Aerobic ruthenium oxide catalyst

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). 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. Masutani et al. [107] described (nitrosyl)Ru(salen) complexes, which can be activated by illumination to release the NO ligand. These complexes demonstrated selectivity for oxidation of the alcoholic group versus epoxidation, which was regarded as evidence for the intermediacy of Ru-oxo moieties. Their excellent alcohol coordination properties led to a good enantiomer differentation in the aerobic oxidation of racemic secondary alcohols (Fig. 19) and to a selective oxidation of primary alcohols in the presence of secondary alcohols [108]. 

In 2000, Yamaguchi et al. [116] synthesized a ruthenium-based hydroxyapatite catalyst, with the formula (RuCl)10(PO4)6(OH)2. This catalyst could also be recycled and displayed a reasonable substrate scope in the aerobic alcohol oxidations (Eq. 30). TOFs reported in this case were generally somewhat lower, on the order of 1 h 1 for 2-octanol to 12 h 1 for benzyl alcohol. The fact that distinct Ru-Cl species are present at the surface points in the direction of a hydridometal mechanism. 

Groves, J. T. Quinn, R. Aerobic oxidation of olefins with ruthenium porphyrin catalysts. j. Am. Chem. Soc. 1985, 107, 5790-5792. 

In recent years there have been important steps towards the development of these ideal aerobic oxidations. Thus, Mizuno and coworkers reported a new generation of ruthenium oxide that acts as a heterogeneous catalyst for the oxidation of alcohols [95, 96]. The main drawbacks of this process were, however, the need for high oxgyen pressure and the fact that the catalyst is moisture sensitive. Both disadvantages constituted a serious limitation of the process. 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. For example, both RUO2 and 5% Ru-on-charcoal catalyze the aerobic oxidation of activated alcohols such... 

The nature of the solvent was found to play an important role for the catalytic activity and selectivity in the aerobic allylic oxidation of a-isophorone to KIP with phosphomolybdic acid (PMA). With 0.43 mol% PMA and potassium tert-butoxide as the additive in dimethyl sulfoxide (DMSO) at 115°C, KIP was obtained in 70% selectivity at 99% conversion.Using a ruthenium-porphyrin complex as the catalyst, the oxidation of a-isophorone with 2,6-dichloropyridine N-oxide in dichloromethane at 40°C afforded KIP at 99% selectivity and 75% con-... 

Nikaidou, F., Ushiyama, H., Yamaguchi, K., et al. (2010). Theoretical and Experimental Studies on Reaction Mechanism for Aerobic Alcohol Oxidation by Supported Ruthenium Hydroxide Catalysts, J. Phys. Chem. C, 114, pp. 10873-10880. 

The reaction of NaBH4 with RuCls dissolved in l-n-butyl-3-methylimidazolium hexafluorophosphate [BM1M][PF6] and l- -butyl-3-methylimidazolium tetrafluoroborate [BMIM] [BF4] as ionic liquid is a simple and reproducible method for the synthesis of stable ruthenium oxide nanoparticles with a narrow size distribution within 2-3 nm. These nanoparticles showed high catalytic activity either in the solventless or liquid-liquid biphasic hydrogenation of olefins and arenes under mild reaction conditions. Nanometric ruthenium oxide confined in a zeolitic framework was recently reported as an efficient catalyst for alcohol oxidation under mild aerobic conditions. The nanofjarticles could be reused in solventless conditions up to 10 times in the hydrogenation of 1-hexene yielding a total turnover number for exposed Ru(0) atoms of 175,000 (Rossi et al., 2004 Rossi et al., 2004). 

He J, Yamaguchi K, Mizuno N (2011) Aerobic oxidative transformation of primary azides to nitriles by ruthenium hydroxide catalyst J Org Chem 76 4606... 

The aerobic dehydrogenative annulation of 2-aryl-substituted pyrroles and indoles for a variety of alkynes, using the system ruthenium(Il) catalyst with oxidant Cu(0Ac)2.H20, was then reported. The reaction was now performed under ambient air as the ideal sacrificial oxidant, thus only 10 mol% of Cu (0Ac)2.H20 could be used for efficient transformations of indoles [(Eq. 89)] [178]. This method could also be applied to synthesize pyrrolo[2,l-a]isoquinolines from 2-arylpyrroles with dialkyl-, diaryl-, or alkylarylacetylenes with an excellent regioselectivity. The competition experiments showed that an electron-deficient alkyne favours this reaction and that the more acidic C-H bond activation is favoured [(Eq. 89)] [178]. 

He J, Yamaguchi K, Mizuno N (2011) Aerobic oxidative transformation of primary azides to nitriles by ruthenium hydroxide catalyst. J Org Chem 76(11) 4606-4610 Zhou W, Xu J, Zhang L, Jiao N (2010) An efficient transformation from benzyl or allyl halides to aryl and alkenyl nitriles. Org Lett 12(12) 2888-2891... 

FigureB Example of a ruthenium(lll) catalyst for the aerobic oxidative dehydrogenation of benzyl alcohols. ...

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

Our initial work on the TEMPO / Mg(N03)2 / NBS system was inspired by the work reported by Yamaguchi and Mizuno (20) on the aerobic oxidation of the alcohols over aluminum supported ruthenium catalyst and by our own work on a highly efficient TEMP0-[Fe(N03)2/ bipyridine] / KBr system, reported earlier (22). On the basis of these two systems, we reasoned that a supported ruthenium catalyst combined with either TEMPO alone or promoted by some less elaborate nitrate and bromide source would produce a more powerful and partially recyclable catalyst composition. The initial screening was done using hexan-l-ol as a model substrate with MeO-TEMPO as a catalyst (T.lmol %) and 5%Ru/C as a co-catalyst (0.3 mol% Ru) in acetic acid solvent. As shown in Table 1, the binary composition under the standard test conditions did not show any activity (entry 1). When either N-bromosuccinimide (NBS) or Mg(N03)2 (MNT) was added, a moderate increase in the rate of oxidation was seen especially with the addition of MNT (entries 2 and 3). 

Chiral (nitrosyl)ruthenium(salen) complexes have been found to be efficient catalysts for aerobic oxidative desymmetrization of mc.vo-diols under photoirradiation to give optically active lactols. With the suitable catalysts, high enantioselectivities up to 93% has been achieved. The kinetics of the oxidation depend on the nature of the ligand. On the basis of kinetic parameters and the kinetic isotope effect, it has been suggested that when a ligand with an apical hydroxy group is used, the hydrogen atom... 

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by /1-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of 02 into the M-H bond with formation of H202. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or /1-hydride elimination. 

Few examples involve the use of dioxygen alone as the primary oxidant. The use of a Ru(III) ethylenediaminetetraacetate complex has been described [28] but this almost certainly involves a free-radical autoxidation pathway and offers no advantages. Following the initial report by Neumann et al. [29] on the use of [WZnRu2(0H)(H20)(ZnW9034)2]11 attention has been focused on the use of ruthenium-containing polyoxometalates (POMs) as catalysts for the aerobic... 

Ruthenium compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

Fig. 14 Ruthenium catalyst in combination with a hydrogen acceptor for aerobic oxidation...

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