A molybdenum based metallomicellar catalyst for controlled and selective sulfoxidation reactions in aqueous medium

Article abstract of DOI:10.1039/c3gc42245c

A surfactant based molybdenum system that exhibits catalytic activity for sulfoxidation reactions of various organic sulfides in aqueous medium has been developed and comprehensively characterized using IR, XRD, NMR, ESI-MS, DLS and TEM. The catalyst showcases remarkable selectivity for the preparation of both sulfoxides and sulfones in the range of good to excellent yields. Furthermore, the catalyst showed a high degree of tolerance towards various sensitive functional groups such as hydroxyl, acetal, aldehyde, amine, imine, oxime, cyano and alkene. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42245c

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DOI: 10.1039/C3GC42245C
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Molybdenum based metallomicellar catalyst for
controlled and selective sulfoxidation reactions in
aqueous medium
Cite this: DOI: 10.1039/x0xx00000x
Rajan Deepan Chakravarthy, Venkatachalam Ramkumar and Dillip Kumar
Chand*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
A surfactant based molybdenum system that exhibits catalytic activity for sulfoxidation
reaction of various organic sulfides in aqueous medium has been developed and
comprehensively characterized with IR, XRD, NMR, ESI-MS, DLS and TEM. The catalyst
showcases remarkable selectivity for the preparation of both sulfoxides and sulfones in the
range of good to excellent yields. Furthermore the catalyst showed high degree of tolerance
towards various sensitive functional groups such as hydroxyl, acetal, aldehyde, amine, imine,
oxime, cyano and alkene.
www.rsc.org/
Disadvantages of such methods could be the requirement of
high temperature and/or long reaction time, poor catalyst
Introduction
Sulfoxidation is one of the most fundamental reactions in
organic synthesis. Many sulfoxides and sulfones are used as
versatile intermediates for various organic transformations.1
Additionally, these compounds are proved as active
pharmaceutical ingredients for several therapeutic drug
molecules.2Sulfoxidation of organic sulfides is a straight
forward method for the selective preparation of sulfoxides or
sulfones. Traditional approaches of sulfoxidation reactions
involve the usage of peracids3 or halogen derivatives4 however
they yield stoichiometric amounts of environmentally
unfriendly byproducts. Alternative approaches involve the
usage of transition-metal based catalysts along with eco-
friendly hydrogen peroxide as an oxidant, predominantly in
common organic solvents.5,6
recyclability, limited scope of substrates, poor functional group
tolerance, and prerequisite to use promoters or involves
complicated experimental procedures. This necessitates the
development of efficient sulfoxidation processes in aqueous
medium.
Molybdenum element is rich in the field of organic,
inorganic and biological chemistry.10 Oxomolybdenum
compounds have been reported for their catalytic function in
several organic reactions.11,12,13,14 In the recent past, our
research has focused on studying oxidation reactions catalyzed
by molybdenum based systems.15 Notably, we have
successfully developed MoO2Cl2 as a mild and selective
catalyst for sulfoxidation reactions featuring high functional
compatibilities in organic solvents16 and its application to
selectively oxidize sulfides has been reported by other
prominent research groups.17 And in order to deliver more
viable “green” protocols for sulfoxidation reactions, our target
has been to design catalysts that can be used in aqueous
medium over the conventional organic solvents. As metal based
surfactant systems are known to have the desirable attributes of
solubilizing the reactants in the aqueous medium and at the
same time catalyze the reactions,18 in the present study we have
designed and characterized a surfactant based molybdenum
system to catalyze sulfoxidation reactions in water. Its catalytic
activity was analyzed in terms of selectivity and yield for
controlled oxidation of various organic sulfides with hydrogen
peroxide oxidant.
Water is undoubtedly a unique solvent that receives
phenomenal attention in the field of “green” or sustainable
chemistry.7 There are a few reports of self-catalyzed8 and metal-
catalyzed sulfoxidation reactions in aqueous medium.9
Department of Chemistry, Indian Institute of Technology Madras, Chennai,
India, 600036. E-mail: dillip@iitm.ac.in; Fax: +91-44-2257-4202; Tel: +91-
44-2257-4224
†
Electronic Supplementary Information (ESI) available: Experimental
procedures, IR, NMR, ESI-MS for all new compounds i.e. 14a 14b 14c
16b including the catalyst as well as powder and single crystal XRD
data of . CCDC reference number 918215.
,
,
,
1
1
see DOI: 10.1039/b000000x/
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Results and discussion
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DOI: 10.1039/C3GC42245C
An
oxodiperoxo
molybdenum
catalyst
(C19H42N)2[MoO(O2)2(C2O4)]·H2O,
1
(Scheme 1) with
surfactant containing Cetyl Trimethyl Ammonium cation
(CTA) as an integral part of the catalyst was prepared for
studying sulfoxidation reactions in pure water. The catalyst
1
could be conveniently synthesized from affordable and readily
available starting materials such as sodium molybdate, oxalic
acid, hydrogen peroxide and CTA bromide (CTAB).
Fig. 2 Crystal structure of (C19H42N)2[MoO(O2)2(C2O4)]∙H2O, 1.
homogeneity of the bulk complex (Fig. 3). The 1H-NMR
spectrum of the complex was recorded in CDCl3 that displays
1
O
O
Mo
O
signature peaks required to establish the presence of CTA
moiety whereas the 13C-NMR spectrum confirms the existence
of bound oxalate group as well. The observation of the 164.9
and 167.8 ppm peaks indicates the asymmetric environment of
the oxalate ligand upon coordination. ESI-MS study (negative
ion mode) substantiated the composition of the anionic
O
Excess H2O2
H2O

H2O
O
(CTA)2
(H2C2O4) 2H2O
Na MoO 2H2O
+
2 CTA-Br
+
2
4
O
O
O
pH
2
O
1
CH3
CH3
N+
CTA =
CH3
molybdenum entity of the compound
1 with correct isotopic
Scheme 1 Synthesis of the catalyst (C19H42N)2[MoO(O2)2(C2O4)]∙H2O, 1.
pattern. The peaks at m/z = 550.21 corresponds to the anion of
-
[
1
-(CTA+H2O)] fragment. Surfactant compounds are known
to form self-assembled aggregates called micelle. The critical
micelle concentration (CMC) of was determined by
The complex was characterised by IR, NMR, ESI-MS and
XRD techniques. The sharp and strong peaks at 948 and 859
cm-1 in the IR spectrum correspond to the characteristic
(Mo=O) and (O-O) stretching frequencies, respectively and the
peaks at 648 and 583 cm-1 are characteristic to the stretching of
(Mo-(O2)) moieties (Fig. 1). These frequencies are in well
agreement with literature values of similar bonds.19 The
structure of the complex was unambiguously confirmed by
single crystal X-ray diffraction technique. Single crystals
suitable for XRD analysis were obtained by slow evaporation
1
conductivity method and was found to be 2.24 mM (see ESI).
The size and shape of the aggregates in water were determined
using TEM and DLS analysis. TEM micrograph (Fig. 4) clearly
demonstrated that the surfactant complex
1
was aggregated as
of a solution of
was crystallized as monohydrate complex
1
in chloroform:ethanol (1:1). The compound
where the
1
asymmetric unit is composed of one dianionic molybdenum
component, two CTA cations and one water molecule (Fig.2).
The axial positions in the coordination sphere are occupied by
the oxo group and one of the oxygen atoms of the oxalate
ligand, whereas the equatorial positions by four oxygen atoms
of the peroxo groups and the remaining oxygen of the oxalate
ligand. The excellent agreement between the simulated and the
experimental powder XRD data certifies the purity and
Fig.3
Powder
XRD
pattern
of
the
bulk
complex
(C19H42N)2[MoO(O2)2(C2O4)]∙H2O compared with the simulated data
Fig.4 TEM image of complex 1.
Fig.1 IR spectrum of complex (C19H42N)2[MoO(O2)2(C2O4)]∙H2O
2 | J. Name., 2012, 00, 1‐3
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spherical micelle with the average size of 46 ± 7 nm. DLS addition to probing the functional group tolerance (Table 1).
analysis showed that the micelles are of narrow size distribution Typical sulfides like methyl -tolylsulfide, methyl 4-
with the average diameter of about 56 nm (see ESI).
methoxyphenylsulfide and 2-chlorophenyl methyVlsieuwlfAirtdicele Ognalvinee
Next, we explored the abilities of as a catalyst for corresponding sulfoxides in 90-94% yields. However, 4-
p
DOI: 10.1039/C3GC42245C
1
sulfoxidation reactions in aqueous conditions. Due to the mild chlorophenyl methyl sulfoxide was obtained at moderate yield
nature of molybdenum catalysts, functional group of 78%. Octyl phenyl sulfide (17a) and tetrahydrothiophene
compatibilities were also anticipated. To begin with,
(18a) also afforded the corresponding sulfoxides under this
thioanisole, 1a was chosen as a model substrate for catalytic catalytic condition. Organic sulfides having various other
sulfoxidation reactions. To a solution of thioanisole (1.5 mmol) functional groups like alkene, oxime, aldehyde, imine and
in 2.5 mL water, 1 equiv. of aqueous H2O2 and 2.5 mol% acetal were further analyzed for their tolerance.
catalyst,
1 were added at 25 °C and the concentration of the
Table 2 Selective oxidation of sulfides to sulfones.a
Table 1 Selective oxidation of sulfides to sulfoxides.a
Reaction conditions: aCatalyst:sulfide:H2O2 = 0.025:1:3 in 0.5 mL water.
bIsolated yields. cCatalyst:sulfide:H2O2 = 0.025:1:2.2 no additional solvent.
dCatalyst:sulfide:H2O2 = 0.15:1:1 in 1 mL water. eCatalyst:sulfide:H2O2
0.025:1:3 in 4 mL water.
=
Reaction conditions: aCatalyst:sulfide:H2O2
= 0.025:1:1 in 2.5 mL
water.bIsolated yields. cCatalyst: sulfide:H2O2 = 0.15:1:1 in 1 mL water.
Sulfides having oxime, acetals and imines groups are
known to undergo deprotection in presence of acid or acidic
impurities under aqueous medium. Interestingly, the
metallomicellar catalyst selectively oxidized such sulfides to
the corresponding sulfoxides with high selectivity. Sulfide
containing aldehydes did not show any functional group
catalyst was maintained well above the CMC (ca.15 mM in
water). To our delight, the sulfoxide 1b was obtained at 95%
yield with reaction time of 30 min. This result prompted us to
investigate the generality and versatility of our catalytic system.
Having the optimized reaction condition for the model
substrate, we explored the substrate scope of the catalyst in
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transformation other than the targeted sulfoxidation. Similarly group compatibility in producing both sulfoxides and sulfones
sulfides bearing amino, hydroxyl and cyano functional groups in good to excellent yields. The reasonable recyclability and the
were also selectively oxidized in excellent yields. The highest usability of this catalyst in aqueous medium catVeirewtAorticalemOnolirnee
DOI: 10.1039/C3GC42245C
yield of 96% obtained thus far for this catalyst was observed in “green” and eco-friendly solution towards sulfoxidations.
the case of 4-(methylsulfinyl)aniline.20
To further highlight the synthetic utility of our protocol, the
selective oxidation of sulfides to sulfones were also carried out
by varying the amount of oxidant. Under similar conditions,
thioanisole was tested with 3 equiv. of aqueous H2O2 for the
preparation of the corresponding sulfone 1c. The reaction gave
only 80% conversion over a period of 3 hours. Subsequent
optimizations lead in reduction of the amount of water from 2.5
mL to 0.5 mL, with the final concentration of the catalyst being
75 mM in water, resulting in significantly improving yield of
(methylsulfonyl)benzene 1c increased to 97% within just 10
min of reaction time. This illustrates the effect of the catalyst
concentration on oxidation reactions. To explore the scope of
substrates several organic substrates were analyzed for the
preparation of sulfones (Table 2). The reaction proceeded with
high chemo-selectivity and we obtained the highest yield of
98% in the case of 2-(phenylsulfonyl)acetonitrile 13c. It is
noteworthy that even in presence of excess oxidant, a high
degree of functional group tolerance was observed for most of
the sensitive groups like cyano, acetal, imine, aldehyde,
alcohol, amine, oxime and alkene (Table 2) proving the catalyst
to be mild yet effective.
Lastly, recyclability of the catalyst being an attractive
feature, we studied the recyclability for the oxidation of
thioanisole to the corresponding sulfoxide 1b. The reaction
mixture was centrifuged after the first run and the obtained
solid residue was washed with ethyl acetate, dried under
vacuum and reused for at least another three more catalytic runs
without any significant loss of activity. The high yields between
the range of 91-94% (Fig. 5) from such runs indicated the
modest recyclability of this newly developed catalyst.
Experimental
Materials and Methods
Thioanisole, 4-(methylthio)benzaldehyde, 4-nitrothioanisole,
methyl
anisole, 4-chlorothioanisole, allyl sulfide, 4-(methyl thio)aniline
4-(methylmercapto)phenol, tetrahydrothiophene and -toluene
p-tolylsulfide, 4-methoxythioanisole, 2-chlorothio
p
sulfonic acid monohydrate were purchased from sigma-aldrich
company.(Phenylthio)acetonitrile and 1,3-propanediol were
purchased from Alfa-Aesar 4-(methylthio)phenyl methanol, 1-
(4-(methylthio)phenyl)ethanol, 4-(methylthio) benzaldehyde
oxime and octyl phenyl sulfide were prepared following
literature protocol.21 Sodium molybdate dihydrate, oxalic acid
dihydrate, CTAB, aniline, sodium hydroxide, ethanol, diethyl
ether and toluene were purchased from SISCO research
laboratories Pvt. Ltd., India and used without purification. The
oxidant hydrogen peroxide was purchased from Merck
specialities Pvt. Ltd., and the concentration was determined by
permanganate titration method. The deuterated solvents were
obtained from Cambridge Isotope Laboratories. 1H and 13C
NMR spectra were recorded on Bruker Avance-400 instrument.
IR spectra of the complexes were recorded on Jasco FT/IR
4100 instrument by KBr pellet method. Single crystal X-ray
analyses were carried out using Bruker X8 Kappa XRD
instrument. Powder XRD was recorded in BrukerD8 advance
instrument. ESI-Mass spectrumwas recorded on Micromass Q-
Tof mass spectrometer in
a
negative ion mode.
A
SYSTRONICS digital bench top conductivity meter (Model
306) was used for the measurements of critical micellar
concentration. Dynamic light scattering (DLS) experiments
were done on a Malvern Zetasizer nano-series 25 °C. The
wavelength of the laser used was 632.8 nm and the scattering
angle was kept at 90. TEM images were obtained on PHILIPS
CM12 transmission electron microscope, using an accelerating
voltage of 120 kV.
Preparation of 2-(4-(methylthio)phenyl)-1,3-dioxane
To a solution of 4-(methylthio)benzaldehyde (3.8 g, 25 mmol)
in toluene (50 ml), was added 1,3-propanediol (4.41 g, 58
mmol) and p-toluenesulfonic acid monohydrate (50 mg, 0.26
mmol). The reaction mixture was taken in a round bottom flask
fitted with a Dean-stark apparatus and reflux condenser. The
solution was refluxed for two days. The solution was then
allowed to cool to room temperature. Next, the reaction mixture
was washed with 0.1 M aqueous NaOH. The organic layer was
separated, dried over anhydrous sodium sulfate and
concentrated under reduced pressure. The crude product thus
obtained was purified by distillation. Isolated as colorless
Fig. 5 (a) Recyclability of the catalyst (b) figure shows the catalyst
recovery after centrifugation.
Conclusion
In conclusion, we have developed, characterized and
demonstrated the first example of surfactant based
molybdenum system for selective and controlled oxidation of
sulfides in aqueous medium. The catalyst shows high functional
liquid; Yield 71% (3.73 g); IR (NaCl plates-neat)
ν(C-O) 1106
cm-1;1H-NMR (CDCl3, 400 MHz):
δ
=1.46-1.42 (m, 1H), 2.15-
2.29 (m, 1H), 2.47 (s, 3H), 4.01-3.94 (m, 2H), 4.28-4.23 (m,
4 | J. Name., 2012, 00, 1‐3
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);
13C-NMR product thus obtained was purified by column chromatography
2H), 5.46 (s, 1H), 7.24 (BB´, 2H), 7.40 (AA´, 2H
(CDCl3, 100 MHz): = 15.95, 25.85, 67.48, 101.41, 126.47, with hexane: ethyl acetate as an eluent.
126.60, 135.80, 139.24.
δ
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DOI: 10.1039/C3GC42245C
General procedure for the catalytic oxidation of sulfides to
sulfones
Preparation of N-(4-(methylthio)benzylidene)aniline
Aniline (2.33 g, 25 mmol) was added into the solution of 4- A mixture of molybdenum complex (2.5 mol%) and sulfide
(methylthio)benzaldehyde (3.8 g, 25 mmol) in ethanol(25 ml) (1.5 mmol) in 0.5 ml of water was stirred at room temperature.
and the resulting mixture was stirred at room temperature. After Then 40% (w/v)hydrogen peroxide (4.5 mmol) was added
3 hours the solvent was removed under reduced pressure to slowly into the reaction mixture. Stirring was continued for 10
obtain the crude product. The N-(4-(methylthio)benzylidene) min to 8 hours as per requirement. Reaction progress was
aniline thus obtained was purified by crystallization from monitored by TLC. After completion, the aqueous phase is
ethanol. Isolated as pale yellow solid; Yield 75 % (4.26 g) ; mp extracted with ethyl acetate 3-4 times. Then the organic extracts
1
92-94°C; IR (KBr pellet)
400 MHz):
8.4), 7.40-7.36 (m, 2H), 7.79 (d, 2H,
NMR (CDCl3, 100 MHz): = 15.18, 120.99, 125.78, 125.94, solvent was removed under reduced pressure. Most of the
ν
(C=N) 1620 cm-1; H-NMR (CDCl3, were dried over anhydrous sodium sulfate. Molybdenum
δ
= 2.50 (s, 3H), 7.23-7.18 (m, 3H), 7.28 (d, 2H,
J
=
catalyst slowly precipitated out on standing. Centrifuged and
J
= 8.4), 8.38 (s, 1H); 13C- decanted the ethyl acetate layer to remove the catalyst. The
δ
129.20, 129.24, 132.97, 143.32, 152.19, 159.70.
cases, pure sulfone was obtained. Further purification was done
by column chromatography with hexane:ethyl acetate as an
eluent.
Synthesis of (C19H42N)2[MoO(O2)2(C2O4)]·H2O
Catalyst (C19H42N)2[MoO(O2)2(C2O4)]·H2O was synthesized by
a slight modification of literature protocol. Solution A: 5ml of Acknowledgements
0.15954 M (0.193 g, 0.8 mmol) sodium molybdate dihydrate
The fellowship of R. D. Chakravarthy was provided by Council
was added into a beaker containing 5 ml water and the acidity
of the solution was adjusted to pH 2 with 0.1 M sulfuric acid.
Then 1 ml of 40% (w/v) hydrogen peroxide (10 mmol) was
added and the resulting solution was diluted to 20 mL with
water. Solution B: Oxalic acid dihydrate (0.106 g, 0.84 mmol)
was added into a beaker containing 2.5 ml of an aqueous
solution of cetyltrimethyl ammonium bromide (0.604 g, 1.66
mmol) and the resulting solution was diluted to 5 mL with
water. In an ice-cold condition with vigorous stirring solution A
was added slowly drop wise into the solution B and the acidity
of the solution was maintained at pH 2 by adding sulfuric acid.
After 5 min the formation of pale yellow precipitate was
observed. The reaction mixture was centrifuged and the
precipitate was washed 2-3 times with water and dried under
vacuum (Yield 82%).mp 264-268 °C (decomposed). Single
crystals suitable for X-ray analysis were obtained by slow
evaporation of1:1 ethanol-chloroform solution of complex.
Anal. Calcd for C40H86MoN2O10: C, 56.45; H, 10.19; N, 3.29.
Found C, 56.40; H, 10.34; N, 3.34.
of Scientific and Industrial Research (09/84(0595)/12-EMR-I).
We acknowledge Department of Metallurgical and Materials
Engineering, IIT Madras for TEM data and Dr. Edamana
Prasad for DLS facility.
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sulfoxides
7
8
A mixture of molybdenum complex (2.5 mol%) and sulfide
(1.5 mmol) in 2.5 ml of water was stirred at room temperature.
Then 40% (w/v) hydrogen peroxide (1.5 mmol) was added
slowly into the reaction mixture. Stirring was continued for 10
min to 6 hours as per requirement. Reaction progress was
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Green Chemistry
View Article Online
DOI: 10.1039/C3GC42245C
Highlights
Surfactant based molybdenum catalyst for eco-friendly sulfoxidation reactions in aqueous
medium.
Graphical Abstract