Low-valent ruthenium complexes

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

Scheme 8 Synthesis of ruthenium polysulfido complexes from low-valent ruthenium complexes...

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

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. 4.59 Low-valent ruthenium complexes in combination with benzoquinone.

Selective oxidative demethylation of tertiary methyl amines is one of the specific and important functions of cytochrome P-450. Novel cytochrome P-450-type oxidation behavior with tertiary amines has been found in the catalytic systems of low-valent ruthenium complexes with peroxides. These systems exhibit specific reactivity toward oxidations of nitrogen compounds such as amines and amides, differing from that with RUO4. It was discovered in 1988 that low-valent ruthenium complex-catalyzed oxidation of tertiary methylamines 53 with f-BuOOH gives the corresponding a-(f-butyldioxy)alkylamines 54 efficiently (Eq. 3.70) [130]. The hemiaminal type 54 product has a similar structure to a-hydroxymethylamine intermediate derived from the oxidation with cytochrome P-450. 

As shown in Scheme 3.8, the catalytic oxidation reactions can be rationalized by assuming the formation of oxo-ruthenium species by the reaction of low-valent ruthenium complexes with peroxides. The C-H activation at the a-position of amines and the subsequent electron transfer gives iminium ion ruthenium complex 55. Trapping 55 with f-BuOOH would afford the corresponding a-ferf-butylhydroxy-amines, water, and low-valent ruthenium complex to complete the catalytic cyde. 

Nonporphyrin-based oxo-metal species can be generated by the reaction of a low-valent ruthenium complex with molecular oxygen in the presence of an aldehyde [141]. Thus, the ruthenium-catalyzed oxidation of alkanes with molecular oxygen in the presence of acetaldehyde gives alcohols and ketones efficiently [155]. These aerobic oxidations can be rationalized by assuming the sequence shown in Scheme 3.10. 

For a review on catalysis using low valent ruthenium complexes, see S.-I. Murahashi and T. Naota, Bull. Chem. Soc. Jpn. 69,1805 (1996). 

Ruthenium complexes also add dioxygen, usually with concurrent expulsion of a neutral ligand. For example, a series of dioxygen complexes having the formula, [RuX(NOXPh3P)2(02)l where X = OH, Cl, Br, I, NCS, NCO, N3 or CN have been prepared by reactions of low valent ruthenium complexes with molecular oxygen. 

The conversion of alkanes into more useful products is one of the most important practical problems in chemistry. The insertion of metals into C-H bonds was first discovered by Chatt and Davidson in 1965 [101] for low-valent ruthenium complexes. Following the discovery of the photochemically induced insertion of a transition metal into alkane C-H bonds [102, 103], a large number of organo-metallic complexes have been shown to activate C-H bonds in alkanes. A typical example is the photochemical oxidative addition of alkanes to complexes such as Rh(Cp)(CO)2- The reaction occurs via an initial loss of CO followed by a-... 

Murahashi SI, Naota T (1996) A new way for efficient catalysis by using low valent ruthenium complexes as redox Lewis acid and base catalysts. Bull Chem Soc Jpn 69 1805... 

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