A facile and efficient procedure for the deoxygenation of sulfoxides to sulfides with the HfCl4/Zn system

Article abstract of DOI:10.1080/17415993.2015.1031234

Treatment of sulfoxides with the HfCl₄/Zn system in acetonitrile at room temperature has been shown to provide the corresponding sulfides in high yields. This new methodology is highly chemoselective, tolerating several functional groups such as-Br,-Cl,-OCH₃,-CHO and double bond.

Full text of DOI:10.1080/17415993.2015.1031234

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A facile and efficient procedure for the
deoxygenation of sulfoxides to sulfides
with the HfCl4/Zn system
Byung Woo Yooa, Bo Ran Yua & Cheol Min Yoona
a Department of Advanced Materials Chemistry, Korea University,
2511 Sejong-ro, Sejong 339-700, Korea
Published online: 24 Apr 2015.
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To cite this article: Byung Woo Yoo, Bo Ran Yu & Cheol Min Yoon (2015) A facile and efficient
procedure for the deoxygenation of sulfoxides to sulfides with the HfCl4/Zn system, Journal of
Sulfur Chemistry, 36:4, 358-363, DOI: 10.1080/17415993.2015.1031234
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Journal of Sulfur Chemistry, 2015
Vol. 36, No. 4, 358–363, http://dx.doi.org/10.1080/17415993.2015.1031234
SHORT COMMUNICATION
A facile and efficient procedure for the deoxygenation of
sulfoxides to sulfides with the HfCl4/Zn system
Byung Woo Yoo∗, Bo Ran Yu and Cheol Min Yoon
Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Korea
(Received 15 February 2015; accepted 15 March 2015)
Treatment of sulfoxides with the HfCl4/Zn system in acetonitrile at room temperature has been shown
to provide the corresponding sulfides in high yields. This new methodology is highly chemoselective,
tolerating several functional groups such as –Br, –Cl, –OCH3, –CHO and double bond.
O
HfCl4/Zn
R1
S
R2
R1
S
R2
CH3CN, r. t.
R1, R2= Aryl and Alkyl
Keywords: deoxygenation; sulfoxide; sulfide; hafnium(IV) chloride; zinc
1. Introduction
The deoxygenation of sulfoxides to the corresponding sulfides is a fundamental reaction that
is of increasing importance in both organic synthesis and biological chemistry.[1] Sulfoxides
are important intermediates in a variety of synthetic transformations, especially as chiral auxil-
iary in asymmetric synthesis. After stereoselective induction the chiral sulfinyl moiety may be
removed by deoxygenation to sulfide followed by the elimination of sulfur.[2–5] This accord-
ingly demands new methods for the removal of the residual sulfoxide moiety to generate the
corresponding sulfide. A good number of methods have so far been developed for the deoxygena-
tion of sulfoxides.[6–18] Although these reducing methods are useful, many of them often are
not practical in some respects, such as low yields, long reaction times and drastic reaction con-
ditions. Therefore, the search for new improved methods had continued with an aim to develop
a mild and efficient procedure, where the presence of various sensitive and reducible functional
groups can be tolerated. The use of low-valent oxophilic d-block metals has become important
in deoxygenation of various types of organic substrates.[19] In this regard, deoxygenations of
sulfoxides are readily performed with low-valent tungsten, molybdenum and titanium generated
from WCl6, MoCl5 and TiCl4 in the presence of metal (Zn, In and Sm).[20–22] In continuation of
our efforts toward low-valent metal reagents for organic transformations,[23–26] we considered
that a combination of HfCl4 with zinc could bring about the deoxygenation of sulfoxides under
*Corresponding author. Email: bwyoo@korea.ac.kr
© 2015 Taylor & Francis

Journal of Sulfur Chemistry 359
O
S
HfCl4/Zn
R1
S
R2
R1
R2
CH3CN, r. t.
R1,R2= Aryl and Alkyl
Scheme 1. Conversion of sulfoxides to sulfides using the HfCl4/Zn system
Table 1. The effect of solvent on the deoxygenation
of diphenyl sulfoxide with the HfCl4/Zn system.
Entry
Solvent
Time(h)
Isolated yield(%)
1
2
3
4
5
PhCH3
CH2Cl2
CH3OH
THF
24
24
24
24
3
45
55
60
65
95
CH3CN
mild conditions. Recently, HfCl4 has been well explored as a Lewis acid in promoting organic
transformation.[27,28] Comparing with other Lewis acids, HfCl4 has some advantages, such as
low toxicity, ease of handling, moisture stability and it is not considered particularly poisonous.
Here, we wish to report a new method for the deoxygenation of sulfoxides to the corresponding
sulfides with the HfCl4/Zn system in high yields under mild conditions.
2. Results and discussion
We have investigated the reactions of the HfCl4/Zn system with a variety of sulfoxides and found
that they can be effectively reduced to the corresponding sulfides in high yields (Scheme 1).
It has recently been reported that the HfCl4/KBH4 system is used as a reagent for reducing
sulfoxides to sulfides.[29] In comparison with the method using the HfCl4/KBH4 system for the
reduction of sulfoxides, the HfCl4/Zn system gave better product yields for the same substrates
in a relatively short time under mild conditions. The new reducing system was readily generated
by the addition of zinc powder to a stirred solution of hafnium tetrachloride in CH3CN. To test if
HfCl4 is responsible for the conversion of sulfoxide to sulfide, the reaction has been performed
without HfCl4 under the present condition. There has been no change in the starting material.
And the reaction was also not effective with HfCl4 without zinc metal. Thus, it is obvious that a
combination of zinc and HfCl4 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, THF and acetonitrile.
Acetonitrile turned out to be the most appropriate solvent for this transformation in terms of
yield and reaction time. The results are listed in Table 1. The effect of different molar ratios of
HfCl4 to zinc on the reduction was investigated to optimize the experimental conditions.
The optimum molar ratio of sulfoxides to HfCl4 to zinc (1:2:4) is found to be ideal for the
complete conversion of sulfoxide into sulfide in CH3CN at room temperature, while with lesser
amounts the reaction remains incomplete.
In order to assess the scope and limitations of this reagent system, the reaction was studied
with a variety of sulfoxides bearing other potentially labile functional groups. The results given
in Table 2 illustrate the high efficiency of this protocol for the deoxygenation of structurally
diverse sulfoxides. Both aromatic and aliphatic sulfoxides were reduced to the correspond-
ing sulfides. 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.[30]

360 B.W. Yoo et al.
Table 2. Deoxygenation of various sulfoxides to sulfides using the HfCl4/Zn system.
Entry
R1
R2
Products
Time(h)
Yield(%)a,b
1
2
3
4
5
6
7
8
Ph
Ph
Ph
CH3
Me
PhSPh
PhSCH3
4-BrC6H4SMe
(4-ClC6H4)2S
(4-CH3OC6H4)2S
4-OHCC6H4SCH3
3.0
1.0
3.0
2.5
3.0
3.0
1.5
0.5
0.5
4.0
3.5
2.0
95
96
93
92
88
89
86
91
93
97
94
90
4-BrC6H4
4-ClC6H4
4-CH3OC6H4
4-OHCC6H4
Ph
4-CH3C6H4
4-CH3C6H4
PhCH2
4-ClC6H4
4-CH3OC6H4
CH3
=
=
CH2 CH
PhSCH CH2
4-CH3C6H4
CH3
(4-CH3C6H4)2S
4-CH3C6H4SCH3
(PhCH2)2S
9
10
11
12
PhCH2
Ph
nC4H9
PhCH2
nC4H9
PhCH2SPh
(nC4H9)2S
O
S
O2N
NC
S
CH3
13
14
4.5
4.0
84
86
O2N
NC
CH3
O
S
S
CH3
CH3
aRefers to isolated yields.
bThe products were indentified by comparison of their spectroscopic data with authentic samples.
The isolated yield was independent of the nature of the substituents. Good to excellent yields
were observed for sulfoxides including electron-withdrawing and electron-donating groups. The
high chemoselectivity of this method is evident from entries 3–7, 13 and 14 of Table 2, which
show that –Br, –Cl, –OCH3, –CHO, –NO2, –CN and double bond functionalities are unaffected
under the reaction conditions. The utility of the HfCl4/Zn system as a new reducing agent is
also demonstrated by the high yields of dibenzyl sulfide (Entry 10) and phenyl benzyl sulfide
(Entry 11) obtained after the reduction of the corresponding sulfoxides. Usually, the sulfoxides
that contain a benzyl group are difficult to reduce by other reagents.[31,32]
Although the precise mechanism of this reduction process is still unclear, it can be rational-
ized as the result of a two-step process. In the first step, it is speculated that the mechanism starts
with the activation of the sulfoxide by the oxygen coordination of the hafnium, yielding the
hafnium–sulfoxide complex. This complex weakens the S–O bond and renders the sulfur atom
more susceptible to the reduction. In the subsequent step, the reduction proceeds by a reduc-
tive cleavage of polarized S–O bond through a single electron transfer from zinc metal to the
hafnium–substrate complex.[33] The reducing property exhibited by metal–metal salt combina-
tions proceeds through transfer of one electron from the metal surface to the substrate. In such
combinations, elementary metal part needs to be more electropositive than metal part of the salt.
The method offers advantages such as mild reaction conditions, high yields, simple experimental
operation and tolerance of several functional groups. Thus, we have been able to demonstrate the
utility of the easily accessible HfCl4/Zn system as a useful reagent for effecting chemoselective
deoxygenation of sulfoxides.
3. Conclusion
In conclusion, the present new methodology using the HfCl4/Zn system may represent a useful
and practical alternative to the existing methods for the deoxygenation of sulfoxides to sulfides.
Further investigations of more useful applications are under way in our laboratory.

Journal of Sulfur Chemistry 361
4. Experimental
4.1. General
All commercial reagents were purchased from Aldrich and Fluka Chemical Company and used
1
without any further purification. The H NMR spectra were recorded on a FT-Bruker AF-300
(300 MHz for 1H NMR; 75 MHz for 13C NMR) using tetramethylsilane as an internal standard.
Mass spectra were recorded on a Shimadzu GC MS-QP 1000 PX at 70 eV. TLC analysis was
performed on silica gel plates (Merck, 60 F-254). All products were purified by flash column
chromatography using silica gel 60 (79–230 mesh, Merck).
4.2. General procedure
Diphenylsulfoxide (101 mg, 0.5 mmol) and hafnium(IV) chloride (320 mg, 1.0 mmol) were
mixed in CH3CN (5 mL) and zinc powder (131 mg, 2.0 mmol) was then added to this solu-
tion. The whole mixture was stirred for 3 h at room temperature and the progress of the reaction
was followed by TLC. On completion, the solvent was removed under reduced pressure and the
residue was extracted successively with ethyl acetate, washed with water and brine. The organic
layer was separated, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The
crude product was purified by column chromatography on a silica gel (hexane:ethyl acetate =
2:1) to afford diphenylsulfide (88 mg, 95%). All of the products were identified by comparison
of their spectroscopic data with authentic samples.[34]
4.3. Spectroscopic data of products
Diphenyl sulfide (Table 2, Entry 1): 1H NMR (300 MHz, CDCl3): δ 7.38–7.25 (m, 10H).
13C NMR (75 MHz, CDCl3): δ 135.7, 131.1, 129.2, 127.1. GC/MS (m/z): 186 (M+).
Methyl phenyl sulfide (Table 2, 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+).
Methyl 4-bromophenyl sulfide (Table 2, Entry 3): 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 2, Entry 4): 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).
1
Di(4-anisole) sulfide (Table 2, Entry 5): H 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+).
4-(Methylthio)benzaldehyde (Table 2, Entry 6): 1H 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+).
1
Phenyl vinyl sulfide (Table 2, Entry 7): H 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+).
Di(4-methylphenyl) sulfide (Table 2, Entry 8): 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+).

362 B.W. Yoo et al.
Methyl 4-tolyl sulfide (Table 2, Entry 9) : 1H 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 2, Entry 10): 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 2, Entry 11): 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+).
Dibutyl sulfide (Table 2, Entry 12) : 1H 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+).
1
4-Nitrophenyl-p-tolyl sulfide (Table 2, Entry 13): H NMR (300 MHz, CDCl3): δ 8.41–7.25
(m, 8H), 2.37 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 147.2, 138.1, 136.9, 132.0, 130.9, 130.0,
128.9, 124.2, 20.4. GC/MS (m/z): 245 (M+).
3-Cyanophenyl-p-tolyl sulfide (Table 2, Entry 14): 1H NMR (300 MHz, CDCl3): δ 7.52–6.86
(m, 8H), 2.34 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 137.0, 135.5, 134.7, 132.7, 131.0,130.7,
129.8, 129.1, 117.5, 114.2, 20.3. GC/MS (m/z): 225 (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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