Hydrogen peroxide activation by fluorophilic polyoxotungstates for fast and selective oxygen transfer catalysis

Article abstract of DOI:10.1039/c6dt01951j

Fluorophilic polyoxotungstates perform the selective epoxidations of internal and terminal double bonds by hydrogen peroxide (H₂O₂) activation in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), under mild temperature conditions. A hybrid synergy of supramolecular interactions, involving the inorganic cluster and the fluorinated solvent, is envisaged to boost H₂O₂ activation and the oxygen transfer mechanism. 1,2-Epoxides have been obtained with >99% selectivity and 98% yield at T = 40-70 °C.

Full text of DOI:10.1039/c6dt01951j

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Hydrogen peroxide activation by fluorophilic
polyoxotungstates for fast and selective oxygen
Cite this: DOI: 10.1039/c6dt01951j
transfer catalysis†
Received 16th May 2016,
Accepted 21st July 2016
DOI: 10.1039/c6dt01951j
Mauro Carraro,* Martino Gardan, Andrea Sartorel, Chiara Maccato and
Marcella Bonchio*
www.rsc.org/dalton
9
Fluorophilic polyoxotungstates perform the selective epoxidations stability and selectivity of the resulting hybrid catalyst. In
of internal and terminal double bonds by hydrogen peroxide particular, functionalized POMs exhibit an improved stability
2 2
(H O ) activation in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), at high temperature, under microwave (MW) irradiation and in
1
0
under mild temperature conditions. A hybrid synergy of supra- diverse solvents.
molecular interactions, involving the inorganic cluster and the In this communication, we describe the unprecedented
fluorinated solvent, is envisaged to boost H activation and the grafting of fluorinated C -alkyl chains onto the lacunary sites
oxygen transfer mechanism. 1,2-Epoxides have been obtained with of Keggin-type polyoxotungstates, enabling their transfer into
2
O
2
8
>
99% selectivity and 98% yield at T = 40–70 °C.
a fluorous-rich environment. In particular, their solubility in
fluorinated alcohols, such as 1,1,1,3,3,3-hexafluoro-2-propanol
Due to its high active oxygen content (47%) and non-toxic
byproduct (H O), hydrogen peroxide (H ) is being con-
sidered as a “green-oxidant” for synthetic chemistry. H
conveniently activated via coordination to high valent (d )
metal complexes of early transition metals, such as Ti, Zr, Hf,
(
HFIP), has been exploited to combine the expected W(VI)-
2
2 2
O
based catalysis with the fluorous-assisted H activation. The
2 2
O
1
2 2
O is
resulting synergy yields an effective protocol for the selective
epoxidation of terminal alkenes, with >99% selectivity and up
to 98% yield, under mild conditions (T = 40–70 °C, air atmo-
sphere), which is a definite improvement with respect to most
tungsten-based catalytic protocols that operate under analo-
0
2
,3
V, Mo, W.
In particular, catalytic protocols involving
W(VI)-peroxides are generally characterized by negligible
decomposition pathways and have advanced into efficient syn-
thetic methods for alkene epoxidation. However, because of
the complexity of H O equilibria with mono- or polynuclear
tungsten precursors, the speciation of the competent inter-
mediates and their dynamic behaviour still represent a major
issue. Based on this, an appealing alternative is offered by
tungsten-based polyoxometalates (POMs) displaying stable
structures under oxidative conditions. Worthy of note is the
catalytic performance of [γ-SiW O (H O) ] , with selectivity
>
bonds (99% yield in 2–10 h at 32 °C) and turnover frequency,
TOF, up to 23 h− . This catalyst is characterized by a “defect”
structure, featuring a tetra-oxygenated nucleophilic site on its
surface, with four W(VI) atoms capable of H
Functionalization of the POM vacant site by the anchoring of
tailored organic groups provides a unique strategy to tune the
7
gous conditions.
4
11
Fluorinated alcohols can indeed act as catalytic solvents,
12
2
2
and were successfully used to perform epoxidations lactoni-
13
14
zations, and sulfoxidations in the presence of H
2 2
O . The
“booster effect”, observed during these oxidative transform-
5
ations, was explained on the basis of a double structural and
electronic effect, mediated by bridging hydrogen bonds, that
allows the electrophilic activation of hydrogen peroxide and,
likely, the stabilization of the transition state, promoting
6
4
−
1
0
34
2
2
99% in the epoxidation of both internal and terminal double
12,15
oxygen transfer to the organic substrate.
In order to achieve the fluorous-phase affinity, perfluoro-
1 7
8
alkyl C chains have been covalently grafted onto the electron-
rich oxygen sites of mono-, di-, and tri-vacant Keggin-type
8
2
O
2
coordination.
8−
8−
silicotungstates
[α-SiW O ]
,
[γ-SiW O ]
and
. Hybrid polyoxometalates 1–3 are obtained in
yield as high as 75% by reaction of the lacunary POM precur-
1
1
39
10 36
10−
9 34
[α-A-SiW O ]
1
6
sors with organosilyl chlorides (RfSiCl3), where Rf corres-
ponds to –(CH (CF CF (Fig. 1).
2
)
2
2
)
7
3
ITM-CNR and Department of Chemical Sciences, University of Padova, via Marzolo
The morphology of the POM lacunary site directs the
functionalization stoichiometry and the arrangement of the
perfluorinated substituents: for the mono and divacant
1
, 35131 Padova, Italy. E-mail: mauro.carraro@unipd.it, marcella.bonchio@unipd.it;
Fax: (+39) 0498275239
Electronic supplementary information (ESI) available: Experimental procedures
with spectroscopic and solid state analyses, computational data. See DOI:
0.1039/c6dt01951j
†
8
−
Keggin-type
precursors,
namely
[α-SiW O ]
and
11 39
8
−
1
[γ-SiW O ] , a bis-substitution pattern is obtained by the
1
0 36
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2
9
the perfluorooctyl chain of 1–3. Si NMR shows the presence
of the Si O, or Si O capping motifs, in addition to the core Si
2
4 3
1
83
heteroatom within the POM structure (Table 1).
W NMR
and FT-IR confirm the integrity of the POM inorganic scaffold,
as well as the functionalization pattern of the perfluorinated
s 3
hybrids. The increasing symmetry of 1 (C ), 2 (C2v), and 3 (C )
is witnessed by a corresponding decrease of the number of
1
83
W NMR signals, being respectively 5, 3 and 2 (Table 1).
The hybrid complexes 1–3 behave as anionic fluoro-surfac-
tants, with a nano-sized polar head corresponding to the in-
organic POM and an asymmetric fluorophilic surface, with 2–4
perfluorinated chains (Fig. 1). The assembly behaviour of com-
plexes 1–3 has been assessed in HFIP solution by Dynamic
Light Scattering (DLS), showing the formation of colloidal
nano-aggregates with diameters of 125 ± 86 nm (1), 116 ± 53
(
2) nm and 141 ± 67 nm (3) (Fig. 2 and S18†).
Fig. 1 Schematic representation of the hybrid complexes:
1
Electron microscopy images provide further insight on the
(
(
(
nBu
CF
nBu
4
N)
(CH
4
[α-SiW11
O
39{CF
3
(CF
(nBu
2
)
7
(CH
N)
2
)
2
Si}
2
O]; 2 (nBu
4
N)
4
[γ-SiW10
(CH
O
36{CF
3
-
].
supramolecular phenomena driven by the fluorophilic POMs
and regulated by the composition of the solvent phase. In par-
ticular, spherical nano-vesicles with diameters in the range of
2
)
7
2
)
2
2
Si} O];
3
4
4
[α-A-SiW
9
O
34{CF
3
(CF
2
)
7
2
)
2
Si}
4
O
3
+
4
N counterions are not shown).
8
0–170 nm are generally detected upon inspection by Trans-
mission Electron Microscopy (TEM) of a HIFP solution
1.33 mM) containing POM-based fluorosurfactants (Fig. 2
grafting of a Rf–Si–O–Si–Rf moiety, while the trivacant surface
(
1
0−
of [α-A-SiW O ]
enables the anchoring of a four-armed
perfluorinated unit (Fig. 1).
Convergent characterization evidence confirms the struc-
17
9
34
and S19†).
Moreover, Scanning Electron Microscopy (SEM) analysis
4 3
(RfSi) O
has been used to investigate the evolution of these nano-
tural identification of hybrids 1–3 (see the ESI and Fig. S1–
S17†). ESI-MS under negative scan mode, in particular, is
useful to assess the number of the appended perfluorinated
chains. Peaks due to the tetraanionic species can be observed
for the POMs 1–3, respectively, centered at m/z values of
4
−
9
10 for [α-SiW11
O
39{CF
3
(CF
2
)
7
4
(CH
,
2
)
2
Si}
2
O]
,
852 for
−
[γ-SiW O {CF (CF ) (CH ) Si} O]
1044 for [α-A-SiW9-
1
0
36
3
2 7
2 2
2
4
−
O
34{CF
3
(CF
2
)
7
(CH
2
)
2
Si}
4
O
3
]
(Fig. S6, S10 and S14 in the
1
9
29 183
ESI†). FT-IR and heteronuclear NMR ( F, Si, W) support
the expected substitution of 1–3 (Table 1 and Fig. S1–S15 in
1
9
the ESI†): F NMR spectra confirm the presence of the
expected multiplets between −86 and −133 ppm, arising from
1
9
29
183
Table 1
F,
Si and
W
NMR data for the hybrid POMs in
CD CN/CH
3
3
CN solutions
1
9
29
183
POM
F NMR, δ (ppm)
Si NMR, δ (ppm)
W NMR, δ (ppm)
1
−86.3 (6F)
−53.8 (2Si)
−80.3 (1Si)
−107.6 (4W)
−113.8 (1W)
−126.9 (2W)
−170.8 (2W)
−248.2 (2W)
−
−
−
−
−
121.4 (4F)
127.2 (12F)
128.1 (4F)
128.8 (4F)
131.6 (4F)
2
3
−86.2 (6F)
−64.4 (2Si)
−88.1 (1Si)
−106.7 (4W)
−133.2 (2W)
−140.5 (4W)
−
120.8 (4F)
127.0 (12F)
127.8 (8F)
131.3 (4F)
−
−
−
Fig. 2 Evolution of fluorophilic POMs to nanovesicles in HFIP solution.
Representative data are shown as follows: (a) DLS of POM 3 in HFIP
−87.4 (12F)
−62.2 (3Si)
−83.7 (1Si)
−88.4 (1Si)
−80.1 (3W)
−154.3 (6W)
1
1
.33 mM; (b) TEM image of POM 3, dropcasted on a copper grid from a
.33 M HFIP solution; (c) SEM image of POM 3 dropcasted on a silicon
−
121.8 (8F)
127.7 (24F)
128.7 (16F)
132.2 (8F)
−
−
−
plate from a 1.33 mM HFIP (with 0.4% v/v H
2
O) solution (image acquired
with 5kV acceleration voltage by using an in-lens detector).
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hybrid structures upon addition of a small water aliquot (0.4% entry 3 in Table 2). Noteworthily, the catalytic leverage of the
v/v H O in HFIP). At the water interface, the supramolecular combined HFIP/POM system is prominent in the epoxidation
2
aggregates grow with diameters reaching the micro-scale and of terminal olefins (entries 4–10 in Table 2), whereby efficient
with an increased polydispersity (Fig. 2 and S20†). Noteworth- epoxidation (>92%) of 1-hexene, 1-octene and 1-decene occurs
ily, the formation of a porous film with a breath-figure (BF) only in the presence of POMs 2–4 at 40 °C in 2–3 hours
pattern, typical of a fluorosurfactant template behavior, has (entries 4, 6 and 9 in Table 2, and comparison of data shown
been also observed for POM 2 (1.33–2.66 mM in HFIP with in Fig. S23 and Table S1†).
1
8,19
0
.4% H O, Fig. S21 and S22†).
The POM catalyzed epoxidation can be accelerated by
The assembly behavior of fluorophilic POMs at the increasing the temperature to 70 °C, with a negligible contri-
fluorous/aqueous interface is relevant for catalytic applications bution of the blank experiment (conversions <6%, entries 5, 7
using aqueous H O as the terminal oxidant.‡ and 10 in Table 2), overriding any detrimental decomposition
Catalytic epoxidation by the fluorophilic POMs has been of H . Under the conditions explored, catalytic epoxidation
explored in HFIP/H solutions, in the presence of cyclic, di- occurs with the maximum turnover number, TON = 125 (moles
substituted, and terminal alkenes (Table 2). As previously of converted H O per mole of catalyst), and turnover
2
2
2
2 2
O
2 2
O
2
2
1
0
−1
observed for its non-fluorinated analogs, the monovacant frequency, TOF = 900–2400 h
POM 1 shows only a negligible contribution to the epoxidation Fig. S24†).
(catalytic cycles per hour,
reactions in HFIP, confirming a crucial role of the POM reac-
tive site (Table S1 in the ESI†). For the sake of comparison, the POMs,
Due to the polyanionic nature of the fluorophilic
1
0a,22
we have explored the possible application of non-
non-covalent fluorophilic ammonium salt (Rf′
3 3 4 10
NCH ) W O32 conventional MW-induced heating to further accelerate the
(
4), (with Rf′ = −(CH ) (CF ) CF , Fig. S16 and S17†), has also catalytic epoxidation of 1-octene. Despite the relatively low
2
3
2 7
3
2
0,21
been screened for catalytic activity.
–4 are collected in Table 2, and compared to the POM-free heating is indeed promoted by the POM catalyst. Quantitative
HFIP solvent-phase. production of 1,2-epoxyoctane is obtained within 20 min in
Quantitative epoxidation of cyclic olefins, such as cyclo- the presence of 3 (entry 8 in Table 2).
Results for compounds HFIP dielectric constant (ε = 16.7), efficient MW-induced
2
hexene and cyclooctene is performed in 30–60 minutes at 25 °C,
As a corollary study, the stability of the fluorophilic POMs
by the combined action of POMs 2–4 with the non-innocent has been assessed by FT-IR analyses performed on the spent
HFIP solvent (entries 1 and 2 in Table 2).§ The key role of the reaction mixture. The recovered catalysts 2–4 were found to
fluorinated alcohol is highlighted by the sluggish H
2
O
2
conver- retain their initial spectral features (Fig. S25–S38†).
To address the nature of the competent oxidant formed in
yield in the range 5–20% at 70 °C, Table S1†). Interestingly, solution, the epoxidation selectivity has been investigated con-
sion which is generally achieved with CH CN as the solvent
3
(
under these conditions the absence of cyclohexene allylic oxi- sidering the competitive reaction of the hexene isomers in the
dation products speaks against possible non-selective homo- presence of catalysts 2–4. Inspection of the results in Table 3
lytic pathways. As expected for acyclic olefins, the catalytic allows the comparison of the selectivity ratios obtained with
epoxidation of trans-2-octene requires a longer time (180 min, and without the POM catalysts. The key aspects are summar-
ized in the following points:
(
i) the POM-free, solvent-assisted epoxidation is mainly
driven by electronic effects with no steric discrimination, as it
results from the 2-hexenes/1-hexene selectivity ratios >18
Table 2 Catalytic epoxidation of alkenes with hydrogen peroxide by
fluorophilic POMs 2, 3, and 4 in HFIP. Reaction yields are calculated with
(
entries 1 and 2 in Table 3), favoring the internal alkene, but
that does not depend on the cis/trans stereochemistry of
-hexene;
ii) the POM involvement in the catalytic epoxidation stems
a
2 2
respect to H O
2
Epoxidation yield, %
(
#
1
Alkene
Cyclohexene
T, °C Time, min HFIP
2
3
4
from an interplay of electronic and steric effects that promote
25
30
16
30
23
40
14
45
25
<5
<1
6
66 >98
70
60
>98 >98 >98
>98 >98 92
>98 >98 >98
42 93
>98 >98 >98
>98
68
98
77
90 >98
92
85
2
3
cis-Cyclooctene 25
15
Table 3 Selectivity ratio determined from the competitive oxidation of
30
a
2 2
hexene isomers with H O in HFIP in the presence of POMs 2–4
trans-2-Octene
1-Hexene
25
60
80
1
4
5
6
7
8
9
1
40
70
40
70
MW
40
180
25
180
25
20
120
25
96 >98
47 73
98 >98
79 >98
67
90
75 >98
HFIP
t = 60′ t = 15′
2/HFIP 3/HFIP 4/HFIP
#
Alkenes
t = 15′
t = 15′
1-Octene
1
2
3
trans-2-Hexene : 1-hexene
cis-2-Hexene : 1-hexene
cis-2-Hexene : trans-2-hexene
19
18
1.4
3.3
7.6
2.0
3.1
7.9
2.4
4.9
6.1
2.0
b
5
<5
5
1-Decene
87
0
70
a
Reaction conditions: catalyst 0.8 µmol; substrate 0.25 + 0.25 mmol;
0.1 mmol; 0.6 ml of HFIP, T = 70 °C. Reaction products were ana-
.1 mmol; 0.6 ml of HFIP. Dielectric heating induced by microwaves, lyzed at complete H consumption. Ratios between the moles of the
00 W irradiation power, Tbulk ≥ 75 °C. epoxides have been reported.
a
Reaction conditions: POM 0.8 µmol; substrate 0.5 mmol; H
O
2 2
2 2
H O
b
0
1
2 2
O
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Conclusions
Fluorous-phase catalysis enhanced by fluorophilic polyoxo-
tungstates has been demonstrated for the selective epoxidation
of both substituted and terminal alkenes using H O as the
2
2
−
1
oxidant with quantitative conversion and TOF up to 2400 h
.
Fluorous-tagged POMs have been conveniently isolated using
either covalent or non-covalent strategies. Catalytic epoxidation
by the resulting systems displays similar stereo-electronic
requirements, which are consistent with their fluorous-driven
evolution to supramolecular aggregates under the reaction
conditions. The novel hybrid POMs act as anionic fluorosurfac-
tants evolving to spherical vesicles under the reaction con-
ditions, so that the solvent-assisted aggregation and peroxide
solvation trigger oxygen transfer catalysis. Future studies will
be addressing the engineering of functional thin-films and
supramolecular nano-reactors to explore POM dependent
fluorous-phase catalysis in confined systems.
Fig. 3 Dependence of the epoxidation rates (Rmax) of cis-cyclooctene
(
2 2
0.5 mmol) by H O (0.1 mmol) in the presence of an increasing concen-
tration of POM 3 in HFIP (0.6 mL) at T = 25 °C.
the epoxidation of the terminal alkenes and discriminate
between the cis/trans stereoisomers of 2-hexene. As a result,
the 2-hexenes/1-hexene selectivity ratios turn out to be in the
range 8–3, while the cis/trans epoxidation selectivity ratio
accounts for a factor as high as 2.5 (entry 3 in Table 3).
Acknowledgements
Financial support from Italo-Israelian Scientific & Technologi-
The epoxidation kinetics have been found to depend on the
POM concentration. In particular, the rate of cis-cyclooctene
epoxidation has been plotted as a function of the amount of
catalysts 2 and 3 (Fig. 3 and S29†) in the HFIP solution. In
both cases, a biphasic behavior is obtained, where an initial
linear increase of the epoxidation rate is followed by a satur-
ation plateau, observed at [POM] > 0.33–0.66 mM. This behav-
ior is consistent with POM aggregation phenomena, being
responsible for levelling off the oxidation rate.
cal Cooperation project “SOGNO”, and COST actions CM1205
“Catalytic routines for small molecule activation (CARISMA)”
and CM1203 “Polyoxometalate chemistry for molecular
nanoscience (PoCheMoN)”, is gratefully acknowledged. Pre-
liminary catalytic tests were performed by Francesca Fabbia-
nesi and Dr Gloria Modugno. We thank Prof. Markus
Antonietti (Max Planck Institute of Colloids and Interfaces,
Potsdam) for helpful discussions and TEM investigation.
Concerning the nature of the competent oxidant, the most
likely scenario involves the formation of W(VI)-based peroxides
1
0a
Notes and references
or hydroperoxides, whose reactivity is triggered by a network
1
5b
of hydrogen bonds involving one or more HFIP molecules.
Fig. 4 shows a simple model of a W(VI)-hydroperoxide, where
‡
Breath figures likely arise by the templating effect of aggregates formed in
HFIP, due to solvophilic/solvophobic interactions with the POM fluorosurfactant.
CN, do not yield porous films.
cis-Cyclooctene epoxidation can also be obtained, with >98% conversion and
2% epoxide selectivity, by stepwise addition of H (0.5 mmol in 5 portions),
donor–acceptor hydrogen-bonding with HFIP induces a small Control experiments, run in CH
W–O and O–O bond elongation, thus increasing the electro-
philic character of the oxidant.¶
3
§
9
2 2
O
according to a 1 : 1 substrate/oxidant stoichiometry (see the ESI†).
1
17
¶
From H and O NMR spectra, it has been deduced that the strong electron-
withdrawing properties of fluorine, along with the hydrogen bonding properties
of the O–H group, lead to the formation of an electrophilically activated inter-
mediate (see ref. 13c).
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3
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(
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