Kinetics and mechanism of the oxidation of alkyl substituted phenols and naphthols with tBuOOH in the presence of supported iron phthalocyanine

Article abstract of DOI:10.1039/b821534k

2,3,5-Trimethylbenzoquinone (precursor of vitamin E) and 2-methylnaphthoquinone (vitamin K₃) were obtained in good yields by oxidation of 2,3,6-trimethylphenol and 2-methyl-1-naphthol, respectively, with ^tBuOOH catalyzed by supported

Full text of DOI:10.1039/b821534k

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Kinetics and mechanism of the oxidation of alkyl substituted phenols and
naphthols with BuOOH in the presence of supported iron phthalocyanine
t
Olga V. Zalomaeva,a Irina D. Ivanchikova,a Oxana A. Kholdeeva*a
and Alexander B. Sorokin*b
Received (in Montpellier, France) 1st December 2008, Accepted 15th January 2009
First published as an Advance Article on the web 12th February 2009
DOI: 10.1039/b821534k
2,3,5-Trimethylbenzoquinone (precursor of vitamin E) and 2-methylnaphthoquinone (vitamin K3)
were obtained in good yields by oxidation of 2,3,6-trimethylphenol and 2-methyl-1-naphthol,
t
respectively, with BuOOH catalyzed by supported iron tetrasulfophthalocyanine. The mechanism
of this heterogeneous oxidation was studied using 18O2 labeling experiments, EPR spectroscopy
with spin traps, kinetic studies, and complete analysis of reaction products including minor ones.
18O2 labeling experiments did not indicate the involvement of O2 in the oxidative process. EPR
study of reaction mixtures of 2,3,6-trimethylphenol and 2-methyl-1-naphthol oxidations in the
presence of 3,5-dibromo-4-nitrosobenzenesulfonic acid spin trap showed no formation of
any radical intermediates. Besides the target quinones, epoxyquinones and formyldimethyl-
1,4-benzoquinones, as over-oxidation minor products have been found. C–C and C–O coupling
products relevant to one-electron oxidation pathways were detected in trace amounts. Based on
the experimental results, a mechanism of oxidation of alkyl-substituted phenols and naphthols
mediated by the supported iron phthalocyanine catalyst has been proposed which involves two
successive electron transfers without escape of radical species in solution.
elaborated structures bearing several different functionalities at
the same molecule are versatile building blocks in fine chemistry.
However, the reason for this high selectivity of oxidation of
Introduction
Phthalocyanine metal complexes are very attractive catalysts
for industrial applications because of their low cost and
functionalized phenols is not yet clear. The mechanistic study of
aromatic oxidation by the FePcS–SiO2–tBuOOH system is
accessibility on the large scale. Homogeneous and supported
metal phthalocyanine complexes have widely been used
as catalysts in the liquid phase oxidation of phenols,1,2
There are two general mechanisms of the oxidation of
therefore of much interest.
hydrocarbons,3 thiols4 (Merox process) and other compounds.5
phenols to hydroquinones and quinones proposed in the litera-
In particular, iron tetrasulfophthalocyanine grafted on SiO2
ture. The first heterolytic mechanism involves a two-electron
pathway operating via electrophilic hydroxylation.9–12 An alter-
native homolytic mechanism involves the formation of radical
(FePcS–SiO2) is very useful for the preparation of functiona-
lized quinones which are important intermediates in fine
chemistry for the preparation of vitamins and drugs.2,6
aryloxyl intermediates.13–15 The homolytic mechanism was
FePcS–SiO2 showed high activity and selectivity in the oxida-
t
proposed for H2O2-based oxidation over hydrophilic mesoporous
titanium–silicates16 and Ti-substituted polyoxometallates.17
tion of 2,3,6-trimethylphenol (TMP) by BuOOH providing
87% yield of 2,3,5-trimethyl-1,4-benzoquinone (TMBQ), a
precursor of vitamin E,2c while 2-methyl-1-naphthol (MNL)
and analysis of minor oxidation products, the formation of
Based on the nature of the EPR spectra, data of kinetic studies
was converted to 2-methyl-1,4-naphthoquinone (MNQ),
vitamin K3, with almost 60% yield.7 It should be noted that
suggested for the oxidation of phenols in the presence of
phenoxyl radicals was suggested. A heterolytic mechanism was
microporous TS-1 and TS-2 titanium-silicate catalysts.9–11
these valuable quinone products are still prepared in industry
by stoichiometric oxidation often using toxic oxidants.8 The
We report here the study of the oxidation of alkyl-substituted
phenols and naphthols, exemplified by TMP and MNL, by the
FePcS–SiO2–tBuOOH catalytic system exhibits a remarkable
catalytic system FePcS–SiO2–tBuOOH. To gain insight into
selectivity in aromatic oxidation to quinones even in the
the mechanism we have used 18O2 labeling experiments, EPR
presence of easily oxidizable olefinic and alcohol groups
to provide an access to functionalized quinones.6 Such
spectroscopy with spin traps, kinetic studies, and complete
analysis of the reaction products.
a Boreskov Institute of Catalysis, Acad. Lavrentiev Av. 5,
630090, Novosibirsk, Russia. E-mail: khold@catalysis.ru;
Fax: +3 3833309573; Tel: +7 3833269433
Results and discussion
b Institut de Recherches sur la Catalyse et l’Environnement de Lyon
´
(IRCELYON), UMR 5256, CNRS-Universite Lyon 1,
Covalent anchoring of metal complexes onto inorganic
supports allows preparation of heterogeneous catalysts with
high stability against leaching of catalyst in the reaction
2 av. A. Einstein, 69626, Villeurbanne cedex, France.
E-mail: alexander.sorokin@ircelyon.univ-lyon1.fr;
Fax: +33 472445399; Tel: +33472445337
ꢀc
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solution. This phenomenon has often been observed in hetero-
geneous catalysis.12 We have found that FePcS supported in
m-oxo dimer form provided better selectivity in the oxidation
of phenols.2 A high potential of dimeric phthalocyanine
complexes as catalysts for oxidation has been demonstrated
in our recent studies. We have discovered that diiron m-nitrido
tetra-tert-butylphthalocyanine catalyzed oxidation of methane
and benzene by H2O2 under very mild conditions.18 This
system represents the first bio-inspired example for the mild
oxidation of methane in water.
signals at 6.28 and 6.72 ppm (J = 7.5 Hz) can be attributed
to signals of vinylic protons of structure III. The downfield
shift of the signal of the CH3 group in MNL from 2.41 ppm to
1.14 ppm in the intermediate is more compatible with structure
III. The effect can be explained by the influence of the
tert-butylperoxo group at the same carbon atom as was found
for the 4-hydroperoxide carbon of 4-methyl-2,6-di-tert-butylphenol,
where the signal was found to be downfield shifted from 2.27
to 1.37 ppm.19 These results can be rationalized by assuming
the formation of a radical or cation intermediate with two
resonance forms leading to para- and ortho-peroxo quinol
derivatives II and III, respectively. Intermediate II should be
significantly less stable and can further be converted to MNQ.
Note that only substituted ortho- and para-quinol derivatives
have been described.20
The covalent grafting of FePcS onto an amino-modified
SiO2 surface was carried out using triphenylphosphine ditri-
flate for direct activation of sulfonate groups for reaction with
surface amines as previously described.6 This is a convenient,
rapid one-pot procedure affording the m-oxo dimer as the main
grafted form according to DR UV-Vis spectroscopy. The
position of the Q band was at 640 nm. The nitrogen sorption
isotherm analysis showed a specific surface area of 185 m2 gꢁ1
while the Fe loading was 43 mmol gꢁ1. FePcS–SiO2 was shown
to be an efficient catalyst for the selective oxidation of alkyl-
substituted phenols and naphthols, as well as phenols with
other functional groups, to the corresponding quinones.2,6,7 In
particular, we have found that oxidation of TMP selectively
afforded 87% TMBQ yield at 96% substrate conversion under
the optimal reaction conditions: [TMP] = 0.02 M, [tBuOOH] =
0.11 M, 1.6 mmol of FePcS–SiO2 in 8 mL of 1,2-dichloro-
ethane, 30 1C, 2 h.2c When MNL was used as the substrate the
yield of MNQ was about 60% at 98% MNL conversion of
MNL under optimal conditions: [MNL] = 0.1 M, [tBuOOH] =
0.5 M, 0.5 mmol of FePcS–SiO2 in 1 mL of 1,2-dichloroethane,
80 1C, 1 h.7 The mechanism of these oxidations was studied
under these reaction conditions.
Oxidation of TMP afforded TMBQ with 65–80% yield
depending on the reaction conditions. Several minor products
1
were also detected and identified by GC-MS and H NMR.
Two isomeric epoxides 2,5,6-trimethyl-2,3-epoxy-1,4-benzo-
quinone IV and 2,3,6-trimethyl-2,3-epoxy-1,4-benzoquinone
V were obtained in 2–7% yields (Scheme 2).
The oxidation of the aromatic phenol ring is quite selective.
Only trace amounts of products of oxidation of methyl groups
were found. Isomeric formyldimethyl-1,4-benzoquinones VII
were obtained in 2–5% total yield. In contrast with the Ti–Si
catalyst –H2O2 system where C–C and C–O coupling products
were the main by-products,21 only a small amount of biphenol,
due to oxidative coupling of phenoxy radicals, was also
detected. Since the yield of biphenol determined by NMR2c
is only 2–5%, one can conclude that phenoxyl radicals are not
long-lived major species. The formation of the tert-butyl-
peroxo derivative of TMP by analogy with MNL oxidation
can also be possible (VI, Scheme 2). This putative compound
VI is less stable then the naphthalene derivative II or III and
undergoes degradation during purification by TLC and during
GC-MS analysis giving only hardly identifiable fragments.
Analysis of reaction products
The products of MNL oxidation were identified and quanti-
fied by GC-MS, GC and 1H NMR methods. The target
quinone MNQ was the principal product with 50–60% yield
depending on the reaction conditions. Importantly, only
0.5 mol% of catalyst was sufficient to achieve these good
yields. A small amount of MNQ was further oxidized to
2-methyl-2,3-epoxy-1,4-naphthoquinone with B5% yield
(Scheme 1, I).
18O2 labeling experiments
To probe the mechanism of the phenol/naphthol oxidation
mediated by FePcS–SiO2 the oxidation of MNL was per-
formed in the presence of 18O2 (99.6% 18O isotope purity).
The reaction flask was filled up with a mixture of 79.6% of Ar
and 20.4% of 18O2, modelling air composition. The isotopic
Along with the two expected products we have detected the
highly reactive tert-butylperoxo derivative of naphthalene
with 25% isolated yield (molecular ion at m/z = 246, corres-
ponding to C15H18O3). This compound was unstable under
1
GC-MS analysis conditions. According to H NMR, ES-MS
and CI-MS we suggested two possible structures II or III
(Scheme 1). Along with the proton signals of the unsubstituted
aromatic ring at 7.36, 7.42, 7.64 and 7.96 ppm, two doublet
Scheme 1 Oxidation of MNL by tBuOOH in the presence of
Scheme 2 Oxidation of TMP by tBuOOH in the presence of
FePcS–SiO2.
FePcS–SiO2.
ꢀc
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composition of the gas phase and the reaction mixture was
analyzed by GC-MS before the reaction started and 2 h after
tBuOOH addition. The isotopic composition of dioxygen
before and after the reaction gas was practically constant,
around 99% 18O2. This finding indicates no appreciable
tBuOOH decomposition via a homolytic mechanism which
raised from 44 to 50%. The same trend was observed when the
catalyst amount was decreased.7 These findings contradict the
one-electron mechanism of MNL oxidation via ArOꢂ radicals,
for which an inverse dependence is expected. For example,
considerable decrease of selectivity to MNQ in the MNL
oxidation was observed upon increasing of the MNL
concentration or the catalyst amount in the Ti–Si catalyst–
H2O2 system.8,23 The formation of radical species in this
system was evidenced and a homolytic mechanism was
established.16c,23
t
would result in 16O2 formation. BuOꢂ radical formed from
homolytic cleavage of tBuOOH should react with tBuOOH to
t
form tBuOH and BuOOꢂ radical. Then, 16O2 should be
formed according to the Russell scheme from tBuOOOOtBu.22
Recently, we have demonstrated a significant increase of 16O2
content in the gas phase in the course of 18O2 labeling
experiments using the catalytic system Ti–Si catalyst–H2O2
due to unproductive decomposition of oxidant during the
oxidation of substituted phenols.23 Thus, these isotopic labeling
data suggest different mechanisms of the phenol/naphthol
oxidation in the presence of iron- and titanium-containing
catalysts.
Kinetic of TMP oxidation
The kinetic study of the oxidation of TMP in the absence and
in the presence of DBNBS showed that the addition of the spin
trap did not affect the rate of TMP consumption and the rate
of TMBQ formation (Fig. 1). This result also indicates that the
formation of TMBQ does not occur via formation of phenoxyl
radicals. For the Ti–Si-catalyst a significant decrease of the
rate of both TMP consumption and TMBQ formation was
observed in the presence of DBNBS.16c
The isotopic compositions of the substrate and products
were constant during the entire reaction course indicating no
blank oxygen exchange with dioxygen. GC-MS analysis of
the final reaction mixture showed that MNQ contained
91.6 ꢃ 0.2% of unlabeled product and only 8.4 ꢃ 0.2% of
labeled quinone with 18O label in one position. This low
incorporation of 18O in quinone is not consistent with a radical
pathway involving alkoxy- or peroxy-radicals which would
rapidly generate phenoxyl radicals from phenol. Phenoxyl
radicals would easily react with 18O2 leading to significant
18O incorporation in the products, as has been previously
observed.5a,b
Typical kinetic curves of TMP consumption and TMBQ
formation show no induction period (Fig. 1). In the absence of
catalyst a non-selective reaction was very slow, the conversion
reached only 20% after 7 h. The apparent activation energy of
the catalytic reaction measured at 25–85 1C was determined as
22 kJ molꢁ1 (Fig. 2).
We detected a small but distinctive increase of the initial rate
with increase of the concentration of water (Table 1). Further
increase of water amount led to decrease of the reaction rate.
The reaction is first order both in tBuOOH (Fig. 3(a)) and in
catalyst (Fig. 3(b)).
EPR study in the presence of a spin trap
It is important to note that under typical reaction conditions
the reaction order in TMP is close to zero (Fig. 4). This result
can be explained by binding of phenol with the iron active
center.
EPR spectroscopy allows the detection of short-living radicals
via their trapping by spin traps to form EPR-detectable
long-lived radicals.24 Based on the characteristics of the EPR
spectrum one can suggest the nature and structure of the
radical. We have recently demonstrated the possibility of
the identification of phenoxyl/naphthoxyl radicals formed in
the Ti–Si catalyst–H2O2 system by EPR spectroscopy using
3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) as a
spin trap.16c
When complete formation of the complex between FePcS
and phenol occurs, the reaction rate should not further depend
When FePcS–SiO2 mediated oxidation of MNL was carried
out in the presence of DBNBS, the EPR spectrum did not
contain the signals of any radical adduct. The signals were
very weak (background level) when TMP was used as sub-
strate. This means that phenoxyl and naphthoxyl radicals were
not formed in appreciable amounts suggesting the non-radical
mechanism of phenol/naphthol oxidation in the presence of
the iron-containing catalyst. This suggestion was confirmed by
the absence of significant amounts of coupling products that
are typical for one-electron processes occurring via formation
of radical intermediates.25 This conclusion is also in accor-
dance with the following observation. While studying the
influence of MNL concentration on the selectivity of oxidation
in the system FePcS–SiO2–tBuOOH, we have found that
selectivity towards MNQ did not decrease with the growth
of MNL concentration.7 When the MNL concentration was
increased from 0.025 to 0.1 M the selectivity in MNQ was
Fig. 1 TMP conversion and TMQ yield vs. time: curves 1, 2 without
DBNBS, curves 10, 20 in the presence of DBNBS. Reaction conditions:
0.1 M TMP, 0.5 M BuOOH, 0.02 M DBNBS (10 and 20), 12 mg
t
FePcS–SiO2, [H2O] = 0.65 M, 1 mL MeCN, 50 1C.
ꢀc
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t
Fig. 2 Temperature dependence of TMP oxidation with BuOOH in
the presence of FePcS–SiO2. Reaction conditions: [TMP] = 0.1 M,
[tBuOOH] = 0.2 M, 3.2 mg of FePcS–SiO2 (0.14 mol%), 1 mL of
MeCN.
on the phenol concentration. To check this hypothesis we have
studied the interaction between TMP and FePcS by UV-Vis
spectroscopy.
UV-Vis study
First, the titration of FePcS by TMP was performed in water
because FePcS is not soluble in acetonitrile or other common
organic solvents. In aqueous solution, even at spectrophoto-
metric concentrations, FePcS exists in a dimeric form.25 After
the addition of TMP solution to the solution of FePcS the
band at lmax = 630 nm, corresponding to the dimer, disappears
while a new band appears at lmax = 655 nm (Fig. 5(a)).
Similar changes have previously been observed during
titration of an aqueous solution of FePcS by 2,4,6-trichloro-
phenol.26 These changes were explained by the cleavage of the
m-oxo dimer upon phenol coordination with formation of the
monomeric [FeIIIPcS]+ form with phenolate as anion. We
propose that the same phenomenon occurs in the case of other
phenols, in particular with TMP and MNL. Addition of
tBuOOH to an aqueous solution of FePcS also resulted in a
red shift, as was previously observed and explained by the
coordination of tBuOOH with the iron site.3g However,
the situation was complicated by the concomitant bleaching
of the phthalocyanine core that made the analysis difficult.
Fig. 3 Dependence of the initial rate Wo on [tBuOOH] (a) and
t
catalyst amount (b) in the TMP oxidation by BuOOH mediated by
FePcS–SiO2. Reaction conditions: [TMP] = 0.1 M, [H2O] = 0.65 M,
1 mL of MeCN, 35 1C; (a) 3.2 mg of FePcS–SiO2 (0.14 mol% Fe);
(b) [tBuOOH] = 0.2 M.
t
Anyway, the first-order dependence in BuOOH oxidant and
near-zero order dependence in TMP substrate allow to suggest
that phenol should be bound to iron phthalocyanine more
strongly than the oxidant.
A different situation should occur in the case of the supported
FePcS–SiO2 catalyst. Covalent grafting of phthalocyanine
onto a support via sulfonate groups should stabilize the
dimeric form against monomerization.2d Indeed, the DR
UV-Vis spectrum of FePcS–SiO2 treated with a TMP solution
in MeCN, separated by filtration and dried at 80 1C was the
same as that of initial material except for a decrease of the
Fig. 4 Dependence of the initial oxidation rate Wo on [TMP].
Reaction conditions: [tBuOOH] = 0.2 M, 3.2 mg of FePcS–SiO2
(0.14 mol% Fe), [H2O] = 0.65 M, 1 mL of MeCN, 35 1C
shoulder at 690 nm (Fig. 5(b)). After the same treatment of
FePcS–SiO2 with MeCN alone the DR-UV spectrum was
unchanged. One can propose that the grafted dimer retains
a
Table 1 The influence of [H2O] on the initial rate (Wo) of TMP oxidation by H2O2 in the presence of FePcS–SiO2
[H2O]/M
0.64
0.73
1.0
0.71
1.5
0.80
2.0
0.93
3.0
0.98
102W0/M minꢁ1
a
Reaction conditions: [TMP] = 0.1 M, [tBuOOH] = 0.2 M, 3.2 mg of FePcS–SiO2 (0.14 Fe mol%)), 1 mL of MeCN, 35 1C.
ꢀc
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t
Scheme 3 Proposed mechanism of oxidation of MNL by BuOOH
mediated by FePcS–SiO2.
Fig. 5 (a) Modification of the visible spectrum of FePcS (water,
10ꢁ5 M) upon addition of TMP; (b) DR UV-Vis spectra of
FePcS–SiO2: (1) initial; (2) treated with a solution of TMP in MeCN
and (3) treated with MeCN.
intermediate C does not occur to a significant extent. Hetero-
lytic cleavage of the O–O bond in B produces FeIIIFeV
intermediate D which should exist mainly in FeIVFeIV form
E due to delocalisation of the charge on two Fe ions. It should
be noted that diiron structural unit should provide the
stabilization of the high-valent species. An intramolecular
one-electron transfer in the intermediate E will lead to the
radical species F. The naphthol radical escaping from F can
undergo trapping by dioxygen which should be observed in the
18O2 trapping experiment. Labeled (in one position) quinone
accounted only for 8.4% of products indicating that this is
only a minor pathway. Further, naphthol radicals should
produce biphenol via bimolecular oxidative coupling, yet only
traces of biphenol (2–5%) were observed in the case of TMP
oxidation. Yet, we were unable to detect MNL coupling
product(s). A second one-electron transfer in F can lead to
cationic species G or G0 which can undergo a nucleophilic
attack of BuOOꢁ to form H or H0. Intermediate II formed
from H should not be stable under the reaction conditions and
gives rise to MNQ obtained with 55–60% yield. In turn,
intermediate H0 should lead to more stable intermediate III
which was indeed obtained in 20–25% isolated yield in MNL
oxidation. Alternatively, the coordinated or escaped MNL
cation might react with water or hydrogen peroxide to
produce a quinol derivative and quinone, respectively. Indeed,
increasing of water concentration up to 2 M led to higher
MNQ selectivity (Table 1). A higher amount of water led to a
decrease of the selectivity, probably via coordination of water
with the diiron complex thus preventing coordination with
its dimeric structure upon coordination of phenol without
significant influence on its UV-Vis spectrum in the Q band
region. The Q band corresponds to p–p* transfer of the
phthalocyanine ligand which is sensitive to p–p interaction of
phthalocyanine cores and less sensitive to ligation of metal iron.
Proposed mechanism of phenol oxidation
Based on the obtained results we propose the following
mechanism of the oxidation of alkyl substituted phenols by
tBuOOH mediated by supported iron phthalocyanine catalyst
using MNL as an example (Scheme 3).
In the first step the coordination of phenol with supported
m-oxo diiron phthalocyanine takes place. Similar coordination
of phenol was previously proposed for iron phthalocyanine2e
and iron porphyrin mediated oxidation.27 Our UV-Vis and
kinetic data (near-zero order in TMP oxidation) support this
hypothesis. The phenolate complex A can coordinate BuOOꢁ
to form intermediate B which can undergo homolytic or
heterolytic O–O cleavage. Homolytic cleavage of the O–O
bond would result in the concomitant formation of a BuOꢂ
radical. However, the low incorporation of 18O in MNQ in the
experiment with 18O2, the increase of the selectivity to MNQ
when increasing starting MNL concentration and, especially,
the absence of detected radicals in the EPR experiments in
the presence of the radical trap DBNBS strongly suggest
that homolytic cleavage of the peroxo bond in B to form
ꢀc
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t
MNL or with BuOOH. All possible quinols can further be
2,5,6-Trimethyl-2,3-epoxy-1,4-benzoquinone. m/z (%) 166
(3) M+, 151 (73) (M ꢁ CH3)+, 138 (79) (M ꢁ CO)+, 124
(100) (M ꢁ CCH3, CH3)+, 109 (20) (M ꢁ CCH3, 2CH3)+.
transformed to unlabeled MNQ. An analogous mechanism
should be operating in the oxidation of TMP where similar
trends have been observed.
Formyldimethyl-1,4-benzoquinone. m/z (%) 164 (58) (M)+,
149 (2) (M ꢁ CH3)+, 136 (71) (M ꢁ CO)+, 121 (100)
(M ꢁ CO ꢁ CH3)+, 108 (13) (M ꢁ 2CO)+, 93 (31)
(M ꢁ 2CO ꢁ CH3)+.
Experimental
2,3,6-Trimethylphenol (Fluka), 2-methyl-1-naphthol (Aldrich),
2-methyl-1,4-naphthoquinone (Aldrich), tBuOOH (Aldrich),
70% aqueous solution, 18O2 (98.5 atom%, Euriso-top) and
3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) were used
as received without further purification.
18O labeling experiments
The oxidation of MNL in the presence of labeling 18O2 was
performed under an atmosphere containing 20.4% of 18O2
(99.6% 18O enrichment) and 79.2% of Ar. The 50 mL reaction
flask containing 0.025 mmol of MNL, 5.2 mg of FePcS–SiO2
(0.25 mmol of Fe) in 1 mL of MeCN was filled up with this gas
mixture. Reaction was started by addition of 0.125 mmol of
tBuOOH. Isotopic composition of products and the gas phase
were determined by GC-MS. Each sample was analyzed three
times and m/z intensities of each peak were obtained by the
integration of all scans of the peak. The degree of 18O
incorporation into MNQ was calculated from ratio of m/z
172/174 intensities in the mass spectra of MNQ.
Iron tetrasulfophthalocyanine was prepared according to
the published procedure.28 Covalently supported FePcS–SiO2
material (0.2–0.3 wt% Fe, SBET = 185 m2
g
ꢁ1, average
particle size 12 nm) was obtained by grafting of FePcS onto
the surface of amino-modified SiO2 (Aerosil) as described.6
Instrumentation
GC analyses were performed using an Agilent 4890D and
Tsvet-500 gas chromatograph equipped with a flame-ionization
detector and a DB-5MS capillary column (30 m ꢄ 0.25 mm)
filled with Durabond phase. CG-MS analyses of organic
products were carried out using HP 5973/6890 and VG-7070
instruments. Electrospray ionization mass-spectrometry
(ESI-MS) and chemical ionization mass-spectrometry
(CI-MS) analyses were performed using ThermoFinnigan
LCQ and ThermoFinnigan MAT 95 XL instruments, respec-
tively. EPR spectra were recorded at room temperature using a
Spin-trap experiments
Experiments were carried out in thermostated vessels under
magnetic stirring at 50 1C (TMP) and 25 1C (MNL). Reaction
was initiated by addition of 0.088 mmol of BuOOH to the
t
mixture containing 0.025 mmol of TMP or MNL, 3–5 mg of
FePcS–SiO2 and 0.005 mmol of DBNBS in 250 mL of MeCN.
After 30 min the reaction mixture was placed into a quartz cell
which is typically used to perform EPR measurements in polar
solvents. The EPR spectra were run at room temperature.
1
Bruker ER-200D. H NMR spectra of the reaction products
were run on MSL-400 Bruker or AM 250 Bruker instruments.
UV-Vis spectra were recorded using a Specord M40 spectro-
photometer. DRS-UV spectra were acquired on a Shimadzu
UV-Vis 2501PC spectrophotometer.
UV-Vis and DR UV-Vis measurements
UV-Vis experiments were performed at room temperature.
The acetonitrile solution containing 10ꢁ6 mol TMP was added
in 10 mL portions (50 equiv.) to a 10ꢁ5 M homogeneous FePcS
solution in 2 mL of MeCN. The spectra were recorded on after
each addition.
Catalytic oxidations
The products of TMP and MNL oxidation were identified
using 1H NMR and mass spectrometric methods (GC-MS,
ESI-MS, CI-MS). The substrate conversion and the yield of
TMBQ and MNQ were quantified by GC using biphenyl as
the internal standard.
For DR-UV analyses of solid catalysts, 0.2 mmol of TMP
was dissolved in 10 mL of MeCN. 200 mg of FePcS–SiO2 was
added to the solution which was stirred for 10 min at room
temperature. Then the catalyst was filtered off, dried at room
temperature, dried at 80 1C for 16 h and analyzed by DR
UV-Vis spectroscopy. In a blank experiment 200 mg of
FePcS–SiO2 was added to 10 mL of MeCN. Preparation for
the other procedures was made in the same manner.
TMP oxidation in the presence of spin trap DBNBS was
performed in a thermostated glass vessel at 50 1C. The reac-
tions were initiated by addition of 0.5 mmol of BuOOH to
t
a mixture of 0.1 mmol of TMP, 12 mg of FePcS/SiO2,
0.02 mmol of DBNBS and internal standard (biphenyl) in
1 mL of MeCN. The course of reaction was followed by GC.
Kinetic study
1,2-Dihydro-2-tert-butylperoxo-2-methyl-1-oxonaphthalene.
=
246 (M)+; 1H NMR (CD3COCD3,
The kinetic studies of TMP oxidation were carried out in a
thermostated glass vessel at 25–85 1C under vigorous stirring
(500 rpm). Reactions were initiated by addition of 0.05–0.3 mmol
of tBuOOH to a solution of 0.05–0.2 mmol of TMP,
3–25 mg of catalyst (0.14–1.00 mmol of Fe), internal standard
(biphenyl) in 1 mL of MeCN. The dependence of the reaction
rate on [tBuOOH] was studied at a constant concentration of
H2O (0.64 M) to make a correction of the reaction conditions
on changing the amount of added aqueous solution of
tBuOOH. The reaction orders were determined from the
ESI-MS: m/z
250 MHz): d 1.14 (s, 3H, CH3), 1.29 (s, 9H, tert-Bu), 6.28
(d, 1H, H3, J = 7.8 Hz), 6.72 (d, 1H, H4, J = 7.8 Hz), 7.36
(d, 1H, H5, J = 7.5 Hz), 7.42 (t, 1H, H6(7), J = 7.5 Hz), 7.64
(t, 1H, H7(6), J = 7.5 Hz), 7.96 (d, 1H, H8, J = 7.5 Hz).
2,3,6-Trimethyl-2,3-epoxy-1,4-benzoquinone. m/z (%) 166
(7) M+, 151 (100) (M ꢁ CH3)+, 138 (17) (M ꢁ CO)+, 124
(33) (M ꢁ CCH3, CH3)+, 109 (13) (M ꢁ CCH3, 2CH3)+, 95
(22) (M ꢁ CCH3, CH3, CHO)+.
ꢀc
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1036 | New J. Chem., 2009, 33, 1031–1037

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dependence of the initial rate of TMP consumption on the
concentration of one of the reagents when the concentrations
of other reagents were kept constant.
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¨
Conclusions
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´
Iron tetrasulfophthalocyanine covalently supported onto
silica in the m-oxo dimeric form in combination with tBuOOH
shows high selectivity in the industrially important oxidation
of 2,3,6-trimethylphenol and 2-methyl-1-naphthol to the
corresponding quinones. The mechanism of this oxidation
has been studied using 18O2 labeling experiments, EPR spectro-
scopy with spin traps, kinetic studies and complete analysis of
reaction products including minor ones. Several lines of
evidence indicate that long-living phenoxyl radicals are not
involved in the reaction as principal reaction species. Based on
the obtained data we propose the initial coordination of
phenol to the diiron center followed by activation of peroxide
oxidant. The principal and minor products are formed via two
successive one-electron transfers in this intermediate. The
selectivity and kinetics of oxidation as well as 18O labeling
results are in accordance with proposed mechanism.
Chem., 2005, 29, 1400; (c) S. L. Kachkarova-Sorokina, P.
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Chimie, 2007, 10, 598.
8 For overview of current production methods of vitamin K3 and
alternative approaches, see: O. A. Kholdeeva, O. V. Zalomaeva,
A. B. Sorokin, I. D. Ivanchikova, C. Della Pina and M. Rossi,
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and H. van Bekkum, Wiley-VCH, Weinheim, 2001.
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Acknowledgements
We thank Dr E. P. Talsi for help and technical support with
EPR experiments. The research was partially supported by the
Russian Foundation for Basic Research (grant 05-03-34760)
and by CNRS in the framework of IRCELYON-BIC
Associated European Laboratory. Dr O. V. Zalomaeva and
Dr I. D. Ivanchikova acknowledge financial support from the
French Embassy in Moscow.
17 O. A. Kholdeeva, T. A. Trubitsina, R. I. Maksimovskaya,
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ꢀc
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