Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 151

alcohol oxidations with 30% aq. and a adduct, respectively.
However, it seems highly likely that the observed catalysis is due to leached
vanadium. Indeed, as we have noted elsewhere 82 , heterogeneous catalysts
based on Mo, W, Cr, V, etc. are highly susceptible towards leaching by
or alkyl hydroperoxides. Hence, in the absence of rigorous experimental
proof, it is questionable whether the observed catalysis is heterogeneous in
nature.
In contrast, titanium silicalite (TS-1), an isomorphously substituted
molecular sieve 108 is a truly heterogeneous catalyst for oxidations with 30%
aq. including the oxidation of alcohols 109 .
Late and first row transition elements can catalyze the oxidation of
alcohols with or via an oxometal pathway. Chromium and
vanadium have already been mentioned (see above). Ruthenium compounds,
e.g. also catalyze the oxidation of alcohols with TBHP 110 ,
presumably involving a high-valent oxoruthenium species as the active
oxidant.
A dinuclear manganese(IV) complex of trimethyl triazacyclononane
(tmtacn) catalyzed the selective oxidation of reactive benzylic alcohols with
hydrogen peroxide in acetone 111 . However, a large excess (up to 8
equivalents) of was required, suggesting that there is substantial non-
productive decomposition of the oxidant. Moreover, we note that the use of
acetone as a solvent for oxidations with is not recommended owing to
the formation of explosion-sensitive peroxides. The exact nature of the
catalytically active species in this system is rather obscure; for optimum
activity it was necessary to pretreat the complex with in acetone.
Presumably the active oxidant is a high-valent oxomanganese species but
further studies are necessary to elucidate the mechanism.

8. CONCLUDING REMARKS

The economic importance of alcohol oxidations in the fine chemical
industry will, in the future, continue to stimulate the quest for effective
catalysts that utilize dioxygen or hydrogen peroxide as the primary oxidant.
Although much progress has been made in recent years there is still room for
further improvement with regard to catalyst activity and scope in organic
synthesis. A better understanding of mechanistic details regarding the nature
of the active intermediate and the rate-determining step would certainly
facilitate this since many of these systems are poorly understood. It may
even lead to the development of efficient methods for the enantioselective
oxidation of chiral alcohols, e.g. the ruthenium-based system recently
described by Katsuki and coworkers 112 .