Efficient and selective oxidation of sulfides in batch and continuous flow using styrene-based polymer immobilised ionic liquid phase supported peroxotungstates

Article abstract of DOI:10.1039/c6ra11157b

Styrene-based peroxotungstate-modified polymer immobilized ionic liquid phase catalysts [PO₄{WO(O₂)₂}₄]@ImPIILP (Im = imidazolium) are remarkably efficient systems for the selective oxidation of sulfides under mild conditions both in batch and as a segmented or continuous flow process using ethanol as the solvent and mobile phase, respectively. The performance of these styrene-based systems has been compared against their ring opening metathesis polymerisation derived counterparts to assess their relative merits. A comparative survey revealed the catalyst supported on N-benzyl imidazolium decorated polymer immobilised ionic liquid to be the most efficient and a cartridge packed with a mixture of [PO₄{WO(O₂)₂}₄]@ImPIILP and silica operated as a segmented or continuous flow system giving good conversions and high selectivity for sulfoxide. The immobilised catalyst remained highly active for the sulfoxidation of thioanisole in ethanol with a stable conversion-selectivity profile for up to 8 h under continuous flow operation; for comparison conversions with a mixture of [NBu₄]₃[PO₄{WO(O₂)₂}₄] and silica dropped dramatically after only 15 min as a result of rapid leaching while [PO₄{WO(O₂)₂}₄]@ImPIILP prepared from commercially available Merrifield resin also gave consistently lower conversions; these benchmark comparisons serve to underpin the potential benefits of preparing the polymer immobilized ionic liquid supports.

Full text of DOI:10.1039/c6ra11157b

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ARTICLE
Efficient and Selective Oxidation of Sulfides in Batch and
Continuous Flow using Styrene-Based Polymer Immobilised Ionic
Liquid Phase Supported Peroxotungstates†
Received 00th January 20xx,
Accepted 00th January 20xx
S. Doherty,*,a J. G. Knight,*,a M. A. Carroll,a A. R. Clemmet,a J. R. Ellison,a T. Backhouse,a N. Holmesb
and R. A. Bourneb
DOI: 10.1039/x0xx00000x
www.rsc.org/
Styrene-based peroxotungstate-modified polymer immobilized ionic liquid phase catalysts [PO4{WO(O2)2}4]@ImPIILP (Im =
imidazolium) are remarkably efficient systems for the selective oxidation of sulfides under mild conditions both in batch and
as a segmented or continuous flow process using either ethanol or acetonitrile as solvent or mobile phase, respectively. The
performance of these styrene-based systems has been compared against their Ring Opening Metathesis Polymerisation
derived counterparts to assess their relative merits. A comparative survey revealed catalyst supported on N-benzyl
imidazolium decorated polymer immobilised ionic liquid to be the most efficient and a cartridge packed with a mixture of
[PO4{WO(O2)2}4]@ImPIILP and silica operated as a segmented or continuous flow system giving good conversions and high
selectivity for sulfoxide. The immobilised catalyst remained highly active for the sulfoxidation of thioanisole in ethanol with
a stable conversion-selectivity profile for up to 8 h under continuous flow operation; for comparison conversions with a
mixture of [NBu4]3[PO4{WO(O2)2}4] and silica dropped dramatically after only 15 min as a result of rapid leaching while
[PO4{WO(O2)2}4]@ImPIILP prepared from commercially available Merrifield resin also gave consistently lower conversions;
these benchmark comparisons serve to underpin the potential benefits of preparing the polymer immobilized ionic liquid
supports.
from low activity and/or selectivity, poor thermal stability,
protocols that require long reaction times and/or complex
handling procedures as well as poor E-factors.12 As such there
Introduction
Sulfoxides and sulfones are technologically important
compounds which find use as intermediates in the synthesis of
fine chemicals, bioactive compounds, agrochemicals,1 as chiral
auxiliaries in asymmetric synthesis2 and most recently as ligands
for transition metal asymmetric catalysis.3 Sulfoxidation is also
the basis for the catalytic oxidative desulfurisation of crude oil
to remove sulfur-based impurities as the resulting sulfones can
be selectively extracted into a polar solvent under milder
conditions than those traditionally required for industrial
catalytic hydrodesulfurisation.4 A variety of powerful oxidants
have been employed for sulfoxidation including m-
has been considerable interest in developing systems that
utilise hydrogen peroxide as the oxidant as it is economical,
environmentally benign and readily available.13 In this regard, a
number of systems based on iron,14 manganese,15 vanadium,16
titanium,17
ruthenium,18
molybdenum,19
tungsten,20
tantalum,21 rhenium,22 zinc,23 tin,24 and copper25 have been
developed. In addition to utilising hydrogen peroxide as the
oxidant, an efficient catalyst must also be highly selective for
either sulfoxide or sulfone, cost effective, straightforward to
prepare and easy to manipulate, operate under mild conditions
across a wide range of substrates, have good long term stability
and be easy to recover and recycle. Even though highly selective
catalysts have been developed there is still a demand to identify
alternative oxidation systems to address the remaining issues
such as low activity and poor thermal stability, complicated and
onerous catalyst recovery procedures and leaching of the active
component as well as the need to improve green credentials.26
Immobilization of an efficient oxidation catalyst onto the
surface of a porous support, metal oxide, magnetic particle or
polymer has been widely explored as a method to facilitate
catalyst separation, recovery and reuse;27 while such systems
often suffer from slow reaction rates there have been reports
of immobilisation resulting in an enhancement in catalyst
chloroperbenzoic acid,5 UHP,6 NaClO,7 NaIO4,8 oxone,9 KMnO4
and dimethyldioxirane11, however, these systems often suffer
10
aNUCAT, School of Chemistry, Bedson Building, Newcastle University, Newcastle
upon Tyne, NE1 7RU, UK.
bInstitute of Process Research & Development, School of Chemistry, University of
Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom
E-mail: simon.doherty@ncl.ac.uk; Tel: +44 (0) 191 208 6537
† This paper is dedicated to the memory of Professor Malcolm H. Chisholm (FRS)
Electronic Supplementary Information (ESI) available: Synthesis and
characterisation of imidazolium-based monomers 1a
immobilized peroxotungstates 3a-e, TGA and DSC curves for 2a-c and 3a-c, SEM
images, FTIR traces and X-ray photoelectron spectra for 3a , characterisation data
for sulfoxides and sulfones, details of catalysis, recycle experiments and graphs
showing conversion-selectivity profiles as function of residence time for
-c, co-polymers 2a-c, polymer
-
c
a
[PO4{WO(O2)2}4]@PIILP-catalysed sulfoxidations under segmented and continuous
flow. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry 20xx
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activity and selectivity compared with its corresponding merits of both reasoning that styrene-based mVoienwoAmrtieclresOnalirnee
homogeneous counterpart.28 easy to prepare and the correspondingDpOoI:l1y0m.1e03rs9/Cw6oRuA1ld115b7eB
Ionic liquids are an intriguing class of solvent that has been more cost effective and have good thermal and mechanical
widely utilized for immobilisation of catalysts under integrity. Herein we report the results of this comparison which
homogeneous, liquid-liquid biphasic and liquid-solid (SILP) demonstrates that styrene-based polymer immobilised ionic
biphasic conditions, in some cases with remarkable success.29 liquid phase supported peroxotungstates give high conversions
Recent endeavours in this area include highly selective and excellent sulfoxide selectivity under mild conditions, both
sulfoxidations catalysed by a SILP system based on imidazolium in batch and under continuous flow operation using ethanol as
modified SBA-15 and [MoO(O2)2(H2O)n],30
a magnetically the solvent or mobile phase, and that the most efficient system
recoverable sulfoxidation catalyst based on magnetic outperforms its ROMP-derived counterpart. Moreover, the
nanoparticles entrapped in a tungstate-functionalised polyionic remarkable stability of the performance-time profile allowed
liquid,31 an eco-friendly protocol for the oxidation of sulfides to continuous flow operation to be maintained over extended
sulfones catalysed by V2O5 in [C12mim][HSO4]32 and efficient and periods of time with only a minor reduction in performance. As
selective sulfoxidation catalysed by peroxotungstates continuous flow processing of sulfoxidation has not been
immobilised on multilayer ionic liquid brushes-modified silica.33 thoroughly investigated this study will provide a valuable
Other recent developments include selective oxidation of benchmark and platform for future developments in this key
sulfides with H2O2 catalysed by heterogeneous ionic liquid- area.
based polyoxometalates,34 selective oxidation of sulfides with a
sulfoacid-hexafluorotitanate(IV) bifunctional ionic liquid,35
ionic liquid-mediated oxidation of sulfides to sulfoxides,36
efficient eco-friendly selective oxidation of sulfides to sulfoxides
with molecular oxygen catalysed by Mn(OAc)2 in
[C12mim][NO3],37 rapid oxidation of sulfides by themoregulated
polyoxometalate based ionic liquids,20b,38 selective and efficient
desulfurization by amphiphilic polyoxometalate-based ionic
liquid supported silica,39 and heterogeneous selective
sulfoxidation with polymeric ionic liquid nanogel-immobilised
tungstate anions.40
Results and Discussion
Catalysts synthesis and batch catalysis
Imidazolium based styrene monomers 1a
-c (Figure 1a) were
prepared by alkylation of the corresponding imidazole with the
appropriate electrophile and isolated as spectroscopically pure
crystalline solids after work up and purification. The immediate
and obvious advantage associated with these styrene-based
supports is the ease of monomer synthesis compared with the
linear 4 step synthesis required to prepare pyrrolidinium-based
norbornene monomers for the corresponding ROMP-derived
We have recently applied the concept of SILP-based
technology to develop peroxotungstate-based Polymer
Immobilised Ionic Liquid Phase (PIILP) oxidation catalysts in
order to combine the favourable and tuneable properties of
ionic liquids with the advantages of a solid porous support.41
Ring Opening Metathesis derived ionic liquid polymers were
used to prepare the corresponding peroxotungstate-based
PIILP catalyst, [PO4{WO(O)2)}4]@PIILP on the basis that the well-
behaved functional group tolerant nature of ruthenium-
catalysed living polymerisation would enable surface
properties, ionic microenvironment, porosity and hydrophilicity
to be modified and thereby catalyst-surface interactions,
substrate accessibility and catalyst efficacy to be optimised in a
rational and systematic manner. Gratifyingly, our initial foray in
this area demonstrated that peroxotungstate immobilised on
pyrrolidinium-decorated norbornene/cyclooctene copolymer
was a remarkably efficient system for the selective oxidation of
sulfides in batch and continuous flow. This was the first report
of continuous flow sulfoxidation and despite the potential
importance of this technology there are still relatively few
examples in the academic literature. In this regard, following
our initial disclosure Alemán and co-workers developed a Pt(II)-
based visible light photocatalyst for the oxidation of sulfides
both in batch and flow; the system gave complete
chemoselectivity for sulfoxide but required long reaction times
(10 h) to reach good converions.42 We have now undertaken a
comparison of the efficiency of our original system against a
range of polystyrene-based polymer immobilised ionic liquid
supported peroxotungstates in order to assess the relative
system. Co-polymers 2a
initiated radical polymerisation of 1a
-
c
(Figure 1b) were prepared by AIBN
with styrene in ethanol
-
c
at 90 °C, isolated by precipitation into diethyl ether and
characterised by a combination of elemental analysis, solution
and solid state NMR spectroscopy, gel permeation
chromatography (GPC), thermogravimetric analysis, scanning
electron microscopy (SEM) and IR spectroscopy.
Fig. 1 (a) Imidazolium-based styrene monomers (b) polystyrene-based ionic co-
polymers (X = Cl-, Br-) used for the preparation of POM@ImPIILP 3a
-
c
(X =
[PO4{WO(O2)2}4]3-) (c) macroreticular resin 2d and POM@PIILP 3d and (d)
imidazolium-modified Merrifield resin and POM@ImPIILP 3e
.
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The molecular weight (Mw) of 2a
permeation chromatography was measured to be 31,600 (2a), with available literature data for tungsten ions in the +6
26,100 (2b) 27,800 (2c) relative to polystyrene standards and oxidation state.44 The tungsten loadings of 32.0-35.0 wt% for
ARTICLE
-
c
determined by gel energies of 37.1 and 39.1 eV, respectively, in good agreement
View Article Online
DOI: 10.1039/C6RA11157B
the polydispersities of 1.32, 1.19, and 1.17, respectively, are 3a
consistent with relatively narrow monomodal molecular weight consistent with complete exchange of the bromide in 2a
distributions. The ratio of imidazolium monomer to styrene the aim of comparing and evaluating the efficacy of in-house
-
c
were determined from elemental analytical data and are
-
c
. With
incorporated into the polymer was determined to be ca. 0.5 synthesised polymer immobilised ionic liquid supports 2a-c
which corresponds to m and n values of 32 and 16, respectively, against commercially available systems, [PO4{WO(O2)2}4]3- was
based on the average molecular weights determined by GPC. also supported on macroreticular resin 2d and imidazolium-
The thermal stability of co-polymers 2a
thermogravimetric analysis and differential scanning
calorimetry. The TGA of 2a showed an initial weight loss at ca. batch conditions to establish optimum conditions for
-
c
was investigated by modified Merrifield resin 2e (Scheme 1c-d).
A series of catalytic reactions were first conducted under
-
c
100 °C, due to removal of physisorbed water and ethanol, comparative catalyst evaluation, substrate screening and
followed by two main degradation pathways, indicating that the recycle experiments as well as to identify potential systems for
polymers are thermally stable up to 300 °C; this is well above use in developing a continuous flow process,45 full details are
the reaction temperature required for liquid phase catalysis. presented in Table 1. Our initial optimisation focused on the
Solution and solid state NMR spectra of 2a
samples do not contain any imidazolium or styrene monomer oxidation has recently been catalysed by peroxometalate-based
as evidenced by the absence of signals at 5.2 and 5.8 ppm systems hosted in layered double hydroxides with enhanced
characteristic of vinylic protons. A reliable assignment of the activity and sulfoxide selectivity,28 polyoxometalate-
signals in the solid state 13C NMR spectrum of 2a hybrids,46
was obtained calix[4]arene thermoregulated Keggin-type
by conducting pairs of measurements, one with full cross- polyoxometalate-based ionic liquids,20b,38 polymeric ionic
polarisation (dipolar dephasing with 0
s delay) and one with a liquids nanogels,40 composite polyoxometalates supported on
50
s dephasing delay to remove the CH and CH2 signals; this Fe2O3,47 poly(ionic) liquid entrapped magnetic nanoparticles,31
enabled the quaternary and CH3 signals to be identified. and peroxometalates immobilised on the surface of ionic liquid
Peroxotungstate-based PIILPs 3a
were prepared by modified silica.33,39 Gratifyingly, good conversions and high
stoichiometric exchange of the halide anion in 2a with sulfoxide selectivity were obtained in methanol and ethanol
-c confirm that the sulfoxidation of thioanisole as the benchmark reaction as this

-
c


-
c
-
c
[PO4{WO(O2)2}4]3-, generated by hydrogen peroxide-mediated after 15 min using a 0.5 mol% loading of 3a at room
decomposition of the heteropolyacid H3PW12O40 (Figure 1b).43 temperature and a H2O2 : S mole ratio of 2.5 (entries 1-2). High
The desired product typically precipitated as an amorphous selectivities were also achieved in propan-2-ol and ethylene
white solid and was characterised by a variety of techniques glycol under the same conditions and even though reactions in
including solid state NMR spectroscopy, IR spectroscopy, TGA, the latter solvent were slower comparable conversions could be
SEM, XPS and elemental analysis. Decomposition of H3PW12O40 achieved at elevated temperatures (entries 3-4). Slightly lower
into [PO4{WO(O2)2}4]3- was confirmed by a signal at 2.9 ppm in conversions were obtained in acetonitrile and 2-Me-THF,
the solid state 31P NMR spectrum; in the case of 3c the spectrum sulfoxide selectivity remained high (entries 5 and 6). For
also showed the presence of minor phosphorus-containing comparison the corresponding ROMP-based POM@PIILP
species previously identified by Hill and co-workers during their system gave a slightly lower sulfoxide selectivity of 84% in
early studies on the formation, reactivity and stability of acetonitrile, under the same conditions and at a similar
[PO4{WO(O2)2}4]3-.44 Surprisingly, TGA analysis revealed that conversion. In this regard, higher sulfoxide selectivity is
thermal decomposition of 3a-c occurred between 250-300 ⁰C generally obtained in protic solvents such as methanol and
which is slightly lower than for 2a-c; this may be associated with ethanol, which has been attributed to their high hydrogen-
a reduction in the binding affinity due to the large size of the bonding capacity,27d,g,48 however, while alcohols are often the
peroxotungstate anion compared with halide and/or initial loss solvent of choice to achieve high sulfoxide selectivity, there
of coordinated peroxide which occurs below 200 °C. A similar have been recent reports in which acetonitrile has been
effect has recently been reported for a polymer ionic liquid identified as the optimum solvent.49 The minor decrease in
nanogel-anchored tungstate which was less thermally stable conversion with increasing alcohol carbon number (entries 1-3)
than the corresponding parent polymeric ionic liquid nanogel.40 may be associated with the different polymer swelling capacity
Scanning electron microscopy revealed a stark difference in of these solvents which could affect access of the substrate to
surface morphology of the polymers after loading of the the active site, however, the differences in conversion are
peroxometalate (supporting information). Specifically, the relatively minor and any interpretation should be treated with
surface of polyoxotungstate loaded 3a
granular texture compared with the smooth flat surface of coupled with its green and sustainable credentials prompted us
polymers 2a . The X-ray photoelectron spectra of 3a each to use this solvent for the remaining optimisation studies.
-c exhibit a rough caution. The high selectivity and conversion obtained in ethanol
-
c
-c
contain characteristic W 4f7/2 and 4f5/2 doublets with binding
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Table 1 Oxidation of thioanisole as a function of catalyst, solvent and hydrogen peroxide ratioa
Journal Name
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DOI: 10.1039/C6RA11157B
entry
solvent
catalyst
H2O2 equiv.
x
sonversionb
% sulfoxideb
% sulfoneb
sulfoxide
selectivityb,c
TOFd
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
MeOH
EtOH
i-PrOH
EG
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
-
3b
3b
3c
3c
3d
2.5
2.5
2.5
2.5
2.5
2.5
2.0
3.0
4.0
5.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
99
94
92
44
81
54
76
95
100
100
0
25
49
36
53
5
95
91
88
43
78
44
74
91
91
83
-
25
48
35
52
5
4
3
4
1
3
2
3
3
9
17
-
0
1
1
1
0
1
1
1
0
96
96
96
98
97
96
98
96
91
83
-
100
98
99
99
100
94
99
99
100
689
654
640
334
564
376
528
661
696
696
-
173
336
234
359
39
MeCN
2-Me-THF
EtOH
EtOH
EtOH
EtOH
EtOHf
EtOH
MeCN
EtOH
MeCN
EtOH
MeCN
EtOH
MeCN
EtOH
3d
3e
3e
18
57
42
2
17
56
41
2
125
403
297
19
2a/H3PW12O40
a Reaction conditions: 0.56–0.58 mol% 3a-e, 1 mmol thioanisole, 1.0–3.0 mmol 35% H2O2, 3 mL solvent, 25 °C, 15 min. b Determined by 1H NMR spectroscopy. c sulfoxide
selectivity = [%sulfoxide/(%sulfoxide+%sulfone)] × 100%. d TOF = moles sulfide consumed per mole catalyst per hour. e Reaction conducted at 50 °C. f Reaction conducted
without catalyst in the presence of 0.5 mol% 2a.
Systematic variation of the H2O2: substrate mole ratio selectivities at room temperature in ethanol under optimum
revealed that the best compromise between conversion and conditions, 3a is the most active with a TOF of 654 h-1 compared
sulfoxide selectivity was obtained for a peroxide to substrate with 173 h-1 and 234 h-1 for 3b and 3c, respectively (entries 2,
ratio of 2.5; below this ratio conversions were markedly lower 12 and 14).
(entry 7) while higher ratios gave complete consumption of
Conversion
Sulfone Selectivity
Sulfoxide Selectivity
sulfide but at the expense of selectivity which was markedly
lower (entries 8-10). As sulfones are a useful class of compound
the conversion-selectivity profile was also monitored as a
function of temperature, with a peroxide to substrate ratio of
2.5, in order to identify conditions for the selective formation of
methyl phenyl sulfone. Figure 2 shows that sulfoxide selectivity
drops dramatically with increased temperature such that
sulfone was obtained as the major product in 93% selectivity
after 15 min at 328 K. A control reaction for the oxidation of
thioanisole conducted in ethanol in the absence of
peroxotungstate but with 0.5 mol% 2a and 2.5 equivalents of
H2O2 gave no conversion, which confirmed the active role of the
catalyst (entry 11).
100
80
60
40
20
0
25
30
35
40
45
55
Temp (oC)
Fig. 2 Influence of temperature on selectivity and conversion for the sulfoxidation of
thioanisole with H2O2 in ethanol using a 0.5 mol% loading of 3a, a H2O2:S ratio of 2.5 and
a reaction time of 15 min.
In order to explore the effect of the imidazolium cation on
catalyst performance the efficiency of 3a-c for the sulfoxidation
of thioanisole in ethanol and acetonitrile was investigated
under the optimum conditions identified above and compared
with the corresponding systems prepared from commercially
available resin 3d-e, details of which are also summarised in
Table 1 (entries 12-19). While 3a-c all gave high sulfoxide
The data in Table 1 also highlights the merits of using
catalyst prepared with in-house synthesised polymer
immobilised ionic liquids as 3d and 3e only reached 5% and 57%
conversion, respectively, in ethanol which correspond to TOF’s
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of 39 h-1 and 403 h-1, respectively, both of which are significantly
lower than that of 654 h-1 obtained with 3a (entries 16 and 18).
In contrast, even though 3a was also more active than either 3b
or 3c in acetonitrile, the difference in performance was not as
marked as in ethanol, as evidenced by the TOF of 564 h-1 for 3a
compared with 336 h-1 and 359 h-1 for 3b and 3c, respectively
(a)
View Article Online
DOI: 10.1039/C6RA11157B
100
80
60
40
20
0
(entries 5, 13, 15). Gratifyingly, 3a-c all outperformed 3d by a
considerable margin, even though the TOF of 125 h-1 obtained
in acetonitrile was a marked improvement on that in ethanol
(entry 17). With the aim of investigating the possibility of
generating [PO4{WO(O2)2}4]@ImPIILP in situ immediately prior
to catalysis, in order to avoid the need to prepare, isolate and
store the catalyst, H3PW12O40 was supported on 2c by wet
impregnation from ethanol-water. Unfortunately, catalyst
generated by treatment of the resulting H3PW12O40/2a with
hydrogen peroxide was essentially inactive for sulfoxidation of
thioanisole in ethanol and only achieved 2% conversion under
the same conditions in the same time (entry 20).
0
50
100
time (min)
150
200
250
(b)
100
80
60
40
20
0
A comparative study of the variation in conversion against
sulfoxide and sulfone as a function of time for the sulfoxidation
of 4-nitrothioanisole catalysed by 3a in ethanol and acetonitrile
at room temperature shows that the composition-time profiles
are qualitatively similar but that oxidation to sulfone is more
rapid in acetonitrile than in ethanol (Figure 3). Approximate rate
0
50
100
150
200
250
time (min)
Fig. 3 Determination of rate constants for the formation of methyl phenyl sulfoxide (ka)
and methyl phenyl sulfone (kb) by fitting the concentration-time profile for the
constants for the formation of methyl phenyl sulfoxide (ka) and consumption of sulfide and the formation of sulfoxide and sulfone in (a) in ethanol and
(b) acetonitrile. Experimental data for sulfide (▪), sulfoxide ( ) and sulfone (●); fitted data
for sulfide (.........), sulfoxide (- - - -) and sulfone (───).
methyl phenyl sulfone (kb) in ethanol and acetonitrile were
extracted by fitting the concentration-time profile for the
consumption of sulfide and the formation of product using
pseudo steady state analysis. It should be noted that 2
Encouraged by the efficacy of 3a-c for the selective
equivalents of H2O2 are consumed during the reaction and as
such the derived rate constants will only be meaningful for this
comparison, even though the data fit is visually very good. The
data confirms that the solvent has a more significant effect on
the second oxidation compared with the first; this may be
associated with the increased hydrogen bond capacity of
ethanol which could solvate the H2O2 effectively and thereby
reduce its availability at the catalyst as it becomes depleted
and/or solvate the sulfoxide and thereby stabilise it with respect
to further oxidation. However, catalyst solvation may also be
responsible for the solvent dependent difference in kb as it
would be reasonable to expect solvation by ethanol to impede
access of sulfoxide to the active centre to a greater extent than
acetonitrile.
oxidation of thioanisole, catalyst testing was extended to
explore their performance across a range of substrates under
the optimum conditions identified above, full details of which
are summarised in Table 3. The tabulated data clearly shows
that 3a outperforms both 3b and 3c across the entire range of
substrates examined, in both ethanol and acetonitrile, as
evidenced from the consistently higher conversions, however,
it is more difficult to use selectivity as a parameter to compare
performance as 3a-c are all highly selective for sulfoxide within
a relatively narrow range between 95-100%, albeit in some
cases at low conversion. Interestingly, 3a gave higher TOFs for
sulfoxidation in ethanol compared with acetonitrile for all but
one substrate; in contrast, 3b and 3c gave higher TOF’s in
acetonitrile than in ethanol for all substrates tested. Moreover,
the performance of 3b and 3c is highly substrate specific with
some quite marked differences in TOF. Interestingly, the
Table 2 Estimated rate constants for the formation of methyl phenyl
sulfoxide (ka) and methyl phenyl sulfone (kb) in ethanol and acetonitrilea
difference in performance between 3a and 3b-c is most clearly
manifested in ethanol as evidenced by the greater disparity in
TOF’s. The contrasting, disparate and solvent dependent
conversions obtained even within this closely related series of
catalysts highlights the complex nature of these PIILP systems,
and, while it is not possible to identify a support-catalyst
performance relationship at this stage, the data in Table 3
suggests that it may well be possible to tailor the ionic
environment on the support to modify and optimise catalyst
efficiency and enhance stability and longevity.
MeCN
EtOH
H2O2
2.5
ka
kb
ka
kb
0.06
0.009
0.068
0.006
a
Data obtained using 4 mmol thioanisole, 12.2 mg 3a, 12 mL solvent, 10 mmol
H2O2 and monitored by analysing 0.2 mL aliquots over 250 min.
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a
Table 3 Selective oxidation of sulfides to sulfoxides with hydrogen peroxide catalysed by [PO4{WO(O2)2}4]@ImPIILP (3a- )
ViewcArticle Online
DOI: 10.1039/C6RA11157B
Ssubstrate
catalyst
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3ae
3be
3ce
3ae
3be
3ce
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3a
3b
3c
3af
3bf
3cf
solvent
EtOH
EtOH
% conversionb
94
% sulfoxideb
91
25
34
74
48
51
82
19
26
73
48
61
73
11
15
67
36
38
36
5
% sulfoneb % sulfoxide selectivityb,c
TOFd
654
173
58
532
337
89
3
0
1
97
100
99
25
34
76
49
52
85
19.5
27
77
49
67
75
11
15
69
38
40
37
5
7
53
23.5
36
65.5
13.5
16.5
72
64.5
45.5
59
11
15
62
48.5
44
100
54.5
69.5
96
89
75
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
EtOH
EtOH
EtOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
2
97
1
98
1
98
3
96
594
76
0.5
1
97
97
182
539
260
436
525
260
102
482
76
4
1
100
99
3
96
2
98
0
0
2
100
100
97
2
96
2
96
271
258
36
1
96
0
0
3
100
100
94
7
52
50
23
35
64
13
16
69
62
44
57
11
15
60
47
43
95
54
69
94
87
74
32
3
376
167
247
459
91
111
499
449
276
474
76
108
436
336
222
697
380
473
675
618
512
143
12
0.5
1
98
97
1.5
0.5
0.5
3
98
96
97
96
2.5
1.5
2
96
97
97
0
0
2
100
100
96
1.5
1
97
99
5
95
0.5
0.5
2
100
100
97
2
98
1
99
41
3
18
9
79
0
5
100
71
13
63
a
b
Reaction conditions: 0.56-0.58 mol% 3a-c, 1 mmol substrate, 2.5 mmol 35% H2O2, 3 mL solvent, 25 °C, 15 min. Determined by 1H NMR spectroscopy using 1,3-
dinitrobenzene as internal standard. c sulfoxide selectivity = [%sulfoxide/(%sulfoxide+%sulfone)] × 100%. d TOF = moles sulfide consumed per mole of catalyst per hour.
Average of 3 runs. e Determined by 13C NMR spectroscopy using 1,3-dinitrobenzene as internal standard. f Reaction conducted at 25 °C for 30 min
6 | J. Name., 2012, 00, 1-3
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Fig. 4
Not surprisingly, high TOFs were obtained for the
sulfoxidation of n-decyl methyl sulfide in ethanol and
acetonitrile with each of the catalysts tested as this substrate is
electron-rich and consequently easy to oxidise; as such it is not
a relaible candidate for differentiating catalyst performance.
The moderate to low conversions obtained for the
View Article Online
DOI: 10.1039/C6RA11157B
[PO4{WO(O2)2}4]@ImPIILP-catalysed
sulfoxidation
of
dibenzothiophene at room temperature in acetonitrile are
consistent with the widely accepted electrophilic pathway and
the lower nucleophilicity of this substrate;
a recent
computational study also supports this pathway50 as do
numerous reports of increasing rates of oxidation with
increasing nucleophilicity of the sulfide.20b,27d,48c,49 The TOF of
143 mol product (mol cat)-1 h-1 obtained with 3a at room
temperature is a significant improvement on that of 9.6 mol
product (mol cat)-1 h-1 for a Merrifield resin supported
peroxomolybdenum(VI) catalyst at 78 °C,27d 25 mol product
(mol cat)-1 h-1 for oxodiperoxomolybdenum(VI) immobilised
onto ionic liquid modified SBA-15,30 4 mol product (mol cat)-1 h-
Proposed mechanism for the peroxotungstate catalysed oxidation of sulfides with
hydrogen peroxide.
Catalyst recycle studies
1
for V2O5 in [C12mim][HSO4] at 45 °C32 and 40 mol product (mol
While ionic liquids have been used as a means to immobilise
and recycle polyoxometalate catalysts this approach is not
always effective since product extraction can lead to leaching of
the catalyst and gradual erosion in the conversion. Reasoning
that the highly ionic microenvironment of polymer immobilised
ionic liquids should efficiently retain the peroxotungstate
during extraction, catalyst recycle experiments were
undertaken using 0.5 mol% 3a for the sulfoxidation of
thioanisole to compare with the corresponding pyrrolidinium-
based ROMP-derived [PO4{WO(O2)2}4]@PIILP system and to
assess the potential for fabricating a continuous flow process.
Ethanol was identified as the solvent of choice for recycle
studies as it combines high selectivity and TOFs with
environmentally green credentials. The reaction time was
reduced from 15 min to 10 min for this study and the catalyst
was recovered by filtration, washed with ethanol, dried and
reused directly without being replenished or reconditioned. The
data in Figure 5 shows that 3a recycled efficiently over 5 runs
with only a minor reduction in conversion and no significant
change in sulfoxide selectivity; thereafter conversions dropped
steadily although selectivity remained at 98% across all twelve
runs.
cat)-1 h-1 for a titanium cyclopentadienyl-silsesequioxane.17e
Unfortunately, it was not possible to obtain reliable data for the
sulfoxidation of dibenzothiophene in ethanol due to its low
solubility in this solvent. Oxidation of allylphenyl sulfide and
homoallylphenyl
sulfide
occurred
with
complete
chemoselectivity for sulfoxide and sulfone with no evidence for
epoxidation of the double bond; this is most likely due to the
mild conditions and short reaction times.19b,20c,27b,d
Reassuringly, the optimum selectivities and TOFs in Table 3
either compete with or are an improvement on those of other
immobilised polyoxo- or peroxometalate-based systems such as
modified Merrifield resin supported peroxomolybdenum(VI),27g
modified SBA-15-based tungstates,27a polyoxometalates hosted
in layered double hydroxides,28 polymeric ionic liquid nanogel-
anchored
tungstates,40
a
divanadium-substituted
phosphotungstate supported on Fe2O3,48 poly(acrylonitrile)-
immobilised peroxotungstate,27d tungstate-based poly(ionic
liquid)
peroxotungstates immobilised on multilayer ionic liquid
brushes-modified silica.27c We believe that catalysts 3a
most
entrapped
magnetic
nanoparticles31
and
-
c
likely operate via a three-step mechanism involving (i) rate
determining attack of sulfide at polymer immobilised ionic
liquid supported peroxotungstate (I) to afford (II), (ii) sulfoxide
conversion
selectivity
100
80
60
40
20
0
dissociation to generate tungsten-oxo (III) and (iii) catalyst
regeneration (Figure 4). As such it should therefore be possible
to control factors that influence catalyst efficacy such as the
accessibility of the active site, the electrophilicity of the active
peroxotungstate and catalyst stability by modifying the ionic
microenvironment of the polymer immobilised ionic liquid
support or introducing additional functional groups and cross
linking.
1
2
3
4
5
6
7
8
9
10 11 12
Recycle Number
Fig. 5 Recycle study for the sulfoxidation of thioanisole in ethanol catalysed by
[PO4{WO(O2)2}4]@ImPIILP (3a).
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Analysis of solvent collected during recovery of the catalyst optimisation studies were conducted using a segmented flow
View Article Online
DOI: 10.1039/C6RA11157B
from the first five runs revealed that the tungsten content was set-up in which 1 mL aliquots of thioanisole (0.2 M) in ethanol
too low to be detected by ICP-OES (i.e. < 1 ppm), a strong and 30% hydrogen peroxide (0.2–0.6 M) were simultaneously
indication that the peroxotungstate was efficiently retained by pumped through a reactor cartridge packed with 2.0 g of silica
the polymer immobilised ionic liquid. Moreover, analysis of (Geduran® Si60 43-60

m) mixed with 0.1
g
of
catalyst recovered after the fifth run gave a tungsten content of [PO4{WO(O2)2}4]@ImPIILP (3a) and using ethanol as the mobile
30.6% which is similar to that of the unused catalyst, a further phase; flow rates were varied with precise control between
indication that leaching was negligible. The IR spectrum of 3a 0.146 and 8.8 mL min-1, which correspond to space velocities
contains bands at 1078 cm-1,

(P-O), 941 cm-1,

(W=O), 588 cm- between 0.033 and 2.0 min-1, respectively, and residence times
1
, 
asym(W-O2) and 529 cm-1,asym(W-O2), which is a close match between 30 and 0.5 min, respectively. The exiting product
to those reported for related systems.51 A sample of catalyst stream was collected in triplicate as 2 mL aliquots, subjected to
recovered after run five contained IR bands that were an aqueous work-up and analysed by either H or 13C NMR
1
essentially superimposable on those of fresh catalyst and an spectroscopy to determine the conversion and selectivity.
SEM image of the sample showed no significant morphological
A survey of the effect of space velocity (sv) on selectivity and
changes, indicating that the peroxotungstate is stable and conversion as a function of the H2O2:thioanisole ratio revealed
remains intact under the reaction conditions; a copy of these IR that the optimum conversion-selectivity profile for the
spectra and the SEM image are provided in the ESI. The gradual sulfoxidation of thioanisole at 25 ⁰C with ethanol as the mobile
erosion in conversion on successive recycles is thought to be phase was obtained with 1.5 equiv. of H2O2 at a space velocity
due to attrition during the filtration and catalyst recovery of 0.1 min-1, details of which are shown in Figure 7. Under these
procedure rather than deactivation as the mass of catalyst conditions, conversions increased gradually with decreasing
recovered after the 12th run (0.011 g) is significantly less than space velocity from 8% for a space velocity of 2 min-1 to 88%
the initial mass of catalyst (0.026 g) used in the first run. To this when this was decreased to 0.1 min-1 while sulfoxide selectivity
end, the turnover frequency of 619 calculated using the mass of decreased slightly from 99% to 94% over the same time. Not
catalyst recovered after run 12 is close to that of 654 obtained surprisingly, when the reactor column was cooled to 0 °C the
in run 1.
space velocity had to be reduced (sv >
0.017 min-1
corresponding to a residence time < 60 min) to reach acceptable
conversions, albeit with no improvement in selectivity which
remained at 94%. Although good conversions were obtained at
shorter residence times when the column was heated to 50 °C
this was at the expense of sulfoxide selectivity which dropped
below 90%; full details of the effect of temperature on the
conversion-selectivity profile are provided in the ESI.
Gratifyingly, the optimum conversion and sulfoxide selectivity
compared favourably with that of 94% and 96% obtained in
batch but with the advantage that a much lower H2O2:substrate
ratio is required. Moreover, the catalyst cartridge could be
stored overnight and reused with only a minor reduction in
performance indicating that the system may be stable and
suitable for use in continuous flow (vide infra).
Segmented and continuous flow
The efficacy of 3a as a catalyst for the selective oxidation of
sulfides under mild conditions coupled with the mechanical
integrity of the system prompted us to extend catalyst testing
to segmented and continuous flow protocols as this would allow
straightforward product separation as well as scale-up and
should overcome the catalyst attrition that occurred during the
batch recycle studies.45 In this regard, there have been
surprisingly few reports of continuous flow sulfoxidation and as
such there is a need to explore this technology to identify
systems that operate under mild conditions and give high
selectivity in short reaction times.
conversion
sulfoxide selectivity
100
80
60
40
20
0
2
1
0.5 0.34 0.25 0.2
space velocity (min-1)
0.1 0.07
Fig. 7 Conversion-selectivity profile as a function of space velocity (sv = volumetric flow
rate/reactor volume) for the [PO4{WO(O2)2}4]@ImPIILP-catalysed sulfoxidation of
thioanisole in ethanol. Reaction conditions: 0.1 g catalyst/2.0 g silica, 1.5 equiv. 35%
H2O2, temp = 25 °C, space velocity 2.0–0.07 min-1.
Fig. 6 Schematic representation of the reactor configuration for segmented and
continuous flow sulfoxidation catalysed by [PO4{WO(O2)2}4]@ImPIILP (3a).
The configuration of the flow system is shown in Figure 6
and is a based on a Uniqsis FlowSyn reactor. Preliminary
The high selectivity and conversion obtained for the
sulfoxidation of dibenzothiophene in acetonitrile under batch
8 | J. Name., 2012, 00, 1-3
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conditions prompted us to explore the potential for developing sulfoxidation of thioanisole in ethanol during the fiVrisetwhAortiuclre aOfntliener
a continuous flow process for oxidative desulfurization of crude which conversions dropped quite dramatDicOaIl:l1y0.w10it3h9/Cti6mRAe11s1u5c7hB
oil as the overwhelming majority of studies involving ionic that the system was completely inactive after 3 h; this was
liquids have focused on batch extraction based protocols.53
A
associated with efficient leaching of the peroxotungstate as
survey of the conversion and selectivity as a function of quantified by ICP analysis (Figure 9b). Having demonstrated that
residence time at 90 °C with acetonitrile as the mobile phase catalyst generated from in-house synthesised polymer
revealed that the concentration of sulfoxide peaked at a space immobilised ionic liquid outperformed that prepared from
velocity of 0.5 min-1, after which sulfone selectivity increased commercially available Merrifield resin modified with
rapidly with increasing conversion, ultimately reaching 96% at a imidazolium ions for sulfoxidations conducted in batch, a
space velocity of 0.07 min-1 as shown in Figure 8. Not performance-time profile was obtained under continuous flow
surprisingly, much lower conversions were obtained at room operation in order to compare the efficiency of this system.
temperature across the range of space velocities examined Under the same conditions, a reactor column packed with
(Figure S91). While this is most likely due to a temperature-rate Merrifield resin-derived 3e on silica showed a steady decrease
affect we cannot rule out temperature dependent changes in in conversion from 65% to 47% together a minor decrease in
the structure of the polymer affecting access of the substrate to selectivity from 94% to 91% (Figure 9c). Although the drop in
the active site.
selectivity was relatively minor, the conversions are markedly
lower than those obtained for 3a under the same conditions
which highlights the advantages of developing polymer
immobilised ionic liquid supports in house.
Sulfide
Sulfoxide
Sulfone
100
80
60
conversion
sulfoxide selectivoity
(a)
100
40
80
60
40
20
0
20
0
2
1
0.5 0.34 0.25 0.2
space velocity (min-1)
0.1 0.07
Fig. 8 Conversion-selectivity profile as a function of space velocity (sv = volumetric flow
rate/reactor volume) for the [PO4{WO(O2)2}4]@ImPIILP-catalysed sulfoxidation of
dibenzothiophene in acetonitrile. Reaction conditions: 0.1 g catalyst/2.0 g silica, 3 equiv.
35% H2O2, MeCN, temp = 90 °C, space velocity 2.0–0.07 min-1.
0
1
2
3
4
5
6
7
8
time-on-stream (hours)
conversion sulfoxide selectivity
(b)
100
80
60
40
20
0
Encouraged by the promising conversion-selectivity profile
achieved under segmented flow, a comparative continuous
flow study was conducted using ethanol as the mobile phase;
parallel reactions were also conducted with freshly prepared
[NEt4]3[PO4{WO(O2)2}4] and Merrifield resin-derived 3e
supported on silica as benchmarks. The continuous flow
sulfoxidation of thioanisole was conducted by purging a catalyst
column packed with a mixture of 3a and silica with a 0.2 M
solution of thioanisole in ethanol and a 0.3 M solution of
peroxide at a rate of 0.44 mL min-1 (sv = 0.1 min-1) at 25 ⁰C and
monitored over an 8 hour period by sampling 5 mL aliquots in
triplicate. The resulting performance-time profile in Figure 9a
shows a slight decrease in conversion with time-on-stream from
87% to 76% while the sulfoxide selectivity remained relatively
stable and constant at 92-94%. Interestingly, this conversion-
selectivity profile is markedly more stable than its ROMP-
derived counterpart in methanol which experienced a 30% drop
in conversion and a concomitant reduction in sulfoxide
selectivity from 77% to 53% after 8 h of continuous
operation.41b A comparative life-time study conducted using a
reactor cartridge packed with [NEt4]3[PO4{WO(O2)2}4] in silica
was also undertaken to further assess the performance of our
optimum POM@PIILP system. Under the same conditions
[NEt4]3[PO4{WO(O2)2}4]/SiO2 was highly active for the
0
1
2
3
4
5
6
7
8
time-on-stream (hours)
conversion sulfoxide selectivity
(c)
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
time-on-stream (hours)
Fig. 9 Conversion-selectivity profile as
a function of time-on-stream (hours) for
continuous flow sulfoxidation of thioanisole catalysed by (a) [PO4{WO(O2)2}4]@ImPIILP
(3a), (b) [NBu4]3[PO4{WO(O2)2}4] and (c) Merrifield-derived [PO4{WO(O2)2}4]@ImPIILP
(3e) each on silica using ethanol as the mobile phase and a residence time of 10 min
(space velocity = 0.1 min-1).
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Finally, the reusability of the catalyst cartridge and the access, improve recyclability and longevity undeVriecwoAnrtticilne uOonluinse
stable conversion-selectivity profile obtained under continuous flow operation and develop aqueousDOpI:h1a0s.1e039c/oCm6RpAa11t1i5b7leB
flow prompted us to conduct a semi-quantitative scale-up and systems, (ii) incorporating coordinating heteroatoms to develop
isolation experiment using ethanol as the mobile phase. Under new supported molecular catalysts and stabilize metal
optimum conditions 2.5 g of thioanisole was processed in 8 nanoparticles and (iii) designing novel architectures such as
hours with a conversion of 82%, a sulfoxide selectivity of 92% nanocapsules and polymeric micelles for use in catalysis.
and a total turnover number (TON) of 12,040; this is a marked
and significant improvement on the 52% conversion obtained
with
ROMP-derived
peroxotungstate-based
Experimental Section
Poly-3-benzyl-1-(4-vinylbenzyl)-1H-imidazol-3-ium bromide-
co-styrene (2a).
[PO4{WO(O2)2}4]@PIILP under similar conditions and in the
same time.
A flame-dried Schlenk flask was charged with AIBN (0.81 g, 4.9
mmol, 5 mol %) followed by 3-benzyl-1-(4-vinylbenzyl)-1H-
Conclusion
Styrene based polymer immobilised ionic liquid supported imidazol-3-ium bromide monomer 1a (11.61 g, 32.8 mmol),
peroxotungstates generated from in-house synthesised styrene (6.8 mL, 66 mmol) and methanol (100 mL) and styrene
imidazolium-decorated styrene co-polymers as well as (6.8 mL, 66 mmol) and the resulting mixture degassed with five
commercially available resins have been evaluated as catalysts freeze/pump/thaw cycles. After reaching ambient temperature
for the selective sulfoxidation of sulfides and their performance the flask was heated to 70 oC and stirred for 72 hours. After this
compared against their ROMP-derived counterparts in order to time the solution was allowed to cool, the volume reduced by
assess the relative merits of each system. Within the limited half and the resulting concentrate added drop-wise into diethyl
range of catalysts tested, performance appears to depend on ether (600 mL) with rapid stirring. The product was isolated by
the nature of the substituents attached to the imidazolium ring filtration, washed with diethyl ether (3 x 50 mL) and dried under
with
in-house
prepared
N-benzyl-based reduced pressure to afford polymer 2a as a white solid (14.0 g,
N-methyl 76%). 1H NMR (400 MHz, CDCl3, δ): 9.67 (br, N-CH-N), 7.89 (br,
[PO4{WO(O2)2}4]@ImPIILP outperforming its
counterparts as well as catalysts prepared from commercially Ar-H), 7.45 (br, Ar-H), 7.38 (br, Ar-H), 7.06 (br, Ar-H), 6.48 (br,
available resins, in most cases by quite some margin. Ar-H), 5.49 (br, Ar-CH2-N), 5.38 (br, Ar-CH2-N), 1.47 (br, CHCH2,
Interestingly, styrene-based [PO4{WO(O2)2}4]@ImPIILP gave polymer backbone). FT-IR (neat, cm-1):
 = 3406, 3057, 3025,
high sulfoxide selectivity in both acetonitrile and ethanol across 2925, 2850, 1601, 1558, 1493, 1452, 1149, 759, 700; Anal. Calc.
the range of substrates examined; this is in stark contrast to for C35H35BrN2 (563.6): C, 74.59; H, 6.26; N, 4.97 %. Found: C,
their ROMP-based counterparts which gave markedly higher 71.69; H, 6.72; N, 5.03%.
sulfoxide selectivities in alcohols compared with acetonitrile.
Ethanol was identified as the solvent of choice for batch Poly-1,2-dimethyl-3-(4-vinylbenzyl)-1H-imidazol-3-ium
reactions on the basis that it gave the optimum balance of chloride-co-styrene (2b).
selectivity and conversion and is in the environmentally Polymer 2b was prepared and purified according to the
preferred class of solvent. The catalyst could be recovered in an procedure described above for 2a and isolated as a white
operationally straightforward procedure and reused in five runs powder in 79% yield. 1H NMR (400 MHz, CDCl3, δ): 7.75 (br, Ar-
before conversions began to decrease. A segmented flow H), 7.06 (br, Ar-H), 6.48 (br, Ar-H), 5.37 (br, Ar-CH2-N), 3.79 (br,
process based on a reactor cartridge packed with the optimum N-CH3), 2.56 (br, N-CHCH3-N), 1.48 (br, CHCH2, polymer
catalyst and silica gave high sulfoxide selectivities and good backbone). FT-IR (neat, cm-1):
 = 3290, 3026, 2923, 2850, 1587,
conversions at short residence times under mild conditions with 1536, 1513, 1493, 1452, 1034, 761, 701; Anal. Calc. for
ethanol as the mobile phase. The catalyst also operated C30H33ClN2 (457.1): C, 78.83; H, 7.28; N, 6.13 %. Found: C, 73.52;
efficiently and with a stable conversion-selectivity profile under H, 6.83; N, 6.57 %.
continuous flow processing with ethanol as the carrier.
Gratifyingly, the performance-time profile over 8h of Synthesis of poly-1-methyl-3-(4-vinylbenzyl)-1H-imidazol-3-
continuous operation was significantly more stable with higher ium chloride-co-styrene (2c).
conversions and sulfoxide selectivities than that for the Polymer 2c was prepared and purified according to the
corresponding ROMP-derived system. We are currently procedure described above for 2a and isolated as a white
exploring the imidazolium-substituent dependent performance powder in 59% yield. 1H NMR (400 MHz, CDCl3, δ): 9.51 (br, N-
of these systems in order to elucidate a composition- CH-N), 7.75 (br, Ar-H), 7.06 (br, Ar-H), 6.49 (br, Ar-H), 5.36 (br,
performance relationship and thereby identify an optimum Ar-CH2-N), 3.87 (br, N-CH3), 1.67 (br, CHCH2, polymer
catalyst-support combination. Future studies will aim to apply backbone), 1.42 (br, CHCH2, polymer backbone). FT-IR (neat,
PIILP technology to a wider range of catalytic transformations cm-1):
 = 3343, 3142, 3056, 3025, 2924, 2849, 1601, 1572,
as well as develop an understanding of how catalyst–support 1493, 1452, 1160, 1031, 760, 700, 619; Anal. Calc. for C29H31ClN2
interactions influence efficiency, this will be achieved by; (i) (443.0): C, 78.62; H, 7.05; N, 6.32%. Found: C, 74.65; H, 6.76; N,
introducing functionality onto the support to modify 6.29 %.
hydrophilicity and porosity in order to facilitate substrate
10 | J. Name., 2012, 00, 1-3
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Synthesis of imidazolium-decorated Merrifield resin (2e)
ARTICLE
functionalised Amberlite as white beads. FT-IR (neat, cm-1):
.
 =
View Article Online
A flame-dried Schlenk flask was charged with imidazole loaded 3401, 3030, 2928, 2362, 2343, 1636, 1614D, 1O4I:7160.,1903294/,C868R5A1,1711575B;
Merrifield resin (1.65 g) and benzyl bromide (2.38 mL, 20.0 Found: C, 44.91; H, 7.66; N, 3.81 %; 16.3 wt% tungsten and a
mmol) in dry acetonitile (20 mL) and the reaction mixture was peroxotungstate loading of 0.223 mmol g-1.
allowed to stir for 72 hours. The reaction mixture was filtered
and washed with acetonitrile (50 mL) and diethyl ether (100 mL) Peroxophosphotungstate loaded imidazolium-decorated
and the resulting solid dried under vacuum to afford the 2e as a Merrifield resin (3e).
white solid (1.15 g). FT-IR (neat, cm-1):
 = 3059, 3025, 2922, Aqueous hydrogen peroxide solution (35% w/w, 4.5 mL, 52
2850, 1601, 1493, 1452, 1151, 1028, 756, 697; CHN Anal. Calc. mmol) was added to a solution of phosphotungstic acid (0.75 g,
based on measured loading of imidazole in 41 N, 2.33%. Found: 0.30 mmol) in water (0.5 mL) and the mixture was stirred at
C, 80.68; H, 7.97; N, 1.43%.
room temperature for 30 minutes. After this time, the solution
was added to a suspension of 2e (0.9 g) in ethanol (47 mL) and
the mixture was stirred for a further 30 minutes after which it
Polymer supported peroxophosphotungstate (3a).
Aqueous hydrogen peroxide solution (35% w/w, 10.2 mL, 118 was added drop-wise into diethyl ether (500 mL) with rapid
mmol) was added to a solution of phosphotungstic acid (1.70 g, stirring. The product was isolated by filtration, washed with
600 µmol) in water (1 mL) and the resulting mixture stirred at diethyl ether (3 × 50 mL) and finally dried under reduced
room temperature for 30 minutes. After this time, a solution of pressure to afford 3e as a white solid (1.2 g, 73%). FT-IR (neat,
2a (1.00 g, 1.80 mmol) in ethanol (50 mL) was added and the cm-1):
 = 3059, 3026, 2922, 2850, 1716, 1602, 1558, 1493,
reaction mixture stirred for a further 30 minutes after which it 1452, 1148, 1029, 960, 814, 755, 697; Anal. Calc. for N6O24PW4
was added drop-wise into diethyl ether (500 mL) with rapid N, 1.86 %. Found: C, 63.46; H, 6.16; N, 0.97 %; 30.2 wt%
stirring. The product was isolated by filtration, washed with tungsten and a peroxotungstate loading of 0.413 mmol g-1.
diethyl ether (3 × 50 mL) and dried under reduced pressure to
afford 3a as an off white solid (1.00 g, 37%). FT-IR (neat, cm-1): General procedure for catalytic sulfoxidation in batch.

= 3140, 3061, 3026, 2925, 1712, 1640, 1602, 1558, 1494, 1453, A flame-dried Schlenk flask was allowed to cool to room
1148, 1029, 943, 887, 814, 756, 700; Anal. Calc. for temperature and charged with sulfide (1.0 mmol), catalyst
C105H105N6O24PW4 (2601.3) C, 48.48; H, 4.07; N, 3.23 %. Found: (0.56-0.58 mol %) and solvent (3 mL). The reaction was initiated
C, 47.45; H, 4.25; N, 3.01 %; 32.3 wt% tungsten and a by the addition of aqueous hydrogen peroxide (35% w/w, 0.21
peroxotungstate loading of 0.414 mmol g-1.
mL, 2.5 mmol) and allowed to stir at room temperature for 15
minutes. The reaction mixture was diluted with
dichloromethane (25 mL), washed with water (50 mL) and the
Polymer supported peroxophosphotungstate (3b).
[PO4{WO(O2)2}4]@ImPIILP 3b was prepared and purified organic extract dried over MgSO4 filtered and the solvent
according to the procedure described above for 3a and isolated removed under reduced pressure. The resulting residue was
1
as a white powder in 49% yield. FT-IR (neat, cm-1):

= 3408, analysed by either H or 13C{1H} NMR spectroscopy to quantify
3140, 3026, 2926, 1614, 1493, 1452, 1422, 1078, 949, 820, 759, the composition of starting material and products; for each
701; Anal. Calc. for C90H99N6O24PW4 (2415.1) C, 44.76; H, 4.13; substrate tested an internal standard of 1,3-dinitrobenzene was
N, 3.48 %. Found: C, 41.29; H, 4.05; N, 3.38 %; 33.9 wt% initially employed to ensure mass balance.
tungsten and a peroxotungstate loading of 0.464 mmol g-1.
General procedure for the catalytic sulfoxidation recycle
Polymer supported peroxophosphotungstate (3c).
studies.
[PO4{WO(O2)2}4]@ImPIILP 3c was prepared and purified A PTFE centrifuge tube containing a magnetic stirrer bar was
according to the procedure described above for 3a and isolated placed in a flame-dried Schlenk flask. The tube was charged with
as a white powder in 29% yield. FT-IR (neat, cm-1):
 = 3411, 3a (0.01146 mmol, 0.58 mol %), sulfide (2.0 mmol) and solvent
3149, 3026, 2925, 1633, 1602, 1562, 1493, 1452, 1425, 1159, (6 mL) and stirred for 2 minutes. The reaction was initiated by
1080, 1029, 956, 869, 836, 756, 700; Anal. Calc. for the addition of aqueous hydrogen peroxide (35% w/w, 0.43 mL,
C87H93N6O24PW4 (2373.0): C, 44.03; H, 3.95; N, 3.54 %. Found: C, 5.0 mmol) and allowed to stir at room temperature for 10
41.04; H, 3.99; N, 3.14 %; 35.0 wt% tungsten and a minutes. After this time the solution was centrifuged (5 min,
peroxotungstate loading of 0.479 mmol g-1.
14,000 rpm), decanted and the remaining PIILP catalyst washed
with the reaction solvent (6 mL), re-centrifuged and the solvent
decanted. The reaction solution was diluted with
Peroxophosphotungstate loaded Amberlite (3d).
Aqueous hydrogen peroxide solution (35% w/w, 11.9 mL, 139 dichloromethane (25 mL), washed with water (50 mL) and the
mmol) was added to a solution of phosphotungstic acid (2.00 g, organic extract dried over MgSO4 filtered and the solvent
700 µmol) in water (1.2 mL) and the resulting mixture stirred at removed under reduced pressure. The resulting residue was
1
room temperature for 30 minutes. After this time, the solution analysed by H NMR spectroscopy to quantify the composition
was passed through
a narrow sinter funnel containing of starting material and products. The residue in the centrifuge
Amberlite IRA 900 chloride form (2.00 g). The Amberlite was tube was re-suspended in solvent and reused without any
then washed with water (50 mL) and Et2O (50 mL) and the further treatment.
solvent removed under reduced pressure to afford the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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Page 12 of 15
ARTICLE
Journal Name
(d) J. Chen, J.-M. Chen, F. Lang, X.-Y. Zhang, L.-F.; Cun, J.; Zhu,
General procedure for the catalytic sulfoxidation kinetic
studies.
View Article Online
J.-G.; Deng and J. Liao, J. Am. Chem. SoDcO.I,: 2100.11003,91/C362R,A411515527B–
4553; (e) Q.-A. Chen, X. Dong, M.-W. Chen, D.-S. Wang, Y.-G.
Zhou and Y.-X. Li, Org. Lett., 2010, 12, 1928–1931; (f) F. Lang,
D.; Li, J.-M. Chen, J. Chen, L.-C. Li, L.-F. Cun, J. Zhu, J.-G. Deng
and J. Liao, Adv. Synth. Catal., 2010, 352, 843–846.
A flame-dried Schlenk flask was allowed to cool to room
temperature and charged with sulfide (4.0 mmol), 3a (0.02
mmol, 0.5 mol %) and solvent (12 mL). The reaction was
initiated by the addition of aqueous hydrogen peroxide (35%
w/w, 0.86 mL, 10.0 mmol) and the resulting mixture stirred at
room temperature for 24 hours during which time 0.2 mL
aliquots were removed for work-up (as above) and analysed by
1H NMR spectroscopy.
4
(a) H. Li, L. He, J. Lu, W. Zhu, X. Jiang, Y. Wang and Y. Tan,
Energy Fuels, 2009, 23, 1354–1357; (b) W. Zhu, H. Li, X. Jiang,
Y. Yan, J. Lu, L. He and J. Xia, Green Chem., 2008, 10, 641–646;
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Eng. Chem. Res., 2008, 47, 6890–6895; (d) H. Li, X. Jiang, W.
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,
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Green Chem., 2011, 13, 1210–1216; (g) W. Zhu, Y. Ding, H. Li,
J. Qin, Y. Yanhong, J. Xiong and Y. Xu, H. Liu, RSC Advances,
General procedure for segmented and continuous flow
catalytic sulfoxidations.
Two reservoirs were charged with sulfide (5.0 mmol) dissolved
in the appropriate solvent (25 mL, 0.2 M) and hydrogen
peroxide (35% w/w) in the same solvent (25 mL, 0.2–0.6 M). A
Uniqsis FlowSyn reactor was used to pump 1.0 mL of each
reagent at total flow rates that varied between 0.146 mL min−1
and 8.8 mL min−1 through a T-piece mixer to combine the two
streams; in the case of segmented flow an additional reservoir
of carrier solvent was also employed. The reaction stream was
then flowed through a OMNIFIT® glass column reactor cartridge
(10 mm id × 100 mm) packed with 0.1 g of [PO4{WO-
(O2)2}4]@PIILP and 2.0 g of SiO2 (Geduran® Si 60) and mounted
in a FlowSyn column heater. The exiting stream was passed
through a back pressure regulator (BPR) and 2 mL fractions
were collected into separate vials followed by a 2 mL post-
collect. Each sample was diluted with dichloromethane (10 mL),
washed with water (ca. 15 mL), the organic extract dried over
MgSO4, the solvent removed under reduced pressure and the
resulting residue analysed by 1H NMR spectroscopy to quantify
the composition of starting material and products.
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Acknowledgements
We gratefully acknowledge Newcastle University for financial
support (A. R. C. and J. R. E). Solid state P-31 and C-13 NMR
spectra were obtained at the EPSRC UK national Solid State
NMR Service at Durham University and high resolution mass
spectra were obtained at the EPSRC National Mass
Spectrometry Service in Swansea.
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DOI: 10.1039/C6RA11157B
Graphical Abstract
Good conversions and high selectivity for sulfoxidation has been achieved under segmented and continuous flow using a
polystyrene-based polymer immobilised ionic liquid phase (PIILP) peroxotungstate.
0.5 mol%
[PO {WO(O ) } ]@ImPIILP
4
2 2 4
SiO2
96% selectivity
EtOH, Rt = 4min