Ruthenium epoxidation, alkenes

In a different type of reaction, alkenes are photooxygenated (with singlet O2, see 14-7) in the presence of a Ti, V, or Mo complex to give epoxy alcohols, such as 180, formally derived from allylic hydroxylation followed by epoxidation. " In other cases, modification of the procedure gives simple epoxidation. " Alkenes react with aldehydes and oxygen, with palladium-on-silica " or a ruthenium catalyst, " " ... 

M. Klawonn, M. K. Tse, S. Bhor, C. Dobler, M. Beller, A convenient ruthenium-catalyzed alkene epoxidation with hydrogen peroxide as oxidant, J. Mol. Catal. A Chem. 218 (2004) 13. 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

Introduction of mesityl groups at the porphyrin ring can prevent the formation of the dimeric products and the reaction with dioxygen now leads to ruthenium(VI)-dioxo complexes of TMP (tetramesitylporphyrin) [35], The tram-Ru(VI)02-TM P species can catalyse the epoxidation of alkenes as well as whole range of other oxidation reactions. After transfer of one oxygen atom to an organic substrate Ru(IV)0-TMP is formed, which disproportionates to an equilibrium of Ru02 and llu ). 

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

Ruthenium complexes catalyse the two main oxidative reactions for alkenes those in which oxygen atoms or hydroxyl groups span the erstwhile double bond without C=C rupture (e.g. epoxidation, ctT-dihydroxylation, ketohydroxylation), and cleavage reactions in which the C=C bond is broken. Although RuO has recently been shown to be effective for c/x-dihydroxylation and ketohdroxylation, epoxidations are in general effected by Ru complexes of lower oxidation states, while RuO excels at cleavage reactions. 

In the presence of excess bipyridyl, ruthenium trichloride is an effective catalyst for the selective epoxidation of alkenes by sodium periodate (equation 297). The epoxidation is syn and stereospecific for cis and trans alkenes.637... 

Cyclic sulfates provide a useful alternative to epoxides now that it is viable to produce a chiral diol from an alkene. These cyclic compounds are prepared by reaction of the diol with thionyl chloride, followed by ruthenium-catalyzed oxidation of the sulfur (Scheme 9.26).166 This oxidation has the advantage over previous procedures because it only uses a small amount of the transition metal catalyst.167168... 

In addition to epoxides, three-membered nitrogen heterocycles, aziridines, can be obtained by means of catalytic asymmetric aziridinations (Eq. 30). To this aim, chiral ruthenium(salen) complexes 67 [56] and 68 [57] were useful (Fig. 1). The former phosphine complexes 67 gave the aziridine from two cy-cloalkenes with 19-83% ee [56]. On the other hand, terminal alkenes selectively underwent aziridination in the presence of the latter carbonyl complex 68 with 87-95% ee [57]. In these examples, N-tosyliminophenyliodinane or N-tosyl azide were used as nitrene sources. Quite recently, catalytic intramolecular ami-dation of saturated C-H bonds was achieved by the use of a ruthenium(por-phyrin) complex (Eq. 31) [58]. In the presence of the ruthenium catalyst and 2 equiv iodosobenzene diacetate, sulfamate esters 69 were converted into cyclic sulfamidates 70 in moderate-to-good yields. 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

Most ruthenium catalysts used in epoxidation reactions are based on bulky porphyrins or other amine ligands and require the use of PhIO and Cl2PyNO as oxidants. For examples see the reviews in Refs. [5,6,45] and some recent examples by Liu and coworkers [46,47] and Jitsukawa et al. [48]. Examples for the aerobic epoxidation of alkenes are the ruthenium mesityl porphyrin complex Ru(TMP)(0)2, where TMP is 5,10,15,20-tetramesitylporphyrinato, of Groves and Quinn [12] in 1985 (Eq. 7), the ruthenium dimethylphenanthroline complex, czs-[Ru(2,9-dimethyl-l,10-phenanthroline)(CH3CN)2]2+ published by Goldstein et al. [23] in 1994 (Eq. 8), and the ruthenium POM catalyst [WZnRu2(0H)(H20)](ZnW9034)2 n of Neumann and Dahan [49] in 1997 (Eq. 9). 

Some examples of asymmetric epoxidations of alkenes using chiral ruthenium porphyrins have been reported for example, the previously reported D4-sym-metrical chiral ruthenium porphyrin complex Run(D4-Por )(CO)(MeOH) [58], which produced (R)-styrene oxide in 57% ee with Cl2PyNO as a donor, was readily converted into the dichloro derivative A [59] (Fig. 11). This di-chlororuthenium porphyrin gave (R)-styrene oxide in 69% ee using Cl2PyNO and was highly active (875 TON in 1.5 h). The use of unsubstituted pyridine N-oxide or NMO as oxidants resulted in low substrate conversions as well as... 

A dinuclear ruthenium complex with both ruthenium in 2+ oxidation states catalyzes the oxidation of adamantane to hydroxy adamantane and alkene to epoxide by dioxygen. Suggest a possible mechanism. 

Peracids themselves produce epoxides and diols from alkenes but are not powerful enough to oxidize these further by cleaving the carbon-carbon bond. However, in the presence of transition metals they will cleave alkenes and diols to give, usually, carboxylic acids. For example, peracids in combination with ruthenium compounds are well known in this capacity (Figure 3.31).146,147 Warwel and co-workers have reported the cleavage of alkenes using peracetic acid and the ruthenium catalyst, Ru(acac)3.148... 

The systems described above all involve peroxometal species as the active oxidant. In contrast, ruthenium catalysts involve a ruthenium-oxo complex as the active oxidant [1]. Until recently, no Ru-catalysts were known that were able to activate H202 rather then to decompose it. However in 2005 Beller and co-workers recognized the potential of the Ru(terpyridine)(2,6-pyridinedicarboxylate) catalyst [63] for the epoxidation of olefins with H202 [64]. The result is a very efficient method for the epoxidation of a wide range of alkyl substituted or allylic alkenes using as little as 0.5 mol% Ru. In Fig. 4.26 details are given. Terminal... 

The epoxidation of alkenes by 02 is catalyzed by polydentate iron and by cobalt porphyrin complexes the latter also operate in a perfluoroalkane/MeCN biphasic system.166 Ruthenium complexes, such as (22-XL), catalyze alkene epoxidations with bis(acetoxy)iodobenzene.167... 

Ruthenium complexes [RuCl2(PNNP)] containing tetradentate hybrid ligands 7.53 with P-and N-donors (PNNP) catalyze the asymmetric epoxidation of unfunctionalized alkenes... 

Where functional groups are present which are more readily oxidized than the ether group, multiple reactions can occur. For example, in their total synthesis of (-i-)-tutin and (-i-)-asteromurin A, Yamada et al. observed concomitant oxidation of a secondary alcohol function in the oxidation of the ether (30) with ruthenium tetroxide (equation 24). The same group successfully achieved the simultaneous oxidation of both ether functions of the intermediate (31) in their related stereocontrolled syntheses of (-)-picrotox-inin and (-i-)-coriomyrtin (equation 25). Treatment of karahana ether (32) with excess ruthenium tetroxide resulted in the formation of the ketonic lactone (33) via oxidation of both the methylene group adjacent to the ether function and the exocyclic alkenic group (equation 26). In contrast, ruthenium tetroxide oxidation of the steroidal tetral drofuran (34) gave as a major product the lactone (35) in which the alkenic bond had been epoxidized. A small amount of the 5,6-deoxylactone (17%) was also isolated (equation 27). This transformation formed the basis of a facile introduction of the ecdysone side chain into C-20 keto steroids. 

In terms of practicality, molecular oxygen is a very attractive terminal oxidant. In this arena, a novel 7V-2 -chlorophenyl-2-pyridinecarboxamide ruthenium complex 29 has been reported to catalyze the efficient epoxidation of cyclic alkenes in the presence of 1 atm of oxygen and isobutyraldehyde, which is believed to coordinate to the catalyst and prevent the formation of unwanted allylic oxidation products. Using this system, cyclooctene 30 is converted to the corresponding epoxide in excellent yield in 9 h at ambient temperature. Interestingly, the reaction is shut down by the addition of 2,6-di-/er/-butyl-4-methylphenol, so that a i ical mechanistic pathway has been postulated <03CC1058>. 

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