Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 136

treating an aqueous solution of and
with a solution of NaOH and followed by heating at 60°C for 18 h 50 .
The resulting slurry was cooled to room temperature, filtered, washed with
water and dried at 110°C for 12 h. The resulting ruthenium-hydrotalcite
showed the highest activity amongst a series of hydrotalcites exchanged with
e.g. Fe, Ni, Mn, V and Cr.
Subsequently, 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 51 . For example,
cinnamyl alcohol underwent complete conversion in 40 min. in toluene at
60°C, in the presence of Ru/Co-HT, compared with 31% conversion under
the same conditions with Ru-HT. A secondary aliphatic alcohol, 2-octanol,
was smoothly converted into the corresponding ketone but primary aliphatic
alcohols, e.g . 1-octanol, exhibited extremely low activity. The authors
suggested that the introduction of cobalt induced the formation of higher
oxidation states of ruthenium, e.g. 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
Ru-HT and Ru-Co-HT are compared in Table 4.
Ruthenium pyrochlore oxides, mixed oxides of ruthenium and bismuth
or lead with the general formula (A = Pb or Bi; 0<x< 1 ;0<y
< 0.5) catalyze the oxidative cleavage of vic-diols to the corresponding
(di)carboxylic acids with NaOCl 52 or dioxygen 53 , 54 as the stoichiometric
oxidant. In the latter case two equivalents of sodium hydroxide are required
to neutralize the carboxylic acid product, otherwise the catalyst is
deactivated. For example, cyclohexane-1,2-diol afforded sodium adipate in
70% yield (Reaction 12).