Ruthenium complex catalysts shift reaction

In 1977 Ford and co-workers showed that Ru3(CO)12 in the presence of a ca. fiftyfold excess of KOH catalyzes the shift reaction at 100°C/1 bar CO (79). The effectiveness of the system increased markedly as temperature was increased (rate of hydrogen formation approximately quadrupled on raising the temperature from 100° to 110°C), and over a 30-day period catalyst turnovers of 150 and 3 were found for Ru3(CO)12 and KOH, respectively. Neither methane nor methanol was detected in the reaction products. Although the nature of the active ruthenium species could not be unambiguously established, infrared data indicated that it is not Ru3(CO)12, and the complexity of the infrared spectrum in the... 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

Ford and co-workers have also recently developed a homogeneous catalyst system for the water-gas shift reaction (95). Their system consists of ruthenium carbonyl, Ru3(CO)12, in an ethoxyethanol solvent containing KOH and H20 under a CD atmosphere. Experiments have been conducted from 100-120°C. The identity of the H2 and CD2 products has been confirmed, and catalysis by both metal complex and base has been verified since the total amount of H2 and COz produced exceeds the initial amounts of both ruthenium carbonyl and KOH. The authors point out that catalysis by base in this system depends on the instability of KHC03 in ethoxyethanol solution under the reaction conditions (95). Normally the hydroxide is consumed stoichiometrically to produce carbonate, and this represents a major reason why a water-gas shift catalyst system has not been developed previously under basic conditions. As has been noted above, coordinated carbonyl does not have to be greatly activated in order for it to undergo attack by the strongly nucleophilic hydroxide ion. Because of the instability of KHC03... 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Trisubstituted furans were obtained via an unprecedented 1,4-shift of the sulfanyl group of allenyl sulfides in high yields employing ruthenium complexes as catalyst, as depicted below. Furan products can also be provided in a one-pot reaction from a-diazocarbonyls and propargyl sulfide using both rhodium- and ruthenium-complexes or only a ruthenium-complex as catalyst <07AGE1905>. 

A very pronounced synergistic effect is found for binary ruthenium-iron carbonyl catalysts in the water-gas shift reaction. Both mixed ruthenium-iron clusters and mixtures of ruthenium clusters with iron complexes are considerably more active in basic solutions. Whereas the water-gas shift activity (moles of H2 per mole of complex per day) of alkaline aqueous ethoxyethanol solutions of Ru3(CO)12 and Fe(CO)j is... 

Ru(CO)5 is less frequently used than Fe(CO)5 for organic synthesis or as a starting material as a zero-valent ruthenium complex because of its ease of decomposition to Ru4(CO),2 [99]. Dodecacarbonyltriruthenium is very useful for these purposes. It has been showm to be an active catalyst for the hydrogenation of olefins [100], carbonyla-tion of ethylene [101], hydroformylation of alkenes [102], water-gas shift reaction [103], and reduction of nitro groups [104], and recently, C—H bond activation [105] and coupling of diynes with CO [106]. 

We also have studied other metal carbonyl complexes in alkaline ethoxyethanol to survey the generality of the shift-reaction catalysis. Under conditions (0.9 atm CO, I00°C) comparable with those used for the ruthenium catalyst described above, iron, rhodium, osmium, and iridium carbonyls all proved active but rhenium carbonyl did not. For systems starting with the listed complexes, the normalized catalytic activities see Table I normalized activity is based on the number of... 

Bearing in mind that iridium complexes are particularly active for the hydrogenation of imines, dual catalysts consisting of rhodium and iridium complexes [RhCl(COD)]2/[IrCl(COD)]2 have been suggested [39, 40]. Also the combined use of Rh(acac)(CO)2/Ru3(CO)j2 in AfAf-dimethylformamide (DMF) proved to be advantageous [41]. If the water gas shift reaction or its reversed version is utilized as sources for CO or H2, ruthenium catalysts give superior results [42-44]. HAM can be speeded up by irradiation with microwaves [45]. 

In general, cerium(in) cannot be oxidized to cerium(IV) by molecular oxygen. An exception is the oxidation of cerium j8-diketonate complexes by O2 (Christoffers and Werner, 2002 Christoffers et al., 2003a, 2003b Rossle et al., 2005). Complex formation shifts the redox potential of the Ce +ZCe " " couple to less positive values. As discussed in section 10.2, aqueous solutions of cerium(IV) are metastable with respect to oxidation of water to oxygen gas. Under normal conditions this reaction will not occur due to the presence of a high kinetic barrier, and acidic solutions of cerium(IV) can be stored for quite a long time. However, cerium(IV) ions can decompose water in presence of a catalyst like platinum or ruthenium(IV) oxide. 

The Water-gas Shift Reaction.—This reaction is catalysed by M(CO) (activity M = W>Mo>Cr) in the presence of base and under phase-transfer conditions these carbonyls, in common with MS(CO)i2 (M =Ru or Os), are also active in the presence of sodium sulphide. The most active catalysts reported are Fe(CO)6 in basic methanol (turnover No. 2000 per day at 180 °C ) and Rh6(CO)i6 with diamine co-catalysts (e.g., en, turnover No. a 25 h at 100 C). Photolysis of [RuCl(CO)(bipy)a]Cl in water under CO produces COa and catalytically the CO2 is produced in a thermal step, whereas the formation of Ha is photo-initiated. Water-gas has also been used to hydroformylate pent-1-ene in the presence of ruthenium complexes similarly, water-gas is used in reaction (9), which is catalysed by a variety of Group VIII metal complexes... 

Ir, Os, Fe, Cu, Mo, Pd, Re, and Rh in water-shift reaction was explored by dissolving them into the silica gel-supported IL [BMMIm][OTf] phase [106]. Finally, several ruthenium complexes with specific structures, that is, [PhNMejllRufCOljClj], [Ru(phen)(CO)3][Ru(CO)3Cl3], and [MMIm][Ru(CO)3Cl2l], were found to be active at 120 °C at ambient pressure. Noteworthy, this catalyst was very stable and without any deactivation after 20 h. Figure 2.37. Furthermore, although it cannot be explained well currently, only ruthenium complexes showed reasonable stability over 24 h time-on-stream. 

A very powerful cascade reaction had been developed by Cho and Lee in their approach to the total synthesis of (3I ,9i ,10/ )-Panaxytriol 179 (Scheme 7.37) [81], which was isolated from Panax ginseng in 1983 [82]. The cascade sequence was initiated by relay metathesis, which is then followed by metallotropic [l,3]-shift and cross-metathesis. This approach has become an efficient way for the synthesis of natural products with highly unsaturated carbon skeletons. Treatment of 174 with Grubbs second-generation catalyst in CH Clj at 40 °C in the presence of 2.0 equiv of alkene 175 generated the expected prodnet 178 in 61% yield as a mixture of Z E-isomers. Surprisingly, ruthenium alkylidene 176 was isolated in 10% yield and could be converted to 178 upon treatment with 175. This confirms that complex 176 is a catalytically viable intermediate in the catalytic cycle. 

Curves for the dependence of the potential shift on the composition of the catalyst are also complex in nature and pass through maxi ma and a minimum. Increase in the ruthenium content of the catalysi changes the reaction mechanism, which is consistent with potentiome trie data, where the variations in activity and potential shift are opposite in form. 

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