Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 134

and is consistent with a heterolytic, two-electron oxidation 44 . Presumably,
the key step involved elimination from a high-valent, e.g.
alkoxyruthenium(VII) intermediate followed by reoxidation of the lower
valent ruthenium by dioxygen. However, as shown in Figure 7, if this
involved the Ru(VII)/Ru(V) couple the reoxidation would require the close
proximity of two ruthenium centres, which would seem unlikely in a
polymer-supported catalyst. A plausible alternative, which can occur at an
isolated ruthenium centre involves the oxidation of a second molecule of
alcohol, resulting in the reduction of ruthenium(V) to ruthenium(III),
followed by reoxidation of the latter to ruthenium(VII) by dioxygen (see
Figure 7).

More detailed mechanistic studies are obviously necessary in order to
elucidate the details of this fascinating reaction. It is worth noting, in this
context, that the reaction of TPAP with 2-propanol was found to be
autocatalytic, possibly due to the formation of colloidal A possible
alternative too is one involving the initial formation of oxoruthenium(VI),
followed by cycling between ruthenium(VI), ruthenium(IV) and possibly
ruthenium(II).
We note, in this context, that James and coworkers 46 showed that a

-dioxoruthenium(VI) complex of meso-tetrakismesitylporphyrin
dianion (tmp) oxidizes isopropanol, in a stoichiometric reaction, with
concomitant formation of a dialkoxyruthenium(IV) tmp complex (Reaction
10).

trans