Kinetic investigation on the highly efficient and selective oxidation of sulfides to sulfoxides and sulfones with t-BuOOH catalyzed by La2O3

Article abstract of DOI:10.1039/c4ra14391d

The selective oxidation of various sulfides to sulfoxides by a simple, efficient, and environmentally benign method is of prime focus. In this paper, we have explored a highly efficient protocol for the oxidation of alkyl aryl sulfides to sulfoxides with high selectivities catalyzed by La₂O₃ in the presence of 70% t-BuOOH solution (water). We obtained predominantly the monooxygenated product. The over oxidation of sulfides to sulfones was not observed under these conditions. The resulting products are obtained in good to excellent yields within a reasonable time without the use of ligands and other additives. The epoxidation of the double bond as well as allylic oxidation are not observed with allyl sulfides. Sulfones can be obtained quantitatively by altering the reaction conditions. The surface morphology and the catalyst reusability were verified by XRD, AFM and SEM techniques. The surface area of the La₂O₃ was measured using BET isotherms.

Full text of DOI:10.1039/c4ra14391d

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Kinetic investigation on the highly efficient and
selective oxidation of sulfides to sulfoxides and
sulfones with t-BuOOH catalyzed by La2O3†
Cite this: RSC Adv., 2015, 5, 12111
*
Mrinmay Mandal and Debashis Chakraborty
The selective oxidation of various sulfides to sulfoxides by a simple, efficient, and environmentally benign
method is of prime focus. In this paper, we have explored a highly efficient protocol for the oxidation of
alkyl aryl sulfides to sulfoxides with high selectivities catalyzed by La2O3 in the presence of 70%
t-BuOOH solution (water). We obtained predominantly the monooxygenated product. The over
oxidation of sulfides to sulfones was not observed under these conditions. The resulting products are
obtained in good to excellent yields within a reasonable time without the use of ligands and other
additives. The epoxidation of the double bond as well as allylic oxidation are not observed with allyl
sulfides. Sulfones can be obtained quantitatively by altering the reaction conditions. The surface
morphology and the catalyst reusability were verified by XRD, AFM and SEM techniques. The surface area
of the La2O3 was measured using BET isotherms.
Received 12th November 2014
Accepted 13th January 2015
DOI: 10.1039/c4ra14391d
www.rsc.org/advances
stoichiometric oxidants such as peracids, dioxiranes, NaIO4,
MnO2, CrO3, SeO2, and PhIO,1h,12 one major environmental
concern is the generation of a large amount of noxious waste.13
1. Introduction
In recent years, the selective oxidation of suldes to the corre-
sponding sulfoxides and sulfones by environmentally benign
and viable synthetic methodologies has been of growing
interest owing to the increasing environmental, ecological and
economic concerns.1 In chemical sciences, pharmaceutical
sciences and biology, sulfoxides have enormous applications in
the preparation of signicant molecules such as drugs, avors,
germicides, catabolism regulators,2 ligands in asymmetric
catalysis,3 and oxo-transfer reagents.4 These organosulfur
compounds are also important building blocks as chiral
auxiliaries in organic synthesis.5 These compounds are very
useful in the extraction and separation of radioactive and less-
common metals.6 Most importantly, chiral sulfoxides are
widely used in asymmetric synthesis.5b In addition, chiral sulf-
oxides are extensively used as therapeutic agents such as anti-
ulcer (proton pump inhibitor), antibacterial, antifungal,
antiatherosclerotic, anthelmintic, antihypertensive, and
cardiotonic agents.3a,5a,7 The most straightforward and conve-
nient synthetic approach for the production of sulfoxides is the
selective oxidation of suldes. Until now various types of
oxidants such as molecular oxygen,8 hydrogen peroxide,9
organic hydroperoxide,10 hypervalent iodine11 and other
halogen derivatives1g have been well documented in the litera-
ture for the preparation of sulfoxides. With respect to the use of
The catalytic oxidation of suldes with H2O2 using some metal
catalysts is important because of the formation of water as the
only by-product.7a,14 Although H2O2, being easily available and
environmentally benign the oxidation process requires an
excess amount of H2O2 (1.1–8.0 equiv. with respect to sulde) to
attain high yields of the sulfoxides.3b In that context, t-BuOOH
appears to be a perfect alternative as it is inexpensive, high
thermal stability and safe to handle even in large quantities for
the efficient and selective production of sulfoxides.15 In addi-
tion, the probability of decomposition by trace metallic impu-
rities is less than H2O2. The by-product of oxidation, t-BuOH can
be easily removed by distillation or rotary evaporation, thus
avoiding the need for aqueous work up.16 It is worth noting that
metal catalysts are essential for the production of sulfoxides in
high yields and selectivities. There are several reports available
based on some metal-containing catalysts, such as V,17 W,3c,d,f,18
Ti,19 Mn,20 Co,14a Cu,21 Ag,22 Re,23 Ru,24 Os,25 Sc,26 Cr,27 Fe,28 and
Bi29 for the oxidation of suldes to sulfoxides. In recent times,
the oxidation of suldes employing vanadium based catalytic
process along with H2O2 in the presence of ionic liquids as
solvent was described.30 However, this procedure was not suit-
able owing to the high cost and the viscosity associated with
ionic liquids. Recently, Zhao et al. reported lanthanum-
containing polyoxometalate for the oxidation of various
substrates including suldes for the production of sulfoxides
and sulfones in high yields with only one equiv. H2O2 as oxidant
at room temperature.31 Previous research has explored that
peroxotungstates immobilized on mono-layered ionic liquid-
Department of Chemistry, Indian Institute of Technology Patna, Patna-800 013, Bihar,
India. E-mail: dc@iitp.ac.in; debashis.iitp@gmail.com; Tel: +91 6122552171
† Electronic supplementary information (ESI) available: AFM images and BET
isotherm. See DOI: 10.1039/c4ra14391d
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modied silica as well as multilayer ionic liquid brushes- using standard methods. La2O3 salt used in this study was
modied silica performs well for the oxidation of suldes to purchased from Aldrich.
sulfoxides and sulfones with high yields, short reaction time
and high substrate conversion.32 Borax-catalyzed selective
oxidation of organic suldes to sulfoxides and sulfones is
2.3. General procedure for sulfoxidation
To a stirred suspension of sulde (1 mmol) and La2O3
(10 mol%) in 2.5 mL of EtOAc, 70% t-BuOOH (water) (2 equiv.)
was added. The mixture was heated to 90 ꢀC and the progress of
the reaction was monitored using TLC periodically until all
suldes were found to be consumed. Aer completion, the
reaction mixture was washed with EtOAc and H2O and the
organic phase was separated and dried with Na2SO4. The EtOAc
was evaporated and the crude residue was puried by chroma-
tography on a silica gel column to obtain the pure products. For
the preparation of sulfone, to a stirred suspension of sulde
(1 mmol) and La2O3 (10 mol%) in 2.5 mL of EtOAc, 70%
t-BuOOH (water) (6 equiv.) was added. The mixture was heated
to 150 ꢀC and the progress of the reaction was monitored using
TLC periodically until all suldes were found to be consumed.
The work-up procedure was same as above. The spectral data of
the various sulfoxides were found to be satisfactory in accor-
dance with the literature.
obtained by controlling the pH values in high yields and selec-
tivities.33 First time the development of imide-catalyzed NaOCl
oxidation of sulde to sulfone and sulfoxide displayed high
selectivity for the formation of products within reasonable time
at room temperature.34 Recently, Fe–porphyrins supported on
multi-walled carbon nanotubes in the heterogeneous oxidation
of suldes have widely investigated. The results showed high
conversion and selectivity for the formation of sulfoxides and
sulfones within very less time.35 Previously, MoO3 catalyzed
chemoselective oxidation of suldes to sulfoxides and sulfones
in the presence of other oxidized functional groups in ethanol at
50 ꢀC is well documented in the literature.36 Later, from our
research group we reported Bi2O3 catalyzed heterogeneous
oxidation of suldes to sulfoxides with high enantioselectivities
(up to 98% ee) and good yields at room temperature.29 Our
primary goal for the current work was the quantitative produc-
tion of both sulfoxides and sulfones with very high yields and
selectivities. We are happy to inform that we were able to
produce both sulfoxides and sulfones up to the 98% selectivity
within reasonable time. To the best of our knowledge the
synthesis of sulfoxides and sulfones by heterogeneous oxidation
of suldes using La2O3 is still unknown. In this aspect, the
commercialization of an efficient protocol for the oxidation of
different suldes using La2O3 with an equimolar amount of
oxidant with respect to the sulde is still challenging.
2.4. Procedure for the recovery of catalyst
The catalyst, La2O3 was recovered from the reaction mixture to
examine the reusability of the catalyst. Aer the completion of
the reaction the residue was ltered and washed with water
followed by ethanol repeatedly. Next the residue was dried
overnight and tested it's activity as a reusable catalyst. We tested
the reusability up to 5 cycles.
In this paper we report for the rst time a new methodology
for the oxidation of sulde to sulfoxides or sulfones in high
yields and selectivities without the use of expensive ligands and
other additives. The over-oxidized product (sulfones) can be
produced quantitatively under controlled reaction conditions.
2.5. X-ray diffraction (XRD)
The diffraction prole was recorded for each sample in order to
examine the diffraction pattern of the material using Rigaku
TTRAX 3XRD machine in the range of 10–50ꢀ (2q) with CuKa
˚
(1.54 A) as radiation source and 50 kV voltage and 100 mA
intensity.
2. Experimental details
2.1. General information
All solvents were used as the commercial anhydrous grade.
Column chromatography was carried out over silica gel
2.6. Atomic force microscopy (AFM)
The AFM measurements were performed on Agilent 5500
scanning probe microscope. AFM images were recorded in non-
contact mode at room temperature. Commercial probes were
used with a spring constant of 21–98 N mꢁ1 and a resonance
frequency of about 146–236 kHz. For AFM analysis of the La2O3
before and aer the reactions, a drop of sample suspension in
EtOAc was allowed to dry on a mica surface.
1
(100–200 mesh). H and 13C NMR spectra were recorded on a
400 MHz spectrometer. Chemical shis for 1H and 13C NMR
spectra were referenced to residual solvent resonances and are
reported as parts per million relative to SiMe4. ESI-MS spectra of
the samples were recorded using Waters Q-Tof micro mass
spectrometer. GC-MS were recorded using Jeol JMS GC-Mate II
instrument. HPLC analysis was done with Waters HPLC
instrument tted with Waters 515 pump and Waters 2487 dual
l absorbance detector.
2.7. Scanning electron microscopy (SEM)
The surface morphology of the samples were studied with the
help of eld emission scanning electron microscope (Hitachi,
S-4800 FESEM) by applying accelerating voltage of 10 kV. Very
2.2. Materials
All the substrates used in this study were purchased from little amount of the sample was put on a conducting carbon
Aldrich and used as received. The solvents, along with t-BuOOH, tape stuck on the stub and coated with platinum by using
were purchased from Ranchem, India. Solvents were puried Hitachi E-1010 Ion Sputter system.
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Fig. 1 Influence of (a) amount of catalyst, (b) amount of oxidant, (c) reaction temperature, and (d) reaction time on the oxidation of thioanisole.
Table 1 Optimization of reaction conditions for the conversion of methyl(4-tolyl)sulfane to 1-methyl-4-(methylsulfinyl)benzene with different
solvents, oxidants with La2O3 (10 mol%)a
Entry
Catalyst
Solvent
Oxidant
Timeb (h)
Yieldc (%)
Selectivityd (%)
1
2
3
4
5
6
7
8
La2O3
La2O3
La2O3
La2O3
La2O3
La2O3
La2O3
La2O3
—
H2O
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
H2O2
6
5
4
5
7
4
7
10
4
10
8
65
78
85
88
75
90
81
79
10
54
68
88
90
94
95
92
98
90
89
65
69
83
CH2Cl2
MeCN
MeNO2
Toluene
EtOAc
THF
EtOAc
EtOAc
EtOAc
EtOAc
9
10
11
t-BuOOH
t-BuOOH
t-BuOOH
LaCl3
La(OAc)3
a
The methyl(4-tolyl)sulfane (1 mmol), 70% t-BuOOH (water) (2 equiv.) and La2O3 (10 mol%) reuxed in EtOAc. b Monitored using TLC until all the
sulde was found consumed. c Isolated yield aer column chromatography of the crude product. d The selectivities of sulde were analyzed by 1H
NMR.
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Table 2 La2O3 catalyzed selective oxidation of sulfides to sulfoxidesa
Entry
1
Substrate
Timeb (h)
4
Yieldc (%)
90
Conversiond (%)
96
Selectivityd (%)
98
2
3
5
4
88
91
94
95
97
98
4
5
5.5
6
86
88
93
94
96
96
6
5.5
87
95
97
7
8
4.5
6
90
86
96
93
97
95
9
4
5
90
88
96
94
97
95
10
11
12
13
8
5
6
83
85
89
93
94
95
95
96
95
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Table 2 (Contd.)
Entry
14
Substrate
Timeb (h)
Yieldc (%)
Conversiond (%)
Selectivityd (%)
7
6
90
86
96
93
97
96
15
a
Reactions performed in EtOAc with sulde (1 mmol), La2O3 (10 mol%) and 70% t-BuOOH (2 equiv.) under reux condition. b Monitored using TLC
until all sulde was found consumed. c Isolated yield aer column chromatography of the crude reaction mixture. d The conversion of sulde and
selectivities were analyzed by H NMR.
1
one parameter while keeping the others xed (Fig. 1). The
kinetic investigation for the determination of reaction order
with respect to each parameter was carried out by varying the
concentration of one component and keeping the rest of the
components constant under standard reaction conditions. First
we carried out four different sets of reaction varying the
concentration of sulde as 1 mmol, 0.5 mmol, 0.2 mmol and 0.1
mmol while keeping the other reagents constant. Then from
each set of reactions we collected crude mixture at different
time intervals. Aer working up the crude mixture, the HPLC
was run and the kinetic plot was obtained from the percent
conversion of reactants into the product (Fig. 7a). Similarly, the
same result was obtained while varying the concentration of
oxidant (t-BuOOH) and catalyst (Fig. 7b and c respectively).
From the percentage conversion with time, a concentration plot
was obtained which shows the disappearance of the starting
material and the appearance of the corresponding products
(sulfoxide and trace sulfone) with time (Fig. 5). From this data,
log(rate) versus log C was plotted (Fig. 6). Again at different
sulde concentration ranging between 0.1 and 1 mmol, we
plotted log(rate) vs. log C (Fig. 8).
2.8. Surface area analyzer (BET)
BET surface area of La2O3 was measured by nitrogen phys-
isorption isotherms at 77 K using the standard multipoint
method using Smartsorb 92/93, India instrument. Prior to the
analysis samples were degassed in a stream of nitrogen at 473 K
for 24 h.
2.9. General procedure for the kinetic studies
To carry out the different kinetic experiments reversed-phase
C18 HPLC (High Performance Liquid Chromatography)
column and HPLC grade methanol as the eluting solvent were
used. Before running the HPLC, the solvent was sonicated for
30 min. The reaction conditions were optimized by changing
3. Results and discussion
Thioanisole was chosen as a model substrate to optimize the
reaction conditions for the oxidation of various suldes in the
presence of different solvents, oxidants and 10 mol% of La2O3.
The results are summarized in Table 1.
The conversion of methyl phenyl sulde to methyl-
sulnylbenzene is extremely facile in the presence of 10 mol%
La2O3 and 2 equiv. 70% t-BuOOH (water) as the oxidant in
EtOAc under reux condition for 4 h without the use of
expensive ligands and other additives. In EtOAc, the reaction
Fig. 2 Influence of the reaction temperature on the yield for methyl
phenyl sulfone in the oxidation of thioanisole with t-BuOOH at
different reaction times.
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Table 3 La2O3 catalyzed selective oxidation of sulfides to sulfonesa
Entry
1
Substrate
Timeb (h)
8
Yieldc (%)
88
Conversiond (%)
94
Selectivityd (%)
96
2
3
9.5
8
85
89
91
93
94
96
4
5
10
83
86
90
92
93
94
10.5
6
10
84
92
94
7
8
9
88
83
94
90
95
92
10.5
9
8.5
9.5
88
85
94
91
95
92
10
11
12
13
13
9.5
11
81
83
86
91
92
92
93
93
92
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Table 3 (Contd.)
Entry
14
Substrate
Timeb (h)
Yieldc (%)
Conversiond (%)
Selectivityd (%)
12
11
88
83
94
90
95
94
15
a
Reactions performed in EtOAc with sulde (1 mmol), La2O3 (10 mol%) and 70% t-BuOOH (6 equiv.) under reux condition. b Monitored using TLC
until all sulde was found consumed. c Isolated yield aer column chromatography of the crude reaction mixture. d The conversion of sulde and
selectivities were analyzed by H NMR.
1
was very fast and high product conversion was observed. In a further oxidation to the sulfone by decreasing the nucleophi-
protic solvent such as water the reaction was slow and resulted licity of the sulfur atom.37 Thus; the reactivity of the sulfoxide
in very low conversion of reactant into product (Table 1, entry 1). towards peroxide oxidation is lowered. The probability of
We observed less yield and prolonged reaction time upon using sulde oxidation is high over the possibility of allylic oxidation
H2O2 instead of t-BuOOH (Table 1, entry 8). The conversion was as well as epoxidation of a double bond (Table 2, entry 15).38
almost negligible (ꢂ10%) in the absence of La2O3. Aer opti-
Later we optimized the reaction conditions kinetically by
mizing the reaction conditions, in order to test the efficiency of changing one specic parameter while keeping the others
the developed catalytic system, various aryl alkyl and aryl benzyl unaltered. The inuences of the reaction parameters such as
suldes were studied for the oxidation reaction. The results are the amount of catalyst, amount of t-BuOOH, reaction temper-
summarized in (Table 2). Various suldes were oxidized to the ature, and time on the catalytic performances are shown in
corresponding sulfoxides in high yields (ꢂ91%) and selectivities Fig. 1. For the efficient and selective production of sulfoxide, the
(ꢂ98%) without much affected by the steric and electronic optimal reaction conditions were found to be 10 mol% La2O3,
effects of the substituents on the aromatic rings (Table 2). The 2 equiv. 70% t-BuOOH (water), 90 ꢀC reaction temperatures, and
solvation effect of the sulfoxide by water deters the rate of 4 h reaction time.
With increases in the amount of the oxidant t-BuOOH,
reaction temperature and time, the selectivity for the formation
of monooxygenated product (sulfoxide) decreases while the
selectivity for the formation of dioxygenated product (sulfone)
increases quickly. Aer this interesting result, we were curious
Table 4 Results for the La2O3 catalyzed selective oxidation of sulfides
to sulfoxides in different cyclesa
Cycle no.
Timeb (h)
Yieldc (%)
1
2
3
4
5
4
4
4.1
4.1
4.2
90
90
89
89
88.5
a
Reactions performed in EtOAc with sulde (1 mmol), La2O3 (10 mol%)
and 70% t-BuOOH (2 equiv.) under reux condition. b Monitored using
TLC until all sulde was found consumed. c Isolated yield aer column
chromatography of the crude reaction mixture.
Fig. 3 Recycling of the catalyst in the oxidation of thioanisole with t-
BuOOH.
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Fig. 4 X-ray diffraction patterns of the catalyst: (a) before reaction, (b) after 3 run, and (c) after 5 run; FESEM images of the catalyst: (d) before
reaction, (e) after 2 run, (f) after 4 run, and (g) after 5 run.
to see whether the sulfone could be produced quantitatively
Next, the catalyst reusability was veried by the XRD, AFM
using more specied conditions. It can be seen from the data in and FESEM techniques. The powder X-ray diffraction (XRD)
Fig. 2 that sulfone could be quantitatively obtained at a higher patterns of the La2O3 before and aer the reaction are presented
reaction temperature of 150 ꢀC and reaction time of 8 h in Fig. 4a–c in the angular range of 10–50ꢀ 2q. By careful
(Table 3). This conversion needed higher amount (6 equiv.) of observation of all the peaks and the 2d-spacing of different
t-BuOOH. These results are well matched with the previous peaks in the XRD diffraction pattern shows that the catalyst is
literature report.39
unaltered and can be conveniently recovered and recycled
We are delighted to inform that the catalyst can be recovered without notable changes in the yield and the catalytic activity
easily aer washing the residue with water and ethanol (Fig. 4a–c). The 2d-spacing of different peaks of fresh La2O3
˚
˚
˚
˚
˚
repeatedly. The residue was dried overnight and recycled (11.32 A, 6.37 A, 4.56 A and 3.74 A) and La2O3 aer 3 (11.30 A,
˚
˚
˚
˚
˚
˚
conveniently. We have tested up to ve cycles choosing thio- 6.38 A, 4.56 A and 3.74 A) and 5 (11.35 A, 6.38 A, 4.56 A and
˚
anisole as the model substrate. The results are depicted in Fig. 3 3.74 A) run and the representative XRD patterns are identical,
and it can be concluded that no signicant loss of conversion thus proving the reusability of the catalyst.31 To further conrm
and selectivity was observed aer ve runs, indicating reus- the reusability of the catalyst, AFM was done in non-contact
ability of catalyst (Table 4).
mode. AFM images of the catalyst, La2O3 before and aer the
Fig. 5 Concentration of sulfide, sulfoxide and sulfone as a function of
reaction time with 10 mol% La2O3 and 2 equiv. 70% t-BuOOH in EtOAc Fig. 6 van't Hoff differential plot for the oxidation of thioanisole with
reflux at 90 ꢀC.
10 mol% La2O3 and 2 equiv. 70% t-BuOOH in EtOAc reflux at 90 ꢀC.
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Fig. 7 Reaction profile with various concentration of (a) sulfide, (b) oxidant, and (c) catalyst (La2O3) (5% error margin).
reactions, are exhibited in Fig. S1a–c (see ESI†). The same rapidly, reached a maximum (ꢂ98%) at 4 h reaction time and
triangle like morphological representation of the fresh and displayed no change with further reaction time (Fig. 5). In
recovered La2O3 proves the reusability of the catalyst employed addition the selectivity for the formation of sulfone, which is
(Fig. S1a–c) (see ESI†). The SEM images of the pure La2O3 and caused by further oxidation of the sulfoxides, remained almost
the recovered La2O3 aer 2, 4 and 5 run represent the same same over the reaction period of 4 h (Fig. 5).
cotton-like porous nature of the fresh and recovered La2O3. The
Next, we wanted to calculate the order (n) and rate constant
images also show the same irregularly shaped agglomerations (k) using van't Hoff differential method. At different concen-
of fresh and recovered La2O3 (Fig. 4d–g). The surface property tration, the rate of the reaction was calculated by estimating the
and the micro structural analysis conrm that the catalytic slope of the tangent at each point on the concentration plot
property of the La2O3 aer 5 run is retained thus can be used as
a recyclable catalyst.
Next, in order to gain some insight about the surface area of
the catalyst used in this study, we carried out Brunauer–
Emmett–Teller (BET) surface area analysis of the catalyst
employed. BET40 surface area results implied a surface area of
ꢂ11.92 m2 gꢁ1. The adsorption–desorption isotherms and BET
surface area results are depicted in Fig. S2 (see ESI†).
Aer that, we explored kinetic studies for the oxidation of
sulde choosing thioanisole as a testing substrate. High Perfor-
mance Liquid Chromatography (HPLC) was used for carrying out
the experiments as it can determine the concentration of various
starting materials and product present as a function of time. The
reaction kinetics was investigated by monitoring the conversion
of sulde to corresponding sulfoxides at different time intervals.
A concentration plot shows the disappearance of the starting
material and the appearance of the corresponding product with
time (Fig. 5). We observed that initially the conversion increased
Fig. 8 Reaction order for sulfides from 0.1–1.0 mmol concentration.
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Scheme 1 Proposed reaction pathway for the oxidation of sulfide.
which shows the disappearance of the starting material and the range of 0.1–1 mmol. The reaction orders appeared to be weakly
appearance of the corresponding product with time. From this positive at lower sulde concentration but strongly negative at
data, log(rate) vs. log C was plotted (Fig. 6) and the order (n) and relatively higher concentrations (Fig. 8).42
rate constant (k) were obtained from the slope of the line and its
The plausible mechanism for the oxidation of sulde
intercept. From the plot we observe it follows second (Scheme 1) can be explained based on the reports available in
order kinetics (n ¼ 2.003 z 2) and the rate constant is the literature.28b The metal rst binds with t-BuOOH and
5.62 ꢃ 10ꢁ5 L molꢁ1 sꢁ1
.
forms La(IV) active oxidant. These species reacts with the
In the next segment of our study, we wanted to investigate sulde and forms oxidant–substrate complex and the transfer
the reaction order with respect to each parameter by varying of oxygen takes place. The reaction rate is dependent on the
the concentration of one component while keeping the rest of concentrations of the La(IV) active oxidant and sulde
the components constant under standard reaction conditions. substrate. This is supported by the kinetic studies
In the case of sulde, the reaction order was found to be nearly providing second order reaction. The active oxidant and
one at the range of 0.1–1 mmol (Fig. 7a). This result was oxidant–substrate complex intermediate species are transient
expected and only shows that sulde is involved in the rate- in nature thereby allowing the unusual oxidation state on the
limiting step, in good agreement with literature reports.41 La metal.
Again, at low sulde concentration product conversion is less
and at higher sulde concentration product concentration is
more. Performing the reaction by varying the concentration of
sulde and keeping the rest constant, the concentration of the 4. Conclusions
monooxygenated product was maximum with 1.0 mmol of
sulde (Fig. 7a). When the amount of oxidant (t-BuOOH) was
In conclusion, we have explored a new method for the selective
oxidation of various aromatic and aliphatic suldes to the cor-
varied from 0.5–2 equiv., the concentration of the resulting
responding sulfoxides with 70% t-BuOOH (water) as the oxidant
product was maximum at 2 equiv. of t-BuOOH. The reaction
catalyzed by La2O3 without the use of ligands and other addi-
tives. The described method in this paper clearly indicate that
the methodology involves mild, environmentally friendly and
order was approximately one over the entire range of 0.5–2
equiv. of t-BuOOH (Fig. 7b).
Next, we were interested to see the effect of catalyst loadings
selective synthesis of sulfoxides from both aromatic and
on the reaction environment. In the case of catalyst (La2O3), the
aliphatic suldes. The resulting products were obtained in good
yields. By changing the reaction conditions sulfones can be
prepared quantitatively. Epoxidation and allylic oxidation
products are hardly observed. Kinetic studies gave information
reaction order was found to be approximately one at the range
of 0.5–10 mol% catalytic loadings (Fig. 7c). Here too, the
maximum product formation was noted with 10 mol% of La2O3
when the amount of catalyst was varied from 0.5–10 mol%. At
about the optimal reaction conditions for the oxidation reaction
low catalytic loadings of La2O3 we observe very less conversion
to occur and rate constant of this reaction. The order of the
of reactants into the product but at higher catalyst loadings
reaction with respect to the different reaction parameters is also
product concentration is more.
studied. The catalyst reusability was veried by careful obser-
Next, we studied the reaction order for sulde. We observed
vation of the surface morphology using XRD, AFM and SEM
techniques. The surface area of the La2O3 was measured using
sulde displayed variable reaction order in the concentration
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BET isotherm. The results of our method are comparable with
the previous metal oxides reported protocol.
7 (a) J. Legros, J. R. Dehli and C. Bolm, Adv. Synth. Catal., 2005,
347, 19–31; (b) P. K. Khatri, S. L. Jain and B. Sain, Ind. Eng.
Chem. Res., 2011, 50, 701–704.
8 T. Chinnusamy and O. Reiser, ChemSusChem, 2010, 3, 1040–
1042.
Acknowledgements
9 (a) F. Shi, M. K. Tse, H. M. Kaiser and M. Beller, Adv. Synth.
Catal., 2007, 349, 2425–2430; (b) R. Ricoux, M. Allard,
R. Dubuc, C. Dupont, J. D. Marechal and J. P. Mahy, Org.
Biomol. Chem., 2009, 7, 3208–3211; (c) C. B. Yang,
H. Zhang, J. Liao, J. Zhu, J. G. Deng, Q. P. Jin and B. Yu,
Green Chem., 2009, 11, 1401–1405.
The authors thank Indian Institute of Technology Patna for
nancial support.
Notes and references
10 S. Kumar, S. Verma, S. L. Jain and B. Sain, Tetrahedron Lett.,
2011, 52, 3393–3396.
1 (a) P. S. Kulkarni and C. A. M. Afonso, Green Chem., 2010, 12,
´
´
1139–1149; (b) M. V. Gomez, R. Caballero, E. Vazquez, 11 (a) J. N. Moorthy, K. Senapati and K. N. Parida, J. Org. Chem.,
´
´
A. Moreno, A. Hoz and A. Dıaz-Ortiz, Green Chem., 2007, 9,
331–336; (c) B. Karimi, M. Ghoreishi-Nezhad and
J. H. Clark, Org. Lett., 2005, 7, 625–628; (d) R. Noyori,
2010, 75, 8416–8421; (b) J. N. Moorthy, K. Senapati,
K. N. Parida, S. Jhulki, K. Sooraj and N. N. Nair, J. Org.
Chem., 2011, 76, 9593–9601.
¨
M. Aoki and K. Sato, Chem. Commun., 2003, 1977–1986; (e) 12 J.-E. Backvall, in Modern Oxidation Methods, ed. J.-E.
¨
Backvall, Wiley-VCH, Weinheim, 2004, pp. 193–222.
K. Sato, M. Aoki and R. Noyori, Science, 1998, 281, 1646–
1647; (f) K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng and 13 (a) D. H. R. Barton, W. Li and J. A. Smith, Tetrahedron Lett.,
R. Noyori, Tetrahedron, 2001, 57, 2469–2476; (g)
P. Kowalski, K. Mitka, K. Ossowska and Z. Kolarska,
Tetrahedron, 2005, 61, 1933–1953; (h) K. Kaczorowska,
1998, 39, 7055–7058; (b) M. Hirano, S. Yakabe, J. H. Clark
and T. Morimoto, J. Chem. Soc., Perkin Trans. 1, 1996,
2693–2698.
Z. Kolarska, K. Mitka and P. Kowalski, Tetrahedron, 2005, 14 (a) M. D. A. Maria, P. Mastrorilli and C. F. Nobile, J. Mol.
61, 8315–8327; (i) B. M. Choudary, B. Bharathi, C. V. Reddy
and M. L. Kantam, J. Chem. Soc., Perkin Trans. 1, 2002,
2069–2074; (j) J. J. Boruah, S. P. Das, S. R. Ankireddy,
S. R. Gogoi and N. S. Islam, Green Chem., 2013, 15, 2944–
2959.
Catal. A: Chem., 1996, 108, 57–62; (b) H. Fakhraian and
F. Valizadeh, J. Mol. Catal. A: Chem., 2010, 333, 69–72; (c)
J. Fujisaki, K. Matsumoto, K. Matsumoto and T. Katsuki,
J. Am. Chem. Soc., 2011, 133, 56–61.
15 K. B. Sharpless and T. R. Verhoeven, Aldrichimica Acta, 1979,
12, 63–73.
2 (a) P. Metzner and A. Thuillier, Sulfur Reagents in Organic
Synthesis, Academic Press, London, 1994; (b) The chemistry 16 P. J. Kropp, G. W. Breton, J. D. Fields, J. C. Tung and
of sulfones, sulfoxides and cyclic suldes, ed. S. Patai, Z. B. R. Loomis, J. Am. Chem. Soc., 2000, 122, 4280–4285.
Rappoport and C. Stirling, Wiley, Chichester, 1994; (c) 17 (a) C. Bolm and F. Bienewald, Angew. Chem., Int. Ed., 1996,
B. Li, A.-H. Liu, L.-N. He, Z.-Z. Yang, J. Gao and
K.-H. Chen, Green Chem., 2012, 14, 130–135.
34, 2640–2642; (b) M. Al-Hashimi, E. Fisset, A. C. Sullivan
and J. R. H. Wilson, Tetrahedron Lett., 2006, 47, 8017–8019;
(c) M. R. Maurya, A. K. Chandrakar and S. Chand, J. Mol.
Catal. A: Chem., 2007, 263, 227–237; (d) I. Khedher,
A. Ghorbel, J. M. Fraile and J. A. Mayoral, J. Mol. Catal. A:
Chem., 2006, 255, 92–96; (e) Y. Jeong, S. Choi, Y. D. Hwang
and K. H. Ahn, Tetrahedron Lett., 2004, 45, 9249–9252; (f)
J. Sun, C. Zhu, Z. Dai, M. Yang, Y. Pan and H. Hu, J. Org.
Chem., 2004, 69, 8500–8503; (g) A. M. Khenkin, G. Leitus
and R. Neumann, J. Am. Chem. Soc., 2010, 132, 11446–11448.
´
3 (a) I. Fernandez and N. Khiar, Chem. Rev., 2003, 103, 3651–
3706; (b) K. Kamata, T. Hiranoa and N. Mizuno, Chem.
Commun., 2009, 3958–3960; (c) R. Dorta, H. Rozenberg,
L. J. W. Shimon and S. Milstein, Chem.–Eur. J., 2003, 9,
5237–5249; (d) R. Tokuno, M. Sodeoka, K. Aoe and
M. Shibasaki, Tetrahedron Lett., 1995, 36, 8035–8038; (e)
¨
A. Thorarensen, A. Palmgren, K. Itami and J.-E. Backvall,
Tetrahedron Lett., 1997, 38, 8541–8544; (f) D. G. I. Petra,
P. C. J. Kamer, A. L. Spek, H. E. Schoemaker and 18 N. N. Mahamuni, P. R. Gogate and A. B. Pandit, Ultrason.
P. W. N. M. van Leeuwen, J. Org. Chem., 2000, 65, 3010–3017. Sonochem., 2007, 14, 135–142.
4 (a) A. J. Mancuso and D. Swern, Synthesis, 1981, 165–185; (b) 19 J. S. Gao, H. C. Guo, S. Z. Liu and M. Wang, Tetrahedron Lett.,
V. Y. Kukushkin, Coord. Chem. Rev., 1995, 139, 375–407; (c) 2007, 48, 8453–8455.
A. M. Khenkin and R. Neumann, J. Am. Chem. Soc., 2002, 20 (a) F. Hosseinpoor and H. Golchoubian, Tetrahedron Lett.,
124, 4198–4199.
5 (a) M. C. Carreno, Chem. Rev., 1995, 95, 1717–1760; (b)
H. Pellissier, Tetrahedron, 2006, 62, 5559–5601; (c)
2006, 47, 5195–5197; (b) J. Brinksma, R. L. Crois,
B. L. Feringa, M. I. Donnoli and C. Rosini, Tetrahedron
Lett., 2001, 42, 4049–4052.
A. L. Nuzhdin, D. N. Dybtsev, V. P. Fedin and 21 R. Das and D. Chakraborty, Tetrahedron Lett., 2010, 51, 6255–
G. A. Bukhtiyarova, Dalton Trans., 2009, 10481–10485. 6258.
6 (a) J. P. Shukla, R. K. Singh, S. R. Sawant and N. Varadarajan, 22 R. Das and D. Chakraborty, Synthesis, 2011, 2, 277–280.
Anal. Chim. Acta, 1993, 276, 181–187; (b) M. H. Khan, 23 J. B. Arterburn and S. L. Nelson, J. Org. Chem., 1996, 61,
S. M. Hasany, M. A. Khan and A. Ali, Radiochim. Acta, 1994,
65, 239–243.
2260–2261.
This journal is © The Royal Society of Chemistry 2015
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View Article Online
RSC Advances
Paper
24 X. T. Zhou, H. B. Ji, Z. Cheng, J. C. Xu, L. X. Pei and 36 M. M. Khodaei, K. Bahrami and M. Khedri, Can. J. Chem.,
L. F. Wang, Bioorg. Med. Chem. Lett., 2007, 17, 4650–4653. 2007, 85, 7–11.
25 B. M. Choudary, C. R. V. Reddy, B. V. Prakash, M. L. Kantam 37 (a) C. G. Overberger and R. W. Cummins, J. Am. Chem. Soc.,
and B. Sreedhar, Chem. Commun., 2003, 754–755.
26 M. Matteucci, G. Bhalay and M. Bradley, Org. Lett., 2003, 5,
235–238.
27 L. Xu, J. Cheng and M. L. Trudell, J. Org. Chem., 2003, 68,
5388–5391.
28 (a) J. Legros and C. Bolm, Angew. Chem., Int. Ed., 2003, 42,
5487–5489; (b) D. Chakraborty, P. Malik and V. K. Goda,
Appl. Organomet. Chem., 2012, 26, 21–26.
1953, 75, 4783–4787; (b) R. Curci, R. A. DiPrete, J. O. Edwards
and G. Modena, J. Org. Chem., 1970, 35, 740–745; (c)
M. Ochiai, A. Nakanishi and T. Ito, J. Org. Chem., 1997, 62,
4253–4259.
38 (a) M. Ochiai, T. Ito, Y. Masaki and M. Shiro, J. Am. Chem.
Soc., 1992, 114, 6269–6270; (b) M. Ochiai, T. Ito,
H. Takahashi, A. Nakanishi, M. Toyonari, T. Sueda, S. Goto
and M. Shiro, J. Am. Chem. Soc., 1996, 118, 7716–7730.
29 P. Malik and D. Chakraborty, Tetrahedron Lett., 2012, 53, 39 (a) P. Tundo, G. P. Romanelli, P. G. Vazquez and F. Arico,
5652–5655.
Catal. Commun., 2010, 11, 1181–1184; (b) G. P. Romanelli,
P. G. Vazquez and P. Tundo, Synlett, 2005, 1, 75–78; (c)
P. Zhao, M. Zhang, Y. Wu and J. Wang, Ind. Eng. Chem.
Res., 2012, 51, 6641–6647.
30 V. Conte, F. Fabbianesi, B. Floris, P. Galloni, D. Sordi,
I. C. W. E. Arends, M. Bonchio, D. Rehder and D. Bogdal,
Pure Appl. Chem., 2009, 81, 1265–1277.
31 S. Zhao, Y. Jia and Y.-F. Song, Appl. Catal., A, 2013, 453, 188– 40 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc.,
194. 1938, 60, 309–319.
32 (a) X. Y. Shi and J. F. Wei, J. Mol. Catal. A: Chem., 2008, 280, 41 (a) J. W. Tye, Z. Weng, A. M. Johns, C. D. Incarvito and
142–147; (b) X. Shi, X. Han, W. Ma, J. Wei, J. Li, Q. Zhang and
Z. Chen, J. Mol. Catal. A: Chem., 2011, 341, 57–62.
33 S. Hussain, S. K. Bharadwaj, R. Pandey and M. K. Chaudhuri,
Eur. J. Org. Chem., 2009, 3319–3322.
J. F. Hartwig, J. Am. Chem. Soc., 2008, 130, 9971–9983; (b)
E. R. Strieter, B. Bhayana and S. L. Buchwald, J. Am. Chem.
Soc., 2009, 131, 78–88; (c) G. O. Jones, P. Liu, K. N. Houk
and S. L. Buchwald, J. Am. Chem. Soc., 2010, 132, 6205–6213.
34 N. Fukuda and T. Ikemoto, J. Org. Chem., 2010, 75, 4629– 42 (a) P.-F. Larsson, C. Bolm and P.-O. Norrby, Chem.–Eur. J.,
4631.
2010, 16, 13613–13616; (b) R. Das, M. Mandal and
D. Chakraborty, Asian J. Org. Chem., 2013, 2, 579–585.
35 A. Rezaeifard and M. Jafarpour, Catal. Sci. Technol., 2014, 4,
1960–1969.
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