Selective oxidation of sulfides to sulfoxides with cetyltrimethylammonium periodate

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Selective Oxidation of
Sulfides to Sulfoxides with
Cetyltrimethylammonium Periodate
a
a
Umesh V. Chaudhari & Pradeep T. Deota
a
Applied Chemistry Department, Faculty of Technology &
Engineering, The Maharaja Sayajirao University of Baroda, Vadodara,
390 001, Gujarat, India
Version of record first published: 01 Aug 2012
To cite this article: Umesh V. Chaudhari & Pradeep T. Deota (2012): Selective Oxidation of Sulfides
to Sulfoxides with Cetyltrimethylammonium Periodate, Organic Preparations and Procedures
International: The New Journal for Organic Synthesis, 44:4, 381-386
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Organic Preparations and Procedures International, 44:381–386, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2012.697729
OPPI BRIEFS
Selective Oxidation of Sulfides to Sulfoxides
with Cetyltrimethylammonium Periodate
Umesh V. Chaudhari and Pradeep T. Deota
Applied Chemistry Department, Faculty of Technology & Engineering, The
Maharaja Sayajirao University of Baroda, Vadodara 390 001, Gujarat, India
1
The selective oxidation of sulfides to sulfoxides is an important transformation, since sul-
2
,3
foxides are useful building blocks especially as chiral auxiliaries. Furthermore sulfoxides
also play an important role for the acquisition of many biologically and chemically im-
4
–6
1
portant compounds. A number of oxidizing agents such as hydrogen peroxide (H2O2),
sodium periodate (NaIO4),7 m-CPBA, ozone, oxygen and t-BuOOH have been em-
,8
9
10
11
ployed. Although, H2O2 is environmentally benign and NaIO4 may be stored and used
7
,8
safely, they oxidize sulfides slowly; for example, NaIO4 requires 12 hours or more, while
12
H2O2 in the absence of catalyst requires 18 hours or more. Moreover H2O2 should be used
in a controlled manner to avoid over-oxidation. Therefore most of the oxidizing agents are
either hazardous peroxy acids or involve toxic heavy metals or rare oxidants that are diffi-
cult to prepare thus detracting their practical application in many cases. Hydrogen peroxide
in the presence of complexes or salts of metals such as vanadium, molybdenum, tungsten
as catalysts selectively furnishes sulfoxides.1 A variety of oxidants in combination with
,13
1
4,15
metal Schiff base complexes as catalysts have also been used for this purpose;
however,
this method has low operational stability due to facile oxidative degradation of porphyrin
rings, phthalocyanines and salens.16 Thus, many of the reagents and catalysts used suffer
from drawbacks such as over-oxidation, long reaction times, low selectivity and low yields.
Oxidizing agents based on quaternary ammonium salts have been exploited in many organic
1
7–21
reactions
due to their solubility in several solvents which provide advantages in terms
of mildness, operational simplicity, selectivity, high reaction rates, low reaction tempera-
tures and absence of side-reactions. However, some of them require catalysts for activation.
For example, tetrabutylammonium periodate (TBAPI) alone is not able to convert sulfides
.
to sulfoxides. In the presence of a Lewis acid (AlCl3 or BF3 Et2O) as catalyst and at reflux
22
temperature, TBAPI oxidizes sulfides to sulfoxides in CH3CN. Cetyltrimethylammonium
Received December 9, 2011.
Dedicated to Prof. Vishwakarma Singh on his 62nd birthday.
Address correspondence to Pradeep T. Deota, Applied Chemistry Department, Faculty of Tech-
nology & Engineering, The Maharaja Sayajirao University of Baroda, Vadodara 390 001, Gujarat,
India. E-mail: deotapt@yahoo.com
381

382
Chaudhari and Deota
bromide (CTAB) is also reported to catalyze the oxidation of sulfides to sulfoxides using
hypervalent iodine reagents.2
3,24
Cetyltrimethylammonium ion (CTA) is well known for its amphipathicity (ability to
make a substance soluble in both aqueous and non-aqueous media). CTA also has rela-
tively small head group and a well-balanced hydrophobic group. Therefore it was thought
worthwhile to explore the oxidation of sulfides using cetyltrimethylammonium periodate
(
CTAPI) which can be easily obtained as a white crystalline solid in excellent yield by
mixing aqueous solutions of sodium metaperiodate and cetyltrimethylammonium bromide
CTAB) at room temperature (Eq. 1). Both CTAB and CTAPI show similar absorption
(
−1
bands except for a strong band at 852 cm in the spectrum of CTAPI which may be
attributed to IO4 stretching.
−
+
+
NalO4
−
−
[
C16H33 N(CH3)3] Br −− −→ [C16H33 N(CH3)3] IO
(1)
4
H
O,r.t
2
−
NaBr
The oxidation of methyl 4-[(methylthio)methyl]benzoate (1a) as a model substrate,
was first investigated by changing the mole proportion of the reagent at room temperature.
Stoichiometry study showed the best molar ratio of sulfide and CTAPI to be 1.0:3.5 in
aqueous methanol as solvent. If the ratio was decreased, the yield was reduced dramatically
along with an increase in reaction times. Several solvents such as water, methanol, H2O-
MeOH mixture, acetonitrile, dichloromethane, ethyl acetate, THF and chloroform were
investigated in order to optimize the reaction conditions; aqueous methanol (H2O-MeOH
8:2) was found to be the most effective.
When methanol was used as a solvent, over-oxidation of sulfide to sulfone was observed
while in water, the reaction proceeded slowly and gave a low yield. In other organic sol-
vent, the reaction was incomplete and in some cases the reaction did not proceed even after
12 hours. The oxidant worked smoothly with a variety of structurally different sulfides such
as alkylaryl sulfides, alkylaralkyl sulfides, diaralkyl sulfides, arylaralkyl sulfides, dialkyl
sulfides, and diaryl sulfides under the optimized reaction conditions (Table 1). The reactions
exhibited very high selectivity for sulfoxides in relatively short reaction time. The oxidation
of the diphenyl sulfide (1e) required 4.5 mole equivalents of CTAPI, perhaps due to its lesser
2
5,26
reactivity.
The selective oxidation of methyl 4-[(methylthio)methyl]benzoate (1a), us-
ing sodium periodate instead of CTAPI, was not at all effective under the optimized reaction
conditions as it gave the over-oxidized product sulfone with longer reaction period. When
the reaction using sodium periodate was stopped after 1 hour a mixture of unreacted sulfide,
sulfoxide and sulfone in the ratio of 0.3:0.35:0.27 was obtained, while the same reaction
using CTAPI gave excellent yield of selectively oxidized product sulfoxide in 1 hour.

Selective Oxidation of Sulfides to Sulfoxides
383
Table 1
Oxidation of Sulfides to Sulfoxides using CTAPI (3.5 equiv.) in Aqueous Methanol
◦
1H NMR (δ)
and MS (EI) m/z
Time Yield
Product (min) (%)
mp ( C)
(lit. mp)
IR (cm−1)
2
7
2a
2b
2c
60
75
45
94
92
89
68–70 (69–72 )
3020, 2960, 1718, 1467,
2.47 (3H, s), 3.93 (3H, s),
4.00 (1H, d, J = 12.8 Hz),
1420, 1240, 1090, 743
4
7
8
.05 (1H, d, J = 12.8 Hz),
.38 (2H, d, J = 8.2 Hz),
.06 (2H, d, J = 8.2 Hz).
+
MS m/z 211.99 (M )
3.99 (1H, d, J = 12.6 Hz),
4.10 (1H, d, J = 12.6 Hz),
120–122
122–124 )
3060, 2980, 1535, 1447,
1070, 740, 692
2
8
(
6
7
7
2
.95–7.00 (2H, m),
.24–7.29 (3H, m),
.37–7.45 (5H, m). MS m/z
16.12 (M )
+
Colorless oil
Colorless oil )
3058, 2986, 2944, 1516,
1468, 1057, 738
1.31 (3H, t, J = 7.5),
2
9
(
2.51–2.67 (2H, m), 3.93
(
1H, d, J = 12.9 Hz), 4.00
(1H, d, J = 12.9 Hz),
7
7
1
.28–7.30 (2H, m),
.32–7.39 (3H, m). MS m/z
+
68.10 (M )
2d
165
91
134–136
135–136 )
3035, 2956, 1605, 1452,
1032, 775, 699
3.87 (2H, d, J = 13 Hz), 3.92
3
0
(
(2H, d, J = 13 Hz),
7
7
2
.25–7.30 (4H, m),
.33–7.40 (6H, m). MS m/z
+
30.18 (M )
2
ea
f
150
210
87
90
70–72 (70–7225)
Colorless oil
3068, 1466, 1440, 1082,
7.41–7.48 (6H, m), 7.63–7.66
1048, 743, 694
(4H, m). MS m/z 202.10
+
(
M )
2
3075, 2960, 2883, 1450,
1086, 1045, 758
0.92 (3H, t, J = 7.4 Hz),
3
1
(Colorless oil )
1.38–1.78 (4H, m),
2
7
7
1
.77–2.84 (2H, m),
.48–7.54 (3H, m),
.61–7.65 (2H, m). MS m/z
+
82.14 (M )
2
8
2
g
225
40
93
92
Colorless oil (oil )
3066, 2927, 1489, 1075,
1.20 (3H, t, J = 7.4 Hz),
1037, 780, 715
2.73–2.95 (2H, m),
7
7
1
.48–7.55 (3H, m),
.60–7.62 (2H, m). MS m/z
+
54.14 (M )
2h
Colorless oil
2982, 2839, 1735, 1263,
1024, 1081, 783
1.25 (3H, t, J = 7.4 Hz), 1.27
2
9
(Colorless oil )
(3H, t, J = 7.1 Hz),
2.81–2.98 (2H, m), 3.78
(
(
(
1H, d, J = 14.1 Hz), 4.02
1H, d, J = 14.1 Hz), 4.21
2H, q, J = 7.1 Hz) MS
+
m/z 164.07 (M )
0.94 (6H, t, J = 7.4 Hz),
1.39–1.54 (4H, m),
2
6
2i
40
93
34–35 (34–34.5 )
2960, 2874, 1465, 1078,
1026, 742
1
2
1
.69–1.77 (4H, m),
.58–2.70 (4H, m). MS m/z
+
62.03 (M )
a4.5 equiv. CTAPI used.

384
Chaudhari and Deota
Experimental Section
Melting points were recorded in open capillary tubes in melting point apparatus and are
uncorrected. Infrared (IR) absorption spectra were obtained on a SHIMADZU FTIR-2450
1
spectrophotometer. H NMR spectra were determined on a Bruker-(200/300/400/500) FT-
NMR spectrometer in CDCl3 or DMSO-d6 (TMS as an internal standard). Mass spectra
were recorded on a Thermo-Fischer DSQ II GCMS instrument. Thin layer chromatography
(TLC) analysis was performed on glass plates using silica gel-G containing 13% calcium
sulfate as a binder. Visualization of spots was achieved by exposure to iodine vapor. Column
chromatography was carried out on Acme’s silica gel (60–120 mesh size) and eluted using
light petroleum (60–80) and ethyl acetate mixtures. Most sulfides were prepared from
corresponding halides and thiols by Williamson-type sulfide synthesis,3
2,33
while sulfides
(1e) and (1i) were purchased from Sigma-Aldrich.
Preparation of Cetyltrimethylammonium Periodate (CTAPI)
To a stirred solution of cetyltrimethylammonium bromide (10g, 0.027 mole) in water
(
200 mL) was slowly added a solution of sodium metaperiodate (5.87g, 0.027 mole) in water
◦
(110 mL) at room temperature (27 C). The reaction mixture was stirred for 15 minutes
and the white precipitate obtained was collected, washed with water and dried in hot air
◦
◦
oven (60 C) for 24 h to give CTAPI (12.7g, 97%) as colorless crystal, mp 126 C. IR (KBr)
−1
1
cm : 2943, 2918, 2876, 2854, 1485, 1477, 852, 721. H NMR (300 MHz, CDCl3): δ 0.88
3H, t, J = 6.5 Hz), 1.26–1.37 (24H, m), 1.76–1.88 (4H, m), 3.24 (9H, s), 3.34–3.39 (2H,
m).
(
Anal. Calcd for C19H42NIO4: C, 48.00; H, 8.84; N, 2.94. Found: C, 47.85; H, 8.82; N,
.88.
2
General Procedure for the Oxidation of Sulfides (1a–i) to Sulfoxides (2a–i)
To a stirred solution of the sulfide (1 mmole) in aqueous methanol (8:2) (5 mL) at room
temperature was added CTAPI (1.66g, 3.5 mmoles) in portions over a period of 15 minutes;
the mixture was then stirred for the appropriate time (Table 1). After completion of reaction
(TLC), the reaction mixture was filtered to remove iodate salt and the filtrate was extracted
with ethyl acetate (4 × 10 mL). The combined organic extracts were washed successively
with water (2 × 4 mL), brine solution (10 mL) and dried over anhydrous sodium sulfate
(
Na2SO4). Removal of solvent under reduced pressure furnished the crude sulfoxides, which
were chromatographed using mixtures of light petroleum (60–80) and ethyl acetate. This
procedure has been scaled up with methyl 4-[(methylthio)methyl]benzoate (5g, 0.025 mole)
to give methyl 4-((methyl sulfinyl)methyl)benzoate in 94% yield.
Acknowledgement
1
The authors thank CDRI, Lucknow and CSMCRI, Bhavnagar for providing H NMR
and elemental analysis. We also thank Dr. Ashutosh V. Bedekar and Head, Chemistry
Department, Faculty of Science, The M. S. University of Baroda, for their help in Mass
Spectra.

Selective Oxidation of Sulfides to Sulfoxides
385
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