Selective oxidation of organic sulfides to sulfoxides using sugar derived cis-dioxo molybdenum(VI) complexes: Kinetic and mechanistic studies

Article abstract of DOI:10.1039/c6ra01087c

Six Mo(vi) complexes of 4,6-O-ethylidene-β-d-glucopyranosylamine derived ligands have been used for the selective oxidation of five organic sulfides to corresponding sulfoxides. The structure of a new sulfide [(((2-(phenylthio)phenyl)imino)methyl)naphthal

Full text of DOI:10.1039/c6ra01087c

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Selective oxidation of organic sulfides to sulfoxides
using sugar derived cis-dioxo molybdenum(VI)
complexes: kinetic and mechanistic studies†
Cite this: RSC Adv., 2016, 6, 28015
*
Noorullah Baig, Vimal Kumar Madduluri and Ajay K. Sah
Six Mo(VI) complexes of 4,6-O-ethylidene-b-D-glucopyranosylamine derived ligands have been used for
the selective oxidation of five organic sulfides to corresponding sulfoxides. The structure of a new sulfide
[(((2-(phenylthio)phenyl)imino)methyl)naphthalen-2-ol] (S5) and its corresponding sulfoxide (SO5) has
been established using single crystal X-ray diffraction studies along with other routine analytical
techniques. The yields of sulfoxides were calculated using HPLC and one of the catalysts was recycled
five times without any appreciable loss in its activity. Kinetic studies on sulfoxide formation was
performed on compound S5 using UV-visible spectroscopy, which suggested overall first order kinetics.
A plausible mechanism for sulfide oxidation has been proposed based on our findings and previous
literature reports.
Received 13th January 2016
Accepted 3rd March 2016
DOI: 10.1039/c6ra01087c
www.rsc.org/advances
conditions for the oxidation of thioanisole (S1) to methyl phenyl
sulfoxide (SO1) with respect to the solvent, catalytic loading,
1. Introduction
reaction timing etc., and the ndings have been already
communicated.18 The best yield of sulfoxide was isolated from
the reaction performed in ethanol, using commercially avail-
able urea–hydrogen peroxide (UHP) as a mild oxidant.
Sulfoxides are important from a biological as well as a chemical
point of view due to their utility in drug development,1 asym-
metric synthesis2 and oxo-transfer reagents like that in Swern
oxidation.3 They are used in the preparation of several biologi-
cally and pharmaceutically important compounds like com-
pactin, ML-236A (antihyperlipidimic agent),4 tacrolimus or FK-
506 (immunosuppressive agent),5 11-oxoequilenin methyl
ether (steroid),6 and 12,13-epoxytrichothec-9-ene (having a wide
range of biological properties)7 etc. In the context of organic
synthesis, sulfoxides are used in carbon–carbon bond forma-
tion,8–10 the Diels–Alder reaction11–13 etc. Due to the versatile use
of sulfoxides, their preparation from corresponding organic
suldes are well documented, however, the methods have
several drawbacks like over-oxidation of the substrate to
sulfone, longer reaction times, selectivity problems, and the use
of strong oxidising agents like hydrogen peroxide (H2O2) and
meta-chloroperoxybenzoic acid (m-CPBA)14–17 etc. Hence, the
need of the current scenario is to develop an environmentally
benign protocol having sustainable and greener reaction
conditions. In this venture, we are developing the methodology
for selective oxidation of organic suldes into corresponding
sulfoxides using glucose derived cis-dioxo Mo(VI) complexes as
the catalyst. At the initial stage, we had optimized the
Molybdenum is one of the essential and less toxic metals,
which easily form complexes with a variety of biologically
important compounds like carbohydrates,18,19 porphyrins,20,21
avins22,23 and amino acids.24 It plays a vital role in the activities
of xanthine oxidase, nitrate reductase, nitrogenase, DMSO
reductase25,26 etc. Our initial success in the selective oxidation of
S1 to SO1 using the glucose derived cis-dioxo molybdenum(VI)
complex under mild conditions, prompted us to make the
methodology sustainable and reproducible for general organic
suldes. Along this line, we have used six complexes (Fig. 1),
including two new ones as a catalyst for selective oxidation of
organic suldes to corresponding sulfoxides (Scheme 1). Five
different suldes were tested for oxidation, and the (%)
formation of sulfoxides were obtained using the HPLC method.
Turn over number and turn over frequency (per hour) were also
calculated for all the reactions and the same were found in the
range of 7.4–19.6 and 12–78 respectively.
Kinetic studies were performed on the oxidation of S5 using
UV-visible spectroscopy and several control reactions were
performed to ascertain the reaction paths of sulde oxidation.
Based on our current studies and the reported literature, we
have proposed a plausible mechanism for this reaction. Hence,
this paper deals with the synthesis of a new sulde, selective
catalytic oxidation of organic suldes to sulfoxides using sugar
derived cis-dioxo Mo(VI) complexes and mechanistic aspects of
this oxidation reaction.
Department of Chemistry, Birla Institute of Technology and Science, Pilani Campus,
Pilani, Rajasthan-333031, India. E-mail: asah@pilani.bits-pilani.ac.in; Fax: +91-
1596-244183; Tel: +91-1596-515662
† Electronic supplementary information (ESI) available: Crystallographic data for
structure S5 and SO5. CCDC 1445051 and 1445052. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c6ra01087c
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Fig. 1 cis-Dioxo molybdenum(VI) complexes of H3Ln.
SHELX-97.29 The crystallographic gures were generated using
ORTEP 3v2 (ref. 30) and mercury 3.3.31
2. Experimental
2.1. General
5-Bromosalicylaldehyde, ortho-vanillin, 2-(phenylthio)aniline
(S4), 2-hydroxy-1-napthaldehyde and UHP were procured from
Sigma-Aldrich India, while diphenyl sulde (S2), benzyl phenyl
sulde (S3) and molybdenum(VI) oxide bis(2,4-pentanedionate)
[MoO2(acac)2] were from Alfa Aeser. The complexes 5 and 6
were synthesized following the procedure reported for complex
1.27 All other chemicals and solvents were purchased from local
vendors and solvents were puried and dried following the
standard methods before use. All the experiments were per-
formed at room temperature under normal atmospheric
conditions.
2.2. Preparation of complex 5
To a stirring solution of H3L5 (0.1176 g, 0.30 mmol) in methanol
(5 mL), MoO2(acac)2 (0.0978 g, 0.30 mmol) was added at room
temperature and the reaction was continued for 13 h. The
workup was done similarly to that reported for complex 2,18 to
isolate the yellow solid product. Yield: 0.134 g (81%); mp:
chared at 208–210 ꢂC; UV-vis [lmax; nm (3; L cmꢁ1 molꢁ1) in
DMSO]: 260 (10 511), 348 (1980); IR (KBr; cmꢁ1): 3371, 1643,
1
1088, 903. H NMR (DMSO-d6, 400 MHz, ppm): d 8.57 (1H, s,
HC]N), 8.04 (1H, br, ArH), 7.61 (1H, d, J ¼ 6.0 Hz, ArH), 6.89
(1H, d, J ¼ 7.6 Hz, ArH), 5.65 (1H, br, glucose OH), 4.76 (2H, m,
glucose H-1, ethylidene CH), 4.13 (4H, m, glucose H-5, meth-
anolic-CH3), 3.80–3.05 (5H, m, glucose, H-2, H-3, H-4, H-6a,b),
1.26 (3H, br, ethylidene CH3); 13C NMR (DMSo-d6, 100 MHz,
ppm): d 161.4 (C]N), 158.7, 137.7, 136.8, 123.1, 122.1, 110.7,
99.3, 91.2, 85.6, 81.0, 73.4, 69.9, 67.6, 49.0, 20.6; HRMS: m/z
calcd for (M + H)+ C15H17BrMoNO8 515.9192; found 515.9358,
without solvent.
UV-visible absorption spectra were recorded on a Shimadzu
UV-260 spectrophotometer and FTIR spectra on an ABB Bomen
1
MB 3000 FTIR machine using a KBr Matrix. H and 13C NMR
spectra were recorded in DMSO-d6 and CDCl3 on a Bruker
Avance spectrometer and data was processed using MestReNova
soware. HRMS and ESI-MS were recorded on a Thermo
scientic Q EXACTIVE and Hewlett-Packard’ HP GS/MS 5890/
5972 mass spectrometer respectively. The yields of sulfoxides
were calculated by HPLC separation using a Waters Sunre C-18
column of 5 mm, 4.6 ꢀ 250 mm, a ow of 0.85 mL minꢁ1
;
2.3. Preparation of complex 6
ꢂ
wavelength of 315 nm; temperature of 28 C; injection volume
of 20 mL; eluent of water/MeOH (v/v) 0.18/0.67. Calibration was
done using 30, 40, 50, 60, 70 and 80 ppm, of sulfoxide solutions.
Single crystal X-ray diffraction data were collected on
a Rigaku XtaLAB mini diffractometer using graphite mono-
chromated Mo-Ka radiation. An empirical absorption correc-
tion was applied on the data and all calculations were
performed using the CrystalStructure28 crystallographic so-
ware package except for renement, which was performed using
This compound was prepared following the procedure adopted
for complex 5, but using (H3L6) (0.1020 g, 0.30 mmol) and
MoO2(acac)2 (0.0978 g, 0.30 mmol). Yield: 0.130 g (89%); orange
solid; mp: chared 233–235 ꢂC; UV-vis [lmax; nm (3; L cmꢁ1
molꢁ1) in DMSO]: 287 (12 336), 361 (2347); IR (KBr; cmꢁ1): 3333,
1643, 1088, 903. 1H NMR (DMSO-d6, 400 MHz, ppm): d 8.52 (1H,
s, HC]N), 7.32 (1H, d, J ¼ 7.2 Hz, ArH), 7.19 (1H, d, J ¼ 7.6,
ArH), 6.91 (1H, m, ArH), 5.63 (1H, s, glucose OH), 4.72 (2H, m,
ethylidene CH, glucose H-1), 4.18 (1H, m, glucose H-5), 3.90–
3.10 (8H, m, glucose H-2, H-3, H-4, H-6a,b, methanolic-CH3),
Scheme 1 Synthetic protocol of sulfoxides using sugar derived Mo(VI) complexes.
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with diethyl ether and dried under vaccum. Yield: 86%, yellow
crystalline solid; mp: 140–142 C; IR (KBr; cmꢁ1) UV-vis [lmax
;
ꢂ
nm (3; L cmꢁ1 molꢁ1) in ethanol]: 315 (5599), 442 (4817), 465
(4531). 1H NMR (CDCl3, 400 MHz, ppm): d 15.24 (1H, d, J ¼ 2.4
Hz, ArOH), 9.40 (1H, d, J ¼ 2.4 Hz, HC]N), 8.13 (1H, d, J ¼ 8.4
Hz, ArH), 7.84 (1H, d, J ¼ 9.2 Hz, ArH), 7.76 (1H, d, J ¼ 7.6 Hz,
ArH), 7.54 (1H, m, ArH), 7.48–7.14 (11H, m, ArH); 13C NMR
(CDCl3, 100 MHz, ppm) d 167.6, 155.8, 145.7, 136.2, 134.0,
133.1, 132.2, 131.5, 131.1, 129.3, 129.3, 128.1, 128.0, 127.6,
127.5, 127.0, 123.6, 121.4, 119.1, 118.1, 109.3. Slow evaporation
of the hexane solution of S5 yielded X-ray suitable single crystals
and the structure was established using single crystal X-ray
crystallography.
Fig. 2 Structure of 4,6-O-ethylidene-N-(2-hydroxybenzylidene)-b-
D-glucopyranosylamine (H3L1) derived cis-dioxo Mo(VI) complex.27
Table 1 Summary of sulfide oxidation using D-glucose derived cis-
dioxo molybdenum(VI) complexes as the catalysta and UHP as the mild
oxidizing agent
2.5. General procedure for selective oxidation of organic
suldes
Entry
Complex
Sulfoxide
Yieldb (%)
TON
TOF/h
To a stirring suspension of molybdenum complexes (1–6; 0.05
mmol) and suldes (1 mmol) in 3 mL of ethanol, UHP (1 mmol)
was added at room temperature. The progress of the reaction
was monitored by thin layer chromatography and upon
completion of the reaction, yields were calculated by HPLC
(Fig. S7–S9, ESI†).
1
2
3
4
5
6
7
8
1
SO2
SO3
SO4
SO5
SO2
SO3
SO4
SO5
SO2
SO3
SO4
SO5
SO2
SO3
SO4
SO5
SO2
SO3
SO4
SO1
SO5
SO2
SO3
SO4
SO1
SO5
90
90
98
50
79
89
96
37c
86
89
94
44c
89
93
98
45c
75
82
91
90
61c
82
83
84
88
66c
17.9
18.1
19.6
10
15.8
17.8
19.2
7.4
17.1
17.7
18.7
8.9
17.7
18.5
19.6
8.9
72
72
78
15
66
72
78
12
66
72
72
12
72
72
78
12
60
66
72
72
18
66
66
66
72
18
2
3
4
5
2.6. Synthesis of [(((2-(phenylsulnyl)phenyl)imino)methyl)
naphthalen-2-ol] (SO5)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
This compound was prepared following the general procedure
described above for sulde oxidation, using S5 (1 mmol), UHP
(1 mmol) and complex 1 (0.05 mmol). Aer stirring the reaction
mixture for 40 minutes at room temperature, pure SO5 was
separated from the reaction mixture using column chroma-
tography on a silica gel solid support and ethyl acetate/hexane
(30/70 v/v) as the eluent. Yield: 46%, yellow crystalline solid;
mp: 142–143 ꢂC; UV-vis [lmax; nm (3; L cmꢁ1 molꢁ1) in ethanol]:
331 (5699), 386 (7598). 1H NMR (CDCl3, 400 MHz, ppm) d 14.37
(1H, s, ArOH), 9.33 (1H, s, HC]N), 8.19 (1H, dd, J ¼ 7.6, 1.7 Hz,
ArH), 8.09 (1H, d, J ¼ 8.4 Hz, ArH), 7.95 (1H, d, J ¼ 9.2 Hz, ArH),
7.83 (1H, d, J ¼ 8.0 Hz, ArH), 7.68 (2H, m, ArH), 7.63–7.54 (3H,
m, ArH), 7.43 (1H, m, ArH), 7.35–7.25 (5H, m, ArH); 13C NMR
(CDCl3, 100 MHz, ppm) d 163.6, 159.9, 145.8, 145.0, 139.1,
136.3, 132.7, 132.2, 131.1, 129.4, 129.2, 128.2, 127.9, 127.6,
125.7, 124.6, 123.9, 119.8, 119.3, 118.6, 109.4. Slow diffusion of
hexane into a dichloromethane solution of SO5 afforded X-ray
suitable single crystals and nally the structure was conrmed
by single crystal X-ray crystallography.
15
16.4
18.1
18
12.2
16.5
16.7
17
6
17.5
13.2
a
Refer to Fig. 1 for the structure of catalysts. b Yield (%) was calculated
c
by HPLC aer 15 min of reaction time. Yield (%) was calculated by
HPLC aer 40 min of reaction time.
1.26 (3H, br, ethylidene CH3); 13C NMR (DMSO-d6, 100 MHz,
ppm): d 159.39 (C]N), 152.3, 149.5, 126.0, 121.3, 119.8, 117.2,
99.3, 91.1, 85.4, 81.1, 73.5, 69.9, 67.7, 56.2, 20.7; HRMS: m/z
calcd for (M + H)+ C16H19MoNO9 467.0114; found 467.0102,
without solvent.
2.7. Catalyst recycling
Recycling of the catalyst was performed using complex 1 (0.05
mmol), S4 (1 mmol) and UHP (1 mmol) in ethanol (3 mL) at
room temperature. At the end of the rst cycle of the reaction,
the solvent was evaporated under reduced pressure and the
pasty mass was extracted with dichloromethane (3 ꢀ 3 mL) to
remove the sulde and its oxidized products. The residue
2.4. Synthesis of [(((2-(phenylthio)phenyl)imino)methyl)
naphthalen-2-ol] (S5)
To the ethanolic solution of 2-hydroxynapthaldehyde (1.01 (complex 1) was freshly treated with an equimolar amount of
mmol), S4 (1 mmol) was added and the reaction mixture was sulde and UHP as mentioned above for the next cycle. The
reuxed for 8 h. The reaction mixture was cooled and the organic portion of the extract was used for isolation of the
resultant yellow crystalline solid product was ltered, washed product using column chromatography.
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FTIR, UV-visible, NMR and Mass spectroscopy. FTIR studies of
both complexes exhibited strong characteristic vibration bands
at 903, 1088, 1643 and 3333–3371 cmꢁ1 corresponding to
nMo]O, nC–O, nC]N and nO–H respectively. No appreciable
changes were observed for nC–O and nC]N stretching frequencies
of complexes with respect to their corresponding free ligands,
but a substantial change was observed in the nO–H region of the
spectra. Sharp bands in the OH region of the ligands converted
into a broad one aer the metalation reaction, indicating the
increment in hydrogen bonding interactions. The peak at 13.01
and 5.33 ppm corresponding to the phenolic and C2-hydroxyl
proton of glucose respectively in the ligand, disappeared from
1
the H NMR spectra of the molybdenum complexes revealing
the metal complexation via the phenolate and alcohalate group.
In both complexes, the signal of glucose C3–OH shied down
eld with respect to the free ligands, which might be attributed
to the change in electronic environment contributed by the
chelated metal ion. All these spectral changes are consistent
with the spectral description of reported similar Mo(VI)
complexes of H3Ln (n ¼ 1–4).18,27
Fig.
3 Graphical representation of catalytic recyclability using
complex 1 on S4.
3.2. Selective oxidation of sulde to sulfoxide
The molecular structure of 4,6-O-ethylidene-N-(2-hydroxy-
benzylidene)-b-D-glucopyranosylamine (H3L1) derived Mo(VI)
complex (Fig. 2) has already been established27 and a similar
complex has also been reported by Zhao et al.,32 using
a glucosamine derived ligand. The literature is rich with the
catalytic reactions of di-oxo Mo(VI) complexes,33–35 however it is
scarce for sugar derived from such molecules. Zhao et al. pub-
lished catalytic epoxidation reactions,32 we have reported the
selective synthesis of a series of bis(indolyl)methanes by
condensing indole derivatives with carbonyl compounds,36
while Mohammadnezhad et al.37 and our group have recently
communicated the oxidation of S1 to SO1.18 We had reported
the optimization of reaction conditions with respect to catalytic
loading, solvent effect, reaction time etc. on a single substrate
S1. In continuation of our previous work, we have tried to
develop a general protocol for sulde oxidation by sugar derived
cis-dioxo Mo(VI) complexes and, in the process, the number of
catalysts as well as substrates have been increased. All the
reactions were performed under optimized conditions devel-
oped by us for sulfoxide formation,18 i.e., ethanol as the reaction
solvent and a shorter reaction time (15 min) using a 1 : 1 molar
ratio of suldes and UHP. In order to better understand the
conversion and purity, we have calculated the yields of product
formation using HPLC and details of the method is stated in the
experimental section. We have achieved the conversion with
good to excellent yields and the same is summarized in Table 1.
The formation of SO1–SO4 was established by comparing the
respective melting points, IR, NMR and ESI-MS (characteriza-
tion data and Fig. S10–S18: ESI†) spectra with the reported
literature.17,18,38,39 Sheikhshoaie et al. have also done similar
Fig. 4 Time dependent UV-visible spectra of the reaction mixture
revealing the gradual decrease in the absorption band at 465 nm due
to the consumption of the substrate S5.
2.8. Kinetic studies
A main reaction of complex 1 (0.005 mmol), sulde S5 (0.11
mmol), and UHP (0.11 mmol) in ethanol 10 (mL) was set at
room temperature. 20 mL of the reaction mixture was diluted to
a nal volume of 5 mL and its absorbance was measured at 465
nm using a UV-visible spectrophotometer. A decrease in the
concentration of S5 during the reaction at 298 K was monitored
using a pre-generated calibration curve and the data was used to
establish the order of the reaction.
3. Results and discussion
3.1. Synthesis and characterization of two new complexes
reactions using
a
2-[(2-hydroxypropylimino)methyl]phenol
H3L5 and H3L6 were reacted with MoO2(acac)2 in methanol to derived Mo(VI) complex as the catalyst and UHP as a mild
yield the corresponding sugar containing molybdenum(VI) oxidant40 and our results are comparable to them. Varma and
complexes. The formation of the complexes were conrmed by Naicker have reported the oxidation of S1 using UHP without
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In order to understand the stability of catalysts during the
catalytic reaction, a recyclability test was performed using
complex 1 and S4 as a substrate. The catalyst was recycled ve
times (methods explained in Experimental section), and the
results are presented in Fig. 3, which clearly supports the reli-
ability of the catalytic process. No appreciable change in the
catalytic efficiency was observed as the sulfoxide formation was
in the range of 98 to 94%, from the rst cycle to the end of the
h cycle respectively.
The literature supports that it is easy to oxidize organic
suldes having electron-donating substituents than one having
electron withdrawing groups.42 Our nding also followed this
trend as we found the lowest yields (37–66%) for the oxidation
of S5, while the highest (84–98%) were found for compound S4.
The salient feature of the present sulfoxidation protocol is its
high selectivity to form only sulfoxide and neither sulfones nor
an oxidized benzylic product with substrate S3.
Fig. 5 Linear fitting plot of ln[S5] vs. time at 298 K.
3.3. Kinetic studies
Most of the commercially available general organic suldes and
their corresponding sulfoxides exhibit a common overlapping
absorption band in the UV-visible spectra. In our case, the
spectral pattern of S1–S4 behaved similarly and the rate of
oxidation was also fast. So, in order to understand the overall
order of the reaction, we planned to develop a sulde which
should have a slow rate of oxidation along with a non-
overlapping absorption band in the UV-visible spectrum for
the substrate and corresponding oxidized product. In this
venture, we synthesized a new sulde S5, which fullled both
criteria and hence the kinetics of the reaction was explored on
this molecule using UV-visible spectroscopy. A comparison of
UV-visible absorption spectra of the reactants (Fig. S-23, ESI†)
revealed that only S5 absorbs at 465 nm and hence the oxidation
reaction was monitored by measuring the absorbance of this
band with course of time at 298 K. A decrease in absorbance at
465 nm was measured for the rst 40 min (Fig. 4) and the data
was analyzed. A linear ln[S5] vs. time plot (Fig. 5) supported the
overall rst order reaction kinetics with a rate constant, k ¼
0.01804 minꢁ1 and a half-life, t1/2 ¼ 38.4 minutes.
Fig. 6 (A) Complex 1 in ethanol and (B) complex 1 in ethanol after UHP
addition.
3.4. Mechanism of reaction
In order to gain insight into the reaction mechanism, sulde S2
was added to the ethanolic solution of the molybdenum
complex 1 while stirring. No visual change in the physical
appearance of the reaction mixture was noticed even aer 15
min of stirring. The reaction mixture was analyzed using a UV-
visible spectrophotometer (Fig. S-24, ESI†), however no inter-
action between complex 1 and sulde S2 was established.
Similar studies performed using an ethanolic solution of sulde
S2 and UHP (Fig. S-25, ESI†) also failed to establish the inter-
Scheme
sulfoxide.
2 Proposed mechanistic pathway for the formation of
any catalyst but at 85 ꢂC and the yield was 80%.41 So, the use of actions. Finally, molybdenum complex 1 was suspended in
a catalyst has not only reduced the reaction temperature but ethanol and UHP was added to that, which resulted in an
also improved the yield and purity of the product as they instantaneous yellowish transparent solution (Fig. 6). The UV-
noticed a 10% mixture of sulfone.
visible absorption studies (Fig. S-26, ESI†) on these sets of
control reactions also supported the interaction via the
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Table 2 Summary of the crystallographic data and structural param-
eters of S5 and SO5
3.5. Crystal structure of S5 and SO5 and their correlation
with the oxidation process
Among all the suldes used in this paper, the rate of oxidation
as well as yields were the lowest for S5. The low yield is
attributed to the formation of sulfoxide at the initial stage
followed by parallel formation of sulde and sulfones,
while the low rate of oxidation might be due to either steric
crowding or the electronic environment or both together
about the sulfur center. In order to understand this chem-
istry, we crystallized both the sulde (S5) and corresponding
sulfoxide (SO5) and their solid state structure was established
using single crystal X-ray crystallography. In both cases,
crystal quality was averaged and the R values (0.0910 and
0.0627) were within the accepted range for publication as
mentioned in the crystallographic summary (Table 2). The
ORTEP plot of S5 shown in Fig. 7(a) indicates the hindrance of
the sulde ion to come closer to the Mo center of the catalyst
and the same can be clearly viewed in the space-lling model
(Fig. 7(b)). The whole molecule is arranged in space such that
three sides of the basal plane of sulfur is covered with naph-
thyl and phenyl rings, while the perpendicular arrangement
of one of the phenyl rings obstructs its exposure to the catalyst
from the vertical side. Such an atomic environment about
sulfur might be responsible for its slow oxidation process as it
might not be able to interact with the catalytic center
appropriately.
Since the environment about sulfur is crowded in S5, it raises
a curiosity to know about the special arrangement of molecular
fragments in SO5. In order to fulll this inquisitiveness, the
solid state structure of SO5 was explored using single crystal X-
ray crystallography. The ORTEP plot of SO5 with an atomic
labeling scheme is presented in Fig. 8(a), while Fig. 8(b) illus-
trates its space-lling model. This structure clearly reveals the
overall change in the atomic environment about sulfur, which
facilitates it to protrude out, which might help it in converting
to sulfone parallel to sulde oxidation. The changes in confor-
mations of sulde and sulfoxide can be easily established by
analyzing the selected torsion angles. The angles C(1)N(1)C(12)
C(17), C(10)C(2)C(1)N(1), C(14)C(13)S(1)C(18) and C(13)S(1)
C(18)C(19) are 15.9(11), 179.4(7), 9.5(8) and ꢁ113.5(7) in sulde,
while ꢁ36.0(7), 170.6(4), ꢁ106.0(4) and 6.6(5) respectively in
sulfoxide.
S5
SO5
Empirical formula
Molecular weight
T (K)
Crystal system
Space group
C
23H17NOS
C23H17NO2S
371.43
100
Orthorhombic
P212121
355.43
100
Orthorhombic
P212121
Cell constants
˚
a (A)
6.132(2)
14.536(5)
19.327(7)
1722.6(11)
4
1.371
11 726
3922
5.6260(10)
12.886(2)
24.399(5)
1768.8(6)
4
1.395
15 463
4029
˚
b (A)
˚
c (A)
3
˚
V (A )
Z value
Dcalcd (g cmꢁ3
)
Total reections
Unique reections
Parameters
Final R (I > 2s(I))
Rw
236
0.0910
0.2135
245
0.0627
0.1231
hypochromic effect along with an overall change in the peak
positions.
Furthermore, the effect of the variation in UHP concentra-
tion was studied, where no change in the kinetics of the reaction
was noticed, however an increase in the conversion of suldes
into sulfones was observed. All these studies concluded that the
binding of UHP with the molybdenum complex occurs at the
initial stage of the sulde oxidation reaction. A similar nding
has also been reported by Kamata et al.,43 Panda et al.44 and
Chakravarthy et al.45 using W, Ti and Mo complexes respec-
tively, however all of them have used H2O2 as an oxidizing
agent. Chakravarthy et al. have proposed the possibility of
sulde binding with either a molybdenum center or peroxo
oxygen.45 The literature reveals that an increase in electron
density about sulfur due to its attachment with electron donor
groups increases the oxidation efficiency and we also noticed
the same trend. This fact is only possible if sulde is interacting
with a metal center and not with peroxo oxygen, as the elec-
tronegative nature of oxygen will repel the electron rich sulfur
centre. Hence, in our opinion, sulde ion binds with the Mo
center rather than peroxo oxygen. On the basis of our results
and the previous literature reports43–45 a plausible oxidation
mechanism has been summarized in Scheme 2.
Fig. 7 (a) ORTEP plot of S5 drawn using 50% probability ellipsoids; (b) space-filling model of S5 showing the hindrance about the sulfur atom.
28020 | RSC Adv., 2016, 6, 28015–28022
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Fig. 8 (a) ORTEP plot of SO5 drawn using 50% probability ellipsoids; (b) space-filling model of compound SO5.
´
2 I. Fernandez and N. Khiar, Chem. Rev., 2003, 103, 3651–3705.
3 A. M. Khenkin and R. Neumann, J. Am. Chem. Soc., 2002, 124,
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4. Conclusions
Selective oxidation of organic suldes to sulfoxides have been
explored using glucose derived cis-dioxo molybdenum(VI)
complexes as catalysts and urea–hydrogen peroxide as a mild
oxidising agent. A total of six catalysts were tested on ve
suldes and good to excellent conversion was achieved under
milder reaction conditions and shorter reaction time. The
conversion was best for compound S4 (84–98%) and least for
compound S5 (37–66%). All the yields were calculated using
HPLC separation and the purity of the products was further
checked by comparing their melting point, FTIR and NMR data
with the literature values. A new sulde S5 and corresponding
sulfoxide SO5 also has been developed and their structures have
been established using the single crystal X-ray diffraction
method. One of the catalysts was tested for recyclability and
only a minute change (ꢃ4% loss) in activity was observed during
ve cycles, revealing the sustainability of the catalyst under the
reaction conditions. Even though similar reactions are reported
using other catalysts and conditions, the advantage of our
report is the greener methodology i.e., catalyst consisting of bio-
viable species (D-glucose and molybdenum), short reaction
time, high selectivity, room temperature reaction and ethanol
as the reaction medium. This paper also deals with the kinetics
and mechanistic aspects of sulde oxidation.
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19 J. Fridgen, W. A. Herrmann, G. Eickerling, A. M. Santos and
We are thankful to Dr Angshuman Roy Choudhury for his help
in crystallographic data collection. A. K. Sah is grateful to
Department of Science and Technology (DST) and University
Grants Commission (UGC) for the nancial support. The
Department of Chemical Sciences, Indian Institute of Science
Education and Research, Mohali is acknowledged for providing
the XtaLABmini X-ray diffractometer facility. We are also
thankful to DST FIST and UGC SAP for their nancial support in
procuring the instruments and developing the research
infrastructure.
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