Rhodium-catalyzed hydrothiolation of alkynes with thiols for construction of sulfur-containing π-conjugated systems

Article abstract of DOI:10.1007/s11164-014-1639-0

To synthesize sulfur-containing π-conjugated polymers, reaction conditions for rhodium-catalyzed hydrothiolation of terminal alkynes with arenethiols are optimized in detail. Under the optimized conditions, rhodium-catalyzed hydrothiolation of terminal al

Full text of DOI:10.1007/s11164-014-1639-0

Res Chem Intermed (2014) 40:2381–2389
DOI 10.1007/s11164-014-1639-0
Rhodium-catalyzed hydrothiolation of alkynes
with thiols for construction of sulfur-containing
p-conjugated systems
•
Aya Yoshimura Akihiro Nomoto Akiya Ogawa
•
Received: 27 January 2014 / Accepted: 27 February 2014 / Published online: 27 April 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract To synthesize sulfur-containing p-conjugated polymers, reaction con-
ditions for rhodium-catalyzed hydrothiolation of terminal alkynes with arenethiols
are optimized in detail. Under the optimized conditions, rhodium-catalyzed hy-
drothiolation of terminal alkynes proceeds regio- and stereoselectively to afford the
corresponding vinyl sulfides via an anti-Markovnikov and syn-addition process.
Then, the rhodium-catalyzed hydrothiolation is applied to polymerization of 2,5-
diethynylthiophene with benzene-1,4-dithiol, which successfully provides sulfur-
containing p-conjugated polymers with excellent regio- and stereoselectivities.
Hydrothiolation ꢀ Terminal alkyne ꢀ Rhodium catalyst ꢀ Anti-Markovnikov
addition ꢀ Sulfur-containing p-conjugated polymer
Introduction
Poly(phenylene sulfide), poly(phenylene vinylene sulfide), and poly(thiophene) are
representative sulfur-containing p-conjugated polymers, and their photochemical
and electrochemical properties have been focused on recently [1–9]. To control
these physical properties, development of highly selective synthetic methods for
sulfur-containing p-conjugated polymers is of great importance. Transition metal-
catalyzed copolymerization methods are the most suitable candidates for highly
selective synthesis of p-conjugated polymers, and indeed poly(phenylene) and
poly(phenylene vinylene) can be prepared by such methods [10–19]. However,
transition metal-catalyzed synthesis of sulfur-containing p-conjugated polymers
such as poly(phenylene sulfide) and poly(phenylene vinylene sulfide) remains
A. Yoshimura ꢀ A. Nomoto ꢀ A. Ogawa (&)
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
e-mail: ogawa@chem.osakafu-u.ac.jp
123

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A. Yoshimura et al.
largely unexplored, most probably because organosulfur compounds are believed to
be catalyst poisons [20].
In contrast, we recently developed a series of highly selective transition metal-
catalyzed reactions of organosulfur compounds with alkynes [21–30]. For example,
organic thiols are revealed to add to alkynes regio- and/or stereoselectively in the
presence of transition metal catalysts to afford a series of vinyl sulfides in good
yields. In particular, in the case of using palladium acetate [Pd(OAc)2] as catalyst,
Markovnikov addition of thiols to terminal alkynes takes place regioselectively
[31], whereas Wilkinson’s catalyst [RhCl(PPh3)3] causes highly selective hydro-
thiolation with anti-Markovnikov and syn-addition selectivities (Scheme 1) [32].
Therefore, these successful results of transition metal-catalyzed hydrothiolation
of alkynes with thiols prompted us to examine construction of p-conjugated
polymers using transition metal-catalyzed reactions of diynes with dithiols
(Scheme 2) [33]. In this paper, we report transition metal-catalyzed hydrothiolation
of terminal alkynes with arenethiols and its application to construction of sulfur-
containing p-conjugated polymers.
Results and discussion
In our previously reported rhodium-catalyzed hydrothiolation of alkynes [32], the
representative reaction conditions were as follows: alkyne (1 mmol), thiol
(1.1 mmol), RhCl(PPh3)3 (3 mol %), EtOH (1 mL), 20–40 °C. This condition is
very useful for synthesis of various anti-Markovnikov-type vinyl sulfides, but it is
not suitable for construction of sulfur-containing p-conjugated polymers because
ethanol as solvent cannot dissolve sulfur-containing p-conjugated polymers,
resulting in low molecular weight of adducts. Therefore, to synthesize sulfur-
containing p-conjugated polymers, we first examined reoptimization of reaction
conditions for rhodium-catalyzed hydrothiolation of terminal alkynes with aren-
ethiols in detail (Table 1). Catalytic hydrothiolation of 1-octyne (1a) with
benzenethiol (5a) in the presence of 5 mol % catalyst was conducted by varying
solvents and temperatures under higher concentration of the substrate. Fortunately, a
variety of solvents are compatible with the present hydrothiolation, and the
corresponding E-isomers of anti-Markovnikov adducts (4a) were obtained stere-
oselectively as the major product. Toluene, chloroform, 2-propanol, and ethanol
exhibited good product selectivity of 4a (entries 1, 6, 7, and 10).
Furthermore, the influence of substituents of arenethiols on the hydrothiolation
was investigated. As indicated in Scheme 3, both electron-withdrawing group (p-Cl)
and electron-donating group (p-OMe) were compatible with the anti-Markovnikov
hydrothiolation, affording the corresponding adducts 4b0 and 4c0, respectively, in
good yields with excellent regio- and stereoselectivities.
Next, we examined rhodium-catalyzed hydrothiolation of phenylacetylene (1c)
as an aromatic acetylene (Table 2). In this hydrothiolation, dichloromethane was
found to be a useful solvent, and excellent regioselectivity was observed to give the
anti-Markovnikov adduct (4) in good yields (entries 2 and 4).
123

Rhodium-catalyzed hydrothiolation of alkynes
2383
cat. Pd(OAc)2
Markovnikov addition
R
SPh
R
PhSH
cat. RhCl(PPh3)3
anti-Markovnikov addition
R
SPh
Scheme 1 Transition metal-catalyzed hydrothiolation of terminal alkynes
cat. RhCl(PPh3)3
anti-Markovnikov addition
Ar
Ar'
Ar
HS Ar' SH
S
S
n
Scheme 2 Rhodium-catalyzed polymerization of diynes with dithiols
Table 1 Rhodium-catalyzed hydrothiolation of 1-octyne (1a) with benzenethiol (5a)
cat. RhCl(PPh3)3
PhSH
nC6H13
nC5H11
nC6H13
nC6H13
1a
SPh
SPh
solvent
SPh
2a
3a
4a
ETN
Entry
Solvent
Temp. (°C)
Yield (%)a
(E/Z)
2a
3a
4a
1
PhCH3
PhH
0.099
102
25
80
35
40
61
40
30
0
0
24
28
15
19
0
5
0
52
56
43
25
53
46
44
53
62
51
49
(100/0)
(100/0)
(99/1)
2
3b
0.111
0.111
0.117
0.207
0.259
0.546
0.654
0.654
0.654
0.762
PhH
5
4
Et2O
0
(100/0)
(100/0)
(96/4)
5
THF
8
6
CHCl3
i-PrOH
EtOH
EtOH
EtOH
MeOH
10
5
7
0
(100/0)
(100/0)
(100/0)
(100/0)
(100/0)
8c
9d
10e
11
24
25
0
0
0
40
65
3
29
0
Reaction conditions: 1-octyne (1.0 mmol), benzenethiol (1.0 mmol), RhCl(PPh3)3 (5 mol % based on
1a), solvent (0.5 mL), 20 h
a
b
c
d
e
Determined by 1H NMR; Catalyst (3 mol %); PhSH (0.8 mmol); PhSH (1.1 mmol); PhSH
(1.4 mmol)
Table 2 also indicates a limitation of the present hydrothiolation. As indicated in
our previous paper, rhodium-catalyzed hydrothiolation of usual terminal alkynes
proceeded in good yields with excellent regio- and stereoselectivity (anti-Markovni-
kov and syn-addition). However, in the cases of some propargyl derivatives such as 1d
and 1e, the regioselectivities of hydrothiolation were somewhat lower (entries 6 and
123

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A. Yoshimura et al.
X
3 mol% RhCl(PPh3)3
EtOH (1 mL), 20 h
nC10H21
X
SH
nC10H21
S
1b, 1.0 mmol
5b, 1.1 mmol
5c
X = MeO
X = Cl
30 °C
40 °C
4b', 72%*
4c', 86%*
*Isolated yield based on 1-dodecyne
Scheme 3 Rhodium-catalyzed hydrothiolation of 1-dodecyne with arenethiols
Table 2 Rhodium-catalyzed hydrothiolation of terminal alkynes
cat. RhCl(PPh3)3
solvent, 20 h
R
R
+
R
PhSH
SPh
SPh
2
4
Entry
1
Alkyne
Cat. (mol %)
Solvent
Temp. (°C)
Yield (%)a
2
4
5
EtOH
20
10
90
Ph
1c
2b
3
4b
5
3
3
1
5
CH2Cl2
EtOH
40
40
40
40
40
8
92
87
80
97
44
3
CH2Cl2
EtOH
4
5
6b
2
CH2Cl2
29
HO
Ph
1d
7
8
5
3
EtOH
EtOH
25
25
31
26
69
51
1e
1f
NC
2
9
3
EtOH
25
50
50
PhS
1g
Reaction conditions: alkyne (1.0 mmol), benzenethiol (1.1 mmol), solvent (0.5 mL), 20 h
a
b
Determined by 1H NMR; PhSH (1.0 mmol)
7), and especially propargyl sulfide (1g) exhibited no regioselectivity (entry 9), most
probably due to coordination of the sulfide group to the catalyst.
To construct sulfur-containing p-conjugated systems, we next examined
rhodium-catalyzed hydrothiolation of 2-ethynylthiophene (1h) with 1,4-benzene-
dithiol (5d) using the optimized reaction conditions. Anti-Markovnikov and syn-
addition took place successfully to yield the corresponding conjugated molecule 6 in
high yield (Scheme 4).
Similar conditions could be employed with bis(4-mercaptophenyl) sulfide (5e),
and the desired sulfur-containing p-conjugated compound 7 was obtained in high
yield regio- and stereoselectively (Scheme 5).
123

Rhodium-catalyzed hydrothiolation of alkynes
2385
5 mol% RhCl(PPh3)3
CH2Cl2, rt, 20 h
HS
SH
5d, 1 equiv.
S
1h
S
S
S
S
S
S
S
S
6, 87%
Scheme 4 Construction of sulfur-containing p-conjugated system
6%
5 mol% RhCl(PPh3)3
CH2Cl2, rt, 20 h
HS
S
SH
S
1h
5e, 1 equiv.
S
S
S
S
S
S
S
S
S
S
7, 91%
Scheme 5 Construction of sulfur-containing p-conjugated system
3%
These successful results prompted us to examine addition polymerization
between 2,5-diethynylthiophene derivative 1i and 1,4-benzenedithiol (5d). The
desired rhodium-catalyzed addition polymerization proceeded successfully to afford
the polymerized adduct 8 (Scheme 6).
1
Measurement of the H nuclear magnetic resonance (NMR) spectrum of the
obtained addition polymerization product 8 clearly indicated that the reaction
proceeded regio- and stereoselectively, producing also small amounts of its
regioisomer A and trace amounts of its stereoisomer B (Fig. 1).
Figure 2 presents the ultraviolet (UV)–visible spectra of the obtained sulfur-
containing p-conjugated polymer (8) in comparison with those of thiophene
derivative 6 and 2-{20-(phenylthio)ethenyl}thiophene (9). As can be seen from
Fig. 2, the absorbance of p-conjugated polymer 8 shifts to a longer-wavelength
region compared with 6 and 9, suggesting longer conjugation through phenylene,
vinylene, and sulfide groups.
In conclusion, highly selective hydrothiolation of alkynes with thiols catalyzed
by rhodium complex leading to formation of anti-Markovnikov and syn-addition
products has been developed. The procedure can be applied for synthesis of sulfur-
containing p-conjugated polymers with excellent regio- and stereoselectivities. We
believe this finding will open up a new field of functional molecules.
Experimental
General methods
Unless otherwise noted, materials were obtained from commercial suppliers and
1
purified by distillation or recrystallization. H NMR spectra were recorded mainly
123

2386
A. Yoshimura et al.
nBu
nBu
nBu
nBu
8 mol% RhCl(PPh3)3
CH2Cl2, rt, 20 h
HS
SH
S
S
S
S
n
1i
1 equiv.
8, Mn > 105
Scheme 6 Rhodium-catalyzed polymerization between 2,5-diethynylthiophene derivative and 1,4-
benzenedithiol
Fig. 1 1H NMR spectrum of 8
on a JEOL JNM-AL-400 spectrometer (400 MHz) using CDCl3 as solvent with
tetramethylsilane (TMS) as internal standard. 13C NMR spectra were taken mainly
on a JEOL JNM-AL-400 (100 MHz) spectrometer using CDCl3 as solvent.
Chemical shifts in 13C NMR were measured relative to CDCl3 and converted to d
(TMS) values by using d (CDCl3) 77.0 ppm. Infrared (IR) spectra were determined
on a Shimadzu FTIR 8400 infrared spectrometer. Mass spectra were obtained on a
JEOL JMS-DX303 in the analytical section of Osaka University, where elemental
analyses were also performed. Molecular weights and molecular weight distribu-
tions of polymers were determined with a Shodex-KF804 (Showa Denko K.K.)
equipped with an RI-2031 from JASCO Ltd.
123

Rhodium-catalyzed hydrothiolation of alkynes
2387
Fig. 2 UV–visible spectra of thiophene derivatives
RhCl(PPh3)3-catalyzed hydrothiolation of phenylacetylene with benzenethiol
in CH2Cl2
In a 20 mL two-necked flask equipped with a reflux condenser were placed
RhCl(PPh3)3 (5 mol %, 0.0463 g), CH2Cl2 (0.5 mL), and phenylacetylene
(1.0 mmol, 0.110 g) under argon atmosphere. Benzenethiol (1.1 mmol, 0.112 g)
was added to the reaction solution at room temperature, and the reaction was stirred
for 20 h with heating at 40 °C. After the reaction was complete, the resulting
mixture was filtered through Celite and concentrated in vacuo. The products were
analyzed by 1H NMR and 13C NMR. Product 4c [registry no. 7214-53-1]; 4d
[registry no. 15148-54-6]. Compounds 4a, 4b0, 4c0, 4e, 4f, and 4g were prepared in
EtOH according to the literature [25]: product 4a [registry no. 75717-41-8]; 4e
[registry no. 29736-81-0]; 4f [registry no. 138942-40-2]; 4g [registry no. 60039-97-
6]; 4b0, 4c0 (known compounds, see, Ref. [32]).
Synthesis of conjugated molecule 6 by RhCl(PPh3)3-catalyzed hydrothiolation
of 2-ethynylthiophene with 1,4-benzenedithiol (Scheme 4)
A mixture of 2-ethynylthiophene (2 mmol, 0.216 g), RhCl(PPh3)3 (5 mol %,
0.0925 g), and 1,4-benzenedithiol (2 mmol, 0.284 g) in CH2Cl2 was stirred for 20 h
at room temperature. Purification was performed by gel permeation chromatography
(GPC, Shodex-KF804, eluent CHCl3), and conjugated molecule 6 was obtained in
87 % yield. Conjugated molecule 6: 1H NMR (270 MHz, CDCl3): d 6.60 (d,
J = 15.3 Hz, 2 H), 6.81 (d, J = 15.0 Hz, 2 H), 6.87 (d, J = 2.7 Hz, 2 H), 6.90 (dd,
J = 1.2, 4.8 Hz, 2 H), 7.10 (d, J = 5.1 Hz, 2 H), 7.19 (s, 4 H); 13C NMR (75 MHz,
123

2388
A. Yoshimura et al.
CDCl3): d 121.5, 124.5, 125.4, 126.1, 127.4, 130.2, 133.9, 141.0; MS (EI), m/
z = 358 (M?, 100.0); Anal. calcd. for C19H30OS: C, 74.46; H, 9.87; S, 10.46.
Found: C, 74.31; H, 9.64; S, 10.61. Similar conditions could be employed with
1
conjugated molecules 7 and 9. Their H and 13C NMR spectra and/or IR and MS
1
spectra were as follows: Conjugated molecule 7: H NMR (300 MHz, CDCl3): d
6.59 (d, J = 15.3 Hz, 2 H), 6.82 (d, J = 15.3 Hz, 2 H), 6.87 (d, J = 3.3 Hz, 2 H),
6.90 (dd, J = 1.5, 5.1 Hz, 2 H), 7.19 (s, 4 H), 7.23 (d, J = 3.0 Hz, 2 H); 13C NMR
(75 MHz, CDCl3): d 121.2, 124.6, 125.5, 126.5, 127.5, 130.0, 131.5, 134.1, 134.5,
1
141.0. 2-(2-(Phenylthio)vinyl)thiophene (9): H NMR (300 MHz, CDCl3): d 6.62
(d, J = 15.0 Hz, 1 H), 6.78 (d, J = 15.0 Hz, 1 H), 6.83 (dd, J = 3.3, 0.6 Hz, 2 H),
6.87 (dd, J = 4.8, 3.6 Hz, 2 H), 7.06 (d, J = 5.1 Hz, 1 H), 7.16–7.34 (m, 5 H); 13
C
NMR (75 MHz, CDCl3): d 122.3, 124.3, 125.2, 125.4, 126.9, 127.4, 129.1, 129.6,
134.9, 141.2; IR (NaCl) 3,067, 1,582, 1,477, 1,438, 1,248, 1,087, 935, 792, 741,
693, 479 cm-1; MS (EI), m/z = 218 (M?, 100.0).
Synthesis of polymerized adduct 8 by addition polymerization between 2,5-
diethynylthiophene derivative and 1,4-benzenedithiol
A mixture of 3,4-dibutyl-2,5-diethynylthiophene, RhCl(PPh3)3 (8 mol %), and 1,4-
benzenedithiol (1 equiv.) in CH2Cl2 was stirred for 20 h at room temperature. The
1
products were analyzed by H NMR (Fig. 1). Molecular weights and molecular
weight distributions of the obtained polymer (8) were determined with a Shodex-
KF804. Polymerized adduct 8: 1H NMR (300 MHz, CDCl3): d 0.40–0.95 (br, 6 H),
0.95–1.80 (br, 8 H), 2.17–2.70 (br, 4 H), 6.30–6.63 (br d, J = 13.5 Hz, 2 H),
6.63–6.96 (br d, J = 13.5 Hz, 2 H). Mn = 1.43 9 106, Mw = 6.02 9 106, Mw/
Mn = 4.21.
Synthesis of 3,4-dibutyl-2,5-diethynylthiophene (1i)
3,4-Dibutyl-2,5-diethynylthiophene (1i) was prepared from 3,4-dibromothiophene
in the following four steps according to the literature [34–36]. Step 1 (synthesis of
3,4-dibutylthiophene): 3,4-Dibromothiophene was allowed to react with n-butyl-
magnesium bromide (5 equiv.) in the presence of NiCl2(dppp) (3 mol %) in Et2O to
afford 3,4-dibutylthiophene in 78 % yield; Step 2 (synthesis of 3,4-dibutyl-2,5-
diiodothiophene): Diiodination of 3,4-dibutylthiophene was carried out using
benzyltrimethylammonium dichloroiodate (PhCH2Me3NICl2, 2.2 equiv.) and ZnCl2
(2.2 equiv.) in acetic acid, and 3,4-dibutyl-2,5-diiodothiophene was obtained in
89 % yield; Step 3 [synthesis of 3,4-dibutyl-2,5-bis(trimethylsilylethynyl)thio-
phene]: Sonogashira coupling reaction of 3,4-dibutyl-2,5-diiodothiophene with
trimethylsilylacetylene (4 equiv.) was carried out in diisopropylamine using
PdCl2(PPh3)2 (2 mol %) and CuI (3 mol %), affording 66 % of 3,4-dibutyl-2,5-
bis(trimethylsilylethynyl)thiophene; Step 4 (synthesis of 1i): Desilylation of 3,4-
dibutyl-2,5-bis(trimethylsilylethynyl)thiophene with KOH in methanol afforded 1i
in 92 % yield.
123

Rhodium-catalyzed hydrothiolation of alkynes
2389
Acknowledgments We thank Dr. Takuma Ikeda and Mr. Kouichi Kimura for their experimental
support.
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