High-Valent Complexes of Ruthenium

High-Valent Complexes of Ruthenium and Osmium Chi-Ming Che and Vivian Wing-Wah Yam... 

There are few well-characterized high-valent peroxo complexes of ruthenium and osmium, presumably because they decompose readily to give oxo 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... 

One example which deserves special mention is the use of a percarboxylic acid such as peracetic acid, generated in situ by autoxidation of the corresponding aldehyde, developed by Murahashi and coworkers, see Eq. (1) [25-27]. These reactions are generally considered to involve high-valent oxoruthenium complexes, generated by reaction of the percarboxylic acid with the ruthenium catalyst, as the active oxidant. 

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]. 

The unique versatility of ruthenium as an oxidation catalyst continues to provide a stimulus for research on a variety of oxidative transformations. Its juxtaposition in the periodic table and close similarity to the biological redox elements, iron and manganese, coupled with the accessibility of various high-valent oxo species by reaction of lower-valent complexes with dioxygen make ruthenium an ideal candidate for suprabiotic catalysis. 

An important factor in the success of these reactions involves chelation-assistance by a heteroatom. Thus, the coordination of the heteroatom to the metal, brings the metal closer to the C-H bond and stabilizes the thermally unstable C-M-H species formed by the oxidative addition of a C-H bond to a low-valent transition metal complex. In addition, the use of the chelation-assistance leads to a high regioselectivity, which is an essential factor in organic synthesis. For reactions, a number of transition metal complexes - including ruthenium, rhodium, and iridium - are used as a catalyst, and ruthenium-catalyzed reactions will be described in this chapter [5]. 

A high-valent ruthenium complex is also reported to cleave the sp C-H bond. RuCl3 -3H20 catalyzes the transformation of cyclic alkanes to the corresponding ketones in the presence of peracetic acid, where oxoruthenium species is considered to act as the active species. Alcohol, as a primary product in this oxidation reaction, is obtained as an intermediate in the presence of trifluoroacetic acid (Scheme 14.11) [25]. 

Other species. In a study by the Che group, it proved possible to prepare a composite complex that spans both categories. Thus, oxidation (either chemical or electrochemical) of the double-helical [Ru2L2(H20)2] (L = quinquepyridine) cation in aqueous solution results in a product containing both a photoactive ruthenium centre and a high-valent Ru=0 centre in the one complex. 

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