Highly selective oxidation of alkylphenols to p-benzoquinones with aqueous hydrogen peroxide catalyzed by divanadium-substituted polyoxotungstates

Article abstract of DOI:10.1021/cs500738e

The catalytic performance of divanadium- and dititanium-substituted γ-Keggin polyoxotungstates, TBA₄H[γ-PW₁₀V ₂O₄₀] (I, TBA = tetra-n-butylammonium), TBA ₄H₂[γ-SiW₁₀V₂O₄₀] (II), and TBA₈[{γ-SiW₁₀Ti₂O ₃₆(OH)₂}₂(μ-O)₂] (III) has been assessed in the selective oxidation of industrially important alkylphenols/naphthols with the green oxidant 35% aqueous H₂O ₂. Phosphotungstate I revealed a superior catalytic performance in terms of activity and selectivity and produced alkylsubstituted p-benzo- and naphthoquinones with good to excellent yields. By applying the optimized reaction conditions, 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate) was obtained in a nearly quantitative yield via oxidation of 2,3,6-trimethylphenol (TMP). The efficiency of H₂O₂ utilization reached 90%. The catalyst retained its structure under turnover conditions and could be recycled and reused. An active peroxo vanadium complex responsible for the oxidation of TMP to TMBQ has been identified using ⁵¹V and ³¹P NMR spectroscopy.

Full text of DOI:10.1021/cs500738e

Research Article
pubs.acs.org/acscatalysis
Highly Selective Oxidation of Alkylphenols to p‑Benzoquinones with
Aqueous Hydrogen Peroxide Catalyzed by Divanadium-Substituted
Polyoxotungstates
Irina D. Ivanchikova,† Nataliya V. Maksimchuk,†,‡ Raisa I. Maksimovskaya,† Gennadii M. Maksimov,†
and Oxana A. Kholdeeva*,†,‡
†Boreskov Institute of Catalysis, Pr. Ak. Lavrentiev 5, Novosibirsk 630090, Russia
‡Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia
S
* Supporting Information
ABSTRACT: The catalytic performance of divanadium- and dititanium-
substituted γ-Keggin polyoxotungstates, TBA4H[γ-PW10V2O40] (I, TBA =
tetra-n-butylammonium), TBA4H2[γ-SiW10V2O40] (II), and TBA8[{γ-
SiW10Ti2O36(OH)2}2(μ-O)2] (III) has been assessed in the selective
oxidation of industrially important alkylphenols/naphthols with the green
oxidant 35% aqueous H2O2. Phosphotungstate I revealed a superior catalytic
performance in terms of activity and selectivity and produced alkylsubstituted
p-benzo- and naphthoquinones with good to excellent yields. By applying the
optimized reaction conditions, 2,3,5-trimethyl-p-benzoquinone (TMBQ,
vitamin E key intermediate) was obtained in a nearly quantitative yield via
oxidation of 2,3,6-trimethylphenol (TMP). The efficiency of H2O2 utilization
reached 90%. The catalyst retained its structure under turnover conditions
and could be recycled and reused. An active peroxo vanadium complex
responsible for the oxidation of TMP to TMBQ has been identified using 51V and 31P NMR spectroscopy.
KEYWORDS: alkylphenols, selective oxidation, hydrogen peroxide, polyoxometalates, benzoquinones
stoichiometric) amounts of CuCl2 are required to achieve high
(>95%) yields of the target quinone. The main drawbacks of
the existing process are the formation of Cl-containing
byproducts and the necessity to use special apparatus, resistant
to corrosion. TMBQ and other quinones can be obtained using
Keggin molybdovanadophosphoric heteropoly acids
H3+nPMo12−nVnO40 (HPA-n, n = 2−6) as catalysts.14,21−24
Their advantages are metal oxide-like structure and, therefore,
thermodynamic stability to oxidation, solubility in various
solvents, and noncorrosiveness, but again, quasi stoichiometric
amounts of the catalyst, HPA-n, are needed to enable high
yields in the desired product. Recently, two groups reported
oxidation of substituted phenols with dioxygen over supported
gold catalysts.25,26 In these systems, dimeric C−C coupling
products predominated among the oxidation products.
In addition to molecular oxygen, hydrogen peroxide (H2O2),
is the most green, atom efficient, and relatively inexpensive
oxidant that gives water as the sole byproduct.27−29 A large
scientific effort has been directed to the creation of catalyst
systems that would activate H2O2 to accomplish selective
oxygen atom transfer to organic substrates. The microporous
titanium silicalite-1 (TS-1) developed by Eni effectively
1. INTRODUCTION
Oxidative transformations of phenolic compounds play a
central role in organic synthesis and many biological
systems.1−4 The selective oxidation of substituted phenols
offers efficient access to functionalized quinones that serve as
building blocks for a wide variety of biologically active
compounds.5−7 Specifically, 2,3,5-trimethyl-1,4-benzoquinone
(TMBQ, vitamin E key intermediate) is produced through
oxidation of 2,3,5- or 2,3,6-trimethylphenol (TMP),8−10
whereas 2-methyl-1,4-naphthoquinone (MNQ, menadione,
vitamin K3) can be obtained from 2-methyl-1-naphthalene11−13
or, alternatively, 2-methyl-1-naphthol (MNL).14−16 Until
recently, stoichiometric oxidations with CrO3, MnO2, HNO3,
and other hazardous reagents were the main route for the
industrial production of valuable oxygenated compounds,
including quinones.11−13 Clearly, such processes do not meet
today’s environmental requirements. Therefore, the develop-
ment of green and sustainable chemical technologies that
eliminate the use of hazardous reactants and reduce generation
of waste has become a widely accepted strategy.17,18
Co(II)-Schiff base complexes have long been known as
highly selective catalysts for oxidation of substituted phenols to
quinones,19,20 but they suffer oxidative destruction of the
organic ligand, leading to low productivity. Nowadays, TMBQ
is produced industrially via oxidation of TMP with molecular
oxygen in the presence of copper halides.8 Large (quasi-
Received: May 29, 2014
Revised: July 9, 2014
Published: July 9, 2014
© 2014 American Chemical Society
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catalyzes hydroxylation of phenol to produce hydroquinone
and cathechol,30,31 but it is a poor catalyst for oxidation of bulky
phenols because of the pore size limitation. On the contrary,
oxidation of substituted phenols and naphthols readily occurs
with H2O2 over mesoporous titanium silicates;32−38 however,
catalyst stability and oxidant efficiency still remain a
problem.39,40
oxidative bromination of alkenes, alkynes, and aromatics;59 and
hydroxylation of alkanes54,55 and alkylbenzenes.56 The latter
reactions become possible only in the presence of 1 equiv (with
respect to I) of a mineral acid. A dititanium-substituted
tetramer, TBA8[{γ-SiW10Ti2O36(OH)2}2(μ-O)2] (III), was
reported to be an effective catalyst for alkene epoxidation and
thioether sulfoxidation using H2O2.60 In this POM, each half of
the γ-Keggin fragment contains a dinuclear titanium center
bridged by two hydroxo groups, and the resulting Ti2(μ-OH)2
core connects to the other one of the paired γ-Keggin subunit
through Ti−O−Ti linkages.60
Recently, titanium di(oligo)nuclear species have been
identified as the active centers responsible for the highly
selective oxidation of alkylphenols to p-benzoquinones.37 This
inspired us to explore catalytic properties of compounds I−III
in the oxidation of TMP and other representative phenolic
compounds. To the best of our knowledge, dimetal-substituted
γ-Keggin POMs (hereafter, γ-XW10M2) comprising a hetero-
metal di-μ-hydroxo (oxo) dimer placed into the lacuna of the
polyoxotungstate [γ-XW10O36]n− (Figure 1) have never been
used for the selective oxidation of phenolic compounds.
Several efficient homogeneous catalytic systems have been
reported for the selective oxidation of phenols/naphthols with
H2O2.41−47 Shimizu et al. developed a method for TMBQ
production in 70−85% yields based on the oxidation of TMP
with H2O2 (60%) in acetic acid catalyzed by Keggin
heteropolyacids HnXM12O40 (M = W, Mo, X = P or Si, n =
3 or 4).41 Methyltrioxorhenium (VII) catalyzed oxidation of
alkylphenols using 83 or 35% aqueous H2O2 in acetic acid or
dimethyl carbonate, respectively, to afford the corresponding p-
benzoquinones in moderate to good yields.42,43 Ruthenium and
iron compounds were successfully employed as catalysts for
oxidation of substituted phenols with H2O2.44−47 Ito et al.
reported 89 and 71% yields in TMBQ for TMP oxidation in
acetic acid as solvent in the presence of RuCl3 and FeCl3,
respectively.44 Ruthenium(2,2′,6′:2″-terpyridine)(2,6-pyridine-
dicarboxylate)-catalyzed oxidation of alkylated phenols and
naphthols led to benzo- and naphthoquinones with 78−83%
yields using 4.4 equiv of H2O2 in MeOH solvent.46 A three-
component system consisting of FeCl3·6H2O, pyridine-2,6-
dicarboxylic acid (H2Pydic), and different benzylamines
(FeCl3/H2Pydic/amine = 1/1/2.2) was applied for the
oxidation of methylated phenols and naphthols.47 TMBQ and
MNQ were obtained in 79 and 55% yields, respectively, using
7.5 mol % of catalyst and 4 equiv of H2O2. The drawbacks of all
these methods are insufficiently high yields (selectivity) of the
target quinones and rather low efficiency of the hydrogen
peroxide utilization; that is, large excesses of the oxidant have to
be used. Yet, conventional transition-metal complexes are
disposed to destruction owing to thermodynamically favorable
oxidation of organic ligands. Therefore, the development of
green catalytic routes for the production of TMBQ, MNQ, and
other important quinones remains a challenging goal of the fine
chemicals industry.
Early transition-metal oxygen anion nanosize clusters or
polyoxometalates (POMs) have attracted significant attention
as oxidation catalysts because (i) they have completely
inorganic, metal oxide-like structure and are not prone to
oxidative and thermal degradation in comparison with
organometallic complexes (generally POMs are stable to at
least 350−450 °C in the presence of molecular oxygen); (ii)
their redox and acid−base properties as well as solubility can be
controlled by changing the chemical composition or structure;
(iii) most POMs possess good hydrolytic stability over a wide
pH range;48−52 and finally, (iv) POMs can be converted to
solid materials using different immobilization/heterogenization
techniques.50−52 Solvolytic degradation of some POMs can be
caused by high excesses of hydrogen peroxide, but susceptibility
to this process is decreased with increasing negative charge of
the polyanion.51
Figure 1. Polyhedral view of polyanions [γ-XW10M2O40]n− (hetero-
metal di-μ-oxo (hydroxo) dimer is in gray).
2. EXPERIMENTAL SECTION
2.1. Materials. 2,3,6-Trimethylphenol (TMP, 97+%) was
obtained from Fluka. 2,6-Di-tert-butylphenol (DTBP, 99%),
2,6-dimethylphenol (DMP, 99%), 2-methyl-1-naphthol
(MNL), and 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ, 98%)
were purchased from Aldrich. Acetonitrile (Panreac, HPLC
grade) was dried and stored over activated 4 Å molecular sieves.
All the other compounds were the best available reagent grade
and were used without further purification. The concentration
of H2O2 (∼35 wt % in water) was determined iodometrically
prior to use.
2.2. Catalyst Preparation and Characterization. Heter-
opolyacid H5PMo10V2O40 was prepared according to the
literature.61 The cesium salt of the divanadium-substituted
phosphotungstate Cs5[γ-PV2W10O40] was synthesized as
described previously.62 TBA salts I, II, and III were prepared
according to the published protocols.53,54,60 Compound I was
purified following the procedure described by Mizuno and co-
workers57 with some modifications. Crude I (1.0 g, ∼90%
purity) was dissolved in MeCN (100 mL), and then the solvent
was removed by evaporation, followed by addition of acetone
(10 mL). The immediately precipitated yellow admixture of β-
isomer (−541 and −555 ppm in 51V NMR;62 −13.26 ppm in
31P NMR) was removed by filtration. After a few days of
storage of the acetone solution at 5 °C, pure (>99.5%,
confirmed by 51V and 31P NMR) yellowish orange crystals of I
were obtained.
Recently, Mizuno and co-workers demonstrated that
divanadium-substituted γ-Keggin polyoxotungstates TBA4H[γ-
PW10V2O40] (I, TBA = tetra-n-butylammonium) and
TBA4H2[γ-SiW10V2O40] (II) are efficient catalysts for a range
of liquid-phase oxidations with aqueous H2O2.53−59 Although II
selectively catalyzes alkene epoxidation,53 I also enables
oxidation of electron-deficient alkenes57 and organic sulfides;58
The protonated form of III (H-III) was prepared by
interaction of an aqueous solution of the potassium salt of III60
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with a cation-exchange resin in H+ form, followed by addition
of TBABr. Typically, TiOSO4 (1.0 g, 6.25 mmol) was dissolved
in 100 mL of water, and then the pH of the solution was
adjusted to 1.3 with 70% HNO3 followed by addition of
K8[SiW10O36]·12H2O (7.5 g, 2.52 mmol) to the resulting
suspension. After stirring for 15 min, a clear solution of K-III
was obtained, and the cation-exchange resin (H+ form; 10 mL)
was added. The resulting mixture was stirred for 15 min, and
then the resin was removed by filtration. TBABr (2.5 g in 15
mL H2O) was added in a single step. The resulting white
precipitate was separated by centrifugation, washed with an
excess of H2O, and dried overnight at 35 °C and then at 100 °C
for 1 h.
The number of TBA cations was determined by ignition of
POMs at 600 °C. Potentiometric titration with TBAOH was
used to determine the number of acid protons in the POMs. In
general, the two methods gave a good cation charge balance. All
POMs were characterized by FT-IR and NMR (51V and 183W)
spectroscopic techniques. The corresponding spectra are given
in the Supporting Information (Figures S1−S3).
2.3. Catalytic Oxidation Tests and Product Analysis.
Catalytic oxidations of alkylphenols with H2O2 in the presence
of POMs were carried out in temperature-controlled glass
vessels at 25−80 °C under vigorous stirring (500 rpm).
Concentrations of the reactants were in the range of [POM] =
0.0015−0.012, [phenol] = 0.1−0.4, and [H2O2] = 0.2−1.4 M.
Typically, the reactions were initiated by the addition of H2O2
to a mixture containing phenolic substrate and POM catalyst in
1 mL of MeCN. Samples were taken during the reaction course
by a syringe and analyzed by GC and GC/MS. The oxidation
products were identified by GC/MS and 1H NMR. The
substrate conversions and product yields were quantified by GC
using biphenyl as the internal standard. Each experiment was
reproduced at least three times. The experimental error in the
determination of the substrate conversions and product yields
normally did not exceed 2%.
2,6-Di-tert-butyl-1,4-benzoquinone (DTBQ). 1H NMR
(CD3CN, 400.13 MHz): δ = 6.48 (s, 2 H), 1.26 ppm (s, 18H).
2,6-Dimethyl-1,4-benzoquinone (DMBQ). GC/MS (EI) m/
z (relative int.): 136 (80) [M]+), 108 (70) [M − CO]+, 96 (25)
[M − CCO or M −HCCCH3]+, 79 (44) [M − 2CO, H]+, 68
1
(100), [M − HCC(CH3)CO]+. H NMR (CDCl3, 400.13
MHz): 6.54 (s, 2 H), 2.04 ppm (s, 6H).
1
2-Methyl-1,4-naphthoquonone (MNQ). H NMR (CDCl3,
400.13 MHz): δ = 8.0−8.15 (m, 2 H), 7.65−7.80 (m, 2 H),
6.84 (m, 1 H), 2.19 ppm (s, 3H).
2.4. Instrumentation. GC analyses were performed using a
gas chromatograph Tsvet-500 equipped with a flame ionization
detector and a quartz capillary column (30 m × 0.25 mm) filled
with BPX5. GC/MS analyses were carried out using an Agilent
7000B system with the triple-quadrupole mass-selective
detector Agilent 7000 and GC Agilent 7890B (quartz capillary
1
column 30 m × 0.25 mm/HP-5 ms). H, 31P, 51V, and 183W
NMR spectra were recorded on a Bruker Avance-400
̈
spectrometer at 400.13, 161.67, 105.24, and 16.67 MHz,
respectively. Chemical shifts, δ, are given relative to
tetramethylsilane, 85% H3PO4, VOCl3, and Na2WO4, respec-
tively. For convenience, secondary external standards were
used: 0.2 M H4PVMo11O40 for 51V (−530 ppm) and 31P
(−3.70 ppm), and 0.4 M H4[SiW12O40] for 183W NMR
(−103.6 ppm). Infrared spectra were recorded for 0.5−1.0 wt
% samples in KBr pellets on a Varian 660 FT-IR spectrometer.
3. RESULTS AND DISCUSSION
3.1. Effect of POM Composition on TMP Oxidation
with H2O2. First, we evaluated the effects of the POM
composition, that is, the nature of the central atom (P versus
Si), heterometal (V versus Ti), and cations (H versus TBA) on
the oxidation of alkylphenols with aqueous H2O2 using TMP as
model substrate. The catalytic tests were carried out under
standard reaction conditions (substrate/oxidant/catalyst
=1:3.5:0.03, 80 °C, MeCN as solvent) that had been previously
established as optimal for TMP oxidation to TMBQ over
mesoporous titanium-silicates.34−38 The main results are
presented in Table 1.
In the absence of any catalyst, only 2% of TMP conversion
was found after 40 min at 80 °C (Table 1, entry 1). All POMs
studied revealed high catalytic activity in TMP oxidation, and
92−100% substrate conversions were attained within 30 min.
However, the selectivity of the TMBQ formation in the
2,3,5-Trimethyl-1,4-benzoquinone (TMBQ). GC/MS (EI):
m/z (relative int.): 150 (100) [M]+, 122 (47) [M − CO]+, 107
1
(64) [M − CO, − CH3]+, 79 (48) [M − 2CO, − CH3]+. H
NMR (400.13 MHz, CDCl3): δ = 6.59 (s, 1 H), 2.05−2.10
ppm (m, 9H).
2,2′,3,3′,5,5′-Hexamethyl-4,4′-bisphenol (BP). GC/MS
(EI) m/z (relative int.): 270 (100) [M]+, 255 (57) [M −
CH3]+, 240 (61) [M − 2CH3]+, 225 (15) [M − 3CH3]+, 210
1
(3) [M − 4CH3]+, 195 (7) [M − 5CH3]+. H NMR (400.13
Table 1. Effects of Pom Composition on Tmp Oxidation
MHz, CDCl3): δ = 6.72 (s, 2 H), 4.57 (s, 2 H), 2.22 (s, 12 H),
1.94 ppm (s, 6H).
a
with H2O2.
b
2,3,6-Trimethyl-2,3-epoxy-1,4-benzoquinone (EQ). GC-
MS (EI) m/z (relative int.): 166 (7) [M]+, 151 (100) [M −
CH3]+, 138 (17) [M − CO]+, 124 (33) [M − CCH3, − CH3]+,
109 (13), [M − CCH3, − 2CH3]+). The choice in favor of this
isomer was made based on the chemical shift (6.7 ppm) of the
time
(min)
TMP conv TMBQ selectivity
entry
catalyst
no catalyst
(%)
(%)
1
2
3
4
5
6
40
30
20
15
5
2
97
III
H-III
II
38
58
c
98
1
92
78
signal of the quinoid proton in the H NMR spectrum.
I
>99
93
>99
97
2,5,6-Trimethyl-2,3-epoxy-1,4-benzoquinone (EQ). GC/
MS (EI) m/z (relative int.): 166 (3) [M]+, 151 (73) [M −
CH3]+, 138 (79) [M − CO]+, 124 (100) [M − CCH3,
−CH3]+, 109 (20) [M − CCH3, − 2CH3]+, 95 (38) [M −
CCH3, − CH3, − CHO]+.
I + 1 equiv of
TBAOH
7
7
8
9
I + 1 equiv of HClO4
H3PW12O40
3
30
30
>99
62
>99
22
H5PMo10V2O40
>99
51
2-Hydroxy-3,5,6-trimethyl-1,4-benzoquinone (OH-TMBQ).
GC/MS (EI): m/z (relative int.): 166 (100) [M]+, 137 (21)
[M − COH]+, 121 (23) [M − 3CH3]+, 109 (19) [M − CCH3,
− 2CH3]+, 95 (30) [M − CCH3, − CH3, − COH]+.
a
Reaction conditions: TMP, 0.1 mmol; POM, 0.003 mmol; H2O2
b
(35% in water), 0.35 mmol; MeCN, 1 mL; 80 °C. GC yield on the
substrate consumed. TBA6.6H1.4[(γ-SiW10Ti2O38H2)2O2].
c
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presence of III was rather poor (Table 1, entry 2). Dimeric
2,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol (BP) along with TMBQ
overoxidation products, that is, two isomeric epoxy derivatives
(EQ) and 2-hydroxy-3,5,6-trimethyl-1,4-benzoquinone (OH-
TMBQ), were identified as the main reaction byproducts
(Scheme 1).
deprotonation of II in the alkene epoxidation,53 and TMBQ
could be obtained in 90% yield after 7 min. In contrast to the
oxidation of alkanes,54 alkylbenzenes,56 and electron-deficient
alkenes,57 the oxidation of alkylphenols does not require one
additional equivalent of acid (HClO4) with respect to I.
Although the acid produces a rate-accelerating effect, without it,
TMBQ is obtained in approximatly quantitative yield after 5
min (Table 1, compare entries 7 and 5). Therefore, we may
conclude that the impact of protons is less pronounced in the
case of the selective oxidation of alkylphenols catalyzed by I
than in the oxidation of less reactive hydrocarbon substrates.
Heteropoly acid H3PW12O40, which according to 31P NMR
was stable under the conditions used, revealed a substantially
lower activity and poor TMBQ selectivity (Table 1, entry 8)
relative to I and II, which proves that vanadium atoms play a
central role in the title reaction. A possible product of its
solvolytic destruction, [PW4O24]3−, was active but not selective:
after 3 h at 40 °C, it converted 90% of TMP, but the TMBQ
yield was only 10%. On the other hand, conventional
H3PMo10V2O40, which possesses two vanadium atoms statisti-
cally distributed over 12 positions of the α-Keggin structure,
also showed much lower selectivity toward TMBQ than the di-
V-substituted γ-Keggin POMs (Table 1, entry 9), indicating
that, namely, the presence of the dimeric vanadium core in the
latter is crucial for the selective formation of the quinone.
3.2. Optimization of Reaction Conditions in TMP
Oxidation Using I. Optimization of the reaction conditions
was carried out for TMP oxidation with H2O2 using I as the
most prospective homogeneous catalyst. First, we evaluated the
effect of solvent nature on the catalytic performance. The
reaction was carried out under standard conditions of Table 1
in MeCN, tert-BuOH, and 1:1 (v/v) mixtures of MeCN with
tert-BuOH and MeOH (Figure 3). In tert-BuOH, the solubility
Scheme 1. TMP Oxidation with Aqueous H2O2 Catalyzed by
III and II
A partial replacement of TBA cations for H+ in the cationic
sphere of III (H-III, extra protons are most likely located at the
Ti−O−Ti bridges linking two γ-Keggin subunits) has led to the
increase of TMBQ selectivity from 38 to 58% (Table 1, entry
3). Lowering the oxidant to substrate molar ratio resulted in the
reduction of both TMP conversion and TMBQ selectivity.
In the presence of II, which has the same central atom as III
but the heterometal is V(V) instead of Ti(IV), the selectivity
toward TMBQ had already reached 78% at 92% TMP
conversion after 15 min (Table 1, entry 4). The main
byproducts were the same as those depicted in Scheme 1.
For both II and III, the carbon mass balance in TMP
oxidation depended on the reaction conditions. At 80 °C and
with a 1.75-fold excess of H2O2 (conditions of Table 1), it was
not complete because of the formation of some unidentified
overoxidation products. Meanwhile, at 40 °C with a
stoichiometric amount of the oxidant, when the substrate
conversion was not complete, the mass balance was close to
96−98% (Figure 2).
Figure 3. Effect of solvent nature on TMP oxidation with H2O2
catalyzed by I. Reaction conditions as in Table 1.
Figure 2. Product distribution in TMP oxidation with 2 equiv of H2O2
at 40 °C catalyzed by compounds II and III.
It is noteworthy that replacement of Si for P in γ-SiW10V2 led
to a significant increase in both the reaction rate and quinone
selectivity. With I, TMP was quantitatively converted to TMBQ
in 5 min (Table 1, entry 5). We also investigated the impact of
protons in the cationic sphere of TBA4H[γ-PW10V2O40] on the
catalytic performance. The addition of 1 equiv of TBAOH with
respect to I resulted in some reduction of both TMP
conversion and TMBQ selectivity (Table 1, entry 6). However,
this effect was not so dramatic as the effect caused by
of I was not complete, and a significant deterioration of all the
reaction parameters took place. In the other cases, I was fully
soluble. In MeCN/tert-BuOH (1:1), the reaction was even
faster than in pure MeCN, and the maximal TMP conversion
(97%) was already achieved after 3 min, TMBQ being the only
product. On the contrary, the reaction rate decreased, and the
selectivity to TMBQ reduced to 55% in MeCN/MeOH (1:1).
Therefore, we performed further optimization of the reaction
conditions using MeCN as solvent.
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Table 2 shows the influence of temperature and concen-
trations of the reactants on the achievable TMP conversion and
utilization efficiency and TMBQ yield can be improved by
lowering the reaction temperature to 40 °C (Figure 4). Further
decrease in the temperature produced a minor improvement of
both parameters but significantly increased the reaction time.
Table 2. Effects of Reaction Conditions on TMP Oxidation
a
by H2O2 in the Presence of I
TMP
conversion
(%)
TMBQ
selectivity
(%)
b
I
TMP H2O2
T
time
entry (mM) (M) (M) (°C) (min)
1
2
3
0.1
0.1
0.2
0.4
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.35
0.35
0.7
80
80
80
80
40
25
80
80
40
40
40
40
5
7
>99
98
>99
99
1.5
6
3
4
>99
>99
99
>99
78
4
12
1.4
2
5
3
3
3
3
3
3
3
3
0.35
0.35
0.25
0.2
30
240
10
10
30
35
40
40
>99
>99
>99
>99
>99
>99
>99
6
98
7
90
8
80
9
0.3
99
10
0.25
0.2
96
11
90
Figure 4. Effect of temperature on TMP oxidation with a
stoichiometric amount of H2O2 in the presence of I. Other reaction
conditions as in Table 2, entry 11.
c
12
0.2
90
65
a
b
c
Solvent: MeCN, 1 mL. GC yield on the substrate consumed. β-
PW10V2 was used instead of γ-PW10V2.
Importantly, isomeric β-PW10V2 separated during purifica-
tion of crude I (see the Experimental Section) gave TMBQ
with only 65% selectivity at 90% TMP conversion under the
optimal reaction conditions (Table 2, entry 12), thus indicating
that the β-isomer is significantly less selective than the γ-isomer
in the title reaction.
3.3. Reaction Scope. With optimal conditions in hand, we
carried out oxidation of other representative alkylphenols as
well as 2-methyl-1-naphthol (MNL) to estimate the reaction
scope (Table 3).
TMBQ selectivity. A 2-fold increase in the substrate-to-catalyst
molar ratio led to a minor decrease in both the conversion and
selectivity, and the target product was obtained in 97% yield
(Table 2, entry 2).
We also attempted to enlarge proportionally concentrations
of all reactants to assess whether higher TMBQ volume yields
are possible. Entry 3 in Table 2 shows that a 2-fold increase in
all concentrations does not impair the product selectivity and
substrate conversion that remain close to 100%; that is, the
volume yield of TMBQ can be increased, at least, twice.
However, a subsequent 2-fold increase in the reactants load
results in the reduction of selectivity (entry 4, Table 2).
Importantly, the reaction temperature can be lowered without a
notable decline of the product yield (Table 2, entries 5 and 6).
Even at room temperature, TMBQ is produced with 98% yield
in a reasonable reaction time (4 h).
Finally, we concentrated on optimization of the oxidant-to-
substrate molar ratio to get a better efficiency of the hydrogen
peroxide utilization. According to the reaction stoichiometry,
TMP oxidation to TMBQ requires 2 equiv of H2O2 (Scheme
2). On the other hand, an excess of H2O2 is often needed to
compensate unproductive decomposition of peroxide that may
occur parallel to the target oxidation reaction.32−38
Table 3. Oxidation of Various Alkylphenols and 2-Methyl-1-
a
naphthol with H2O2 Catalyzed by I
H2O2
T
time
substrate
conv (%)
product
b
substrate product
(M) (°C) (min)
select. (%)
2,3,5-
TMP
TMBQ
0.2
40
80
40
40
40
40
80
60
10
40
40
15
15
10
60
95
62
66
98
90
73
80
82
0.35
0.2
DMP
DTBP
MNL
DMBQ
DTBQ
MNQ
90
0.2
98
0.2
>99
>99
0.35
0.35
>99
a
b
Substrate, 0.1 mmol; I, 0.003 mmol; MeCN, 1 mL. GC yield on the
substrate consumed.
Scheme 2. Stoichiometry of TMP Oxidation to TMBQ with
H2O2
At 40 °C with 2 equiv of the oxidant, isomeric 2,3,5-TMP
was oxidized to TMBQ with a moderate yield (37%) that could
be increased up to 63% by increasing the reaction temperature
and H2O2 concentration. On the other hand, 2,6-dimethyl- and
2,6-di-tert-butylphenols gave corresponding p-benzoquinones
with ∼90% yields using only stoichiometric amounts of the
oxidant. Oxidation of MNL produced MNQ in 73−82% yields,
depending on the reaction conditions. Hence, the I/H2O2
catalytic system can be applied to oxidation of various kinds of
phenolic compounds.
3.4. Catalyst Reuse and Recycling. To verify the
possibility of the catalyst reuse and recycling, we performed
two types of experiments. First, after the oxidation of TMP in
the presence of I for 40 min under the reaction conditions of
entry 11, Table 1, additional portions of the reactants (0.1
Indeed, comparison of entries 1, 7, and 8 (as well as entries 5
and 9−11) in Table 2 shows that the decrease in H2O2
concentration resulted in the reduction of TMP conversion.
With the stoichiometric amount of the oxidant (2 equiv of
H2O2), the maximal TMP conversion attained 80 and 90%, at
80 and 40 °C, respectively. Therefore, hydrogen peroxide
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mmol of TMP and 0.2 mmol of 35% aqueous H2O2) were
added into the reaction solution. As one can judge from Figure
5, the oxidation reaction again proceeded with the same
Figure 7. Recycling of I. Reaction conditions as in Table 2 (entry 1).
Figure 5. Reuse of catalyst I. Reaction conditions as in Table 2 (entry
11).
conversion, selectivity to TMBQ, and reaction time as those
observed for the first run (90% conversion and ≥99%
selectivity). In the third use, there was only a slight
deterioration of the catalytic performance, indicating that the
catalyst system is intrinsically reusable. Although water
accumulated in the system as a byproduct, no precipitation of
the catalyst occurred.
Importantly, 51V and 31P NMR spectra of I run before and
after the catalytic reaction were very close (Figure 6), pointing
Figure 8. FT-IR spectra of I: initial sample and after the catalytic
reaction. Reaction conditions as in Table 2 (entry 1).
3.5. Active Peroxo Vanadium Species. Attempts to
identify an active peroxo vanadium complex responsible for the
oxidation of TMP to TMBQ were performed under quasi-
stoichiometric conditions using multinuclear NMR spectrosco-
py. In agreement with the data reported by Mizuno et al.,54 the
51V NMR spectrum of pure I in CH3CN revealed a single line
at −581 ppm and the 31P NMR spectrum exhibited a peak at
−14.2 ppm (Figure 9). The signals were broadened, most likely
because of a slow proton exchange on the NMR time scale in
the MeCN solution of I that contains one H+ as countercation.
After treatment of I with 1−2 equiv of 35% H2O2, a new signal
(δ −557 ppm) appeared in the 51V NMR spectrum (Figure 9)
that had previously been assigned to a vanadium μ-hydroxo-μ-
hydroperoxo complex V−(OH)(OOH)−V (IV).54 Simulta-
neously, a new signal emerged in the 31P NMR spectrum at
−14.8 ppm (Figure 9). Importantly, no signals that could be
attributed to lower-nuclearity peroxotungstates, such as
[PW2O14]3− or [PW4O24]3−, were observed. After subsequent
addition of a 10-fold excess of TMP, both 51V and 31P NMR
signals promptly disappeared (Figure 9), which allowed us to
assign them to the same active peroxo vanadium species. GC
analysis confirmed the formation of TMBQ in the reaction
mixture; no BP was found. It is noteworthy that the rate of the
peroxo complex decomposition in the absence of the phenolic
substrate was significantly slower (2 h versus 20 min in the
presence of TMP; see Figure S4 in Supporting Information).
In the range of concentrations studied, TMP does not bind
to I, as evidenced by the absence of new signals in both 51V and
31P NMR spectra upon addition of TMP to a MeCN solution
of I. All these facts collectively indicate that hydroperoxo
complex IV formed from I and H2O2 is the active species
responsible for the selective oxidation of TMP to TMBQ.
Figure 6. 51V and 31P NMR spectra of I: initial and after catalytic
reaction. Reaction conditions as in Table 2 (entry 11).
out that the catalyst structure is fairly stable under the turnover
conditions. After the second use, minor peaks of the β isomer
(−541 and −555 ppm in 51V NMR and −13.3 ppm in 31P
NMR) appeared, which is consistent with some deterioration of
the catalytic performance (Figure 5).
In another experiment, I was separated from the reaction
mixture by precipitation with diethyl ether, dried in air, and
then reused in the oxidation of TMP under the conditions of
entry 1, Table 2. Again, the reaction proceeded at the same rate
and produced TMBQ with nearly the same conversion and
selectivity (Figure 7). The FT-IR spectrum of I recovered from
the reaction mixture revealed all the main vibrations present in
the spectrum of the initial POM (Figure 8), which corroborated
that the recycled catalyst maintained its structure.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization details (FT-IR, 183W, 31P, and 51V NMR
spectra of POMs). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: (+7) 383-326-94-33. E-mail: khold@catalysis.ru.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Russian Foundation for Basic
Research (Grant No. 13-03-12042). The authors thank Dr. M.
V. Shashkov for identification of the oxidation products by GC/
MS technique and Dr. A. V. Golovin for 1H NMR
measurements.
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