Table 3 Oxidation of 2-methylcyclohexanone, 2, to 6-oxoheptanoic acid, 3
e
e
Run
Catalyst precursor
Oxidant
H O
TBHP
O2
Conv. (%)
Yield (%)
a
1
2
3
1
80
92
100
99
55
67
90
97
2
2
b
[VO(OO-t-Bu)(dipic)(H O]
2
c
IV
V
HPA-4 (V /V )
HPA-3
d
4
O2
a
b
Reaction conditions: precursor, 0.076 mmol; 2, 7.6 mmol; H O , 15 mmol; MeCN, 5 mL; temperature, 40 ЊC; reaction time, 24 h. Reaction
2
2
c
conditions: precursor, 0.76 mmol; 2, 7.6 mmol; TBHP in decane, 15 mmol; decane, 5 mL; temperature, 50 ЊC, reaction time, 24 h. Reaction
conditions: precursor, equivalent to 0.076 mmol of vanadium; 2, 7.6 mmol; methanol, 5 mL; dioxygen pressure, 0.1 Mpa; temperature, 60 ЊC;
reaction time, 24 h. Reaction in water (5 mL) for 8 h; see (c) for the other parameters. Conversions (% of substrate consumed) and yield ([mmol
d
e
of product per mmol of substrate] × 100) were determined by GC analysis (OV 1701) after addition of an ethereal solution of diazomethane.
2
Ϫ
37
ϩ
Ϫ
exhibits some one-electron (V()–()) redox chemistry, intro-
ducing free-radical character into its reactions.
(O ) }) O] with M = Mo or W). [PPh ] [Ph SiO{WO(O ) }]
2 2 2 4 3 2 2
is one of the rare examples of “six-fold phenyl embrace” (6PE)
between a –SiPh group and a PPh4 cation with Si ؒ ؒ ؒ P =
6.61 Å, O–Si ؒ ؒ ؒ P–C colinearity = 175.4Њ. 1 gives another
example of multiple phenyl interactions (Si ؒ ؒ ؒ P = 6.03–6.75 Å
and O–Si ؒ ؒ ؒ P–C colinearity = 172.0Њ). Peroxo complex 1 and
the alkylperoxo complex [VO(OO-t-Bu)(dipic)(H O)], catalyse
oxidative C–C bond cleavage in 2-methylcyclohexanone, 2, and
a number of compounds under study. Oxoperoxo derivatives
are also probably intermediates in the reactions which involve
ϩ
A final practical note is that, owing to the one-electron
chemistry, much more decomposition of excess H O is caused
3
44
2
2
by vanadium systems than with W, Mo and Re. Our work is
devoted to the development of catalysts for C–C bond cleavage
by dioxygen, hydrogen peroxide and alkyl hydroperoxide with
purely inorganic precursors (without any oxidizable ligand)
easy to prepare and to apply. These precursors or catalysts
are a trump card for homogeneous, two- or even three-phase
40
2
ϩ
systems. Here, the ligand Ph Si–O is sensitive to “oxidative
HPA-n catalysts, [VO ] , cycloalkanone and dioxygen giving
3
2
degradation” and we cannot consider a thorough study on this
system as a priority.
peroxygen species which decompose by homolytic fission of
O–O bonds, with simultaneous or concerted C–C bond
cleavage, as proposed recently. Rates and selectivity of oxi-
1
With the discovery that solvents, acidity function, counter-
ions and experimental conditions (for the addition of dioxygen
or H O ) have a major effect on rates and selectivity of
dation of varied substrates by HPA-n or oxoperoxo complexes
42,45–46
are significantly affected by several parameters;
their
2
2
reactions (involving HPA-n and dioxygen or peroxovanadates
impact will be further investigated with the study of novel
catalysis systems.
and H O ) there is a need for thorough studies on the reaction
2
2
of vanadium- and tungsten-based systems with H O2 as a
2
means of preparing adipic acid from cyclohexene as well as
from cyclohexane and dioxygen; they are under study in our
group.
Acknowledgements
5,42,43
The authors thank Dr J. S. Lomas for correcting the English
51
for this manuscript, Dr R. Thouvenot for V NMR spectra
and Dr J.-Y. Piquemal for computing and Ms F. Sarrazin for
preparing the manuscript.
Conclusion
As part of a general programme concerned with modelling
heteropolyacid catalysts, H3ϩn[PMo12ϪnV O ] (where n = 1–9),
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ϩ
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43
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21,22
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21
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1
1
1
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46
axial carboxylate oxygen and terminal oxo ligand. In 1b, two
Ϫ
oxygen atoms of the Ph SiO ligands replace O and N from
3
1
1
1
the hydroxyl and histidine, respectively. Steric constraints of the
Ϫ
ϩ
Ph SiO ligands and PPh4 are certainly responsible for this
3
structure. This anion also stabilizes novel structural types
(
e.g. pentagonal monopyramidal arrangement in mono- and
Ϫ
dianionic species, [Ph SiO{MO(O ) }] and [(Ph SiO{MO-
3
2
2
2
9
12
D a l t o n T r a n s . , 2 0 0 4 , 9 0 8 – 9 1 3