Reduction of sulfoxides to sulfides in the presence of copper catalysts

Article abstract of DOI:10.1007/s10562-011-0590-6

Copper complexes catalyze the reduction of aliphatic and aromatic sulfoxides in the presence of silanes as reducing reagent. The influence of different reaction parameters on the catalytic activity is investigated in detail. The scope and limitations of the described catalyst is demonstrated in the reduction of various sulfoxides. In most cases, high conversion and excellent chemoselectivity are obtained. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0590-6

Catal Lett (2011) 141:833–838
DOI 10.1007/s10562-011-0590-6
Reduction of Sulfoxides to Sulfides in the Presence of Copper
Catalysts
•
Stephan Enthaler Maik Weidauer
Received: 25 February 2011 / Accepted: 23 March 2011 / Published online: 12 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Copper complexes catalyze the reduction of ali-
phatic and aromatic sulfoxides in the presence of silanes as
reducing reagent. The influence of different reaction param-
eters on the catalytic activity is investigated in detail. The
scope and limitations of the described catalyst is demon-
strated in the reduction of various sulfoxides. In most cases,
high conversion and excellent chemoselectivity are obtained.
molybdenum-, iron-, and zinc-based systems the applica-
tion of copper can be beneficial. Indeed, the potential of
copper catalyst in reduction techniques has been explored
in the reduction of various functionalities [46–58]. How-
ever, the catalytic reduction of sulfoxides in the presence of
copper has been so far unreported, while the converse
process has been studied [59]. Herein we report, based on
our initial work with zinc, iron and molybdenum-catalysts
the first copper-catalyzed deoxygenation of sulfoxides to
generate the corresponding sulfide under non-inert condi-
tions with inexpensive reductant [60–62].
Keywords Catalysis Á Copper Á Reduction Á Sulfoxide Á
Sulfide
1 Introduction
2 Results and Discussion
The deoxygenation of sulfur-based compounds is an
essential method in biological processes e.g., to circumvent
oxidative stress. Those enzymatic processes are largely
based on molybdenum as active site, e.g. dimethylsulfox-
ide-, biotin-, and methionine sulfoxide reductase [1–4].
Moreover, the deoxygenation of sulfoxides is of signifi-
cance in chemistry since a quantity of technologies has
been attributed to those reactions [2–40]. Within these
transformations the application of catalysts has been
demonstrated as key technology, since high atom effi-
ciency, reduced amounts of waste as well as energy, and in
consequence advantageous economics are feasible [41–43].
Despite the obvious advantages of catalysts current atten-
tion is directed to the application of inexpensive, abundant
and low toxic metals as catalyst core [44, 45]. Besides
Initially, the reduction of diphenyl sulfoxide 1 in the
presence of catalytic amounts of commercially available
copper(II) acetylacetonate was studied as model reaction
to explore appropriate reaction conditions. The pre-cata-
lyst (5.0 mol%) was dissolved in toluene and subsequently
substrate 1 and diphenyl silane were added and the mix-
ture was heated to 100 °C. After 24 h the mixture was
cooled to room temperature and a sample was subjected to
GC–MS analysis to determine the reaction outcome.
Indeed, an excellent yield ([99%) for the formation of the
sulfide 1a was monitored accompanied by excellent che-
moselectivity ([99%) (Table 1, entry 3). To examine the
influence of the catalyst the reaction was performed in the
absence of the metal resulting in a marginal yield (\5%),
which demonstrated the need of the copper catalyst to
enhance the reaction (Table 1, entry 2). Performing the
reaction without silane no product formation was observed
(Table 1, entry 1). Besides, the effect of different silanes
on the product formation was investigated. Excellent
performance was observed for Ph2SiH2 and PhSiH3 as
S. Enthaler (&) Á M. Weidauer
¨
Technische Universitat Berlin, Department of Chemistry, Cluster
of Excellence ‘‘Unifying Concepts in Catalysis’’, Straße des 17.
Juni 135/C2, 10623 Berlin, Germany
e-mail: stephan.enthaler@tu-berlin.de
123

834
S. Enthaler, M. Weidauer
Table 1 Copper-catalyzed reduction of diphenyl sulfoxide 1
Entry
Copper loading (mol%)
Silane (equiv)
Solvent
T (°C)
Yield (%)a
1
–
5
5
5
5
2.5
1
5
5
5
5
5
5
5
5
5
1
Ph2SiH2 (1.1)
–
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
H2O
100
100
100
70
\5
\5
[99
7
2
3
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Et3SiH (1.1)
PMHS (5)
4
5
r.t.
7
6
100
100
100
100
100
100
100
100
100
100
70
[99
[99
\1
9
7
8
9
10
11
12
13
14
15
16
17
(EtO)3SiH (1.1)
Me2PhSiH (1.1)
PhSiH3 (1.1)
Ph3SiH (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
Ph2SiH2 (1.1)
7
7
99
\5
84
70
85
\1
Dioxane
THF
THF
70
Reaction conditions: 1 (0.72 mmol), catalyst (1–5 mol%), silane (0.79 mmol or 3.6 mmol), solvent (2.0 mL), 24 h, r.t.-100 °C
a
Determined by GC methods using biphenyl as an internal standard
reducing reagent (Table 1, entries 7 and 12), while other
silanes (Et3SiH, PMHS (polymethylhydrosiloxane),
(EtO)3SiH, Me2PhSiH, Ph3SiH) resulted in low yields
(Table 1, entries 8–11, 13). Additionally, the effect of the
reaction temperature was studied. Decreasing the temper-
ature exhibited a diminished yield of diphenyl sulfide (1a),
while at 100 °C best performance was noticed (Table 1,
entries 3–5). Besides, the catalyst loading was decreased.
Excellent catalyst abilities were observed for a loading of
1.0 mol%, while at lower loadings a significantly dimin-
ished yield of product 1a was exhibited. Along with tol-
uene as solvent water, dioxane and THF were applied
(Table 1, entries 14–17). Good results were obtained for
water (84%) and dioxane (70%) at 100 °C, while the
reaction mixture using THF as solvent were refluxed at
70 °C resulting in a yield of 85%.
Next, other copper sources were tested in combination
with diphenylsilane in accordance to the reaction condi-
tions described in Table 1, entry 3, but with a reaction time
of 12 h. Best performance was observed for Cu(acac)2 and
copper(II) triflate (Table 2, entries 1 and 6). A comparison
of copper(II) triflate and copper(I) triflate showed an
enhanced yield for copper(II) compounds, while cop-
per(I) resulted in lower amounts of product 1a. In tendency
this fact is also true for copper(I) and copper(II) halides.
Moreover, the copper(II) acetylacetonate was modified
by either P- or N-ligands including monodentate, bidentate
and tridentate systems (Table 3). However, no improve-
ment of the reaction outcome was observed in all cases in
contrast to unmodified copper(II) acetylacetonate. In gen-
eral, best performance was noticed for the copper(II) ace-
tylacetonate and dppf (3) as bidentate phosphane ligand
with 36% (catalyst loading 1.0 mol%).
Once suitable reaction conditions were established, the
scope and limitations of the copper-catalyzed deoxygen-
ation of various sulfoxides, including aromatic and ali-
phatic sulfoxides, using Ph2SiH2 as reducing reagent were
examined (Table 4). Excellent yields ([99%) were
achieved in the reduction of substituted diaryl sulfoxides
after 12 h at 100 °C with a catalyst loading of 5.0 mol%
(Table 4, entries 2 and 3). Excellent chemoselectivities
were noticed in the presence of nitrile and nitro function-
alities, since no reduction of these groups have been
monitored (Table 4, entries 6 and 7). Applying aryl alkyl
sulfoxide a slightly reduced yield was observed.
Moreover, excellent selectivity ([99%) for the reduc-
tion of S=O bond was observed in the presence of various
additives (Table 5). Noteworthy, in the presence of addi-
tives containing ester, amide, sulfone and oxo-functional-
ities the sulfoxide substrate was selectively reduced to the
123

Reduction of Sulfoxides to Sulfides in the Presence of Cu Catalysts
835
Table 2 Copper-catalyzed reduction of diphenyl sulfoxide 1—
influence of the copper source
Table 3 Copper(II) acetylacetonate pre-catalyst modified by N- or P-
ligands for the reduction of diphenyl sulfoxide 1
Yield (%)a
Entry
Copper source
Yield (%)a
Entry
Ligand
Metal:ligand
[99 ([99)b
35
1
Cu(acac)2
CuCl
[99
9
1
2
2
1:0
1:1
2
3
CuI
9
4
CuF2
79
12
[99
44
52
59
85
39
3
1:1
[99 (36)b
5
CuCl2
6
Cu(OTf)2
CuOTfÁC6H6
Cu(OAc)2
Cu(NCMe)4PF6
Cu(NCMe)4BF4
Cu2O
7
8
4
5
6
7
PPh34
1:2
1:2
1:2
1:2
49
33
17
44
9
P(2,6-(MeO)2Ph)35
PBu36
10
11
Reaction conditions: 1 (0.72 mmol), catalyst (5 mol%), Ph2SiH2
(0.79 mmol), toluene (2.0 mL), 100 °C, 12 h
a
Determined by GC methods using biphenyl as an internal standard
8
1:2
11
sulfide, while the functional group of the additive was
unaffected. However, in some cases a diminished yield of
the sulfide was observed compared to the reaction without
additive. In the presence of an alkynyl group to some
extent the corresponding alkene is formed (Table 5, entry
2).
9
1:1
1:1
67
55
10
3 Conclusion
Reaction conditions: 1 (0.72 mmol), Cu(acac)2 (5 mol%), ligand (5-
10 mol%) Ph2SiH2 (0.79 mmol), toluene (2.0 mL), 12 h, 100 °C
In the present study we demonstrated the catalytic potential
of copper complexes in the reduction of sulfoxides to the
corresponding sulfide. The application of cheap and
abundant copper(II) acetylacetonate and silanes as hydride
source resulted in a highly active system under non-inert
conditions. Furthermore, the reduction of various aryl and
alkyl sulfoxides can be performed with broad functional
group tolerance.
a
Determined by GC methods using biphenyl as an internal standard
b
In brackets with 1.0 mol% catalyst
solvents as reference. GC–MS measurements were carried
out on a Shimadzu GC-2010 gas chromatograph (30 m
Rxi-5 ms column) linked with a Shimadzu GCMA-QP
2010 Plus mass spectrometer.
4.2 General Procedure for the Deoxygenation
of Sulfoxides
4 Experimental Section
4.1 General
A pressure tube was charged with an appropriate amount
of Cu(acac)2 (0.036 mmol, 5.0 mol%), the corresponding
sulfoxide (0.72 mmol), and the Ph2SiH2 (0.72 mmol).
After addition of toluene (2.0 mL) the reaction mixture
was stirred in a preheated oil bath at 100 °C for 12 h.
The mixture was cooled on an ice bath and biphenyl
(internal standard) was added. The solution was diluted
All compounds were used as received without further
purification. THF and toluene were dried applying standard
procedures. H, and 13C NMR spectra were recorded on a
1
Bruker AFM 200 spectrometer (1H: 200.13 MHz; 13C:
50.32 MHz) using the proton signals of the deuterated
123

836
S. Enthaler, M. Weidauer
Table 4 Scope and limitations of the copper(II) acetylacetonate pre-
catalyst in the reduction of sulfoxides
Table 5 Copper-catalyzed reduction of sulfoxide 1 and 11 in the
presence of substrates sensitive towards reduction
Entry
1
Substrate
Yield (%)a
Entry
1
Substrate/additive
Yield (%)a
[99
45 (\1)
2
3
4
[99
[99
93
2
3
59 (19)b
[99 (\1)
4
5
6
40 (\1)
[99 (\1)
51 (\1)
5
6
7
89
73
Reaction conditions: 1 or 11 (0.72 mmol), 21–26 (0.72 mmol),
Cu(acac)2 (5 mol%), Ph2SiH2 (0.79 mmol), toluene (2.0 mL), 24 h,
100 °C. Yield determined by GC methods
a
Yield of 1a or 11a. In brackets the yield of the reduced additive is
stated
b
[99
cis-Stilbene was observed
8
9
62
74
Diphenylsulfide (1a): Rf = 0.75 (n-hexane:ethyl acetate
1:5); 1H NMR (CDCl3, 200 MHz): d = 7.26–7.47 ppm
(m, 10H); 13C NMR (CDCl3, 50 MHz): d = 135.8, 131.0,
129.2, 127.0 ppm; MS (ESI) m/z = 186 (100, M?), 152
(11), 92 (19), 77 (27), 65 (20), 51 (43).
10
11
71
86
Di(p-methylphenyl)sulfide (11a): Rf = 0.79 (n-hex-
ane:ethyl acetate 1:5); 1H NMR (CDCl3, 200 MHz):
d = 7.06–7.22 ppm (m, 8H), 2.30 (s, 6H, CH3); 13C NMR
(CDCl3, 50 MHz): d = 137.1, 132.8, 131.3, 130.1,
21.3 ppm; MS (ESI) m/z = 214 (100, M?), 199 (34), 184
(20), 105 (18), 91 (33), 65 (19).
Reaction conditions: substrate (0.72 mmol), Cu(acac)2 (5 mol%),
Ph2SiH2 (0.79 mmol), toluene (2.0 mL), 12 h, 100 °C
Products 17a and 20a have been only characterized by GC–MS
Di(p-chlorophenyl)sulfide (12a): Rf = 0.67 (n-hex-
ane:ethyl acetate 1:10); 1H NMR (CDCl3, 200 MHz):
d = 7.23–7.33 ppm (m, 8H); 13C NMR (CDCl3, 50 MHz):
d = 133.9, 133.4, 132.3, 129.4 ppm; MS (ESI) m/z = 254
(M?, not detected), 219 (33), 184 (100), 139 (11), 108 (29),
91 (18), 75 (24).
a
1
Determined by GC methods and H NMR
with dichloromethane and an aliquot was taken for
GC-analysis (30 m Rxi-5 ms column, 40–300 °C). The
solvent was removed and the residue was purified by
column chromatography. The analytical properties of the
corresponding sulfides are in agreement with literature
[63–71].
Phenyl methylsulfide (13a): Rf = 0.88 (n-hexane:ethyl
1
acetate 1:5); H NMR (CDCl3, 200 MHz): d = 7.19–7.35
(m, 5H), 2.53 ppm (s, 3H); 13C NMR (CDCl3, 50 MHz):
123

Reduction of Sulfoxides to Sulfides in the Presence of Cu Catalysts
837
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8.07–8.12 (m, 2H), 7.24–7.29 (m, 2H), 2.53 ppm (s, 3H);
13C NMR (CDCl3, 50 MHz): d = 148.8, 144.5, 124.8,
123.7, 14.8 ppm; MS (ESI) m/z = 169 (100, M?), 139
(50), 123 (13), 111 (16), 108 (33), 82 (14), 77 (38).
Dibenzylsulfide (16a): Rf = 0.74 (n-hexane:ethyl ace-
tate 1:5); 1H NMR (CDCl3, 200 MHz): d = 7.31–7.37 (m,
10H), 3.65 ppm (s, 4H); 13C NMR (CDCl3, 50 MHz):
d = 138.0, 128.9, 128.4, 126.9, 35.5 ppm; MS (ESI)
m/z = 214 (59, M?), 123 (31), 91 (100), 65 (17).
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(36, M?), 57 (82), 41 (100).
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d = 2.28 (t, 4H, J = 7.20 Hz, S(CH2CH2CH2CH3)2),
1.14–1,49 (m, 8H, S(CH2CH2CH2CH3)2), 0.75 ppm (t, 6H,
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32.0, 22.3, 13.8 ppm; MS (ESI) m/z = 146 (M?, 40), 103
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