Oxidation of sulfides with pyridinium tribromide in the presence of hydrated silica gel

Article abstract of DOI:10.1080/00397910600619044

A variety of sulfides have been oxidized to sulfoxides utilizing pyridinium tribromide in the presence of hydrated silica gel in a non-aqueous media. A combination of pyridinium tribromide and hydrated silica gel releases molecular bromine slowly in the r

Full text of DOI:10.1080/00397910600619044

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Oxidation of Sulfides with Pyridinium
Tribromide in the Presence of Hydrated Silica
Gel
Mohammed Hashmat Ali a & Susan Stricklin a
a Chemistry Department , Southeast Missouri State University, Cape
Girardeau , Missouri , USA
Published online: 22 Aug 2006.
To cite this article: Mohammed Hashmat Ali & Susan Stricklin (2006) Oxidation of Sulfides with Pyridinium
Tribromide in the Presence of Hydrated Silica Gel, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 36:12, 1779-1786, DOI: 10.1080/00397910600619044
To link to this article: http://dx.doi.org/10.1080/00397910600619044
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Synthetic Communicationsw, 36: 1779–1786, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600619044
Oxidation of Sulfides with Pyridinium
Tribromide in the Presence of Hydrated
Silica Gel
Mohammed Hashmat Ali and Susan Stricklin
Chemistry Department, Southeast Missouri State University,
Cape Girardeau, Missouri, USA
Abstract: A variety of sulfides have been oxidized to sulfoxides utilizing pyridinium
tribromide in the presence of hydrated silica gel in a non-aqueous media. A combi-
nation of pyridinium tribromide and hydrated silica gel releases molecular bromine
slowly in the reaction, affecting the oxidation. Hydrated silica gel also promotes
decomposition of the bromosulfonium intermediate to the product. This procedure
employs non-aqueous media for the first time in such a reaction.
Keywords: Oxidation, pyridinium tribromide, silica gel, sulfide, sulfoxide
We and others have reported oxidation of sulfides to sulfoxides with molecular
bromine.[1–5] It has been postulated that a reaction between an organic sulfide
1 and bromine produces unstable bromosulfonium intermediate 2 or 20
(Scheme 1).[5] In the presence of water, intermediate 2 and/or 20 decompose
to the sulfoxide 3.
To avoid toxic molecular bromine, various stable solid quaternary
ammonium tribromides have been introduced as in situ sources of
molecular bromine.[6–10] In the presence of water, quaternary ammonium tri-
bromides slowly release molecular bromine. Handling solid stable tribromides
is much safer than handling molecular bromine itself. Also, the low concen-
tration of bromine in the reaction mixture during the course of the reaction
Received in the USA December 7, 2005
Address correspondence to Mohammed Hashmat Ali, Department of Chemistry,
Southeast Missouri State University, Cape Girardeau, Missouri 63701, USA.
Tel.: (573) 651-2983; Fax: (573) 986-6433; E-mail: mhali@semo.edu
1779

1780
M. H. Ali and S. Stricklin
Scheme 1.
makes these procedures safer and minimizes side reactions that would
otherwise be promoted by higher concentrations of bromine.
Benzyltrimethylammonium tribromide in aqueous sodium hydroxide-
dichloromethane media or in dichloroethane with reflux,[6] cetyltrimethylam-
monium tribromide in aqueous acetonitrile,[7] and phenyltrimethylammonium
tribromide in aqueous pyridine have been used in the oxidation of various
sulfides to the corresponding sulfoxides.[8] In addition to the aqueous
reaction media, these procedures utilize a base to neutralize the hydrogen
bromide produced as a by-product.
Some time ago, we reported a procedure for oxidation of sulfides to sulfox-
ides using molecular bromine in the presence of hydrated silica gel in dichlor-
omethane media.[5] In this reaction, hydrated silica gel plays several important
roles. It provides the water necessary to decompose bromosulfonium salt inter-
mediate to the sulfoxides, removes the hydrogen bromide by-product produced
in this reaction, and acts as a heat sink, which allowed us to carry out this
exothermic reaction without controlling the reaction temperature.
Encouraged by the excellent results of our procedure, we became inter-
ested in investigating the oxidation of sulfides with pyridinium tribromide
in the presence of hydrated silica gel utilizing similar reaction conditions.
The result of this investigation was excellent. Oxidation of dialkyl, aryl-
alkyl, and diaryl sulfides produced excellent yields of the corresponding sulf-
oxides (Tables 1 and 2). Hydrated silica gel provides water needed to
decompose the pyridinium tribromide that generates the oxidizing agent
molecular bromine and also helps decompose the intermediate bromosulfo-
nium salt to the sulfoxide product. This newly developed procedure is very
simple and safer than handling hazardous molecular bromine. Other benefits
of this procedure include high yields, nonaqueous reaction media, easy
workup, and elimination of the utilization of a base to remove the hydrogen

Oxidation of Sulfides with Pyridinium Tribromide
1781
Table 1. Oxidation of sulfides 1 to sulfoxides 3
Entry
Sulfide (1)
Rxn. time (min)
80
Yield (%)
99
a
b
c
75
55
75
87
98
91
d
e
f
75
83
43
130
g
40
91
99
h
120
i
150
60
86
97
j
bromide by-product. Silica gel absorbs hydrogen bromide, which is then
removed from the reaction mixture by filtration. By-products of pyridinium
tribromide can be removed from the product by passing the crude product
through a short silica-gel column or washing a dichloromethane solution of
the crude product with water. Both of these two workup procedures gave us
pure product.
The selectivity of this procedure is excellent. Oxidation of 4-thiomethyl-
benzyl alcohol 1o proceeded smoothly to produce the corresponding sulfoxide

1782
M. H. Ali and S. Stricklin
Table 2. Oxidation of sulfides 1 to sulfoxides 3
Entry
Sulfide (1)
Rxn. time (min)
Yield (%)
92
k
20 hours
Mix of
products
l
12 hours
25
m
100
n
o
p
30
35
20
97
92
95
q
r
130
135
89
89
3o without interference from the alcohol functional group. Sulfide 1i produced
the expected sulfoxide 3i without producing any a-bromination product. Also,
we have not observed any bromination at the benzylic position in sulfides 1g,
j, n, and o.
The results of oxidation of the thioanisoles clearly indicate that the
presence of an activating group at the 4-position of the benzene ring
makes thioanisoles 1n–p more reactive, whereas a deactivating group at
this position makes thioanisoles 1q–r less reactive toward the oxidation
reaction.
In conclusion, we have developed a simple and efficient procedure
for oxidation of a wide variety of sulfides utilizing pyridinium tribromides
in nonaqueous media.

Oxidation of Sulfides with Pyridinium Tribromide
EXPERIMENTAL
1783
All reaction mixtures were stirred with a Teflon-coated magnet. Sulfides were
purchased or prepared using literature procedures. Solvents were used as
received from the supplier without any further purification. NMR spectra
were recorded on a Bruker DPX-300 NMR instrument (1H at 300 MHz,
13Cat 75 MHz). Samples for NMR were dissolved in CDC13. Proton
chemical shifts are expressed as parts per million (ppm) relative to tetra-
methylsilane, and 13C chemical shifts were referenced to the solvent signal.
IR spectra were recorded on a Perkin-Elmer Spectrum-1000 FT-IR instrument
and are reported in wavenumbers (cm21). Chromatographic separations were
carried out by preparative centrifugal thin-layer chromatography (TLC) with
silica gel (Merck #7749) on a Chromatotron model 7924T. Analytical TLC
was done on precoated silica-gel plates with 254 nm fluorescent indicator
(Merck #5715) and developed in a 1:9 mixture of ethyl acetate–hexan.
Compounds were visualized by UV and/or stained either with a p-anisaldehyde/
sulfuric acid or phosphomolybdic acid.
General Procedure
Silica gel (5 g) placed in a round-bottom flask were hydrated by adding
2.00 mL of distilled water drop by drop with stirring. The content of the
flask was stirred until a free-flowing solid was obtained (less than 5 min).
Dichloromethane (20 mL) was added to the flask. The sulfide (2.0 mmol)
was then added to the flask. While stirring, 2.1 mmol of solid pyridinium
tribromide were added in small portions to the reaction flask. Progress of
the reaction was monitored by TLC. After the reaction was complete, the
reaction mixture was filtered using a fritted glass funnel. The solid residue
was washed with 70–75 mL of dichloromethane. The combined dichloro-
methane solution was then transferred into a separatory funnel and washed
with distilled H2O three times (20 mL each time) and once with 20 mL of a
brine solution. The orgnic layer was dried over anhydrous magnesium
sulfate, the drying agent was removed by filtration, and the filtrate was concen-
trated using a rotary evaporator. All products, except 3k and 3r, were purified by
radial chromatography. Products 3k and 3r were purified by recrystallization.
Identity of the sulfoxides was established by NMR and IR data.
Data
Dibutylsulfoxide (3a).[11] 1H NMR (CDCl3) 0.95 (t, J ¼ 7.1 Hz, 6H)
1.30–1.60 (m, 2H), 1.40–1.65 (m, 2H), 1.70–1.82 (m, 4H). 2.68 (t, J ¼
7.5 Hz, 4H). 13C NMR (CDC13) 13.30, 21.66, 24.20, 51.70 IR (film) 731,
922, 1027, 1380, 1409, 1465, 2873, 2932, 2959 cm21
.

1784
M. H. Ali and S. Stricklin
2-(Methylsulfinyl)butane (3b). (mixture of two diasteromers, produced
1
identical NMR data as compared to commercially available product.) H
NMR (CDC13) 0.87 (t, J ¼ 7.45 Hz, 3H), 0.92 (t, J ¼ 7.5 Hz, 3H), 1.06
(d, J ¼ 6.9 Hz, 3H), 1.20 (d, J ¼ 6.9 Hz, 3H), 1.25–1.42 (m, 2H), 1.60–1.80
(m, 2H), 2.42–2.44 (m, 1H), 2.46 (s, 2CH3, 6H), 2.64 (m, 1H). 13C NMR
(CDC13) 10.81, 11.06, 11.50, 11.60, 22.82, 23.50, 34.35, 34.84,
57.71, 58.63. IR (film) 942, 1029, 1064, 1382, 1424, 1462, 2878, 2935,
2968 cm21
.
Tetrahydrothiophene oxide (3c).[12] 1H NMR (CDC13) 1.96–2 20 (m, 2H),
2.30–2.50 (m, 2H), 2.70–2.86 (m, 4H). 13C NMR (CDC13) 24.61, 53.50. IR
(film) 1009, 1095, 1305, 1410, 1446, 2874, 2948 cm21
.
4-Oxo-1,4-thioxane (3d).[11] 1H NMR (CDC13) 2.66–2.76 (m, 2H), 2.86–
2.95 (m, 2H), 3.80–3.85 (m, 2H), 4.33–4.42 (m, 2H), 13C NMR (CDC13)
46.36, 59.20. IR (film) 960, 1010, 1058, 1098, 1150, 1215, 1321, 1385,
1404, 1463, 2868, 2927, 2967 cm21
.
Methyl t-butyl sulfoxide (3e).[12] 1H NMR (CDC13) 1.37 (s), 2.42 (s), 13C
NMR (CDCl3) 26.60, 36.68, 57.80 IR (film) 1158, 1356, 1381, 1452, 1467,
2863, 2892, 2927, 2967 cm21
.
Di-tert-butyl sulfoxide (3f).[13] 1H NMR (CDCl3) 1.31 (s). 13C NMR (CDCI3)
26.60, 57.80. IR (film) 1161, 1362, 1388, 1456, 1470, 2860, 2896, 2921,
2961 cm21
.
Benzyl methyl sulfoxide (3g).[11] 1H NMR (CDCl3) 2.42 (2, 3H), 3.96
(d, J ¼ 6 Hz, 2H), 7.20–7.45 (m, 5H). 13C NMR (CDCl3) 37.21, 59.90,
128.33, 128.90, 129.70, 130.10. IR (film) 1030, 1423, 1454, 1495, 29.15,
3029, 3061 cm21
.
Ethyl phenyl sulfoxide (3h).[11] 1H NMR (CDC13) 1.20 (t, J ¼ 7.3 Hz, 3H),
2.74–2.92 (m, 2H), 7.30–7.40 (m, 3H), 7.42–7.60 (m, 2H). 13C NMR
(CDCl3) 5.53, 49.80, 123.80, 128.80, 130.60, 142.96. IR (film) 692, 748,
1021, 1044, 1086, 13.74, 1443, 1478, 2874, 2934, 2978, 3057 cm21
.
Thiooxochroman-4-one S-oxide (3i).[14] 1H NMR (CDC13) 2.85–2.90
(m, IH), 3.38–3.42 (m, 3H), 7.65–7.70 (m, lH), 7.92 (d, J ¼ 7.4 Hz, lH),
8.10 (d, J ¼ 7.4 Hz, 2H). 13C NMR (CDC13) 29.71, 46.0, 127.80, 128.28,
128.75, 131.65, 134.10, 145.50, 191.65. IR (film) 784, 1048, 1176, 1346,
1450, 1292, 2916, 2956, 2980, 3029, 3065 cm21
.
Benzyl phenyl sulfoxide (3j)[11] 1H NMR (CDCl3) 3.98 (d, J ¼ 12.5 Hz, lH),
4.08 (d, J ¼ 12.5 Hz, lH), 6.90–7.40 (m, l0H). 13C NMR (CDC13) 63.18,
124.38, 128.20, 128.28, 128.79, 128.88, 130.12, 131.10, 142.06. IR (film)
692, 748, 1037, 1085, 1441, 1493, 2959, 3057 cm21
.
1
Thioxanthanone S-oxide (3k).
H NMR (CDCl3) 7.40–7.50 (m, 4H),
7.57–7.66 (m, 4H), 8.60–8.65 (m, 2H). 13C NMR (CDCl3) 125.97, 126.30,

Oxidation of Sulfides with Pyridinium Tribromide
1785
129.20, 129.84, 132.25, 137.25, 180.10. IR (film) 927, 1072, 1161, 1320,
1435, 1591, 1644, 3061 cm21. Mp 199–202.58C (reported 200–2028C).[15]
Diphenyl sulfoxide (3I).[11] 1H NMR (CDC13) 7.38–7.44 (m, 6H), 7.55–7.60
(m, 4H). 13C NMR (CDC13) 124.65, 129.25, 131.00, 145.14. IR (film) 707,
768, 1047, 1121, 1456, 1481, 3063 cm21
.
Methyl phenyl sulfoxide (3m).[11] 1 H NMR (CDCl3) 2.73 (s, 3H), 7.50–7.54
(m, 3H), 7.60–7.67 (m, 2H). 13C NMR (CDC13) 44.30, 123.83, 129.73,
131.42, 145.90. IR (film) 690, 752, 957, 998, 1037, 1089, 1297, 1415, 1444,
1477, 2913, 2998, 3057 cm21
.
Methyl p-methylphenyl sulfoxide (2n)[11] 1H NMR (CDC13) 2.42 (s, 3H),
2.70 (s, 3H), 7.32 (d, J ¼ 8.3 Hz, 2H), 7.55 (d, J ¼ 8.3 Hz, 2H); 13C NMR
(CDCl3) 21.10, 43.50, 123.26, 129.85, 141.27, 142.45. IR (film) 810, 966,
1048, 1087, 1426, 2881, 2956, 3006, 3051 cm2l.
4-Hydroxymethylphenyl methyl sulfoxide (20).[16] 1 H NMR (CDCl3) 2.63
(s, 3H), 3.60 (bs, OH), 4.64 (s, 2H), 7.36 (d, J ¼ 8.4 Hz, 2H), 7.52
(d, J ¼ 8.4 Hz, 2H). 13C NMR (CDCl3) 43.20, 63.58, 123.40, 127.45,
141.08, 145.50. IR (film) 811, 958, 1036, 1088, 1180, 1302, 1413, 1495,
2920, 2995, 3052, 3410 cm21
.
p-Methoxyphenyl methyl sulfoxide (2p)[17] 1 H NMR (CDC13) 2.65 (s, 3H),
3.80 (s, 3H), 7.02(d, J ¼ 9.10 Hz, 2H), 7.60 (d, J ¼ 9.10 Hz, 2H). 13C NMR
(CDC13) 43.75, 55.65, 114.82, 125.10, 136.40, 161.72. IR (film) 827, 956,
1084, 1259, 1299, 1474, 1587, 2925, 2987, 2990, 3065 cm21
.
4-Bromophenyl methyl sulfoxide (3q).[10] 1H NMR (CDC13) 2.73 (s, 3H),
7.58 (d J ¼ 8.8 Hz, 2H), 7.70 (d, J ¼ 8.8 Hz, 2H). IR (film) 722, 814,957,
1007, 1047, 1086, 1385, 1417, 1472, 2871, 2911, 3040cm21
.
Methyl p-nitrophenyl sulfoxide (3r)[10] 1H NMR (CDC13) 2.83 (s, 3H), 7.88
(d, J ¼ 8.7 Hz, 2H), 8.44 (d, J ¼ 8.7 Hz, 2H). I3C NMR (CDC13) 44.20,
124.91, 125.00, 150, 10, 153.46. IR (film) 746, 859, 969, 1038, 1088, 1340,
2861, 2920, 3026 cm21
.
ACKNOWLEDGMENT
M. H. A. expresses his gratitude to Bjorn Olesen for help during the prep-
aration of this manuscript.
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