Chromium(VI) oxide catalyzed oxidation of sulfides to sulfones with periodic acid

Article abstract of DOI:10.1021/jo030031n

A highly efficient and selective oxidation of sulfides to sulfones with periodic acid catalyzed by CrO₃ is described. A variety of electron-rich and electron-deficient sulfides were oxidized to sulfones with 2 mol% CrO₃ in acetonitrile at room temperature in excellent yields. Sulfides with other readily oxidized functional groups were selectively oxidized to sulfones in high yields with 10 mol% CrO₃ in ethyl acetate/acetonitrile at -35 °C.

Full text of DOI:10.1021/jo030031n

available for the oxidation of electron-deficient sulfides
Ch r om iu m (VI) Oxid e Ca ta lyzed Oxid a tion
of Su lfid es to Su lfon es w ith P er iod ic Acid
to sulfones. Typical reagents include HOF‚CH3CN
16
complex,6,15 H2O2 in (CF3CO)2O
or HOAc,17 RuO4,18
Liang Xu,† J ie Cheng,‡ and Mark L. Trudell*,‡
KMnO419 and CrO3 in HOAc,20 or HNO3.21 However, most
of these methods either give low yields of products or use
sophisticated reagents that are difficult to handle. There-
fore, convenient and efficient reagents are still desired
for this transformation. The strong oxidative power of
the H5IO6/CrO3 system9b prompted an investigation of
this reagent with electron-deficient sulfides. Herein, we
describe the successful use of the H5IO6/CrO3 system as
a method to efficiently oxidize electron-deficient sulfides
to sulfones and selectively oxidize sulfides containing
other easily oxidized groups.
Department of Chemistry, University of New Orleans,
New Orleans, Louisiana 70148, and
St. Charles Pharmaceuticals, Inc., P.O. Box 850815,
New Orleans, Louisiana 70185
mtrudell@uno.edu
Received J anuary 24, 2003
Abstr a ct: A highly efficient and selective oxidation of
sulfides to sulfones with periodic acid catalyzed by CrO3 is
described. A variety of electron-rich and electron-deficient
sulfides were oxidized to sulfones with 2 mol % CrO3 in
acetonitrile at room temperature in excellent yields. Sulfides
with other readily oxidized functional groups were selectively
oxidized to sulfones in high yields with 10 mol % CrO3 in
ethyl acetate/acetonitrile at -35 °C.
Initially, we examined the influence of catalyst loading,
oxidant concentration, reaction time, and solvent on the
oxidation of methyl phenyl sulfide (1a ) to the correspond-
ing methyl phenyl sulfone (2a ) (Table 1).
The reactions were performed by addition of sulfide 1a
into a stirred solution of H5IO6 and CrO3 in acetonitrile
at room temperature. Acetonitrile was determined to be
the most appropriate solvent due to poor solubility of
CrO3 in most other organic solvents (CH2Cl2, EtOAc).
Relatively low catalyst loading (1-5 mol %) resulted in
low yields of the sulfone 2a with only 2.1 equiv of H5IO6.
In those cases (Table 1, entries 1-3), the intermediate
sulfoxide could not be converted completely into sulfone.
Longer reaction times did not improve the conversion.
The reaction mixture turned green quickly indicating the
generation of CrIII species. It is believed that the catalyst
was converted into an inactive low oxidation state
chromium species and that the concentration of H5IO6
(2.1 equiv) was not sufficient to effectively regenerate the
catalyst and drive the reaction to completion. Alterna-
tively, the oxidation proceeded quickly (12 min) and
cleanly with 2.1 equiv of H5IO6 and 10 mol % CrO3 at
room temperature and gave the optimal yield of 2a (Table
1, entry 5). It is noteworthy that the oxidation also readily
occurred at -60 °C using EtOAc as a cosolvent with
acetonitrile (Table 1, entry 6). The reaction rate was
Sulfone moieties are useful synthetic intermediates in
the preparation and functionalization of chemically and
biologically significant compounds.1-4 Among the differ-
ent procedures to prepare sulfones, the direct oxidation
of sulfides to sulfones is the most straightforward method
in organic synthesis.5 Although various oxidizing re-
agents have been developed for this oxidation, efficient
reagents that can also be used for the oxidation of
electron-deficient sulfides and that are tolerant of other
easily oxidized groups are rare.6
Oxidations with H5IO6 alone or with catalysts have
been well documented.7,8 In recent years, H5IO6 has been
employed to oxidize a variety of substrates with catalysts,
such as CrO3,9 TEMPO,10 MnIV complex,11 CrO2(OAc)2,12
and FeCl3.13 We recently reported a mild and efficient
oxidation of alcohols to aldehydes and ketones with
H5IO6 catalyzed by Cr(acac)3.14 Only a few methods are
† St. Charles Pharmaceuticals, Inc.
‡ University of New Orleans.
(1) (a) Nakayama, J .; Sugihara, Y. Top. Curr. Chem. 1999, 205, 131.
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10.1021/jo030031n CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003
5388
J . Org. Chem. 2003, 68, 5388-5391

TABLE 1. Oxid a tion of Meth yl P h en yl Su lfid e to Meth yl
P h en yl Su lfon e w ith H5IO6/Cr O3
TABLE 2. H5IO6/Cr O3 Oxid a tion of Electr on -Deficien t
Su lfid es to Su lfon es
entry
H5IO6 (equiv)
CrO3 (mol %)
time (h)
yielda (%)
1
2
2.1
2.1
2.1
2.1
2.1
2.1
3.0
3.0
4.0
4.0
0
48
48
24
3
0.2
2
15
7
6
2
13
60
71
90
96
96b
96
96
96
96
1.0
5.0
7.5
10
3
4
5
6
10
7
1.0
2.0
1.0
2.0
8
9
10
a
b
Isolated yield. Reaction performed at -60 °C in EtOAc/
CH3CN (2:1).
slower at -60 °C and formation of the intermediate
sulfoxide could be monitored by TLC. However, the
overall oxidation to the sulfone was complete after 2 h
and gave the same yield as the room-temperature reac-
tion (Table 1, entry 5).
Further investigation of the H5IO6/CrO3 stoichiometry
revealed that the oxidation proceeded equally well with
less CrO3 (1-2 mol %) provided the concentration of
H5IO6 (3 or 4 equiv) was increased (Table 1, entries
7-10). These catalytic systems gave equivalent yields to
systems with 10 mol % CrO3 but required longer reaction
times. It was determined that the H5IO6/CrO3 system
that employed 4 equiv of H5IO5 with 2 mol % CrO3 (Table
1, entry 10) would be further explored since this system
afforded an optimum yield of 2a as well as offered the
advantages of both low catalyst loading (2 mol %) and
relatively short reaction times (2 h). The low catalyst
loading was particularly attractive since the disposal of
toxic chromium waste was minimized.
Other solvents were also investigated as cosolvents in
an effort to improve catalytic efficiency. When used as a
cosolvent, dichloromethane and chloroform gave similar
yields while acetone, ether, and THF gave very poor
yields of the desired sulfones. The effect of moisture on
catalyst loading and activity was also investigated. In all
cases, independent of H5IO6/CrO3 concentration, reac-
tions performed with wet acetonitrile (5-10 vol %)
required much longer reaction times.
The results of the oxidation of electron-deficient sul-
fides (1b-i) to the corresponding sulfones (2b-i) with
H5IO6/CrO3 are summarized in Table 2. The reactions
were performed in a fashion similar to the above oxida-
tion of 1a employing 2 mol % CrO3 and 4 equiv of H5IO6.
The oxidations were generally completed within 2.5 h. A
simple workup then furnished the sulfones cleanly and
in high yields. In addition, the oxidations were performed
with 10 mol % and 2.1 equiv of H5IO6. The yields of these
reactions were equivalent to the 2 mol % CrO3 system;
however, the oxidations were typically completed within
minutes. There are several noteworthy examples in Table
2 that demonstrate the oxidative strength of the H5IO6/
CrO3 system. The 4-fluorophenyltrifluoromethyl sulfone
(2c)22,23 was readily prepared from commercial 4-fluo-
rophenyl trifluoromethyl sulfide (1c) at room tempera-
ture within 1-2.5 h and in 96% yield. The binuclear
a
b
Isolated yields. Conditions: H5IO6 (2.1 equiv), CrO3 (10 mol
%), rt, 1 h. c Conditions: H5IO6 (8 equiv), CrO3 (2 mol %), rt, 3.5
d
h. Conditions: H5IO6 (10 equiv), CrO3 (80 mol %),CH3CN, reflux,
1 h.
manganese complex [MnIV-MnIV(µ-O)3L2](PF6)2 has been
reported to be a catalyst for the oxidation of sulfides to
sulfones by using H5IO6 as oxidant but it required 3 days
to convert dibenzothiophene (1f) into dibenzothiophene-
5,5-dioxide (2f).11 The oxidation of 1f with H5IO6/CrO3
was completed within 1-2 h in 96% yield. In addition,
thianthrene 5,5,10,10-tetraoxide (2g) was obtained by
oxidation of thianthrene (1g) with 8 equiv of H5IO6 and
2 mol % of CrO3. The extremely electron-deficient sulfide,
pentafluorophenyl sulfide (1h ), was oxidized to the cor-
responding sulfone 2h with 10 equiv of H5IO6 and 80 mol
% CrO3 in refluxing acetonitrile for 1 h. To our knowl-
edge, there is only one other report of this transformation
in the literature and it employed the HOF‚CH3CN
complex.6 It has been reported that 1,2-benzothiazole-3-
(22) J ones, T. R.; Varney, M. D.; Webber, S. E.; Lewis, K. K.;
Marzoni, G. P.; Palmer, C. L.; Kathardekar, V.; Welsh, K. M.; Webber,
S.; Matthews, D. A.; Appelt, K.; Smith, W. W.; J anson, C. A.;
Villafranca, J . E.; Bacquet, R. J .; Howland, E. F.; Booth, C. L. J .;
Herrmann, S. M.; Ward, R. W.; White, J .; Moomaw, E. W.; Bartlett,
C. A.; Morse, C. A. J . Med. Chem. 1996, 39, 904.
(23) Beaumont, A. J .; Clark, J . H. J . Fluorine Chem. 1991, 52, 295.
J . Org. Chem, Vol. 68, No. 13, 2003 5389

TABLE 3. H5IO6/Cr O3 Ch em oselective Oxid a tion of Su lfid es to Su lfon es
a
b
Isolated yields. Conditions: 0 °C. c Conditions: H5IO6 (4.5 equiv).
one (1i) is resistant to oxidation and that there are no
effective reagents for the transformation of 1i into
saccharin (2i).24 However, oxidation of 1i with H5IO6/CrO3
afforded 2i in 91% yield. These results demonstrate that
the H5IO6/CrO3 catalyst system is very effective for the
oxidation of electron-deficient sulfur atoms.
oxidation a variety ancillary readily oxidized functional
groups were unaffected.
Many reagents have been developed for the selective
oxidation of sulfides to sulfones [e.g., H2O2/catalyst,25
m-chloroperbenzoic acid (m-CPBA),26 dimethyldioxirane
(DMD),27 tert-butyl hydroperoxide (TBHP),28 and potas-
sium hydrogen persulfate (OXONE)29]. However, there
have been no previous reports that describe the clean
oxidation of phenolic sulfides to the corresponding phe-
nolic sulfones. The oxidations of both 4-(methylthio)-
phenol (1j) and 4-methylthio-3-methylphenol (1k ) were
achieved with H5IO6/CrO3 to furnish the corresponding
sulfones 2j and 2k in exceptionally pure form and in
excellent yields (g92%).
Chemoselective oxidation of a variety of sulfides (1j-
z) to sulfones (2j-z) in the presence of a variety of other
easily oxidized functional groups was examined, and the
results are summarized in Table 3. The reactions were
performed by the addition of a solution of H5IO6/CrO3 in
acetonitrile to a stirred solution of sulfide in ethyl acetate
at -35 °C. Ethyl acetate was employed as the solvent in
order to access lower reaction temperatures. The chemo-
selectivity of the oxidation is due in part to control of
both temperature and stoichiometry. In most cases, if
higher reaction temperatures or excess oxidant (>2.1
equiv) was employed the chemoselectivity was lost and
the additional oxidation of ancillary functional groups
became evident. To this end, reaction conditions that
employed H5IO6 (2.1 equiv)/10 mol % CrO3 were found
to be superior to conditions that employed more H5IO6
with less CrO3. Under these conditions of the H5IO6/CrO3
In addition to phenolic sulfides, sulfides containing a
primary alcohol or a formyl group were selectively
oxidized to the corresponding sulfones 2l and 2m , re-
(25) (a) Yamada, Y. M. A.; Tabata, H.; Takahashi, H.; Ikegami, S.
Synlett. 2002, 2031. (b) Alonso, D. A.; Najera, C.; Varea, M. Tetrahe-
dron Lett. 2002, 43, 3459. (c) Choi, S.; Yang, J . D.; J i, M.; Choi, H.;
Kee, M.; Ahn, K. H.; Byeon, S. H.; Baik, W.; Koo, S. J . Org. Chem.
2001, 66, 8192. (d) Sato, K.; Hyodo, M.; Aoki, M.; Zheng, X.; Noyori,
R. Tetrahedron 2001, 57, 2469. (e) Van Dort, P. C.; Fuchs, P. L. J .
Am. Chem. Soc. 1994, 116, 5657.
(26) (a) Loughlin, W. A.; Rowen, C. C.; Healy, P. C. J . Chem. Soc.,
Perkin Trans. 2 2002, 296. (b) Kurtner, A.; Chodynski, M.; Halkes, S.
J . Synth. Commun. 1996, 26, 1175. (c) Davis, F.; Zhou, P. Tetrahedron
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(24) Zlotin, S. G.; Kislitsin, P. G.; Podgursky, A. I.; Samet, A. V.;
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65, 8439.
5390 J . Org. Chem., Vol. 68, No. 13, 2003

spectively, in high yields. However, benzylic alcohol and
secondary alcohol moieties were not tolerated under these
conditions and gave poor yields of the desired sulfone
due to competing oxidation of the hydroxyl group. Even
at lower reaction temperatures (-60 °C), competing
oxidation was problematic. The oxidation of 4-methyl-
thio benzyl alcohol (1n ) with the H5IO6/CrO3 afforded
4-methylsulfonyl benzyl alcohol (2n ) in only 41% yield,
with a 28% yield of 4-methylsulfonyl benzaldehyde.
It is known that aromatic rings can be oxidized to
quinones,9a but the oxidation of the sulfur atom is much
faster. The 2-methylthionaphthalene (1o) was selectively
oxidized to the sulfone 2o in excellent yield leaving the
aromatic ring unaffected. In addition, benzyl phenyl
sulfide (1p ) and methyl p-tolylsulfide (1q) were selec-
tively oxidized to the corresponding sulfones 2p and 2q
without formation of benzylic oxidation products.
Periodic acid is a weak acid (pK1 ) 3.29, pK2 ) 8.3,
pK3 ) 11.6),7b and it has recently been reported that
H5IO6 can selectively oxidize sulfur acetals to aldehydes
in the presence of acid sensitive groups.30 Sulfides
containing a tert-butyl group (1r and entry 1s) and a
methoxymethyl (MOM) group (1t) were selectively oxi-
dized to sulfones 2r -t in yields greater than 90%.
Moreover, 2-phenyl-1,3-dithiane (1u ) was oxidized to
the 1,3-dithiane-2-phenyl-1,1,3,3-tetraoxide (2u ) in 91%
yield by using 4.5 equiv of H5IO6 and 10 mol % of CrO3
at 0 °C.
Both carbon-carbon double bonds and triple bonds
were unaffected by the H5IO6/CrO3 oxidation conditions.
Phenyl vinyl sulfide (1v), allyl phenyl sulfide (1w ), and
phenyl propargyl sulfide (1x) were converted into the
corresponding sulfones 2v-x in excellent yields. Similar
selectivity was observed for the heterocyclic sulfides 1y
and 1z. The sulfide moieties were oxidized selectively to
the corresponding sulfones without formation of any
other oxidation products. It is noteworthy that the
thiazole ring sulfur atom of 2-methylthiobenzothiazole
(1z) was not oxidized, and only the 2-methylsulfonyl-
benzothiazole (2z) was obtained. Although amide and
heterocyclic nitrogen atoms (entries 10, 16, and 17) were
not affected by the conditions of this oxidation giving high
yields of the corresponding sulfones, the amino sulfide
derivatives (e.g., 2-methylthioaniline) gave dark intrac-
table mixtures.
new protocol is superior to other methods by furnishing
sulfones in high yield and high purity as well as requiring
lower reaction temperatures and shorter reaction times.
Exp er im en ta l Section
All starting materials unless otherwise noted were com-
mercially available. All products unless otherwise noted were
identified by comparison of NMR spectral and physical data with
the data reported in the literature and with authentic samples
when available. 1H and 13C NMR spectra were recorded on a
Varian-400 MHz spectrometer at ambient temperature in CDCl3
or DMSO-d6 (Cambridge Isotope Laboratories, Inc.). C, H, N
analyses were determined by Atlantic Microlabs, Inc., Norcross,
GA. Melting points were recorded on
apparatus and are uncorrected.
a Hoover Mel-Temp
Gen er a l P r oced u r e for Oxid a tion of Sim p le Su lfid es
a n d Electr on -Deficien t Su lfid es to Su lfon es. H5IO6 (4.6 g,
20 mmol) was dissolved in acetonitrile (80 mL) by vigorous
stirring at room temperature for 30 min, and then CrO3 (10 mg,
0.1 mmol, 2 mol %) was added to the solution. The mixture was
stirred at room temperature for 5 min to give a clear orange
solution. To this solution was added a solution of sulfide (5 mmol)
in CH3CN (10 mL) at room temperature. The exothermic
reaction formed a precipitate immediately. The reaction mixture
was stirred at room temperature until the oxidation was
completed (monitored by TLC), the mixture was then filtered,
and the filter cake was washed with CH3CN (35 mL). The filtrate
was concentrated under reduced pressure at room temperature,
and the residue was extracted with ethyl acetate or chloroform.
The combined extracts were washed, respectively, with saturated
aqueous Na2SO3 solution and brine and then dried over MgSO4.
Removal of the solvent under reduced pressure afforded the
crude sulfones. Most of the crude sulfones were of sufficient
1
purity (>95%) for further use on the basis of H NMR spectra
and melting points.
Gen er a l P r oced u r e for Selective Oxid a tion of F u n c-
tion a lized Su lfid es to Su lfon es. H5IO6 (2.40 g, 10.5 mmol)
was dissolved in CH3CN (30 mL) by vigorous stirring at room
temperature for 1 h. Then CrO3 (50 mg, 0.5 mmol, 10 mol %)
was added to the solution. The mixture was stirred at room
temperature for 5 min to give a clear orange solution. The H5-
IO6/CrO3 solution was then added dropwise over a period of 45
min to a solution of sulfide (5 mmol) in ethyl acetate (60 mL) at
-35 °C. After the addition was completed, the reaction mixture
was stirred at -35 °C for 1 h. The reaction was quenched by
addition of saturated Na2SO3 solution (5 mL) and filtered, and
the solids were washed with ethyl acetate (60 mL). The filtrate
was washed, respectively, with saturated aqueous Na2SO3
solution and brine and then dried over MgSO4. Removal of the
solvent under reduced pressure afforded the crude sulfones. Most
of the crude sulfones were of sufficient purity (>95%) for further
1
use on the basis of H NMR spectra and melting points.
2-(Met h ylsu lfon yl) ter t-Bu t yl Nicot in a m id e (2s). 2-
(Methylsulfonyl) tert-butyl nicotinamide was prepared from
2-(methylthio) tert-butyl nicotinamide (1s)31 using the general
procedure for funtionalized sulfides described above: mp 163-5
In conclusion, we have found H5IO6/CrO3 to be an
excellent catalytic system for promoting the efficient
oxidation of sulfides to sulfones. Low catalyst loading (2
mol %) afforded the advantage of reduced chromium
waste while providing high yields of a variety of electron-
deficient sulfones. Alternatively, higher catalyst loading
(10 mol %) led to shorter reaction times and the highly
chemoselective oxidation of sulfides to sulfones in the
presence of other easily oxidized functional groups. This
1
°C; H NMR (400 MHz, DMSO-d6) δ 8.74-8.75 (d, 1H, J ) 4.4
Hz), 8.14 (s, 1H, NH), 7.92-7.94 (d, 1H, J ) 7.2 Hz), 7.73-7.76
(m, 1H), 3.27 (s, 3H, MeSO2), 1.34 (s, 9H); 13C NMR (75 MHz,
DMSO-d6) δ 164.5, 153, 149.0, 138.1, 133.6, 127.4, 50.9, 41.5,
28.1; IR (liquid film) 2923, 1656, 1556, 1464 cm-1; MS(CI, NH3)
m/z 257 (24, M + 1), 242 (45), 185 (100). Anal. Calcd for
C11H16N2O3S: C, 51.56; H, 6.25; N, 10.93. Found: C, 51.47; H,
6.30; N, 10.80.
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compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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J . Org. Chem, Vol. 68, No. 13, 2003 5391