Rapid and efficient deoxygenation of sulfoxides to sulfides with tantalum(V) chloride/sodium iodide system

Article abstract of DOI:10.1080/17415993.2017.1337770

TaCl₅/NaI system converts a wide range of sulfoxides to the corresponding sulfides in high yields with short reaction times, under mild conditions. It is worth mentioning that this protocol is chemoselective and tolerates various functional groups (such as –Br, –Cl, –OCH₃, –CHO, and –NO₂) and double bond.

Full text of DOI:10.1080/17415993.2017.1337770

Journal of Sulfur Chemistry
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Rapid and efficient deoxygenation of sulfoxides to
sulfides with tantalum(V) chloride/sodium iodide
system
Byung Woo Yoo, Jeeyeon Park, Hyo Jong Shin & Cheol Min Yoon
To cite this article: Byung Woo Yoo, Jeeyeon Park, Hyo Jong Shin & Cheol Min Yoon (2017):
Rapid and efficient deoxygenation of sulfoxides to sulfides with tantalum(V) chloride/sodium iodide
system, Journal of Sulfur Chemistry, DOI: 10.1080/17415993.2017.1337770
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Date: 23 June 2017, At: 06:11

JOURNAL OF SULFUR CHEMISTRY, 2017
https://doi.org/10.1080/17415993.2017.1337770
Rapid and efficient deoxygenation of sulfoxides to sulfides
with tantalum(V) chloride/sodium iodide system
Byung Woo Yoo, Jeeyeon Park, Hyo Jong Shin and Cheol Min Yoon
Department of Advanced Materials Chemistry, Korea University, Sejong, Korea
ABSTRACT
ARTICLE HISTORY
Received 8 February 2017
Accepted 26 May 2017
TaCl5/NaI system converts a wide range of sulfoxides to the corre-
sponding sulfides in high yields with short reaction times, under mild
conditions. It is worth mentioning that this protocol is chemoselec-
tive and tolerates various functional groups (such as –Br, –Cl, –OCH3,
–CHO, and –NO2) and double bond.
KEYWORDS
Deoxygenation; sulfoxide;
sulfide; tantalum(V) chloride;
sodium iodide
O
TaCl5/NaI
R1
S
R2
R1
S
R2
CH3CN, r. t.
R1, R2 = Aryl and Alkyl
1. Introduction
The deoxygenation of sulfoxides to sulfides constitutes a process of importance in both
organic synthesis and biochemistry [1]. Sulfoxides are important intermediates in a vari-
ety of synthetic transformations, especially as chiral auxiliaries in asymmetric synthesis.
After the asymmetric transformation, the sulfoxide moiety is removed from the target
molecule through deoxygenation followed by desulfidation [2–5]. There are many avail-
able methods that can be used to convert sulfoxides to their corresponding sulfides [6–18].
However, most of these methods often suffer from drawbacks, including functional group
incompatibility, long reaction times, harsh reaction conditions or complex experimental
procedures. For these reasons, the search for mild and efficient methods based on eas-
ily accessible reagents and operationally simple procedures to overcome the limitations
remains an important challenge in organic synthesis. The uses of oxophilic d-block metals
have become important in deoxygenation of various types of organic molecules [19]. In
this regard, the chemistry of TaCl5 is one of the current interest in organic synthesis and
has been extensively studied [20–24]. Fortunately, tantalum(V) compounds generally have
low toxicity and are not considered particularly poisonous [25]. Attracted by the strong
oxophilic character of TaCl5, we decided to investigate its reactivity with sulfoxides. As a
CONTACT Byung Woo Yoo
bwyoo@korea.ac.kr
Department of Advanced Materials Chemistry, Korea
University, 2511 Sejong-ro, Sejong 339-700, Korea
© 2017 Informa UK Limited, trading as Taylor & Francis Group

2
B. W. YOO ET AL.
result we are gratified to report herein that the TaCl5/NaI system efficiently deoxygenates
various sulfoxides in high yields under mild conditions.
2. Results and discussion
We have investigated the reactions of the TaCl5/NaI system with a broad range of sulfox-
ides. All the reactions proceeded almost instantaneously upon addition of sodium iodide
to a suspension of TaCl5 and sulfoxide in acetonitrile at room temperature and the corre-
sponding sulfides were obtained in high yields. We have found that the optimized molar
ratio of the substrate with respect to TaCl5 and NaI was 1/0.5/2 which under mild condi-
tions the reaction was completed in a short time (3–5 min). The route for the synthesis of
sulfides is shown in Scheme 1.
To ensure the role of NaI, a control experiment was carried out with TaCl5 in the absence
of NaI under the present condition, which failed to yield any desired product. It is obvious
that a combination of TaCl5 and NaI is essential for the efficient reduction of sulfoxides. To
evaluate the solvent effect, the deoxygenation of diphenyl sulfoxide was carried out using
different organic solvents such as toluene, dichloromethane, methanol, tetrahydrofuran,
and acetonitrile. Among the solvents that were examined, acetonitrile turned out to be the
most suitable solvent for this transformation in terms of reaction time and yield.
In order to assess the generality and scope of this reagent system, we examined the
deoxygenation of a variety of sulfoxides bearing other potentially labile functional groups.
An inspection of the data in Table 1 shows that diaryl, dialkyl, and aryl alkyl sulfoxides are
all reduced smoothly, giving sulfides in yields exceeding 89%. The isolated yield was inde-
pendent of the nature of the substituents. Thus, high yields were observed for sulfoxides
with either electron-withdrawing or electron-releasing substituents on the aromatic ring
with no obvious preference on the reactivity. However, the attempted reduction of diphenyl
sulfone resulted in the recovery of starting material, suggesting that the polarized nature
of sulfoxides is critical for reactivity [26].
The functional group tolerance of this method is evident from entries 8–13, which show
that the sulfoxides possessing bromo, chloro, methoxy, aldehyde, vinyl, and nitro function-
alities are chemoselectively reduced to the corresponding sulfides without affecting these
groups under the reaction conditions. Thus we have been able to demonstrate the util-
ity of easily accessible TaCl5/NaI system as a useful reagent for effecting chemoselective
deoxygenation of sulfoxides.
A plausible mechanism for the reductive S–O bond cleavage of the sulfoxide can be
rationalized as illustrated in Scheme 2. The reaction presumably proceeds via initial com-
plexation of a sulfonyl oxygen with a Lewis acid, TaCl5, which facilitates stepwise reduction
of the sulfoxide. In the first step, the coordination of tantalum(IV) chloride to the oxygen
O
TaCl5/NaI
R1
S
R2
+
I2
TaO2Cl +
R1
S
R2
CH3CN, r. t.
Scheme 1. Conversion of sulfoxides to their corresponding sulfides.

JOURNAL OF SULFUR CHEMISTRY
3
Table 1. Deoxygenation of various sulfoxides to sulfides using the TaCl5/NaI system.
Entry
R1
R2
Products
Time (min)
Yield (%)a,b
1
2
3
4
5
6
7
8
Ph
Ph
nC4H9
4-CH3C6H4
4-CH3C6H4
PhCH2
Ph
CH3
nC4H9
4CH3C6H4
CH3
PhCH2
Ph
CH3
PhSPh
PhSCH3
(nC4H9)2S
(4-CH3C6H4)2S
4-CH3C6H4SCH3
(PhCH2)2S
3
3
5
3
3
5
3
3
5
5
5
3
95
96
94
93
94
96
94
91
93
92
90
89
PhCH2
PhCH2SPh
4-BrC6H4
4-ClC6H4
4-CH3OC6H4
4-OHCC6H4
Ph
4-BrC6H4SCH3
(4-ClC6H4)2S
(4-CH3OC6H4)2S
4-OHCC6H4SCH3
9
4-ClC6H4
4-CH3OC6H4
CH3
10
11
12
=
=
CH2 CH
PhSCH CH2
O
O2N
S CH3
O2N
S CH3
13
5
90
aRefers to isolated yields.
bAll the products were characterized by comparison of their spectral data with authentic samples.
Cl
Cl
TaCl4
TaCl3
_
O
O
+
S
NaI
NaI
I2
R
+
R
R
R
S
R
S
+
R
R
S
+
R
I-
-Cl-
-TaOCl3
I-
I
Cl
Cl
TaOCl2
TaOCl
_
O
O
+
S
NaI
NaI
I2
R
+
R
S
R
R
R
S
+
R
I-
R
S
+
R
-Cl-
-TaO2Cl
I-
I
Scheme 2. Proposed mechanism for the deoxygenation of sulfoxide.
proceeds to make the attack of iodide anion feasible. In a subsequent step, the resultant iod-
inated species is in turn attacked by another iodide anion to give the deoxygenated sulfide
and concomitantly I2. The immediate development of a deep brown color was observed,
which is consistent with the generation of molecular iodine in the reaction mixture. We
have also found that half a molar equivalent of TaCl5 and two molar equivalents of NaI
were required to complete the reaction. On the basis of our observations, we have pro-
posed a mechanism in which reduction of sulfoxides with TaCl5 produces TaOCl3, which
would react with another equivalent of sulfoxide to give sulfide and TaO2Cl.
The utility of the present protocol as a new reducing agent is also demonstrated by the
high yields of dibenzyl sulfide (entry 6) and benzyl phenyl sulfide (entry 7) obtained after
the reduction of the corresponding sulfoxides. Usually the sulfoxides which contain a ben-
zyl group are not reduced or give very poor yields with other reducing agents and can
undergo C–S bond cleavage reactions [27,28].

4
B. W. YOO ET AL.
3. Conclusion
In conclusion, we believe that the present procedure will present a practical and useful
alternative to the conventional methods for the deoxygenation of sulfoxides to sulfides.
Further investigations are currently in progress to extend the scope of this methodology in
our laboratory.
4. Experimental
4.1. General
The starting materials and solvents were purchased from commercial suppliers (Fluka
and Sigma-Aldrich Chemical Companies) and used without further purification. 1H and
13C nuclear magnetic resonance (NMR) spectra were determined on a FT-Bruker AF-300
NMR spectrometer in CDCl3 using tetramethylsilane as internal standard. Chemical shifts
were reported in ppm (δ) and coupling constants (J) in Hz. GC/MS measurements were
carried out on a Shimadzu GC/MS-QP 1000. Thin-layer chromatography (TLC) analy-
sis was performed on silica gel plates (Merck, 60 F-254). All products were purified by
flash column chromatography on silica gel 60 (79-230 mesh, Merck) with ethyl acetate and
hexane. 1H and 13C NMR spectra of the products agreed with the reported data.
4.2. General procedure
In a 10 mL round-bottom flask, to a solution of diphenylsulfoxide (202 mg, 1.0 mmol) in
CH3CN (4 mL), tantalum (IV) chloride (179 mg, 0.5 mmol) and sodium iodide (300 mg,
2.0 mmol) were added at room temperature. The mixture turned dark brown almost imme-
diately and the progress of the reaction was followed by TLC. After completion of the
reaction (3 min), the reaction mixture was diluted with water and then extracted with ethyl
acetate. The combined organic extracts were washed successively with 10% aq Na2S2O3
and H2O. The organic layer was separated and dried over anhydrous Na2SO4 and con-
centrated under reduced pressure. The resulting crude product was purified through silica
gel column chromatography (hexane:ethyl acetate = 2:1) to afford diphenylsulfide (88 mg,
95%). All the products were characterized by comparison of their spectral data with those
reported in the literature [29–31].
4.3. Spectroscopic data of products
1
Diphenyl sulfide (Table 1, Entry 1): H NMR (300 MHz, CDCl3): δ 7.42–7.39 (m, 4H),
7.38–7.34 (m, 3H), 7.33–7.26 (m, 3H). 13C NMR (75 MHz, CDCl3): δ 135.7, 131.1, 129.2,
127.1. GC/MS (m/z): 186 (M+).
Methyl phenyl sulfide (Table 1, Entry 2): 1H NMR (300 MHz, CDCl3): δ 7.17–7.15 (m, 5H),
2.49 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 138.5, 128.8, 126.7, 125.0, 15.7. GC/MS (m/z):
124 (M+).
1
Dibutyl sulfide (Table 1, Entry 3): H NMR (300 MHz, CDCl3): δ 2.61–2.68 (m, 4H),
1.68–1.73 (m, 4H), 1.42–1.52 (m, 4H), 0.95 (t, 6H, J = 7.2 Hz). 13C NMR (75 MHz,
CDCl3): δ 32.0, 30.4, 22.2, 13.8. GC/MS (m/z): 146 (M+).

JOURNAL OF SULFUR CHEMISTRY
5
Di(4-methylphenyl) sulfide (Table 1, Entry 4): 1H NMR (300 MHz, CDCl3): δ 7.01–7.45 (m,
8H), 2.31 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 139.8, 132.8, 131.3, 129.8, 21.4. GC/MS
(m/z): 214 (M+).
1
Methyl 4-tolyl sulfide (Table 1, Entry 5): H NMR (300 MHz, CDCl3): δ 7.54 (d, 2H,
J = 11.1 Hz), 7.33 (d, 2H, J = 8.1 Hz), 2.70 (s, 3H), 2.41 (s, 3H). 13C NMR (75 MHz,
CDCl3): δ 134.9, 134.6, 129.6, 127.4, 20.9, 16.6. GC/MS (m/z): 138 (M+).
1
Dibenzyl sulfide (Table 1, Entry 6): H NMR (300 MHz, CDCl3): δ 7.35–7.25 (m, 10H),
3.61(s, 4H). 13C NMR (75 MHz, CDCl3): δ 138.2, 129.2, 128.6, 127.2, 35.7. GC/MS (m/z):
214 (M+).
Benzyl phenyl sulfide (Table 1, Entry 7): 1H NMR (300 MHz, CDCl3): δ 7.33–7.19 (m, 10H),
4.13 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 137.4, 136.3, 129.9, 128.8, 128.4, 127.8, 127.1,
125.4, 39.1. GC/MS (m/z): 200 (M+).
Methyl 4-bromophenyl sulfide (Table 1, Entry 8): 1H NMR (300 MHz, CDCl3): δ 7.40 (d,
2H, J = 8.5 Hz), 7.12 (d, 2H, J = 8.6 Hz), 2.47 (3H). 13C NMR (75 MHz, CDCl3): δ 138.5,
132.0, 128.9, 119.6, 15.0. GC/MS (m/z): 202 (M), 204 (M+2).
Di(4-chlorophenyl) sulfide (Table 1, Entry 9): 1H NMR (300 MHz, CDCl3): δ 7.28–7.46 (m,
8 H). 13C NMR (75 MHz, CDCl3): δ 134.0, 132.9, 130.8, 129.7. GC/MS (m/z): 254 (M), 258
(M+2).
Di(4-anisole) sulfide (Table 1, Entry 10): 1H NMR (300 MHz, CDCl3): δ 6.84–7.28 (m, 8H),
3.79 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 162.2, 137.4, 127.3, 115.1, 55.9. GC/MS (m/z):
246 (M+).
1
4-(Methylthio)benzaldehyde (Table 1, Entry 11): H NMR (300 MHz, CDCl3): δ 9.93 (s,
1H), 7.78 (d, 2H, J = 8.4 Hz), 7.33 (d, 2H, J = 8.0 Hz), 2.54 (s, 3H). 13C NMR (75 MHz,
CDCl3): δ 191.0, 145.1, 133.2, 130.0, 127.1, 14.7. GC/MS (m/z): 152 (M+).
Phenyl vinyl sulfide (Table 1, Entry 12): 1H NMR (300 MHz, CDCl3): δ 7.42–7.27 (m, 5H),
6.55 (dd, 1H, J = 15.7 Hz, J = 10.4 Hz), 5.39–5.33 (m, 2H). 13C NMR (75 MHz, CDCl3):
δ 135.2, 132.3, 129.6, 129.1, 125.6. 114.7. GC/MS (m/z): 136 (M+).
Methyl 4-nitrophenyl sulfide (Table 1, Entry 13): 1H NMR (300 MHz, CDCl3): δ 8.09 (d, 2H,
J = 8.9 Hz), 7.25 (d, 2H, J = 8.9 Hz), 2.52 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 149.1,
144.5, 124.9, 123.8, 14.7. GC/MS (m/z): 169 (M+).
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was financially supported by Korea University.
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