyl alcohol as an alternative to the metal chlorates. Although catalytic cis dihydroxylation by using perchlorates or hydrogen peroxide usually gives good yields of diols, it is difficult to avoid overoxidation, which with some types of olefins becomes a serious limitation to the method. Superior cooxidants that minimize overoxidation are alkaline t-butylhydroperoxide, introduced by Sharpless and Akashi [10], and tertiary amine oxides such as A - rn e t h y I rn o r p h o I i n e - A - o x i d e (NMO), introduced by VanRheenen, Kelly, and Cha (the Upjohn process) [11], A new, important addition to this list of cooxidants is potassium ferricyanide, introduced by Minato, Yamamoto, and Tsuji in 1990 [12]. 

Selective chemical transformations can also be performed on the natural product sanglifehrin A (1) itself [19]. It is surprising that C26-C27 of 1 can be chemoselectively cis-dihydroxylated (Sharpless conditions) in a yield of 70%. The natural product can afterwards be reassembled by Julia-Kocienski olefination. The success of this operation indicates that total syntheses of sanglifehrin A (1) alternative to those by Nicolaou et al. and Paquette should be worth pursuing. 

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. 

Density fimctional theory (DFT) calculations show that this mechanistic hypothesis is energetically reasonable. The generation of the HO-Fe =0 oxidant from the [(TPA)Fe -OOH(OH2)] intermediate was found to have a thermodynamic cost of only 5 kcal/mol and a kinetic barrier of 20 kcal / mol (Fig. 18.5) [52]. Furthermore, the HO-Fe =0 oxidant could carry out either epoxidation or cis-dihydroxylation of the olefin, depending on which oxygen atom of the oxidant initiated attack of the substrate [53]. Thus, epoxidation occurs by 0x0 attack on the olefin, forming the first C-O bond and an intermediate carbon-based radical, which then reboimds to form the second C-O bond. On the other hand, czs-dihydroxylation is initiated by hydroxo attack to form the first C-O bond and an intermediate carbon-based radical, followed by reboimd with the 0x0 group to form the second C-O bond. 

In the last few years, several non-heme iron complexes have been identified as functional models for non-heme iron dioxygenases (85-88). These model complexes are able to catalyze the cis-dihydroxylation of olefins as well as the epoxidation of olefins using H2O2 as the primary oxidant. Table V presents the results of olefin oxidation by some representative mononuclear and dinuclear non-heme iron complexes in combination with H2O2. 

Fig. 31. Proposed cis-dihydroxylation mechanism for the oxidation of olefins catalyzed by [Mn203(Me3tacn)2](PF6)2/gmha 155).

The present review has outlined the efforts to develop biomimetic non-heme iron and manganese catalysts for alkane hydroxylation, olefin epoxidation, and cis-dihydroxylation reactions. However, the examples reviewed here are mostly presented as reported in the literature, since the various reaction conditions involved in the catalytic oxidations hamper a direct comparison and critical evaluation of the data. The survey has not only illustrated a rich variety of iron and manganese complexes that lead to the successful structural modeling of important non-heme iron and manganese enzymes, but also significant features of the oxidation reactions catalyzed by these complexes in combination with dihydrogen peroxide. 

With regard to the biomimetic non-heme iron complexes, the work devoted to develop catalysts that perform catalytic alkane hydroxylation has resulted in a large number of iron complexes, which generate Fe =0 iron-oxo species characterized by different spectroscopic techniques. There is now direct evidence that the involvement of high-valent iron-oxo species leads to stereospecific alkane hydroxylation, while hydroxyl radicals contribute to non-selective oxidations. The impressive work performed by Que and co-workers has demonstrated that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe -OOH intermediate, whose spin state can be modulated by the electronic and steric properties of... 

The results of quantum chemical DFT and ab initio calculations of the reaction mechanism of the olefin epoxidation with diperoxo complexes and cis-dihydroxylation with oxo complexes show clearly that the organometallacyclic compounds which were previously suggested as intermediates in the reaction are not formed. The epoxidation reactions do not proceed via formation of metalla-2,3-dioxolanes as intermediate but rather via direct attack of the metal peroxo moiety to the olefin. Likewise the reaction of neutral metal oxides in high oxidation states with olefins occurs not as a [2+2] addition via metalla-... 

Oxidative rearrangements, via oxythallation, have been improved in yield and selectivity by the use of thallium(iii) nitrate supported on clay rather than in methanolic solution. Thus, cyclohexene gave an 85% yield of dimethoxymethyl-cyclopentane while 1-tetralone, which normally gives a complex mixture of products, gave a 1 1 mixture of methyl indane-l-carboxylate and 2-methoxytetralone. An efficient, large-scale procedure for the direct cis-dihydroxylation of olefins has been reported. The oxidant is t-butyl hydroperoxide and the catalyst osmium tetroxide, with the reaction conducted under alkaline conditions (E%N OH ), so facilitating a rapid turnover of catalyst via enhanced hydrolysis of the osmate esters. The method appears to be more advantageous for the more substituted olefins than the Hofmann and Miles procedure. 

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.

The high-valent iron-oxo sites of nonheme iron enzymes catalyze a variety of reactions (halogenation and hydroxylation of alkanes, desaturation and cyclization, electrophilic aromatic substitution, and cis-dihydroxylation of olefins) [lb]. Most of these (and other) reactions have also been achieved and studied with model systems [Ic, 2a-c]. With the bispidine complexes, we have primarily concentrated on olefin epoxidation and dihydroxylation, alkane hydroxylation and halogenation, and sulfoxidation and demethylation processes. The focus in these studies so far has been on a thorough analysis of the reaction mechanisms rather than the substrate scope and catalyst optimization. 

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