A novel and efficient zinc-catalyzed thioetherification of aryl halides

Article abstract of DOI:10.1039/c5ra03869c

The first zinc-catalysed protocol for the C-S cross-coupling reaction is reported. Zinc catalysis has an undeniable advantage over other catalytic systems as it is non-toxic, and the catalysts are easily available and cheap, with environmentally benign properties. This novel, efficient, palladium- and triphenylphosphine-free protocol yielded a variety of aryl and alkyl sulfides in moderate to excellent yields. This journal is

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DOI: 10.1039/C5RA03869C
A Novel and Efficient Zinc-catalyzed Thioetherification of
Aryl Halides
Amrutha P Thankachan,a K S Sindhu,a K Keerthi Krishnan a and Gopinathan
Anilkumar a*
X
S
R2
Zn-catalyst
HS
+
R2
R1
R1
X = I/ Br/ Cl
R1 = -OMe, -NO2, -CN,-COCH3
R2 = Aryl/ Alkyl
The first Zn-catalyzed protocol for C-S cross-coupling reactions for the synthesis of substituted aryl and alkyl
sulfides with good yields under mild reaction is described.

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ARTICLE TYPE
A Novel and Efficient Zinc-catalyzed Thioetherification of Aryl Halides
Amrutha P Thankachan, a K S Sindhu, a K Keerthi Krishnan a and Gopinathan Anilkumar a*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
along with L-proline as ligand for the C-S cross-coupling reaction
in perspective manner that it may perform chemical
50 transformations similar to that of enzymatic catalysis. Compared
to the traditionally used phosphine-based ligands, L-proline is
very cheap, non-toxic and readily available.
We herein report a novel and efficient zinc-catalyzed C-S
cross-coupling reaction between aryl halides and thiophenols. At
55 first the reaction involving 4-iodoacetophenone and thiophenol
was chosen as a model reaction. The reactions are conducted in a
previously dried sealed tube in the presence of K2CO3 in DME at
80 oC under nitrogen atmosphere ( Scheme 1).
The first zinc-catalyzed protocol for the C-S cross-coupling
reactions is reported. Zinc catalysis has an undeniable
significance over other catalytic systems due to its non-toxic,
easily available, cheap and environmentally benign
10 properties. This novel, efficient, palladium- and
triphenylphosphine-free protocol yielded a variety of aryl and
alkyl sulfides having moderate to excellent yields.
a
Aryl Sulfides and their derivatives are very important reactive
intermediates in synthetic organic chemistry due to their
15 biological and pharmacological activity.1 Over the last few years
significant growth has been realized in the field of transition
metal catalyzed carbon-heteroatom bond formation. In this
context, numerous protocols have been reported for carbon-
nitrogen and carbon-oxygen bond forming reactions while that
20 for carbon-sulfur bond formation is moderate.2 This is due to the
deactivation of the metal catalyst by organosulfur reactant
because of the strong coordination capacity of sulfur with the
metal catalysts. The usual methods for carbon-sulfur bond
formation reactions are highly inefficient due to the pre-requisite
25 of extremely harsh reaction conditions such as elevated
temperature, extensive reaction time, use of more polar solvents
and multistep reactions. One of the major challenges associated
with the transition metal-catalyzed Carbon-Sulfur bond formation
is its greater tendency for oxidative S-S coupling. A large number
30 of fine reports are available for palladium,3 copper,4 cobalt,5
nickel,6 iron,7 indium8 and rhodium9 catalyzed C-S cross-
coupling reactions. The first report on C-S cross-coupling
reaction was published by Migita et al. in 1978 using catalytic
amount of tetrakis(triphenylphophine)palladium.10
To the best of our knowledge, no Zn-catalyzed C-S bond
forming reaction is reported so far. There are a few reports on
zinc-mediated transition metal-catalyzed C-S coupling reactions
where zinc is used either as a reducing agent11 or as an
electrophilic buffer12 to protect the transition metal catalyst. The
40 bio-catalytic ability of zinc is well established and there exist a
large number of reports in which zinc is used as a catalyst in
organic synthesis.13 This prompted the idea of using zinc as a
catalyst in carbon-heteroatom bond formation reactions. Even
though Zinc-catalysts used in carbon-carbon coupling reactions
45 showed great tolerance towards many functional groups,14 to our
surprise, zinc-based catalytic systems have not been used for C-S
cross-coupling reactions. In our study we used zinc catalyst
Zn-catalyst (10 mol %)
L-Proline (20 mol %)
HS
I
S
+
K2CO3 (2equiv.)
DME, 80 oC, 20 h
H3COC
H3COC
2a
3a
1a
60
Scheme 1. Zn-catalyzed C-S cross-coupling of 4-
iodoacetophenone and thiophenol.
Screening of different zinc sources revealed that only Et2Zn
along with L-proline showed catalytic ability (Table 1, entry 4).
65 The structure of the product 3a was assigned based on nuclear
magnetic resonance and mass spectrometric analyses. Having
obtained the product in moderate yield, we decided to perform
the optimization studies in detail.
Table 1. Screening of different zinc-sources for the C-S cross-coupling
70 reaction.a)
Entry
Catalyst
Yieldb)
35
1
2
3
4
Zn-granules
Zn-powder
Zn(OAc)2
ZnEt2
ndc)
nd
nd
54
a) Reaction conditions: aryliodide (1mmol), thiophenol (1.1 mmol),
K2CO3 (2 equiv.), Zn-source (10 mol %), L-proline (20 mol %), DME (3
ml), 80 oC, 20 h. b) isolated yield. c) not detected
First, we tried to find out the influence of the quantity of Et2Zn
75 catalyst for the C-S cross-coupling reaction. The results revealed
that the amount of catalyst loading has great control over the
yield, and the optimum catalyst loading was found to be 8 mol %
(Table 2, entry 2). Further decrease in catalyst loading reduced
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Entry
Base
(2 equiv.)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
NaOtBu
NaH
Solvent
(3 ml)
DME
THF
CH3CN
DMF
Tempereture
Yieldb)
(%)
54
the yield.
(oC)
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
0
Table 2. Effect of the amount of Et2Zn catalyst for the C-S cross-coupling
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21d)
22e)
23
24f)
25g)
reaction.a)
34
81
Zn (Et)2 (X mol %)
L-Proline (Y mol %)
ndc)
34
I
S
HS
+
tBuOH
DMSO
Toluene
1,4-dioxane
DME
K2CO3 (2equiv.)
DME (3 ml)
80 oC, 20 h
L-proline
(mol %)
H3COC
H3COC
nd
nd
nd
75
64
62
nd
14
75
20
85
95
nd
nd
54
43
3a
2a
1a
Entry
Zn (Et)2
(mol %)
Yieldb)
DME
DME
DME
DME
1
2
3
4
5
10
8
6
4
2
20
16
12
8
54
53
35
19
10
Et3N
K3PO4
KOtBu
NaH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
4
Cs2CO3
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
-
5
a) Reaction conditions: aryliodide (1mmol), thiophenol (1.1 mmol),
K2CO3 (2 equiv.), DME (3 ml), 80 oC, 20 h. b) isolated yield
rt
Next we examined the influence of solvents, bases and
temperature on Zn-catalyzed thioetherification. The solvent effect
analysis revealed that the preferred solvents are DME (Table 3,
125
80
80
80
80
80
traces
nd
traces
traces
10 entries 1, 9, 10, 11) and acetonitrile (Table 3, entries 3, 14, 16,
17). Lower conversions were observed when THF (Table 3, entry
2) and t-BuOH were used as solvents (Table 3, entry 5). The base
optimization studies on thiolation showed that inorganic bases
such as K2CO3, Cs2CO3, NaOtBu, KOtBu and NaH are better
15 compared to triethylamine. The dependence of the amount of
bases on C-S cross-coupling indicated that on decreasing the
amount of base there is considerable decrease in the amount of
product formation (Table 3, entry 21). Studies on the influence of
temperature on thioetherification revealed that no reaction took
NaOtBu
NaOtBu
a)Reaction conditions: aryliodide (1mmol), thiophenol (1.1 mmol), Base
45 (2 equiv.), Et2Zn (8 mol %), L-proline (16 mol %), Solvent (3 ml), Temp.
(oC), 20 h. b)isolated yield. c)not detected. d)1.5 equiv. of NaOtBu.
e)Absence of Et2Zn. f)Absence of L-proline. g)Absence of inert
atmosphere.
To explore the scope of the reaction, we carried out the
50 thiolation reaction of electronically and structurally diverse aryl
iodides and thiols with Zn-proline catalytic system at 80 C in
o
20 place at 0 C and at room temperature (Table 3, entries 18, 19).
o
o
But at 80 C the product could be isolated. On further increasing
acetonitrile. A variety of substrates are transformed to their
corresponding diarylsulfides in good to excellent yields under the
optimized reaction conditions. It is observed that electron
55 withdrawing substituents in the aryl iodides increased the yield of
the desired product compared to electron releasing substituents
(Table 4, entries 4, 6). Attempts to extend the catalytic system to
alkyl thiols were successful, and under the optimized reaction
conditions both benzyl and butyl thiols reacted with various aryl
60 iodides affording the products in good yields (Table 4, entry 16,
17, 18).
the temperature to 125 oC, the yield of the coupled product
decreased presumably due to the decomposition of the product at
the elevated temperature (Table 3, entry 20).
25
As part of control experiment, the reaction was performed at
optimized conditions in the absence of base, ligand and catalyst.
In the absence of base no product could be detected (Table 3,
entry 23). In the absence of catalyst and ligand only trace
amounts of the product were detected (Table 3, entries 22, 24).
30 Carrying out the reaction in the absence of inert atmosphere
afforded the diphenyldisulfide as the major product along with a
trace amount of the desired C-S coupling product (Table 3, entry
25). The optimization studies revealed that the yield was vastly
dependent upon the base, solvent, reaction temperature and the
35 amount of catalyst. The optimum reaction condition for the
desired zinc-catalyzed C-S cross-coupling reaction was found to
be 8 mol % of Et2Zn, 16 mol % of L-proline and 2 equivalents of
NaOtBu at 80 oC in acetonitrile to obtain 95 % of 1-(4-
phenylsulfanyl-phenyl)-ethanone (Table 3, entry 17).
Table 4. Substrate scope of Zn-catalyzed C-S cross-coupling reactions.a)
Zn (Et)2 (8 mol %)
L-Proline (16 mol %)
I
S
R2
HS
R2
+
NaOtBu (2 equiv.)
CH3CN (3 ml), 80 oC
R1
R1 = -CN, -OCH3, -NO2, -COCH3
R2 = aryl/alkyl
40 Table 3. Optimization of reaction conditions for Zn-catalyzed
thioetherification.a)
Zn (Et)2 (8 mol %)
L-Proline (16 mol %)
I
S
HS
+
Base (2 equiv.)
Solvent (3 ml)
Temp. oC, 20 h
H3COC
H3COC
2a
3a
1a
2
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DOI: 10.1039/C5RA03P86a9gCe 4 of 5
Entry
1
Aryl halide
Thiol
Product
Yieldb)
(%)
higher reactivity of aryl iodides over bromides and chlorides
affording the coupling products in 95 %, 58 % and 33 % yield
respectively (Table 4, entries 1, 2, 3).
In conclusion we have developed the first Zn-catalyzed
I
HS
S
95
3a
O
O
O
15 C-S cross-coupling reactions of aryl halides with thiophenols
Br
HS
HS
HS
S
2
3
58
33
under mild conditions using in situ generated Et2Zn-proline in
o
CH3CN in the presence of NaOtBu at 80 C. The Et2Zn-proline
3b
O
Cl
S
catalytic system showed moderate to excellent yield on a variety
of electronically diverse aryl halides for the C-S cross-coupling
20 reactions. The newly developed Zn-proline catalytic system is an
efficient and successful combination for the production of aryl
sulfides in high yields with 8 mol % of catalyst loading, and
shows high functional group tolerance. We hope that the new
procedure will finely substitute the existing methodologies for the
25 C-S cross-coupling of aryl halides with both aryl and alkyl thiols.
Further studies of this zinc-catalyzed reaction of C-S coupling are
in progress.
3c
O
O
I
S
4
5
97
60
61
70
3d
NC
Br
NC
I
NC
NC
S
HS
HS
3e
S
6
O
3f
H3CO
I
S
HS
HS
7
Acknowledgements
3g
OCH3
OCH3
OCH3
OCH3
I
S
8
30 GA thanks the Kerala State Council for Science, Technology and
O
86
70
86
53
65
47
82
85
OCH3
3h
Environment
(KSCSTE),
Trivandrum
(Order
no.
O
HS
HS
S
I
341/2013/KSCSTE dated 15.03.2013) for financial support. APT
thanks the KSCSTE for a junior research fellowship. SKS thanks
UGC for a junior research fellowship. K K thanks Ministry of
35 Social Justice and Empowerment for RJNF. We thank the Inter
University Instrumentation Centre (IUIC) and Institute for
Intensive Research in Basic Sciences (IIRBS) of Mahatma
Gandhi University for HRMS and NMR facilities respectively.
9
3i
S
O2N
OCH3
OCH3
O2N
I
10
OCH3
NC
3j
NC
HS
I
S
11
3k
O
O
S
I
HS
3l
NC
NC
Br
NC
I
40 Notes and references
12
13
14
15
16
17
18
S
HS
HS
a,* School of Chemical Sciences, Mahatma Gandhi University, PD Hills
P O., Kottayam INDIA-686 560. Fax: +91-481-2731036 E-mail:
anilgi1@yahoo.com
† Electronic Supplementary Information (ESI) available: [details of any
45 supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
3m
NC
S
3n
O2N
O2N
HS
S
I
‡ Typical Experimental Procedure for the Synthesis of 1-(4-
phenylsulfanyl-phenyl)-ethanone (3a): A dry sealed tube was charged
with 1mmol (246 mg) of 4-iodoacetophenone, 16 mol% of L-proline (18
50 mg) and 2 equiv. of NaOtBu (192 mg) under nitrogen. To the above
mixture was added 8 mol % of Et2Zn (1M in hexane, 0.08 ml) and 3 ml of
acetonitrile followed by the addition of 1.1 mmol of thiophenol (0.11 ml)
under nitrogen. The sealed tube was heated in an oil bath which was
3o
F
O2N
F
O
I
HS
HS
S
90
70
59
O
3p
O
o
I
preheated to 80 C and the reaction mixture was stirred under the same
S
55 conditions for 20 hours. The reaction mixture was then cooled and
extracted with ethyl acetate (3 x 15 ml) and the ethyl acetate layer was
washed with saturated aqueous NaCl solution. The organic layer was
dried over anhydrous Na2SO4 and the solvent was removed under reduced
pressure in a rotary evaporator. The crude residue was purified by column
60 chromatography using EtOAc-hexane as the eluent to get 217 mg (95 %)
NC
3q
NC
I
S
HS
O
3r
O
o
of the product as a colourless solid. M. P: 67 C; 1H NMR (400 MHz,
‡a) Reaction conditions: aryliodide (1mmol), thiophenol (1.1 mmol),
CDCl3): δ 7.83(d, J = 8.4 Hz, 2H), 7.51-7.48 (m, 2H), 7.41-7.39 (m, 3H),
7.22 (d, J = 8.4 Hz, 2H), 2.55 (s, 3H); 13C NMR (100 MHz, CDCl3): δ
197.10, 144.92, 134.55, 133.87, 132.16, 129.69, 128.91, 128.79, 127.52,
65 26.46; IR (neat): 3060, 1669, 1555, 1182, 819, 616 cm-1; HRMS (QToF):
[M+H]+ calculated for C14H12OS is 229.0687; found 229.0675
.
NatBu (2 equiv.), Et2Zn (8 mol %), L-proline (16 mol %), CH3CN (3 ml),
80 oC), 20 h. b) isolated yield
To extend the scope of the reaction further, the Zn-proline
catalytic system was then applied to aryl bromides and chlorides.
As expected, the new catalytic system was found to be
compatible with aryl bromides and chlorides and yielded the
products albeit in low yields (Table 4, entries 2, 3, 5, 13). The
coupling reaction of thiophenol with 4-iodoacetophenone, 4-
5
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