Mild and efficient deoxygenation of sulfoxides to sulfides using HFCL 4/KBH4 system

Article abstract of DOI:10.1080/00397910903161819

HfCl₄/KBH₄ was found to be a facile, efficient, convenient, and chemoselective system for the deoxygenation of dialkyl, diaryl, and aryl alkyl sulfoxides, especially for the reduction of dibenzyl sulfoxide to the corresponding sulfides under mild conditions. In addition, the HfCl ₄/KBH₄ system could be used in reduction of some other sulfur-bearing substrates to the corresponding sulfides, such as 2,2'-dibenzothiazolyl disulfide, but this reducing system could not reduce sulfolane, diphenyl sulfone, p-toluenesulfonic acid, and p-toluenesulfonyl chloride to their corresponding thiophenols. Copyright

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Mild and Efficient Deoxygenation of
Sulfoxides to Sulfides Using HfCl /KBH
4
4
System
a
a
a
a
Jianghua Zhang , Xinqin Gao , Chengyi Zhang , Cheng Zhang
,
a
a
Jiming Luan & Defeng Zhao
a
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, China
Published online: 17 May 2010.
To cite this article: Jianghua Zhang , Xinqin Gao , Chengyi Zhang , Cheng Zhang , Jiming Luan &
Defeng Zhao (2010) Mild and Efficient Deoxygenation of Sulfoxides to Sulfides Using HfCl /KBH4
4
System, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 40:12, 1794-1801, DOI: 10.1080/00397910903161819
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1
Synthetic Communications , 40: 1794–1801, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903161819
MILD AND EFFICIENT DEOXYGENATION OF
SULFOXIDES TO SULFIDES USING HfCl /KBH
SYSTEM
4
4
Jianghua Zhang, Xinqin Gao, Chengyi Zhang,
Cheng Zhang, Jiming Luan, and Defeng Zhao
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian, China
4 4
HfCl /KBH was found to be a facile, efficient, convenient, and chemoselective system for
the deoxygenation of dialkyl, diaryl, and aryl alkyl sulfoxides, especially for the reduction
of dibenzyl sulfoxide to the corresponding sulfides under mild conditions. In addition, the
HfCl /KBH system could be used in reduction of some other sulfur-bearing substrates to
4
4
0
the corresponding sulfides, such as 2,2 -dibenzothiazolyl disulfide, but this reducing system
could not reduce sulfolane, diphenyl sulfone, p-toluenesulfonic acid, and p-toluenesulfonyl
chloride to their corresponding thiophenols.
4 4
Keywords: Deoxygenation; HfCl =KBH ; reduction; sulfides; sulfoxides
INTRODUCTION
The deoxygenation of sulfoxides to the corresponding sulfides is an important
functional-group transformation, which has found considerable utility in organic
[
1]
[2]
synthesis and in biochemical reactions. Accordingly, numerous methods for
the deoxygenation of sulfoxides have been developed, such as using metal hydride
[
3]
reagents [e.g., LiAlH , diisobutylaluminum hydride (DIBAH), I =NaBH ],
4
4
2
[
hydrogen halides’ reductive halogenations, phosphorus compounds (e.g., PBr3,
4]
[5]
Ph P=TiCl ),
3
silicon compounds (e.g., Si Cl , PhSiH =MoO Cl , Et SiH=
2 6 3 2 2 3
4
[6]
[7]
MoO Cl , Ph SiH=MoO Cl ), borane agents, and other miscellaneous systems
2
2
3
2
2
[
8]
[9]
[
CpTiCl =In,
e.g., Cs Mo Cl H, (NH ) Mo Cl ꢀ NH Cl ꢀ H O,
titanocene methylidenes,
3
2
10]
8
4 4
2
[11]
8
4
2
[
[12]
and tetracyanoethylene oxide ]. More recent
oxalyl chloride,
2
research in this area has demonstrated the use of [catecholborane=MoO Cl
2
2
[13]
[14]
[15]
[16]
(
H O) =BH ꢀ THF=MoO Cl ],
MoCl =In,
WCl =In,
silica=PSCl3,
for the deoxygenation of sulfoxides. Although these reducing
and
2
2
3
2
2
5
6
[17]
Woollins’ reagent
methods are useful, many of them suffer from various drawbacks, such as using
toxic, hazardous (e.g., I =NaBH
2
[
3]
[7]
and borane agents ), and not readily available
4
[
8]
reagents [(NH ) Mo Cl ꢀ NH Cl ꢀ H O],
functional group incompatibility
4
4
2
8
4
2
Received April 22, 2009.
Address correspondence to Dr. Xinqin Gao, Institute of Chemical Technology, Dalian University
of Technology, State Key Laboratory of Fine Chemicals, 158 Zhongshan Road, Dalian 116012, China.
E-mail: gaoxq@dlut.edu.cn
1
794

DEOXYGENATION OF SULFOXIDES TO SULFIDES
3]
diisobutylaluminum hydride (DIBAH)], harsh reaction conditions (e.g., titanocene
1795
[
[
[9]
methylidenes ), or difficult workup procedures. Furthermore, some of these meth-
ods are associated with limitations regarding low pH (hydrogen halides’ reductive
[
4]
[6]
halogenations), prolonged reaction time (Ph SiH=MoO Cl , tetracyanoethylene
3
2
2
[
12]
[17]
oxide ), or elevated temperature (Woollins’ reagent ). Therefore, there still exists
a need for new methods based on using facile reagents, by which some functional
groups can be tolerated. In this article, the reduction of sulfoxides to the correspond-
ing sulfides by HfCl =KBH in good yields under mild conditions is reported.
4
4
RESULTS AND DISCUSSION
In our last article, we reported that HfCl =KBH could efficiently and chemo-
4
4
selectively reduce carboxylic acids and their derivatives to the corresponding alcohols
under mild conditions. In ongoing research into the utilities of HfCl =KBH , it was
4
4
found that this novel reducing system could reduce methyl phenyl sulfoxide 1 to
methyl phenyl sulfide 2 in excellent yield (96%) under mild conditions (Scheme 1).
It was anticipated that the HfCl =KBH system would efficiently reduce some other
4
4
sulfoxides to their corresponding sulfides, so the effect of different molar ratios of
=HfCl =KBH on the reduction was investigated to optimize the experimental
conditions. The results are listed in Table 1.
1
4
4
As shown in Table 1, the optimum experimental conditions for the reduction of
to 2 were as follows: (1) the molar ratio of 1=HfCl =KBH was 0.8=1.0=4.0, (2) the
4 4
1
reaction mixture should be heated under reflux, and (3) the reduction should be
carried out under nitrogen. Then various sulfoxides were investigated according to
4 4
Scheme 1. Deoxygenation of methyl phenyl sulfoxide to methyl phenyl sulfide by HfCl =KBH .
Table 1. Deoxygenation of methyl phenyl sulfoxide 1 with different molar ratios of 1=HfCl
4
=KBH
4
a
b
Yield (%)
Entry
1=HfCl
4
=KBH
4
(molar ratio)
Conditions
1
2
3
4
5
6
7
8
0.8=0=4.0
0.8=0.2=4.0
0.8=0.3=4.0
0.8=0.6=5.0
0.8=0.8=4.0
0.8=1.0=4.0
0.8=1.0=4.0
0.8=1.0=4.0
Reflux, 28.8 h
Reflux, 31.0 h
Reflux, 31.0 h
Reflux, 30.3 h
Reflux, 30.3 h
Trace
36
66
73
85
ꢁ
40 C, 30.3 h
Reflux, 12.5 h
Reflux, 31.8 h
81
96
c
87
a
4 4
Dry THF was added to the mixture of HfCl and KBH under nitrogen and stirred for 3 h at room
temperature, and then a solution of 1 in dry THF was added dropwise and heated to the reaction
temperature.
b
1
Isolated yield; all products were identified by H NMR and mass spectrometry.
c
The preparation of reducing agent was not under nitrogen.

1796
J. ZHANG ET AL.
Table 2. Reduction of sulfoxides and some other sulfur-bearing substrates using the HfCl
4
=KBH
4
system
a
b
Yield (%)
Entry
Substrate
Conditions
Reflux, 6.5 h
Reflux, 8.5 h
Reflux, 14.5 h
Sulfide or thiophenol
c
1
2
3
100
89
90
4
5
6
7
Reflux, 13.5 h
Reflux, 13.0 h
Reflux, 12.0 h
Reflux, 12.5 h
87
93
83
97
8
9
Reflux, 13.3 h
Reflux, 14.5 h
91
71
10
Reflux, 11.5 h
93
1
1
1
2
Reflux, 9.5 h
Reflux, 4.5 h
98
97
1
1
3
4
Rt, 15 min
81
—
d
Reflux, 28.8 h
NR
15
Reflux, 28.8 h
NR
—
(
Continued )

DEOXYGENATION OF SULFOXIDES TO SULFIDES
1797
Table 2. Continued
a
b
Yield (%)
Entry
Substrate
Conditions
Sulfide or thiophenol
NR
1
6
7
Reflux, 9.5 h
—
1
Reflux, 10.5 h
NR
—
a
4 4
Dry THF was added to the mixture of HfCl and KBH under nitrogen and stirred for 3 h at room
temperature, and then a solution of substrate in dry THF was added dropwise and heated to the reaction
temperature.
b
1
Isolated yield; all products were characterized by H NMR and mass spectrometry.
c
Conversion.
d
NR, no reaction.
these experimental conditions to establish the generality of the present reducing
agent and to examine the tolerance of some other functional groups present in the
substrates. The process of these reactions was monitored by thin-layer chromato-
graphy (TLC), and the results are shown in Table 2. The methodology was suitable
for the deoxygenation of dialkyl, diaryl, and aryl alkyl sulfoxides, especially for the
reduction of dibenzyl sulfoxide (entry 12), which was difficult to reduce by other
[
18]
reducing systems.
from entries 4–9, which showed that the -OMe, -F, -Cl, -Br, and -NO groups were
The functional-group tolerance of this method was evident
2
unaffected under these experimental conditions. Methyl phenyl sulfoxide derivatives
with electron-withdrawing groups, such as the -F (entry 6), -Cl (entry 7), and
-Br (entry 8) groups on the aromatic rings, were reduced faster than those with
electron-donating groups, such as the -Me (entry 3) and -OMe (entries 4 and 5)
groups. Besides, the reduction of dimethyl sulfoxide (entry 1) and dibutyl sulfoxide
(
(
entry 2) by HfCl =KBH was faster than that of methyl phenyl sulfoxide derivatives
4 4
entries 3–11). Methyl 2-naphthyl sulfoxide (entry 11) was reduced more smoothly
than methyl phenyl sulfoxide (1).
In addition, the action of HfCl =KBH4 on some other sulfur-bearing
4
0
substrates, such as sulfolane, diphenyl sulfone, 2,2 -dibenzothiazolyl disulfide, p-
toluenesulfonic acid, and p-toluenesulfonyl chloride, was also investigated to enlarge
0
the synthetic utility of HfCl =KBH . As listed in Table 2, 2,2 -dibenzothiazolyl
4
4
disulfide (entry 13) could be easily reduced to 2-mercaptobenzothiazole in good
yield, which would provide HfCl =KBH a potential reducing system for the
4
4
reduction of disulfides. However, the attempted reduction of sulfolane (entry 14)
and diphenyl sulfone (entry 15) resulted in only the recovery of starting materials.
p-Toluenesulfonic acid (entry 16) and p-toluenesulfonyl chloride (entry 17) could
not be reduced to the corresponding thiophenols.
CONCLUSION
In summary, HfCl =KBH is a facile, efficient, convenient, and chemoselective
4
4
reducing agent for the reduction of dialkyl, diaryl, and aryl alkyl sulfoxides,
especially for the reduction of dibenzyl sulfoxide to the corresponding sulfides,

1798
J. ZHANG ET AL.
and it also features potential reduction of disulfides. We believe this procedure using
the HfCl =KBH system is a useful and efficient alternative to the existing reducing
4
4
methods. Further investigation is currently under way to extend the scope of this
methodology into other reductions, the results of which will be reported in due
course.
EXPERIMENTAL
General Procedure for the Reduction by HfCl /KBH4
4
Freshly distilled tetrahydrofuran (THF; 3 mL) was syringed into a Schlenk
flask with a stirrer bar that contained HfCl (0.32 g, 1 mmol) and KBH (0.216 g,
4
4
4
mmol) under nitrogen in an ice-water bath; it immediately formed a yellowish sus-
pension. In the case of reducing sulfolane, diphenyl sulfone, p-toluenesulfonic acid,
and p-toluenesulfonyl chloride, THF was syringed into a Schlenk flask with a stirrer
bar that contained HfCl (0.64 g, 2 mmol) and KBH (0.432 g, 8 mmol) under nitro-
4
4
gen in an ice-water bath. Then this mixture was stirred for 0.1 h and allowed to warm
to room temperature for 3 h. A solution of substrate (0.8 mmol) in dry THF (3 mL)
then was added dropwise and stirred for a further time at the reaction temperature.
After this time, the reaction mixture was poured into water (3 mL) with ice-cooling
and extracted with ethyl acetate (3 ꢂ 5 mL). The combined extract was washed with
saturated NaHCO solution and brine, dried over anhydrous Na SO , and then
3
2
4
evaporated. The crude product was purified by column chromatography.
DATA
Dibutyl Sulfide
1
H NMR (400 MHz, CDCl ): d 1.04 (t, 6H, J ¼ 7.2 Hz), 1.55 (m, 4H, J ¼ 7.2 Hz),
3
1
3
1
1
.71 (m, 4H, J ¼ 7.2 Hz), 2.65 (t, 4H, J ¼ 7.2 Hz). C NMR (400 MHz, CDCl ): d
3
þ
3.8, 22.2, 32.0. EI-MS m=z: [M] 146.
4-Methoxyphenyl Methyl Sulfide
1
H NMR (400 MHz, DMSO-d ): d 2.42 (s, 3H), 3.73 (s, 3H), 6.91 (d, 2H,
6
þ
J ¼ 8.4 Hz), 7.25 (d, 2H, J ¼ 8.4 Hz). EI-MS m=z: [M] 154. IR (neat) 3400, 2998,
2
6
919, 2833, 1594, 1571, 1494, 1462, 1438, 1284, 1243, 1179, 1097, 1032, 969, 822,
37, 624 cm
ꢃ1
.
Methyl p-Bromophenyl Sulfide
ꢁ
[19a]
1
Mp 39–40 C (lit.
39–40). H NMR (400 MHz, DMSO-d ): d 2.46 (s, 3H),
6
þ
7
.21 (d, 2H, J ¼ 8.8 Hz), 7.48 (d, 2H, J ¼ 8.8 Hz). EI-MS m=z: [M] 203.
Methyl 4-Nitrophenyl Sulfide
1
H NMR (400 MHz, DMSO-d ): d 2.68 (s, 3H), 7.57 (d, 2H, J ¼ 8.8 Hz), 8.24
6
1
3
(
d, 2H, J ¼ 8.8 Hz). C NMR (400 MHz, CDCl ): d 15.0, 124.1, 125.2, 144.9, 149.0.
3

DEOXYGENATION OF SULFOXIDES TO SULFIDES
1799
IR (KBr) 3429, 3092, 3000, 2915, 1640, 1584, 1509, 1476, 1423, 1400, 1338, 1093,
ꢃ1
9
77, 967, 853, 831, 819, 740, 679 cm
.
Diphenyl Sulfide
1
13
H NMR (400 MHz, CDCl ): d 7.11–7.30 (m, 6H), 7.29 (d, 4H, J ¼ 7.2 Hz).
C
3
NMR (400 MHz, DMSO-d ): d 127.3, 129.4, 130.6, 134.7. HRMS (EI) m=z found:
M , 186.0507; calcd. for C H S: 186.0503.
6
þ
1
2
10
Methyl 2-Naphthyl Sulfide
ꢁ
[19b]
1
Mp 60–61 C (lit.
m, 3H), 7.51 (s, 1H), 7.66 (m, 3H). HRMS (EI) m=z found: M , 174.0502; calcd. for
61–62). H NMR (400 MHz, CDCl ): d 2.49 (s, 3H), 7.33
3
þ
(
C H S: 174.0503.
1
1
10
Dibenzyl Sulfide
1
13
H NMR (400 MHz, DMSO-d ): d 3.64 (s, 4H), 7.22–7.34 (m, 10H). C NMR
6
þ
(
400 MHz, CDCl ): d 35.6, 127.1, 128.6, 129.1, 138.2. HRMS (EI) m=z found: M ,
3
2
14.0810; calcd. for C H S: 214.0816.
14 14
2-Mercaptobenzothiazole
ꢁ
[19c]
1
Mp 180–182 C (lit.
177–179). H NMR (400 MHz, DMSO-d ): d 7.27–7.32
6
(
m, 2H, ArH), 7.40 (t, 1H, J ¼ 7.2 Hz, ArH), 7.70 (d, 1H, ArH), 13.77 (s, 1H, ArH).
þ
HRMS (EI) m=z Found: M , 166.9860. Calcd. for C H NS : 166.9863.
7
5
2
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