Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 135

The oxoruthenium(VI) complex was prepared by exposing a benzene
solution of to air at 20°C. Addition of isopropanol
to the resulting solution, in the absence of air, afforded the
dialkoxyruthenium(IV) complex, in quantitative yield, within 24 hours. In
the presence of air, benzene solutions of the dioxoruthenium(VI) or the
dialkoxyruthenium(IV) complex effected catalytic oxidation of isopropanol
at room temperature, albeit with a modest rate (1.5 catalytic turnovers per
day). Interestingly, with the dialkoxyruthenium(IV) complex, catalytic
oxidation was observed with air but not with dry oxygen, suggesting that
hydrolysis to an oxoruthenium(IV) complex is necessary for a catalytic
cycle.
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 and
5% Ru-on-charcoal catalyze the aerobic oxidation of activated alcohols such
as allylic alcohols 47 and

e.g. Reaction 11.

More recently, Kagan and coworkers 49 have described the use of
ruthenium supported on ceria, as a catalyst for the aerobic oxidation of
alcohols. Primary and secondary alcohols are oxidized to the corresponding
aldehydes (carboxylic acids) and ketones, respectively, at elevated
temperatures (>140°C). Surprisingly, allylic alcohols, such as geraniol, and
some cyclic alcohols, e.g. menthol, are unreactive. The former result
suggests that low-valent ruthenium species are possibly involved and that
coordination of ruthenium to the double bond inhibits alcohol oxidation.
Ruthenium-exchanged hydrotalcites were shown, by Kaneda and
coworkers 50 , to be heterogeneous catalysts for the aerobic oxidation of
reactive allylic and benzylic alcohols. Hydrotalcites are layered anionic clays
consisting of a cationic Brucite layer with anions (hydroxide or carbonate)
situated in the interlayer region. Various cations can be introduced in the
Brucite

layer by ion exchange. For example, ruthenium-exchanged

hydrotalcite with the formula, was prepared by