Thioether Oxidation with H2O2 Catalyzed by Nb-Substituted Polyoxotungstates: Mechanistic Insights

Article abstract of DOI:10.1002/ejic.201801108

Nb-monosubstituted polyoxotungstates of the Lindqvist and Keggin structures, (Bu₄N)₃[Nb(O)W₅O₁₈] (1) and (Bu₄N)₄[PW₁₁NbO₄₀] (2), respectively, catalyze oxidation of or

Full text of DOI:10.1002/ejic.201801108

A Journal of
Accepted Article
Title: Thioether oxidation with H₂O₂ catalyzed by Nb-substituted
polyoxotungstates. Mechanistic insights
Authors: Olga V. Zalomaeva, Nataliya V. Maksimchuk, Gennadii M.
Maksimov, and Oxana A. Kholdeeva
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
FULL PAPER
Thioether oxidation with H2O2 catalyzed by Nb-substituted
polyoxotungstates. Mechanistic insights
Olga V. Zalomaeva,[a] Nataliya V. Maksimchuk,[a,b] Gennadii M. Maksimov,[a] and Oxana A. Kholdeeva
[a,b]
*
Abstract: Nb-monosubstituted polyoxotungstates of the Lindqvist
and Keggin structures, (Bu4N)3[Nb(O)W5O18] (1) and
(Bu4N)3[Nb(O2)W5O18] (3), (Bu4N)4[PW11Nb(O2)O39] (4) and
(Bu4N)2[HNb(O2)W5O18] (5) toward a range of organic S-
compounds under stoichiometric conditions. The oxidation
mechanism has been investigated using kinetic and
spectroscopic tools, product studies on the test substrate
thianthrene 5-oxide, competitive experiments, and Hammett
correlations.
(Bu4N)4[PW11NbO40] (2), respectively, catalyze oxidation of organic
sulfides with aqueous H2O2 in acetonitrile. The corresponding
peroxocomplexes
(Bu4N)3[Nb(O2)W5O18]
(3)
and
(Bu4N)4[PW11Nb(O2)O39] (4) formed upon interaction of 1 and 2 with
H2O2 are able to oxidize sulfides to sulfoxides and sulfones under
stoichiometric conditions. The product analysis of the oxidation of
thianthrene 5-oxide, competitive sulfide-sulfoxide oxidation and
Hammett plot implicated electrophilic oxidative properties of 4 and a
complex oxidative nature of 3. Protonation of peroxo complex 3
Results and Discussion
leading
to
the
formation
of
the
peroxo
species
Catalytic oxidations
(Bu4N)2[HNb(O2)W5O18] (5) strongly increases its electrophilicity,
which has a great impact on its reactivity and sulfoxidation selectivity.
The oxidation of methyl phenyl sulfide (MPS) with 1 equiv. of
H2O2 was studied in the presence of catalytic amounts (2 mol%)
of tetrabutylammonium (TBA) salts of Nb-substituted polyoxo-
and peroxopolyoxotungstates with the Lindqvist (1, 3 and 5) and
Keggin (2 and 4) structures (Scheme 1). The stoichiometry of
MPS oxidation with H2O2 to produce sulfoxide (MPSO) and
sulfone (MPSO2) is 1:1 and 1:2, respectively.
Introduction
In recent years, various niobium-containing materials have
attracted increasing attention as catalysts for important H2O2-
based selective oxidations, such as epoxidation of alkenes[1] and
2
]
sulfoxidation of thioethers.[
While several attempts of
rationalizing the catalytic performance of Nb(V) in alkene
epoxidation have been published,[2j, 3 ] the mechanism of Nb-
catalyzed thioether sulfoxidation remains practically unexplored.
Transition-metal-substituted polyoxometalates (POMs) are
well-known homogeneous catalysts for a range of selective
oxidations[4] and they can also serve as tractable soluble models
for heterogeneous catalysts in mechanistic studies.[5] Our recent
work showed that Nb-monosubstituted tungstates of the
Lindqvist structure mimic well the catalytic performance of
mesoporous Nb-silicate catalysts in epoxidation of alkenes with
H2O2 and that protons play a crucial role in heterolytic activation
of the oxidant and selectivity of epoxidation.[6]
Scheme 1. Catalytic oxidation of MPS in the presence of Nb-POM.
The results on the catalytic MPS oxidation are presented in
Table 1. Catalytic activity of the Lindqvist 1 and Keggin 2 POMs
having terminal oxo group (Nb=O) was found to be close: TOF
0.4 and 0.3 min-1, respectively (Table 1, entries 1 and 2).
However, in the presence of 1, the overoxidation product MPSO2
was obtained with a higher yield (16 vs 10% for 2). Both 2 and
its peroxocomplex 4 revealed similar activity and selectivity
(Table 1, compare entries 2 and 4). For Lindqvist 1 and its
peroxocomplex 3, the product yields were also close: 66-68%
MPSO and 16% MPSO2 (Table 1, entries 1 and 3). However, the
activity of corresponding peroxo derivative 3 was significantly
higher (TOF 2 vs 0.4 min-1 for 1). At the same time, protonated
peroxocomplex 5 was more active than 3 and revealed higher
selectivity toward MPSO (Table 1, compare entries 3 and 5).
Recently, some of us reported that 5 is formed from µ-oxo dimer
(Bu4N)4[(NbW5O18)2O] (6) upon hydrolysis and interaction with
H2O2.[6] Indeed, if we compare catalytic properties of the
protonated peroxo species 5 and dimer 6, we can see that both
compounds demonstrate practically the same activity and
selectivity in MPS oxidation (Table 1, entries 5 and 6), which
In the present work, we first explored catalytic properties of
the
polyoxotungstates,
(Bu4N)4[PW11NbO40] (2), in the oxidation of thioethers with H2O2
and also studied reactivity of peroxo complexes
Lindqvist
and
Keggin
type
Nb-monosubstituted
(Bu4N)3[Nb(O)W5O18]
(1) and
Dr. O.V. Zalomaeva, Dr. N.V. Maksimchuk, Dr. G.M. Maksimov, Prof. Dr.
O.A. Kholdeeva
[a]
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk
630090, Russia
E-mail: khold@catalysis.ru
http://en.catalysis.ru/
[b]
Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090,
Russia
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
FULL PAPER
indicates that 5 is most likely the active species responsible for
the catalysis observed with 6.
The values of the equilibrium constant K and rate constant
k2 can be estimated using a plot of 1/W0 versus 1/[H2O2] (Figure
1d). The following values were found: K = 14 ± 5 and k2 = 0.06 ±
0.01 M-1·s-1.
Table 1. Catalytic oxidation of MPS with H2O2 in the presence of Nb-POMs.[a]
Entry
POM
MPS
conversion [%]
Yields [%][b]
MPSO
Time
[h]
TOF,
[min-1][c]
MPSO2
16 (20)
10 (11)
16 (19)
11 (13)
11 (13)
11 (13)
1
2
3
4
5
6
1
2
3
4
5
6
82
87
84
91
87
85
66 (80)
77 (89)
68 (81)
79 (87)
76 (87)
74 (87)
3
0.4
0.3
2
4
1
4
0.4
4
0.5
0.5
4
[a] Reaction conditions: [MPS] 0.2 M, [H2O2] 0.2 M, [Nb-POM] 0.004 M, MeCN 1
mL, 60 °C. [b] Yield based on initial MPS. Yield based on converted MPS is
given in paretheses. [c] TOF = (moles of substrate consumed)/(moles of Nb x
time), determined from initial rates of substrate consumption.
Reaction kinetics
Kinetics of sulfide oxidation with H2O2 in the presence of 1 was
investigated using 4-bromothioanisole (Br-MPS) as a model
substrate. Typical kinetic curves showed no induction period,
autocatalysis or inhibition behavior. The reaction rate was not
affected by light and by the presence of molecular oxygen. All
these indicate that neither photochemical nor autoxidation
processes are involved. The first-order dependence of the
reaction rate on the concentration of sulfide and catalyst was
observed (Figures 1a and 1b). The linier dependence of the
reaction rate on the concentration of 1 confirms stability of the
Nb-POM under the turnover conditions, which was also
corroborated by IR and 93Nb NMR techniques.The order in the
oxidant changed from first to zero with increasing H2O2
concentration (Figure 1c).
Such kinetic behavior implies Michaelis-Menten-type
kinetics and suggests a two-step mechanism: a) the first step is
a reversible interaction of H2O2 with 1 with the production of
peroxocomplex 3 (Equation 1) and b) the second step is an
interaction between 3 and organic substrate (Equation 2).
Figure 1. Initial reaction rates vs concentrations of (a) Br-MPS, (b) 1, and (c)
H2O2. Reaction conditions: [H2O] 0.7 M, MeCN 1 mL, 60 °C; (a) [H2O2] 0.05 M,
[1] 0.002 M; (b) [Br-MPS] 0.1 M, [H2O2] 0.05 M; (c) [Br-MPS] 0.1 M, [1] 0.002
M. (d) Plot of 1/W0 vs 1/[H2O2].
In the framework of this mechanism, the reaction rate law
can be described by Equation (3), which is consistent with the
experimental data.
Stoichiometric oxidations
[
] [
] [
]
O
2
ퟏ ∙ MPS ∙ H
2
W0 = 푘2 ∙ 퐾 [
(3)
]
O + 퐾∙[H O ]
2 2
H
Sulfides, which are strong nucleophiles, can be oxidized to
corresponding sulfoxides by using only electrophilic oxidants.
2
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
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Sulfoxides in its turn have biphilic nature; therefore sulfoxide
oxidation to sulfones can be accomplished with both nucleophilic
and electrophilic oxidants. Typically, the rate-determining step in
organic sulfide oxidation is either (1) a single electron transfer
(SET) from sulfide to metal ion[7] or (2) an electrophilic oxygen
atom transfer from oxidant (active form of a catalyst) to the sulfur
atom of sulfide (concerted mechanism).[7a,b,h,k,8] The consequent
oxidation of sulfoxide to sulfone, besides electrophilic
paths,[7a,e,f, 9 ] can also occur through a nucleophilic attack of
oxidant on the sulfur atom of sulfoxide. [8f,10]
To confirm the oxidizing capability of the Nb-POM
peroxocomplexes, the interaction of 3 and 4 with MPS was
studied by 93Nb NMR (3 and 4) and 31P NMR (4). After the
addition of MPS to the solution of 3 in MeCN, we observed a
simultaneous disappearance of the 93Nb NMR signal at -1048
ppm corresponding to the Lindqvist Nb peroxocomplex,[6] and
appearance of the signal at -898 ppm, corresponding to 1
(Figure 2). After the reaction of 4 with MPS, thebroad 93Nb NMR
signal at -910 ppm attributed to 4 revealed a significant shift to
ca. -958 ppm (Figure 3a), indicating the formation of 2. In turn,
the 31P NMR signal slightly shifted from -13.1 to-13.2 ppm
(Figure 3b), which also corresponds to the fransfornation of 4 to
2 (see Supporting Information, Table S1).
To verify the nature of the active species in Nb-POM-
catalyzed thioether oxidation and to investigate their electrophilic
properties, we studied MPS oxidation with 3 in stoichiometric
conditions ([MPS]/[3] = 5/1) without addition of H2O2. Indeed,
peroxo complex 3 was able to oxidize MPS to MPSO and
MPSO2 with selectivities of 95 and 5%, respectively (Table 2,
entry 1). Unlike the oxidation of alkenes,[6] additional protonation
of the peroxocomplex was not necessary, most likely,because
sulfides are more nucleophilic compounds than olefins. With 3,
MPS conversion achieved a maximum of the possible value
(19%) in 24 h. However, the addition of 1 equiv. of H+ or the use
of the protonated peroxo complex 5, which is more
electrophilic,[6] led to a significant increase in the reaction rate
and the formation of sulfoxide as the sole product (Table 2,
entries
2 and 3, respectively). In sharp contrast to the
epoxidation of alkenes,[6] peroxocomplex 4 with the Keggin
structure was also active in MPS oxidation under stoichiometric
conditions. In this case, MPSO formed with 99% selectivity
(Table 2, entry 4), thus pointing to electrophilic behavior of the
oxidizing species. The addition of H+ to 4 also led to increasing
the reaction rate but did not affect significantly the product
distribution (Table 2, entry 5). All these results collectively
allowed us to suggest that sulfide oxidation with 4 and 5 occurs
by an electrophilic concerted mechanism, while the oxidation
with 3 may have a more complicated nature. The difference in
the oxidation properties of the Lindqvist and Keggin polyanions
is most likely related to the difference in their charge density,
which is higher for the former, making Lindqvist peroxo complex
more prone to protonation.[6]
Figure 2. 93Nb NMR spectra of 3 after the addition of 5 equiv. of MPS.
Reaction conditions: [Nb-POM]/[MPS] = 1/5, MeCN, [3] 0.04 M; 20 °C.
Table 2. Stoichiometric oxidation of MPS with Nb-POMs.[a]
Entry
POM
MPS
conversion [%]
Selectivity [%][b]
MPSO2
Time
[h]
Figure 3. 93Nb NMR and 31P NMR spectra of 4 before and after the reaction
with 5 equiv. MPS along with the corresponding spectra of 2. Reaction
conditions: [Nb-POM]/[MPS] ratio 1/5, MeCN, [4] 0.01 M, 60 °C.
MPSO
1
2
3
4
5
3
19
95
5
0
0
1
2
24
3+H+
5
20
100
100
99
0.08
0.08
24
18
4
14.5
15
4+H+
98
0.5
[a] Reaction conditions: [MPS] 0.05 M, [Nb-POM] 0.01 M, MeCN 1 mL, 30 °C.
[b] Yield based on converted MPS; calculated as the average of 2-3
experiments.
Figure 4. UV-vis spectra of 5 before (black line) and after addition of 5 equiv.
of MPS (red line). Reaction conditions: [MPS] 0.0013 M, [5] 0.00026, MeCN,
30 °C, 5 min.
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
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The reaction between 5 and MPS was monitored by UV-
vis (Figure 4). Since the reaction is very fast, immediately after
the addition of 5 equiv. of MPS to the solution of 5, we observed
disappearance of the characteristic absorption band attributed to
O2Nb ligand-to-metal charge transfer (310–350 nm).[6]
has moderate electrophilic properties. An intermediate value of
XNu 0.55 was obtained for nonprotonated Lindqvist
peroxocomplex 3. Previously, Ballistreri et al.[12] observed similar
results for a picolinate vanadium oxoperoxo complex, which is
[7b, 13 ]
known as a typical radical oxidant.
Therefore, we also
cannot exclude a contribution of one-electron oxidation in the
Oxidation of thianthrene 5-oxide
case of 3.
In 1984 Adam et al.[ 11 ] suggested the use of oxidation of
thianthrene 5-oxide (SSO) as a mechanistic probe for the
determination of the oxidant electronic character. The molecule
of SSO contains electrophilic sulfoxide and nucleophilic sulfide
sites. Therefore, an electrophilic oxidant would react with sulfide
moiety to produce thianthrene 5,10-dioxide (SOSO), whereas a
nucleophilic oxidant would react with sulfoxide site with the
formation of thianthrene 5,5-dioxide (SSO2). Both products could
be further oxidized to thianthrene 5,5,10-trioxide (SOSO2) via
both mechanisms (Scheme 2).
Competitive oxidation of sulfide and sulfoxide
Another method for evaluation of the oxidant character is a
competitive oxidation of a sulfide and a sulfoxide with different p-
substituents in one run, under conditions when both substrates
are in competition for the oxidant.[12] A considerable formation of
either sulfoxide or sulfone indicates an electrophilic or
nucleophilic character of the oxidant, respectively. We
performed such competitive oxidation of Br-MPS and MPSO
with three peroxocomplexes 3, 4 and 5 (Table 4). In the
presence of nonprotonated peroxocomplex 3, a relatively high
yield of sulfone with respect to sulfoxide was observed: the ratio
of sulfoxide/sulfone was 1.4 (Table 4, entry 1). On the other
hand, in the presence of 4 and especially 5, the formation of
sulfone was much less pronounced: the ratio of sulfoxide/sulfone
attained the values of 6.4 and 100, respectively (Table 4, entries
2 and 3). Note that the reaction with 4 was performed at a higher
temperature because of the low solubility of the peroxocomplex
at 30 °C. Again, it is difficult to distinguish the type of the
oxidation processin the case of 3. On the other hand, the result
acquired for the protonated Lindqvist peroxo complex
5
unambiguously supports strongly electrophilic character of the
oxidant.
Scheme 2. Oxidation of SSO.
Table 4. Competitive oxidation of Br-MPS and MPSO with Nb-POMs.[a]
The nucleophilicity of the oxidant can be estimated by
calculating value of XNu parameter from equation XNu
=
Entry
POM
Yields [%][c]
Br-MPSO MPSO2
Br-MPSO/MPSO2
ratio
Time
[h]
(nucleophilic oxidation)/(total oxidation), where nucleophilic
oxidation is a sum of SSO2 and SOSO2 yields and total
oxidation is (SSO2 + SOSO + 2SOSO2). It is widely accepted
that XNu ≤ 0.3 indicates electrophilic character of the oxidant
while XNu ≥ 0.7 is typical of nucleophilic ones. The results of
stoichiometric oxidation of SSO in the presence of 3, 4 and 5
are presented in Table 3. The value of XNu for three Nb-POM
peroxo complexes differs significantly. Thus, protonated
Lindqvist peroxocomplex 5 is the most electrophilic oxidant
showing XNu 0.05 while Keggin peroxocomplex 4 with XNu 0.29
1
2
3
3[b]
4[c]
5[b]
5.5
7.7
11
4
1.4
6.4
110
24
1.2
0.1
0.08
0.08
[a] Reaction conditions: [Br-MPS] = [MPSO] 0.1 M, [Nb-POM] 0.0125 M,
MeCN 1 mL, [b] 30 °C; [c] 60 °C. [d] Yield based on initial substrate.
Table 3. Oxidation of thianthrene 5-oxide in the presence of Nb-POMs.[a]
Hammett correlations
POM
SSO conversion
[%]
Yields [%][b]
SOSO
XNu
The rate of the oxidation of aryl methyl sulfides with 3 revealed a
complicated dependence on the nature of p-substituents in the
aryl moiety. The correlation of log(WX/WH) with Hammett σ
constants[14] showed acurved plot (Figure 5). More precisely, the
curved plot consists of two intersecting linear correlations with
opposite slopes: one correlation with ρ = -0.48 (r = 0.989) for
electron-donating substituents (σ < 0) and the other correlation
with ρ = +0.42 (r = 0.720) for electron-withdrawing substituents
(σ > 0). Previously, Bonchio et al. obtained an analogous
correlation in the oxidation of p-substituted aryl methyl sulfoxides
SSO2
SOSO2
3
4
5
15
14
19
6
4
5
0.55
0.29
0.05
2
9
3
0.3
18
0.7
[a] Reaction conditions:[SSO] 0.025 M, [Nb-POM] 0.005 M, MeCN 1 mL,
60 °C, 24 h. [b] Yield based on initial SSO.
with
a
Ti(IV)-(R,R,R)-tris(2-
phenylethoxy)aminealkylperoxocomplex.[9c] They assumed that
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
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was obtained from Lancaster, thianthrene (97%) was purchased from
Aldrich. Acetonitrile (HPLC–grade, Panreac) was dried and stored over
activated 4 Å molecular sieves. The concentration of H2O2 (ca. 35 wt %
in water) was determined iodometrically prior to use. All the other
compounds were the best available reagent grade and used without
further purification. Thianthrene 5-oxide was prepared from thianthrene
following the literature protocol,[16] and its purity was confirmed by 1H
NMR.
the reaction occurred by two parallel pathways: electrophilic and
nucleophilic. The negative ρ indicates domination of electrophilic
oxidative process for substrates with electron-donating
substituents, while the positive ρ shows that the oxidation of
substrates with electron-withdrawing substitution occurs mainly
by the nucleophilic pathway. A similar curved Hammett plot was
also obtained by Espenson et al. for oxidation of
thiobenzophenone S-oxides with methyltrioxorhenium.[ 15 ] We
also found that, for sulfides with electron-donating p-substituents,
log(WX/WH) correlates better with σ than with σ+ (r = 0.966).
According to the literature, this is more consistent withthe
oxygen transfer mechanism rather than with electron transfer
one. [7d,e,g,i,l,8a-c]
Instrumentation: GC analyses were performed using
a
gas
chromatograph Chromos GC-1000 equipped with a flame ionization
detector and a quartz capillary column BPX5 (30 m × 0.25 mm). GC–MS
analyses were carried out using an Agilent 7000B system with the triple-
quadrupole mass-selective detector Agilent 7000 (HP-5ms quartz
capillary column 30 m × 0.25 mm). HPLC measurements were performed
using HPLC Agilent Technologies 1220 Infinity LC using ZORBAX
Eclipse Plus C-18 column (4.6 × 150 mm, 5-Micron, H2O-iPrOH = 40:60,
1mL/min, 25 °C). Aliquots of 2.5 µL of the reaction mixture were taken
periodically by a syringe and diluted withiPrOH (100 µL) before analysis.
1H, 31P, 93Nb, and 183W NMR spectra were recorded at 400.130, 161.67,
97.94, and 16.67 MHz, respectively, on
a Brüker AVANCE-400
spectrometer using high-resolution multinuclear probe head with 10 mm
o.d. (3 mL solution volume) sample tubes. Chemical shifts for 31P,93Nb,
and 183W, δ, were determined relative to 85% H3PO4, NbCl5, and
Na2WO4, respectively. For convenience, secondary external standards
were used: 0.2 M solution of H4PVMo11O40 in water for 31P (−3.70 ppm),
0.05 M H5SiW11NbO40 (-975 ppm) for 93Nb NMR, and 0.4 M H4SiW12O40
for 183W NMR (−103.6 ppm). Infrared spectra were recorded as 0.5–2.0
wt % samples in KBr pellets on an Agilent Cary 600 FTIR spectrometer.
Electronic absorption spectra were run on a Cary–50 spectrophotometer
using a 0.2 cm quartz cells.
Figure 5. Hammett plot for the oxidation of p-substituted methyl phenyl
sulfides with 3. Reaction conditions: [X-MPS] 0.05 M, [3] 0.01 M, MeCN 1 mL,
60 °C.
Synthesis and characterization of POMs: The synthetic procedures
The oxidation of p-substituted sulfides with 5 proceeds
very fast (within 1-2 minutes), making impossible the
determination of the initial reaction rates and investigation of
Hammet correlations. However, we should mention that, for all
the sulfide substrates studied, the corresponding sulfoxides
were the sole oxidation product. This finding agrees well with our
conclusion made on the basis of the other techniques about the
strong electrophilic character of 5 and electrophilic oxygen
transfer mechanism in the reaction with organic sulfides.
for(Bu4N)3[Nb(O)W5O18
]
(1),
(Bu4N)4[PW11NbO40
]
(2),
and
(Bu4N)3[Nb(O2)W5O18
]
(3),
(Bu4N)2[HNb(O2)W5O18
]
(5),
(Bu4N)4[(NbW5O18)2O] (6) were reported elsewhere. [6] Characterization of
all the compounds is provided in the Supporting Information, Table S1
(NMR data) and Table S2 (IR data).
(Bu4N)4[PW11Nb(O2)O39
] (4): H3PW12O40·6H2O (9 g, 3 mmol) was
dissolved in 20 mL of water. Then 0.9 mL of 0.325 М H3PO4, 50 mL of
hot aqueous solution of Na7HNb6O19·15H2O (0.702 g, 0.5 mmol), and 1
mL of 30% Н2О2 (11 mmol) were subsequently added and the reaction
mixture was stirred upon heating during 1 h. TBA salt was precipitated by
the addition of a solution of ТBАBr (4.5 g in 20 mL of water). The yellow
residue was isolated by filtration, washed with THF, dried at 80-90 °C,
and then recrystallized from MeCN and dried at air. Yield ca. 70 %. The
number of TBA cations (4) was determined by ignition. The presence of
0.8 peroxo group per molecule of 4 was confirmed by titration with
triphenylphosphine (PPh3) followed by monitoring with 31P NMR. IR (KBr,
1100-400 cm-1): 1068, 965, 894, 812, 606, 590, 516. 31P NMR (ppm, in
MeCN): -13.1. 183W NMR (ppm, in MeCN): -83, -90, -93, -101, -108.5, -
110. 93Nb NMR (ppm, in MeCN): -910.
Conclusions
The Nb-substituted polyoxotungstates with Lindqvist and Keggin
structures are effective catalysts in the H2O2-based oxidation of
organic sulfides. Their peroxo complexes are able to oxidize
sulfides under both turnoverand stoichiometric conditions. Nb-
POM peroxocomplexes of the Lindqvist and Keggin structures
reveal different electrophilic properties in the oxidation of S-
General method for thioanisoles oxidation: All reactions were
performed in thermostated glass vessels under vigorous stirring (500
rpm) in MeCN. Each experiment was reproduced at least two times. All
products were identified by the comparison of GC retention time with
those of the authentic samples as well as by GC-MS. Substrate
conversion and product yields were determined by GC or HPLC using
biphenyl as an internal standard for both methods.
compounds. While Keggin peroxocomplex
4 has medium
electrophilic nature, Lindqvist peroxocomplex 3 reveals more
complex oxidative properties, which can not be associated with
one type of oxidation mechanism. The protonation greatly
increases electrophilicity of 3, which is manifested by significant
increase in the thioether oxidation rate and sulfoxidation
selectivity.
Catalytic and stoichiometric oxidation of MPS in the presence of
Nb-POMs: Catalytic reactions were initiated by addition of H2O2 (0.2
mmol) into the solution of MPS (0.2 M) and Nb-POM (0.004 M) in 1 mL of
MeCN at 60 °C. In stoichiometric conditions, reactions were started by
addition of 0.5 mL of a Nb-POM solution in MeCN (0.02 M) to 0.5 mL of
MPS solution (0.1 M) to achieve concentrations [MPS] = 0.05 M and [Nb-
Experimental Section
Materials: Methyl phenyl sulfide (99%) and methyl phenyl sulfoxide
(98%) were purchased from Acros, 4-bromothioanisole (98+%, Br-MPS)
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10.1002/ejic.201801108
European Journal of Inorganic Chemistry
FULL PAPER
POM] = 0.01 M in 1 mL of MeCN at 30 °C. MPS conversion and product
yields were determined by GC.
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Kinetic study: The reactions were initiated by addition of H2O2 (0.02-0.2
M) into the solution of Br-MPS (0.02-0.2 M), catalyst 1 (0.001-0.01 M)
and H2O (0.18-1.8 M) in 1 mL of MeCN at 60 °C. The concentration of
H2O in the experiments to determine the reaction order in H2O2 was kept
constant (0.7 M) by the addition of corresponding amounts of water. Br-
MPS consumption was determined by HPLC. Initial rates were
determined from the reaction profiles at low conversions.
[2]
Catalytic and stoichiometric oxidation of thianthrene 5-oxide in the
presence of Nb-POMs: Catalytic reactions were initiated by addition of
H2O2 (0.025 mmol) into the solution of thianthrene oxide (0.025 M) and
Nb-POM (0.0005 M) in 1 mL of MeCN at 60 °C. Stoichiometric reactions
were carried outat 60 °C by mixing 0.5 mL of the solution of Nb-POM
(0.01 M) with 0.5 mL of the solution of thianthrene oxide (0.05 M) to
achieve the following concentrations: [Nb-POM] = 0.005 M and [SSO] =
0.025 M. After 24 h, the reaction mixtures were cooled down to the room
temperature and evaporated. CHCl3 was added to the solid to dissolve
reaction products. The insoluble catalyst was filtered off. CHCl3 was
evaporated once more and deuterated CDCl3 was added to the residue
for 1H NMR analysis of the product composition. Thianthrene 5-oxide
conversion and products yields were calculated from 1H NMR spectra by
integration of corresponding signals taking into consideration that total
amount of all products is 100%.
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(Bu4N)3[Nb(O2)W5O18] (3) solution in MeCN (0.02 M) to 0.5 mL of the
solution of p-substituted sulfide (X-MPS) in MeCN (0.1 M) at 60 °C.
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HPLC. Initial rates were determined from the reaction profiles at low
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Acknowledgments
The authors thank Dr. I. E. Soshnikov and Dr. R. I.
Maksimovskaya for 1H and 183W NMR measurements,
respetively; V. Yu. Evtushok for collecting IR spectra, and V. A.
Utkin for product identification by GC–MS. This work was carried
out in the framework of budget project No. АААА-А17-
117041710080-4 for Boreskov Institute of Catalysis and partially
supported by the Russian Foundation for Basic Research
(RFBR grant N 16-03-00827).
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Keywords: sulfoxidation • niobium • polyoxometalate • Lindqvist
structure • hydrogen peroxide
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