1
4
M. Vennat, J.-M. Brégeault / Applied Catalysis A: General 386 (2010) 9–15
4. Conclusion
The above results show the formation of novel ruthenium (II)
and/or (III)–dioxygen systems to promote carbon–carbon cleavage
in a tentatively assigned dioxygenase mode, although some minor
competing processes can occur. “Radicals are far more frequently
involved in oxygenation reactions than originally assumed; in fact,
they appear almost omnipresent” [46], and short-lived radicals can
be transient species [10,11] for facile catalysis with appropriate
substrates such as ␣-ketols [6,7,9]. It is difficult to establish by
EPR the intermediates formed in the Ru(II)/HOAc–H O/O catalysis
2
2
system, which can involve several equilibria.
Under anaerobic conditions, there is no C–C bond cleavage of
the cycloalkanones. To the best of our knowledge, there is no firm
6
evidence for Ru(IV)-peroxide formation from d Ru(II) precursors,
III
III
while the peroxo-bridged Ru O Ru species are still to be detected
2
[
17].
Current work in our laboratory is aimed at expanding the scope
and applications of this new C–C bond cleavage methodology [43]
and to preparing ruthenium phosphates and [LiRuPO ], an analogue
4
of [LiFePO ], with high specific surface area, to develop heteroge-
4
neous catalysts with site isolation, stabilized anionic species and
supported Rux catalysts with low-leaching. What is clear from this
work is that there are many ways of cleaving cycloalkanones with
oxidizing agents, and that options for catalyst development are far
greater than previously recognized.
Scheme 2. Proposed mechanism for the catalytic C–C bond cleavage of 2-
methylcyclohexanone by ruthenium (III/II) systems.
Acknowledgments
We thank Dr. L. Beaunier (SEM-EDS), Dr. B. Morin (EPR) for fruit-
ful contributions, and Dr. J.S. Lomas for correcting the text and for
fruitful discussions, Madame F. Sarrazin is thanked for preparing
the manuscript.
water. The reaction starts with the coordination of the enol to pro-
duce the enolate (I). The “free proton” and one-electron transfer
from the carbon atom to the ruthenium moieties give rise to the
formation of (II) which is a ruthenium (II) donor–acceptor com-
plex. Proton transfer occurs through deprotonation–protonation
with participation of protic solvent, substrate or water. There is an
unpaired electron on the organic ligand; in the presence of dioxy-
gen (II) would react with O2.
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ꢁ
•
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[
[
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[
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A
[
similar
mechanism
has
been
proposed
for
2001, Berlin/Postdam, 2001, pp. 321–326.
FeCl /O /MeOH/cycloalkanones [44]. These reactions are reminis-
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3
2
[
[
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[
[
[
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[
[
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dioxygenases have also been proposed with RuCl (PPh ) [45]
maximum turnover was about 20, but with formation of OPPh ).
2
3 3
(
3