Ruthenium(VIII) Oxide1

RuO4

[20427-56-9]  · O4Ru  · Ruthenium(VIII) Oxide  · (MW 165.07)

(strong oxidant for many functional groups; can cleave double bonds, aromatic rings, and diols1)

Alternate Name: ruthenium tetroxide.

Physical Data: yellow form: mp 25.5 °C; bp 40 °C; d 3.29 g cm-3. Brownish orange form: mp 27 °C; bp 108 °C (dec).

Solubility: slightly sol water; highly sol CHCl3, CCl4.

Form Supplied in: although the reagent is commercially available either in solid form or stabilized aqueous solution, it is usually prepared in situ from black solid RuO2 (mw 133.07) [120236-10-1;.xH2O, 32740-79-7] or dark brown (or black) RuCl3 (mw 207.42) [10049-08-8;.xH2O, 14898-67-0], in stoichiometric or catalytic amounts, and an oxidation agent; both of the above Ru salts are widely available.

Handling, Storage, and Precautions: handle in a fume hood only. Inhalation should be avoided; vapors irritating to eyes and respiratory tracts, since it readily oxidizes tissue, leaving a deposit of ruthenium dioxide.2 It attacks rubber and reacts explosively with paper filter and alcohol, and violently with ether, benzene, and pyridine.3 However, the use of the catalytic system greatly minimizes the risk in its manipulation, and its usage is strongly recommended.

Introduction.

When RuO4 was introduced into organic synthesis it was generally used in stoichiometric amounts, usually prepared by oxidation of Ruthenium(III) Chloride or RuO2 with aqueous periodate or hypochlorite and then extracted into carbon tetrachloride. This yellow solution could be roughly analyzed by treating an aliquot with ethanol to reduce the tetroxide to the black dioxide, which was collected and weighed.4

However, since ruthenium compounds are expensive and occasionally it is difficult to separate the products from precipitated ruthenium dioxide, it is more convenient to use a system formed by a catalytic amount of the ruthenium compound (RuO2 or RuCl3) along with an appropriate co-oxidant, usually in a biphasic solvent system. These reagents actually act as catalysts, because they are reoxidized after the reaction with organic compounds by one of the oxidants previously mentioned. Ruthenium tetroxide is usually prepared in situ from ruthenium dioxide or ruthenium trichloride by oxidation with Sodium Hypochlorite,5 Sodium Bromate,6 Peracetic Acid,6 periodic acid,7 Sodium Periodate,8 Oxygen,9 cerium sulfate,10 Potassium Permanganate,10 electrochemically generated Chlorine,11 or Potassium Monoperoxysulfate (Oxone®).12

It appears that contact between RuO4 and the material to be oxidized takes place in the organic phase, where they are both most soluble. The ruthenium dioxide produced when oxidation occurs is insoluble in all solvents and migrates to the interface where it contacts the co-oxidant (in the aqueous layer) and is reoxidized. Thus best results are obtained when the mixture is shaken or stirred vigorously throughout the course of the reaction to achieve good contact between all components.

It has been pointed out that RuCl3.H2O is not a good Ru source under acidic conditions (pH < 5), because it initially gives an orange RuIV chloro aquo complex, which is slowly oxidized to RuO4. In such cases, RuO2 is the alternative recommended.10 In the reactions carried out at pH > 9, any RuO4 produced in the aqueous phase is unstable, being reduced to green perruthenate (RuO4-), and subsequently to orange ruthenate (RuO42-). Both species are insoluble in CCl4. Although in most of the cited mixtures RuO4 is considered the oxidant, it cannot be ruled out that the real oxidant is another lower valent ruthenium species. The only way to ensure that RuO4 is really the oxidizing agent is if it is isolated after its preparation, as cited in the early literature.2,4 UV analysis of the RuO4 solutions can provide some information about this, because RuO4 gives absorption bands at lmax 310 nm (strong) and 380 nm. These bands are replaced by others at lmax 310 nm (strong), 385 nm (strong), and 460 nm when base is added to the solution, corresponding to the formation of the perruthenate ion (RuO4-) and subsequently to absorption bands at lmax 385 nm and 460 nm (strong), produced by RuO42-.1,13

RuO4 is a strong oxidant. However, conditions for ruthenium-catalyzed reactions are very mild; usually a few hours (or less) at room temperature (or below) is sufficient. A thorough study of oxidations with RuO4 generated in situ from RuO2.xH2O and RuCl3.xH2O shows the importance of the presence of water in the reaction.10,14 Thus many ruthenium-catalyzed reactions have been performed in the CCl4-H2O solvent system. The addition of Acetonitrile to the system greatly improves yields and reaction times,8 especially when carboxylic groups are present or generated in the reaction. MeCN probably disrupts the insoluble carboxylate complexes and returns the ruthenium to the catalytic cycle, acting as a good ligand for the lower valent (III/II) ruthenium present.15

An important feature of RuO4-catalyzed oxidations is that the stereochemistry of the stereocenters close to the reaction site (eqs 1 and 2)16,17 remains unaffected.

In a typical procedure,8 to a stirred mixture of 2 mL of CCl4-2 mL of MeCN-3 mL of H2O/mmol of organic compound are added 4.1 mmol of Sodium Periodate/mmol of organic compound and 2.2 mmol% of RuCl3.xH2O (RuO2.xH2O is equally effective) sequentially. The mixture is stirred vigorously at 0-25 °C until the end of the reaction (TLC or GC monitoring). Then 20 mL of diethyl ether are added and the vigorous stirring is continued for 10 min to precipitate black RuO2. The reaction mixture is then dried (MgSO4) and filtered through qualitative Whatman filter paper 2. The solid residue is then washed with diethyl ether (3 × 5 mL). The combined organic phases are concentrated to yield the crude oxidation product.

Functional Group Oxidations without Bond Cleavage.

One of the most common synthetic uses of ruthenium-catalyzed oxidations is the reaction with alcohols. A mixture of RuO2 or RuCl3 with strong co-oxidants converts primary alcohols to carboxylic acids18 (eq 3),19 including epoxy alcohols (eq 4),8 but under milder (Iodosylbenzene,20 molecular oxygen,21 Calcium Hypochlorite (eq 5),22 or amine N-oxides23) or controlled conditions,19 aldehydes are obtained (eq 6).24

Secondary alcohols are transformed into ketones.1,3,25 The yields obtained from the oxidation of secondary alcohols are usually excellent. Since the reactions are carried out under very mild conditions, there is little danger of the product undergoing secondary reactions. Ketones can also be prepared using a great variety of other oxidants26 (see, for example Chromium(VI) Oxide and Dimethyl Sulfoxide based oxidant reagents), which in some cases are more readily available and less expensive. However, RuO4 is recommended for reactions which require a vigorous oxidant under mild conditions. Thus it can be used to oxidize alcohols which are resistant to other oxidants: (1) is successfully oxidized with RuO4, while attempts made with 15 other standard oxidizing procedures failed (eq 7);27 oxidation of (2) is successful in good yield using RuO4 (eq 8), being fruitless with CrO3 in either Pyridine, Acetone, Acetic Acid, or Al(O-i-Pr)3, KMnO4, Lead(IV) Acetate in acetone, with CrO3 in i-BuOH giving the ketone in very low yield.28 Numerous examples of the advantages of RuO4 over Dipyridine Chromium(VI) Oxide for the oxidation of carbohydrates have been cited.29

RuO4 is also reported to provide higher yields when other oxidizing procedures give poor yields. As examples can be cited the conversions of cyclobutanols to cyclobutanones (eq 9)30 and the transformation of lactones into the corresponding ketocarboxylates under basic conditions (60-97%) (eq 10). Significantly higher yields are obtained compared with KMnO4 under alkaline conditions.31

Vicinal diols can be oxidized to diketones but only in low yields, the principal reaction being the oxidative cleavage of the C-C bond.1 If the hydroxyl groups are not adjacent, then diketones can be prepared (eq 11).32

The catalytic procedure is also applicable to the oxidation of aldehydes to carboxylic acids,1,2,23,33 primary alkyl iodides to carboxylic acids (eq 12),34 aromatic hydrocarbons to quinones,25 and sulfides to sulfones,25 including an improved and simple method to obtain water-soluble sulfones using periodic acid as the co-oxidant in high concentration conditions (eq 13).35

Oxidation of 1,2-cyclic sulfites to 1,2-cyclic sulfates with RuO4 has been reported as a part of a method to activate diols for further nucleophilic attack (eq 14).36

Along with these types of oxidations, RuO4 is used to carry out transformations that involve oxidation of methylene groups a to heteroatoms such as oxygen and nitrogen. Thus it is possible to convert acyclic ethers into esters, such as methyl ethers into methyl esters (eq 15),8 ethyl ethers into acetates,37 and benzyl ethers into benzoates (eq 16).38 It is possible to avoid the benzyl-benzoyl group transformation by carrying out the reaction at 0 °C and/or in the presence of base (eq 17).39

Cyclic ethers are also oxidized, yielding lactones (eq 18).40 Although secondary positions are usually more reactive towards oxidation than tertiary positions, the regioselectivity of RuO4 can be strongly dependent on steric factors (eq 19).41 In those cases in which lactones are unstable under aqueous conditions (notably d- and ε-lactones), the corresponding diacids are the final products, presumably via the intermediacy of lactols (eq 20).42

RuO4 also oxidizes alkyl amines to mixtures of nitriles and amides,43 cyclic amines to lactams,44 and amides (cyclic or acyclic) to imides (eq 21),45 including an improved procedure that uses ethyl acetate as the organic solvent in the biphasic solvent system, enhancing both the solubility of the substrates and the rate of reaction.46

RuO4 usually reacts with unsaturated systems, giving cleavage of the C-C bonds. Although epoxide formation has been detected in small amounts (ca. 1%),47 it can be the principal reaction when the double bond is located in a very hindered position (eq 22).48 With 1,5-dienes, unexpected oxidation results are obtained. Thus oxidation of geranyl acetate leads to a tetrahydrofuran mixture, instead of the cleavage products (eq 23).8 Nonterminal alkynes are also oxidized without cleavage, yielding vicinal diketones (eq 24).49

RuO4 is also capable of oxidizing C-H bonds in bridged bicyclic and tricyclic alkanes to alcohols by insertion of oxygen (eq 25).50 Although epoxides survive RuO4 oxidations, when such functionality is located in this kind of bridged system a tandem ruthenium-catalyzed rearrangement/oxidation occurs (eq 26).51

Most of the common protecting groups used in organic synthesis are stable under RuO4 oxidation conditions (eq 27).52 Generally it is only necessary to carry out the reaction at 0 °C or perform it under buffered conditions when acid-sensitive groups are present, such as tetrahydropyranyl (eq 28)33 or silyl ethers (eq 29).53

Functional Group Oxidations with Bond Cleavage.

Carbon-carbon double bonds are readily cleaved by RuO4 to give ketones and aldehydes or carboxylic acids. In this respect the greater vigor of RuO4 as an oxidant stands in marked contrast to that of Osmium Tetroxide (eq 30),54 which also reacts with C-C double bonds but does not cleave them. While carboxylic acids are usually the final products, sometimes under neutral conditions aldehydes can be obtained from double bonds that are not fully substituted.55 The cleavage of such double bonds proceeds by the route: alkene -> dialdehyde -> diacid.5a RuO4 is also indicated to carry out oxidations of substrates with double bonds resistant to other oxidizing agents, such as OsO4, Potassium Permanganate, and Ozone (eq 31).56 Degradative oxidations of unsaturated C-C bonds with loss of carbon atoms occur with terminal alkynes (eq 32),5b cyclic allylic alcohols (eq 33),57 and a,b-unsaturated ketones.57

RuO4 also cleaves a-chloroenol derivatives obtained from a,a-dichlorocyclobutanones to give dicarboxylic acids through successive treatment with n-Butyllithium, Acetic Anhydride, and Sodium Periodate-RuO2 (eq 34).58,59

RuO4-catalyzed oxidation of arenes can proceed in two ways: (a) the phenyl ring can be cleaved from R-Ph to R-CO2H (eq 35);8 (b) the phenyl ring can be degraded to form a dicarboxylic acid in polycyclic aromatic hydrocarbons (eq 36).60 An electron-donating substituent favors cleavage of the substituted ring, while an electron-withdrawing substituent favors cleavage of the unsubstituted ring. Thus selective oxidation of the more activated ring can be performed with high selectivity. When acid-sensitive groups are not present, an improved procedure that utilizes periodic acid instead of sodium periodate can be used, preventing the problems associated with the precipitated sodium iodate, allowing the reaction to go to completion, and permitting oxidation reactions to be run on larger scales (eq 37).61

Furan62 (eq 38),63 thiophene (eq 39),64 and benzopyridine rings (eq 40)65 are also cleaved by catalytic RuO4 to carboxylic acids. When pyridine derivatives are not oxidized, they can be transformed into their N-oxides prior to the oxidation to decrease the ability of the nitrogen to complex with ruthenium, albeit with low yields (eq 41).64

Vicinal diols are cleaved to give carboxylic acids (eq 42) following the route: glycol -> a-ketol -> diacid. A diketone is apparently not an intermediate in this oxidation.5a The mildness of the reaction conditions is underscored by the lack of epimerization shown in eq 43.8 This feature has been proved to be general when RuO2-NaIO4 is used to oxidize chiral diol benzoates, this being a useful method to synthesize chiral a-benzoylcarboxylic acids (eq 44).66

Other vicinal dioxygenated functionalities present (eq 45)5a or generated in situ undergo oxidation with C-C bond cleavage by RuO4 to give carboxylic acids, with (eq 46)67 or without (eq 47)68 loss of carbon atoms.

RuO4 is also used to cleave oxidatively carbon-boron bonds in cyclic alkylboranes, presenting advantages over the usage of CrVI for the same purpose or even the oxidation of the corresponding alcohols (eq 48).69

Another formal carbon-heteroatom bond cleavage occurs in the oxidation of cyclic ethers that give cyclic products unstable to the oxidation conditions (eq 49).42

As pointed out, ketones are stable under RuO4 oxidation conditions, although cyclic ketones can undergo Baeyer-Villiger reaction when Sodium Hypochlorite is used as co-oxidant (eq 50).70

Other Ruthenium-Based Oxidation Reagents.

Less reactive oxidants are obtained by lowering the oxidation state of ruthenium. One example is the ruthenate ion (RuO42-) which, as mentioned above, is formed when RuO4 is treated with alkaline solutions. The most important synthetic applications of such ions is in the oxidation of alcohols in basic media to give carboxylic acids or ketones.71 In general, RuO42- does not appear to oxidize isolated C-C double bonds at room temperature (eq 51).72

When there is no reductive pathway for the elimination of ruthenate esters, RuO42- has been used as an alternative to RuO4 (eq 52).40

The perruthenate ion RuO4- is also useful for the oxidation of primary alcohols, nitroalkanes, primary halides, and aldehydes to acids.73 When tetraalkylammonium salts are added to the RuO4- solutions, stable tetraalkylammonium perruthenates are obtained. Tetra-n-butylammonium perruthenate (TBAP) and tetra-n-propylammonium perruthenate (TPAP) are used to oxidize successfully alcohols to carbonyl compounds (eq 53),23 and sulfides to sulfones74 under very mild conditions, employing as co-oxidant N-Methylmorpholine N-Oxide (NMO) (see Tetra-n-propylammonium Perruthenate).

The behavior of inorganic transition metal oxidizing agents can be modified by the introduction of ligands. Electron-rich ligands, which increase the basicity of the metal and moderate its oxidizing power, have been used to improve the selectivity of these oxidation reactions. Thus porphyryl-75 and bipyridyl-Ru complexes epoxidize alkenes, instead of cleaving the double bond (eq 54).47

Several other Ru complexes, along with co-oxidants or hydrogen acceptors, are used as catalysts in oxidation reactions, RuCl2(PPh3)3 and RuH2(PPh3)4 being the most commonly utilized. The conversion of alkanes and alcohols to aldehydes or ketones is achieved with RuCl2(PPh3)376 and molecular oxygen,77 Bis(trimethylsilyl) Peroxide,78 Iodosylbenzene,20 N-Methylmorpholine N-Oxide (eq 55),79 t-Butyl Hydroperoxide,80 and hydrogen acceptors.81 Selective oxidations of primary vs. secondary alcohols are possible (eq 56),78 and it is also possible to stop the oxidation of primary alcohols at the aldehyde stage by simply controlling the co-oxidant equivalents and reaction times.20 a-Diketones can be obtained from vicinal diols (eq 57)82 and nonterminal alkynes (eq 58).83

The combination of RuH2(PPh3)4 plus a hydrogen acceptor, like benzalacetone or Acetone, converts unsymmetrically substituted 1,4- and 1,5-diols into b-substituted g-lactones and g-substituted d-lactones, respectively (eqs 59 and 60),84 also allowing the oxidative condensation of alcohols, or aldehydes and alcohols, to give esters (eq 61).85

Oxoruthenium species are also useful in organic oxidations. Thus oxoruthenium(V) complexes obtained from lower valent ruthenium species effect a-oxygenation of tertiary amines86 and b-lactams (eq 62).87 [PPh4][RuO2(OAc)Cl2].2AcOH generated from RuO4 is used to oxidize alcohols and benzyl halides to carbonyl compounds.88


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Victor S. Martín, José M. Palazón & Carmen M. Rodríguez

University of La Laguna, Tenerife, Spain



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