I 2 as a mild and efficient catalyst in deoxygenation of sulfoxides with thioacetic acid

Article abstract of DOI:10.1080/17415993.2012.693491

An effective procedure for deoxygenation of sulfoxides to sulfides using thioacetic acid as a reducing agent and a catalytic amount of I2 in MeCN at ambient temperature has been developed. Using this protocol, a variety of sulfoxides including benzyl, allyl, alkyl and aryl sulfoxides have been successfully reduced to the corresponding sulfides in excellent yields.

Full text of DOI:10.1080/17415993.2012.693491

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I2 as a mild and efficient catalyst in
deoxygenation of sulfoxides with
thioacetic acid
Arida Jabbari a , Morteza Zarei a & Azar Jamaleddini b
a Department of Chemistry , Islamic Azad University , Qeshm
Branch, Qeshm , 79515-1393 , Iran
b Department of Chemistry , Islamic Azad University , Firoozabad
Branch, Firoozabad , 74715 , Iran
Published online: 01 Aug 2012.
To cite this article: Arida Jabbari , Morteza Zarei & Azar Jamaleddini (2012) I2 as a mild and
efficient catalyst in deoxygenation of sulfoxides with thioacetic acid, Journal of Sulfur Chemistry,
33:4, 413-418, DOI: 10.1080/17415993.2012.693491
To link to this article: http://dx.doi.org/10.1080/17415993.2012.693491
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Journal of Sulfur Chemistry
Vol. 33, No. 4, August 2012, 413–418
I2 as a mild and efficient catalyst in deoxygenation of sulfoxides
with thioacetic acid
Arida Jabbaria*, Morteza Zareia and Azar Jamaleddinib
aDepartment of Chemistry, Islamic Azad University, Qeshm Branch, Qeshm 79515-1393, Iran;
bDepartment of Chemistry, Islamic Azad University, Firoozabad Branch, Firoozabad 74715, Iran
(Received 18 March 2012; final version received 10 May 2012)
An effective procedure for deoxygenation of sulfoxides to sulfides using thioacetic acid as a reducing agent
and a catalytic amount of I2 in MeCN at ambient temperature has been developed. Using this protocol, a
variety of sulfoxides including benzyl, allyl, alkyl and aryl sulfoxides have been successfully reduced to
the corresponding sulfides in excellent yields.
O
Thioacetic acid, I2 (cat.)
S
S
R1
R2
R1
R2
MeCN, r.t.
Keywords: deoxygenation; sulfoxides; sulfides; iodine; thioacetic acid
1. Introduction
Sulfoxides (1) and sulfides (2) have significant roles in organic and medicinal chemistry.
The conversion of sulfoxides to the corresponding sulfides is a valuable reaction that has found
considerable utility in organic (3) and biochemical reactions (4). A large number of methods
have been published for this transformation. These methods use a variety of reagents such as
metal hydride reagents (5), low-valent metallic species or halide ions (6), phosphines combined
with Lewis acids (7), Woollins’ reagent (8), tetracyanoethylene oxide (9), catecholborane (10),
1,3,5-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-tetrachloride (11) and 1,3-dithiane in the presence
of electrophilic bromine and other sulfur compounds (12). However, in terms of functional group
tolerance, most of these reagents have one or more drawbacks. As a result, the choice of reducing
agent is important for the success of the reaction. Thioacids have been widely used to reduce
sulfoxides (12d,e, 13); nevertheless, the reported procedures are not synthetically useful because
they have long reaction times and low yields. For example, DMSO and methyl phenyl sulfoxide
were reduced with thioacetic acid in moderate yields but only after 1 week (12d). Nevertheless,
*Corresponding author. Email: arida_jabbari@yahoo.com
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2012 Taylor & Francis
http://dx.doi.org/10.1080/17415993.2012.693491
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414 A. Jabbari et al.
the advantage of this method is that thioacetic acid is an inexpensive and common laboratory
reagent. Accordingly, we are interested in exploring a modified procedure for deoxygenation of
sulfoxides with thioacetic acid under mild conditions using a suitable catalyst. In recent years,
the use of molecular iodine has drawn considerable attention as a cheap, non-toxic and a readily
available catalyst for diverse organic transformations (14). Iodine had been previously utilized
for catalytic deoxygenations of sulfoxides with NaBH4 (15) and 3-mercaptopropionic acid (13).
Consequently, as a corollary to this work, we decided to explore the efficacy of molecular iodine
as a catalyst in deoxygenation of sulfoxides with thioacetic acid.
2. Results and discussion
First of all, we carried out a study of the proper conditions to perform the reaction between methyl
phenyl sulfoxide 1f and I2/thioacetic acid. The optimization of reaction conditions was carried
out at different temperatures and with different solvents (Table 1).
Solvents, such as CH3CN, THF, dichloromethane and chloroform (Table 1, Entries 1–8), were
used in this study.A comparison of these four solvents demonstrated that acetonitrile was superior
to the other solvents at room temperature (rt) or under reflux (Table 1, Entry 7 and 8). The
reductions in dichloromethane and chloroform required much longer reaction times, and these
solvents were not suitable for the proposed reaction (Table 1, Entries 1–4). We also studied the
effect of different sulfoxide/thioacetic acid/I2 ratios in CH3CN for the conversion of methyl
phenyl sulfoxide 1f to methyl phenyl sulfide 2f. Employing the ratio of 1/2/0.01 in CH3CN gave
the best result and produced methyl phenyl sulfide 2f after 30 min in 98% isolated yield. During
the reaction, diacetyl disulfide was formed as a co-product (as evidenced by Gas chromatography-
mass spectroscopy (GC–MS) analysis) and isolated by column chromatography, then identified by
mass spectroscopy (m/e = 150[M+]) and elemental analysis. With the above standard conditions
in hand, we screened a representative range of structurally different sulfoxides. The results are
listed in Table 2.
The results obtained here clearly demonstrate that this procedure is an effective method for
the deoxygenation of sulfoxides. A wide range of diaryl, aryl-alkyl and dialkyl sulfoxides were
Table 1. Deoxygenation of methyl phenyl sulfoxide 1f with thioacetic acid/I2.
O
S
Thioacetic acid (2 mmol), I2 (cat.) (0.01 mmol)
T (°C),Solvent
S
CH3
CH3
2f
1f (1 mmol)
Temperature
(◦C)
Entry
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CHCl3
CHCl3
THF
THF
CH3CN
CH3CN
rt
Reflux
rt
Reflux
rt
Reflux
rt
Reflux
7
6.5
9
8
3
2.5
1
1
89
88
85
85
74
79
98
96
aIsolated yields based on the starting sulfoxides.

Journal of Sulfur Chemistry 415
Table 2. Reduction of various sulfoxides using I2and thioacetic acid in CH3CNa.
Time
(min)
Yield
Entry
1
Sulfoxide
Sulfide
(%)b,c
Ref.
S
S
O
1a
1b
2a
2b
60
96
93
6d
O
S
S
2
3
140
8
O
S
S
1c
2c
250
480
89
75
6d
H3C
CH3
H3C
CH3
O
S
S
4
1d
2d
6b
O2N
O2N
O
S
S
S
5
6
1e
1f
2e
2f
350
30
89
98
16
3b
Cl
Cl
Cl
Cl
O
S
CH3
CH3
O
S
S
CH3
CH3
7
1g
1h
1i
2g
2h
2i
40
50
96
94
84
91
6d
3b
3b
3b
H3C
H3C
O
S
S
S
8
O
S
9
180
60
O
S
S
10
1j
2j
S
O
S
11
12
1k
1l
2k
2l
140
40
90
95
6f
8
S
O
S
O
S
S
13
1m
2m
50
92
16
aSulfoxide (1 equiv.), iodine (0.01 equiv.) and thioacetic acid (2.2 mmol) in CH3CN at rt.
bIsolated yields.
cData of products are consistent with reported values.

416 A. Jabbari et al.
reduced to give the corresponding sulfides in good to excellent yields at rt. The reaction with
dibenzyl sulfoxide is particularly noteworthy (Table 2, Entry 1) since reduction of this sulfoxide
fails or gives very poor yields using several other published reduction methods (16, 17).
Diphenyl sulfoxide 1b (Table 2, Entry 2) and related substrates 1c–1e (Table 2, Entries 3–5)
provided good yields of the corresponding products 2b–2e (75–93%). In addition, deoxygenation
of diphenyl sulfoxide was found to be more efficient than deoxygenation of electron-poor sulfox-
ides such as bis(4-chlorophenyl)sulfoxide (by comparing Entry 2 with Entry 5). This substituent
effect is especially stark with the NO2 group as revealed in the reduction of 1d that gave low
yields even at long reaction times (Table 2, Entry 4). Alkyl-aryl and dialkyl sulfoxides 1f–1l were
also examined under these reaction conditions and our catalytic system, I2/thioacetic acid, was
found to be effective furnishing the corresponding sulfides 2f–2l in excellent yields (Table 2,
Entries 6–12). The yields of 2f–2j from sulfoxides 1f–1j illustrate the effect of steric size of the
substrate and/or substituents in the substrate on the reduction reaction. The yields were highest
in reactions of substrates 1f and 1h (alkyl group = Me, Et), which have no branching at the alkyl
group. The yield dropped to 84% in the reaction of 1i that carries the secondary alkyl group, iPr.
This reaction is also compatible with the vinyl functional group (Table 2, Entry 13). Moreover,
the reduction of dibenzyl and benzyl phenyl sulfoxides to their sulfides without the cleavage of
sensitive C–S bond shows the usefulness of the presented protocol (Table 2, Entries 1 and 11).
A feasible pathway that explains many aspects of the reactions is illustrated in Scheme 1. The
formation of diacetyl disulfide revealed significant evidence for the support of our mechanism.
O
O
S
S
2HI
R1 R2
S
O
O
S
R1 R2
+ H2O
I2
2
SH
Scheme 1. A possible pathway for deoxygenation of sulfoxides with thioacetic acid catalyzed
by molecular iodine.
3. Conclusion
In summary, we have developed a facile and high yielding method for the reduction of sulfoxide
to the corresponding sulfides using thioacetic acid and a catalytic amount of molecular iodine in
excellent yields. In comparison with literature methods using thioacid reagents, this procedure is
faster, more efficient and gives higher yields.
4. Experimental
ChemicalswereeitherpurchasedfromFluka, MerckandAldrichChemicalCompaniesorprepared
in our laboratories. Most of the products were purified by column chromatography and were
identified by comparing their melting points (mps) and IR and NMR spectra with those reported
for authentic samples. Progress of the reactions was monitored by TLC using silica gel polygrams
SIL G/UV 254 plates. Fourier transform infrared spectroscopy were recorded on a Shimadzu DR-

Journal of Sulfur Chemistry 417
8001 spectrometer. NMR spectra were recorded on a BrukerAvance DPX 250 MHz instrument in
CDCl3 using TMS as internal standard. Chemical shifts were reported in ppm (δ), and coupling
constants (J) in Hz. Mass spectra were recorded on a Shimadzu GC MS-QP 1000 PX at 70 eV.
Elemental analyses were determined in our department using Thermo Finnigan Flash EA 1112
Series. Mps were determined in open capillaries with a GalenKamp mp apparatus and are not
corrected.
General procedure for deoxygenation of sulfoxides 2a–2m: To a stirred solution of sulfoxide
(1 mmol) and thioacetic acid (0.16 ml, 2.2 mmol) in CH3CN (2.0 ml), I2 (2.54 mg, 0.01 mmol)
was added and the resulting solution was stirred at rt. The progress of the reaction was monitored
by TLC and GC–MS. After completion of the reaction, the reaction was quenched with a 5% aq
solution of NaOH (25 ml) and extracted with Et2O (3 × 20 ml). The combined organic extracts
were dried over anhydrous Na2SO4 and concentrated in vacuo. Purification on preparative silica
gel TLC (n-hexane as an eluent) gave sulfide 2a–2n.
Acknowledgement
The authors wish to acknowledge the Azad Islamic University of Qeshm for financial support of this work.
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