Oxidation with Low-Valent Ruthenium Catalysts and Oxidants

Unactivated alkanes can also be oxidized with the RuCl3/NaI04 system. Tertiary carbon-hydrogen bonds undergo chemoselective hydroxylation to afford the corresponding tertiary alcohols (Eq. (7.17)) [31]. 

Oxidation with Low-Valent Ruthenium Catalysts and Oxidants 

The treatment of a low-valent ruthenium catalysts with an oxidant generates middle-valent Ru = O species, which often show a different reactivity from that of the RUO4 oxidation. 

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Oxidation with Low-Valent Ruthenium Catalysts and Oxidants 245 RUO2 (cat.)... 

Oxidation with Low-valent Ruthenium Catalysts and Oxidants... 

When the oxidation of alkenes was performed with low-valent ruthenium (generated in situ from peracetic acid) in an aqueous medium, the formation of a-ketols was observed. For this interesting transformation, for which ruthenium was found to be the best catalyst, the reader is referred to the reviews of Naota et al. [5] and Murahashi and Komiya [6]. 

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... 

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... 

Subsequently, in 1999 the same group showed that the activity of the ruthenium hydrotalcite was significantly enhanced by the introduction of cobalt(II), in addition to ruthenium(III),in the Brucite layer [115]. For example, cinnamyl alcohol underwent complete conversion in 40 min in toluene at 60 °C, in the presence of ruthenium/cobalt hydrotalcite, compared with 31% conversion under the same conditions with ruthenium hydrotalcite. A secondary aliphatic alcohol, 2-octanol, was smoothly converted into the corresponding ketone but primary aliphatic alcohols, for example, 1-octanol, exhibited extremely low activity. The authors suggested that the introduction of cobalt induced the formation of higher oxidation states of ruthenium, for example, Ru(IV) to Ru( VI), leading to a more active oxidation catalyst. However, on the basis of the reported results it is not possible to rule out low-valent ruthenium species as the active catalyst in a hydridometal pathway. The results obtained in the oxidation of representative alcohols with ruthenium hydrotalcite and ruthe-nium-cobalt-hydrotalcite are compared in Table 5. 

In some cases, notably ruthenium, the aerobic oxidation of alcohols is catalyzed by both low- and high-valent forms of the metal (see later). In the former case the reaction involves (see Scheme 4.5) the formation of a hydridometal species (or its equivalent) while the latter involves an oxometal intermediate (see Scheme 4.6), which is regenerated by reaction of the reduced form of the catalyst with dioxygen instead of a peroxide. It is difficult to distinguish between the two and one should bear in mind, therefore, that aerobic oxidations with high-valent oxometal catalysts could involve the formation of low-valent species, even the (colloidal) metal, as the actual catalyst... 

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. 

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