Switchable Ni-catalyzed bis-thiolation of acetylene with aryl disulfides as an access to functionalized alkenes and 1,3-dienes

Article abstract of DOI:10.1016/j.apcata.2018.12.018

The article provides the first example of metal-catalyzed aryl disulfide addition to unsubstituted acetylene. The use of inexpensive Ni(acac)₂ precatalyst with phosphine ligands results in competitive formation of (Z)-1,2-bis(arylthio)ethenes a

Full text of DOI:10.1016/j.apcata.2018.12.018

AppliedCatalysisA,General571(2019)170–179
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Switchable Ni-catalyzed bis-thiolation of acetylene with aryl disulfides as an
access to functionalized alkenes and 1,3-dienes
Evgeniya S. Degtyareva1, Kirill S. Erokhin1, Alexey S. Kashin, Valentine P. Ananikov⁎
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, Moscow, 119991, Russia2
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalysis
Nickel
Acetylene
The article provides the first example of metal-catalyzed aryl disulfide addition to unsubstituted acetylene. The
use of inexpensive Ni(acac)2 precatalyst with phosphine ligands results in competitive formation of (Z)-1,2-bis
(arylthio)ethenes and (Z,Z)-1,4-bis(arylthio)buta-1,3-dienes. The process with the PPhCy2 as a ligand results in
selective formation of diene molecular skeletons. Replacement of PPhCy2 with the PPh3 switches the reaction
toward formation of alkenes. The use of substituted phenyl disulfides does not affect the selectivity and allows
obtaining alkenes or dienes in good to high yields. Mechanistic investigations reveal major differences on the
catalyst activation stage depending on the nature of phosphine ligand. Key novel point is to carry out video-
monitoring of catalyst evolution with electron microscopy, which revealed the dynamic nature of the catalytic
system and showed that the ligand played a prominent role in formation of the catalytically active phase. For
PPh3, the development of catalytically active species proceeds through nickel thiolate [Ni(SAr)2]n formation,
which renders the system heterogeneous. In contrast to PPh3, the PPhCy2 ligand promotes direct activation of
the catalyst in its molecular form without disturbing the homogeneous state of the system.
Diene
Sulfur derivatives
Atom-economic addition
Calcium carbide
1. Introduction
conjugated systems [18] and their optional modification with sulfur
moieties.
Modern chemistry of catalytic reactions is like a vortex, which
permanently swallows new starting materials to transform them into
complicated substances. Increase of selectivity and yields always ac-
companies the development of new catalytic systems. Despite the
variety of readily available building blocks for laboratory use, search
for the reactions with the simplest and cheapest molecules are of utmost
practical importance and represents a hot topic. Saturated and un-
saturated hydrocarbons keep the first lines in rating of the cheap and
easily available materials among the others bulk chemicals, as they are
the main products of chemical industry.
Heterofunctionalization of hydrocarbons allows discovery of sub-
stances with completely new properties. In a major focus is the C–S
bond construction [1–8] due to the great demand for organosulfur
compounds, which find application in material, polymer, pharmaceu-
tical, and chemical sciences and industries [7–13]. Conjugated sulfur-
containing substances are of special demand in bio-, opto-, and alter-
native energy electronics because of their enhanced mechanical and
optical properties and the outstanding charge conductivity [12–17].
Acetylene is a simple and convenient building block for the synthesis of
In this work, we explore the possibility of S-functionalization of
unsubstituted acetylene. Despite being the simplest alkyne, difficulties
to handle and special precautions exclude C2H2 from the list of typical
substrates. For comparison, in substituted alkynes the prevailing
methods of C–S bond formation are metal-catalyzed syntheses with
thiols, sulfides, and disulfides [19–22]. Wide substrate scopes along
with high yields and high selectivities are typical for the transition
metal complexes-catalyzed transformations. The chemistry of addition
of thiols to alkynes is studied fairly well [1,23–31], while the reactions
with disulfides are less studied [19,32]. Nevertheless, utilization of
disulfides seems to have some advantages over thiols, as disulfides are
nonvolatile, relatively odorless, and do not require storage under inert
atmosphere. Several systems, based on Rh [33], Ni [34,35], Pd [36,37],
Cu [38], or Ga [39] complexes, were reported to catalyze the addition
of disulfides to various alkynes, but the issue of utilizing acetylene in
this reaction was not considered.
Ni(acac)2/PR3 catalytic systems, which have been previously de-
veloped in our laboratory, are efficient for construction of the C–S bond
[34,35]. A fascinating feature of these systems is that, depending on the
⁎ Corresponding author.
E-mail address: val@ioc.ac.ru (V.P. Ananikov).
1 These authors contributed equally
2 http://Ananikovlab.ru
https://doi.org/10.1016/j.apcata.2018.12.018
Received 11 October 2018; Received in revised form 12 December 2018; Accepted 15 December 2018
Availableonline17December2018
0926-860X/©2018ElsevierB.V.Allrightsreserved.

E.S. Degtyareva et al.
AppliedCatalysisA,General571(2019)170–179
phosphine ligand, they selectively promote formation of either 1,2-bis
(alkylthio)alkenes or 1,4-bis(arylthio)-1,3-dienes. However, the unique
physical and chemical properties of C2H2 strongly distinguish it from
other alkynes, thereby specific optimization of the catalytic system and
reaction conditions is required for C2H2 involvement in the reaction.
In this study, we report an efficient catalytic system to obtain 1,2-bis
(arylthio)ethenes or 1,4-bis(arylthio)-1,3-butadienes from acetylene in
a highly selective manner. Controlled switching of the reaction toward
one particular product is accomplished by specific choice of the ligand.
As a result, two products can be obtained on demand in good to high
yields and with high selectivity. Derivatives from acetylene are valuable
for obtaining new materials and there is a particular practical demand
for selective catalysts efficient in for synthesis of complex structures
from small available molecules (acetylene).
0.18 ml) was added to the second one. Both vessels were closed im-
mediately. The vessel with reagents was placed into a heating block
(t = 60 °C). The reaction was carried out under stirring for 1 h.
2.2. Characterization data
The 1a [41] product was identified according to the published data.
2.2.1. (Z)-1,2-bis(4-tolylthio)ethene (1b)
White solid. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.32 (d,
J = 8.2 Hz, 4 H), 7.21 (d, J = 8.1 Hz, 4 H), 6.58 (s, 2 H), 2.32 (s, 6 H).
13C NMR (75 MHz, Acetone-d6) δ, ppm: 137.84, 132.48, 130.86,
130.26, 125.41, 20.97. HRMS (ESI-TOF): m/z calcd for [M + Ag]+
378.9739, found 378.9758 (Δ = 5 ppm).
:
2. Experimental
2.2.2. (Z)-1,2-bis(3-tolylthio)ethene (1c)
Yellow oil. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.30 – 7.20 (m,
6 H), 7.15 – 7.07 (m, 2 H), 6.67 (s, 2 H), 2.34 (s, 6 H). 13C NMR
(75 MHz, Acetone-d6) δ, ppm: 140.06, 135.79, 130.32, 130.04, 128.57,
2.1. General procedures
126.88, 125.32, 21.26. HRMS (ESI-TOF): m/z calcd for [M]+
273.0766, found 273.0746 (Δ = 7.3 ppm).
:
Unless noted otherwise, the synthetic work was carried out under
argon atmosphere. All synthetic procedures were designed for solvents
presaturated with acetylene at 0 °C, which is mandatory to reach the
best yields. The disulfides were prepared according to the previously
published procedure [40]. The other reagents were obtained from
Sigma Aldrich and used as supplied (checked by NMR before use). The
solvents were purified according to the published methods. 1H and 13C
chemical shifts are reported relative to the corresponding solvent sig-
nals used as an internal reference.
2.2.3. (Z)-1,2-bis((4-bromophenyl)thio)ethene (1d)
White solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.45 (d, J = 8.6 Hz,
4 H), 7.26 (d, J = 8.5 Hz, 4 H), 6.49 (s, 2 H). 13C NMR (75 MHz, CDCl3)
δ, ppm: 134.31, 132.39, 131.12, 125.35, 121.21. HRMS (ESI-TOF): m/z
calcd for [M + Ag]+: 506.7636, found 506.7637 (Δ = 0.2 ppm).
2.2.4. (Z)-1,2-bis((3-bromophenyl)thio)ethene (1e)
2.1.1. General Synthetic Procedure for synthesis of (Z)-1,2-bis(arylthio)
ethene (1)
White solid. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.60 (s, 2 H),
7.51 – 7.41 (m, 4 H), 7.35 (t, J = 7.9 Hz, 2 H), 6.84 (s, 2 H). 13C NMR
(75 MHz, Acetone-d6) δ, ppm: 138.37, 131.93, 131.74, 130.75, 128.42,
125.89, 123.53. HRMS (ESI-TOF): m/z calcd for [M + Ag]+: 506.7636,
found 506.7629 (Δ = 1.4 ppm).
Acetylene was bubbled through acetonitrile in a water-ice bath for
30 min. The solid reagents including disulfide (0.2 mmol), Ni(acac)2
(6 × 10−3 mmol, 0.0015 g), and PPh3 (6 × 10-2 mmol, 0.0157 g) were
added into a screw-capped test tube. The test tube with the solid re-
agents was flushed with argon, and the presaturated MeCN (1 ml) was
added. The tube was immediately closed with a screw cap. The reaction
was carried out at 60 °C under stirring for 2 days. After completing the
reaction, the solvent was evaporated on a rotary evaporator and the
product was purified by flash chromatography on silica gel 60 (0.015-
0.040 mm) with hexane/CH2Cl2 gradient elution. After drying under
vacuum, the pure products were obtained.
2.2.5. (Z)-1,2-bis((2-bromophenyl)thio)ethene (1f)
Light oil. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.60 (dd, J = 8.0,
1.4 Hz, 2 H), 7.40 (dd, J = 7.9, 1.7 Hz, 2 H), 7.31 (td, J = 7.6, 1.4 Hz,
2 H), 7.15 – 7.05 (m, 2 H), 6.64 (s, 2 H). 13C NMR (75 MHz, CDCl3) δ,
ppm: 136.53, 133.37, 130.03, 128.12, 126.23, 124.03. HRMS (ESI-
TOF): m/z calcd for [M + Ag]+
(Δ = 0.8 ppm).
:
506.7636, found 506.7640
2.1.2. General synthetic procedure for (Z,Z)-1,4-bis(arylthio)buta-1,3-
diene (2)
2.2.6. (Z)-1,2-bis((4-methoxyphenyl)thio)ethene (1g)
Yellow oil. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.38 (d,
J = 8.9 Hz, 4 H), 6.96 (d, J = 8.9 Hz, 4 H), 6.43 (s, 2 H), 3.81 (s, 6 H).
13C NMR (75 MHz, Acetone-d6) δ, ppm: 160.42, 132.81, 126.33,
Acetylene was bubbled through DMF in a water-ice bath for 30 min.
The disulfide (0.2 mmol), Ni(acac)2 (6 × 10−3 mmol, 0.0015 g), and
PPhCy2 (6 × 10-2 mmol, 0.0165 g) were added into a screw-capped
test-tube. The test tube with the solid reagents was flushed with argon,
and the presaturated DMF (3 ml) was added. The test tube was im-
mediately closed with a screw cap connected to an acetylene balloon
via an adapter. The reaction was carried out at 60 °C under stirring for
1 h. After completing the reaction, the solvent was evaporated on a
rotary evaporator and the product was purified by flash chromato-
graphy on silica 60 (0.015-0.040 mm) with hexane/CH2Cl2 gradient
elution. After drying under vacuum, the pure products were obtained as
light yellow solids.
125.63, 115.81, 55.74. HRMS (ESI-TOF): m/z calcd for [M + Ag]+
:
410.9637, found 410.9648 (Δ = 2.7 ppm). CCDC-1864715 contains the
supplementary crystallographic data for this substance. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data request/cif
2.2.7. (Z)-1,2-bis((4-chlorophenyl)thio)ethene (1h)
White solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.38 – 7.28 (m,
8 H), 6.50 (s, 2 H). 13C NMR (75 MHz, CDCl3) δ, ppm: 133.64, 133.32,
130.94, 129.47, 125.38. HRMS (ESI-TOF): m/z calcd for [M + Ag]+
418.8646, found 418.8648 (Δ = 0.5 ppm).
:
2.1.3. Synthesis of (Z,Z)-1,4-bis(phenylthio)-1,3-butadiene (2a) using
CaC2
The reaction was carried out in a screw-capped H-type test-tube.
One vessel was charged with Ph2S2 (0.2 mmol, 43.6 mg), Ni(acac)2
(6 × 10−3 mmol, 1.5 mg), and PPhCy2 (6 × 10-2 mmol, 16.4 mg).
Calcium carbide (2.5 mmol, 160.2 mg) and DMF (1 ml) were loaded
into another vessel. Afterwards, DMF presaturated with C2H2 (3 ml)
was quickly added to the first vessel, and distilled water (10 mmol,
2.2.8. (1Z,3Z)-1,4-bis(phenylthio)buta-1,3-diene (2a)
White solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.43 – 7.37 (m,
4 H), 7.37 – 7.29 (m, 4 H), 7.30-7.19 (m, 2 H), 6.76 – 6.64 (m, 2 H),
6.45 – 6.33 (m, 2 H). 13C NMR (75 MHz, CDCl3) δ, ppm: 135.69,
129.57, 129.27, 127.13, 126.97, 125.03. HRMS (ESI-TOF) m/z calcd for
[M + Ag]+: 376.9582, found 376.9579 (Δ = 0.8 ppm).
171

E.S. Degtyareva et al.
AppliedCatalysisA,General571(2019)170–179
2.2.9. (1Z,3Z)-1,4-bis(3-tolylthio)buta-1,3-diene (2b)
[M + Ag]+: 436.9794, found 436.9779 (Δ = 3,4 ppm).
White solid. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.31 – 7.19 (m,
6 H), 7.16-7.07 (m, 2 H), 6.73 – 6.61 (m, 2 H), 6.56 – 6.48 (m, 2 H),
2.37 – 2.32 (m, 6 H). 13C NMR (75 MHz, Acetone-d6) δ, ppm: 140.08,
135.85, 130.42, 130.04, 128.60, 127.80, 126.97, 124.97, 21.24. HRMS
3. Results and discussion
(ESI-TOF) m/z calcd for [M + Ag]+
(Δ = 0,5 ppm).
: 404.9895, found 404.9893
3.1. A model reaction with acetylene: the choice of the catalytic system and
optimization of the reaction conditions
Selective catalytic addition of organic disulfides to unsubstituted
acetylene is complicated by the fact that one of the reagents – acetylene
– in its pure form at room temperature, is a gas. The easiest way of its
addition to reaction is to involve the C2H2 presaturated solvent. It is a
convenient method, which avoids application of complicated equip-
ment (acetylene can be explosive under high pressure in the presence of
metals). The main drawback is that the list of suitable solvents is limited
to ones effectively dissolve acetylene [42]. Commonly disulfides addi-
tion to alkynes is carried out in MeCN [35], DMF [38,43], benzene or
toluene [34,39,44], acetone [33,34] or without a solvent [34,37].
Concentration of dissolved acetylene in the solutions utilized in the
reactions was measured at two representative temperatures at 22 °C and
at 60 °C (Table S1). Under conditions studied here toluene dissolves
only 0.02 mmol/ml of C2H2, which makes it unsuitable for the synth-
esis; the other solvents dissolve about 0.1 mmol/ml of C2H2 at 60 °C.
Among the efficient catalysts for disulfides addition to alkynes our
attention was caught by Ni-based catalytic systems. For the model re-
action (Scheme 1) of acetylene with phenyl disulfide the Ni(acac)2 was
chosen as the catalytic precursor, nevertheless reaction cannot be car-
ried out without a ligand (Table 1, entry 1). Evaluated efficiencies of
the system for different phosphine ligands are given in Tables 1 and S2.
The majority of the ligands were unable to promote catalytic process
(Table 1, entry 2). Some interesting results were obtained for 1,1-bis
(diphenylphosphino)methane (DPPM), with a good yield of the desired
(Z)-1,2-bis(phenylthio)ethene (1a, 48%, Table 1, entry 3). However,
the reaction was not selective and accompanied by formation of a
mixture of poorly identifiable byproducts. Changing the solvent to
acetone or DMF did not help to reduce the amount of byproducts
(Table 1, entries 4 and 5). The competitive formation of 1a and (Z,Z)-
1,4-bis(phenylthio)buta-1,3-diene 2a was observed for PPh3 (Table 1,
entries 6, 7 and 8), with the maximal ratio of 1a: 2a = 10 : 1 obtained
in MeCN (Table 1, entry 6). Attempts to carry out the reaction under
slight acetylene overpressure led to reaction suppression (Table 2, entry
1). The product 1a yield was successfully increased to 76% by pro-
longation of the reaction time (Table 2, entries 2 and 3). The tem-
perature effect was also tested; changing the temperature up to 80 °C
and down to 40 °C led to decreased yield or selectivity of 1a formation
(Table 2, entries 4 and 5). The amount of the PPh3 was varied in the
range from 12 mol% to 60 mol% (Table S3) and it was found that best
selectivity and yield was obtained for 30 mol% of PPh3. The effect of
basic additives was tested, and it was found that the introduction of
inorganic bases such as Cs2CO3, K2CO3, or KOH affected the process
selectivity, yielding a 3:2 mixture of 1a:2a. The utilization of organic
bases (such as pyridine, Et3N, DBU, or PhNMe2) had no significant ef-
fect on the reactivity (Table S4).
2.2.10. (1Z,3Z)-1,4-bis(4-tolylthio)buta-1,3-diene (2c)
White solid. 1H NMR (600 MHz, CDCl3) δ, ppm: 7.29 (d, J = 8 Hz,
4 H), 7.14 (d, J = 8 Hz, 4 H), 6.66 – 6.60 (m, 2 H), 6.36 – 6.30 (m, 2 H),
2.34 (s, 6 H). 13C NMR (151 MHz, CDCl3) δ, ppm: 137.12, 132.05,
130.03, 127.81, 124.28, 21.19. HRMS (ESI-TOF) m/z calcd for [M]+
298.0844, found 298.0829 (Δ = 5 ppm).
:
2.2.11. (1Z,3Z)-1,4-bis((3-bromophenyl)thio)buta-1,3-diene (2d)
Yellow solid. 1H NMR (300 MHz, Acetone-d6) δ, ppm: 7.58 (t, J
= 1.8 Hz, 2 H), 7.51 – 7.40 (m, 4 H), 7.35 (t, J = 7.8 Hz, 2 H), 6.83 –
6.72 (m, 2 H), 6.68 – 6.56 (m, 2 H). 13C NMR (75 MHz, Acetone-d6) δ,
ppm: 138.71, 131.92, 131.72, 130.67, 128.38, 126.78, 126.44, 123.51.
HRMS (ESI-TOF): m/z calcd for [M + Ag]+: 532.7793, found 532.7788
(Δ = 0.9 ppm).
2.2.12. (1Z,3Z)-1,4-bis((4-bromophenyl)thio)buta-1,3-diene (2e)
Yellow solid. 1H NMR (300 MHz, CD2Cl2) δ, ppm: 7.47 (d,
J = 8.5 Hz, 4 H), 7.26 (d, J = 8.5 Hz, 4 H), 6.80 – 6.61 (m, 2 H), 6.44 –
6.28 (m, 2 H). 13C NMR (75 MHz, CD2Cl2) δ, ppm: 135.34, 132.75,
131.32, 126.98, 125.94, 121.23. HRMS (ESI-TOF): m/z calcd for
[M + Ag]+: 532.7793, found 532.7679 (Δ = 21 ppm).
2.2.13. (1Z,3Z)-1,4-bis((2-bromophenyl)thio)buta-1,3-diene (2f)
Yellow solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.57 (dd, J = 7.9,
1.0 Hz, 2 H), 7.37 – 7.27 (m, 4 H), 7.14 – 7.05 (m, 2 H), 7.00 – 6.89 (m,
2 H), 6.43 – 6.31 (m, 2 H). 13C NMR (126 MHz, CDCl3) δ, ppm: 137.15,
133.21, 129.48, 128.15, 127.76, 127.69, 125.73, 123.41. HRMS (ESI-
TOF): m/z calcd for [M + Ag]+
:
532.7793, found 532.7790
(Δ = 0.6 ppm). CCDC-1864716 contains the supplementary crystal-
lographic data for this substance. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data request/cif
2.2.14. (1Z,3Z)-1,4-bis((4-chlorophenyl)thio)buta-1,3-diene (2g)
White solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.30 (s, 8 H), 6.75 –
6.64 (m, 2 H), 6.39 – 6.27 (m, 2 H). 13C NMR (75 MHz, CDCl3) δ, ppm:
134.12, 133.11, 130.79, 129.43, 126.87, 125.50. HRMS (ESI-TOF): m/z
calcd for [M + Ag]+: 444.8803, found 446.8801 (Δ = 0.4 ppm).
2.2.15. (1Z,3Z)-1,4-bis((4-hydroxophenyl)thio)buta-1,3-diene (2h)
1H NMR (300 MHz, Acetone-d6) δ, ppm: 8.61 (s, 2 H), 7.33 – 7.27
(m, 4 H), 6.90 – 6.84 (m, 4 H), 6.54 – 6.42 (m, 2 H), 6.39 – 6.28 (m,
2 H). 13C NMR (75 MHz, Acetone-d6) δ, ppm: 158.23, 133.37, 129.97,
124.77, 123.04, 117.24. HRMS (ESI-TOF): m/z calcd for [M + Ag]+
408.9481, found 408.9494 (Δ = 3,2 ppm).
:
The following conditions were finally accepted for (Z)-1,2-bis(ar-
ylthio)ethene 1 synthesis: Ni(acac)2 with PPh3 in MeCN at 60 °C. The
reaction was set up in screw-capped test-tube by mixing all solid re-
agents, followed by addition of C2H2-presaturated MeCN. To prevent
the loss of C2H2 and, accordingly, to get better yields, the test-tube
should be immediately closed. The developed reaction procedure was
successfully scaled up to 1 mmol of Ph2S2, where the product 1a was
2.2.16. (1Z,3Z)-1,4-bis((4-methoxyphenyl)thio)buta-1,3-diene (2i)
White solid. 1H NMR (300 MHz, CDCl3) δ, ppm: 7.35 (d, J = 8.5 Hz,
4 H), 6.88 (d, J = 8.5 Hz, 4 H), 6.62 – 6.49 (m, 2 H), 6.33 – 6.22 (m,
2 H), 3.81 (s, 6 H). 13C NMR (75 MHz, CDCl3) δ, ppm: 159.38, 132.48,
128.90, 126.04, 123.30, 114.92, 55.52. HRMS (ESI-TOF): m/z calcd for
Scheme 1. The model reaction of diphenyl disulfide addition to
acetylene.
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E.S. Degtyareva et al.
AppliedCatalysisA,General571(2019)170–179
Table 1
Screening of ligands using a selection of solventsa.
Entry
Ligand
Solvent
Ph2S2 conversion, %
0
Yield of 1a (%)b
Yield of 2a (%)b
1
2
–
MeCN
MeCN
0
< 5
0
< 5
P(n-C8H17),
P(o-Tol)3, [Ph2PCH2CH2]2PPh, Cy2PCH2PCy2, PPhMe2, PPh2Me, PCy2(o-Tol), PCy3
3
4
5
6
7
8
DPPMc
DPPMc
DPPMc
PPh3
PPh3
PPh3
MeCN
Acetone
DMF
MeCN
Acetone
DMF
100
100
100
22
22
62
48
29
28
20
18
39
0
0
0
2
4
23
a
Reaction conditions: Ph2S2 (0.2 mmol), Ni(acac)2 (3 mol%), phosphine (30 mol%), C2H2-saturated solvent (1 ml), 3 h, 60 °C.
The yields were determined by 1H NMR.
Full conversion to a complex mixture of unidentified products.
b
c
3.2. Notes on switching between bis(arylthio)ethene and bis(arylthio)-1,3-
butadiene products during the addition of disulfides to acetylene
Table 2
Optimization of the reaction time and temperature for the model reactiona.
Entry
Temperature (°C)
Time (h)
Yield of 1a (%)b
Yield of 2a (%)b
During screening for the optimal reaction conditions for the Ni
(acac)2-catalyzed addition of disulfides to acetylene, a competitive
formation of two different products was observed: 1,2-bis(arylthio)
ethene (1) and 1,4-bis(arylthio)-1,3-butadiene (2, Scheme 2). The de-
veloped systems for selective formation of a single product have much
in common; their selectivities are defined by particular ligands and the
acetylene pressure. When using PPh3 in MeCN, the reaction switches to
formation of 1. For successful synthesis of 2, it is required to replace
PPh3 with PPhCy2 and to use the excess of acetylene, so the best yields
of 2 are obtained in DMF. The use of solvents presaturated with C2H2 is
essential for both syntheses. It is important to note that the synthesis of
2 can be completed in just 1 h.
1
2
3
4
5
60c
60
60
80
40
3
6
0
7
14
15
0
17
24
17
17
56
76
63
17
a
Reaction conditions: Ph2S2 (0.2 mmol), Ni(acac)2 (3 mol%), PPh3 (30 mol
%), MeCN saturated with C2H2 (1 ml).
b
The yields were determined by 1H NMR.
The reaction was carried under 0.045
c
0.005 atm C2H2 overpressure.
isolated in a 74% yield (82% as determined by 1H NMR).
During the screening for optimal ligands, the competitive substrate
transformation yielding 1,3-butadiene 2a was taken into consideration
as an alternative route worth studying in more details. In earlier study
carried out in our laboratory it was discovered that PPhCy2 ligand
specifically favors the formation of 1,3-dienes [35], thus all investiga-
tions of this transformation were carried out with this particular ligand.
Utilization of the reaction conditions earlier optimized for the 1a
synthesis resulted in formation of 10% product mixture (1a:2a = 3 : 7,
Table 3, entry 1). Considering that formation of a diene requires 2
equiv. of acetylene, we increased the volume of the C2H2-saturated
solvent and carried out the reaction under connection to a latex balloon
filled with acetylene (see Section 3.6 for details). This modification
raised the 2a yield to a remarkable 41% (Table 3, entry 2). Replacement
of MeCN with acetone decreased the yield (Table 3, entry 3), but using
DMF instead of MeCN increased the yield to 81% (Table 3, entry 4).
Thus, the optimal reaction conditions for synthesis of 1,4-bis(phe-
nylthio)-1,3-butadiene 2a are Ni(acac)2 with PPhCy2 in C2H2-pre-
saturated DMF under C2H2 pressure at 60 °C. The procedure was suc-
cessfully scaled up to 1 mmol of the disulfide, where the isolated yield
of 2a was 64% (83% as determined by 1H NMR).
With the optimized switchable procedures at hand, the scopes of
both reactions were examined.
3.3. Synthesis of (Z)-1,2-bis(arylthio)ethenes (1)
Performances of various aromatic disulfides in the reaction of
ethene 1 synthesis are shown in Scheme 3. The products were isolated
in moderate to high yields (up to 85%). The directing effect of a sub-
stituent in the aromatic ring of disulfide is significant in the case of 1
synthesis, as the best yields of the products were invariably obtained
with para-substituted disulfides. A shift of the substituent to ortho- or
meta-position dramatically decreases the reactivity potential of dis-
ulfide. For example, despite the 85% yield of (Z)-1,2-bis(4-tolylthio)
ethene 1b, the yield of (Z)-1,2-bis(3-tolylthio)ethene 1c was only 28%,
even with the reaction time extension to 6 days. Moreover, no con-
version was detected in the reaction with (2-TolS)2. The same trend was
observed in the case of electron-withdrawing bromine group, where the
disulfides reactivity deacreased in the following order: p- > > m- > o-
bromophenyl disulfide (products 1d > > 1e > 1f). The products with
electron-donating groups in para-position (1b, 1g) were obtained in
higher yields than the corresponding products with electron-with-
drawing groups (1d, 1h). Interestingly, the utilization of the aliphatic
disulfide (n-butyl disulfide) did not result in the product formation.
Molecular structure of 1g was determined by X-ray crystallography;
Table 3
Optimization of the 2a synthesisa.
Entry Solvent
Yield of 1a (%)b Yield of 2a (%)b Acetylene pressure
1
2
3
4
MeCN (1 ml)
3
7
2
15
7
NO
MeCN (3 ml)
Acetone (3 ml)
DMF (3 ml)
41
15
81
Balloon
Balloon
Balloon
a
Reaction conditions: Ph2S2 (0.2 mmol), Ni(acac)2 (3 mol%), PPhCy2
(30 mol%), C2H2-saturated solvent, 1 h, 60 °C under C2H2 pressure
0.045 0.005 atm.
The yields were determined by 1H NMR.
Scheme 2. Switching between products 1 and 2 of the disulfide catalytic ad-
dition to acetylene.
b
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Scheme 3. The scope of 1,2-bis(arylthio)ethenes (1). Isolated yields (the yields determined by 1H NMR are shown in parentheses). [a] The reaction proceeded for 6
days. [b] The reaction was carried out in acetone due to the low solubility of disulfide in MeCN.
chloride, hydroxyl and methoxy substituents in various position of
phenyl disulfide were studied. No considerable influence of the elec-
tronic properties of a substituent in the aromatic ring on the reactivity
of disulfide was observed. The effect of substituent position in the
synthesis of 2 was even stronger than in the synthesis of 1. The best
yields were obtained in reactions with meta-substituted phenyl dis-
ulfides. Relocation of a substituent from meta- to para- position gen-
erally resulted in lower yields. The presence of a substituent in ortho-
position was fatal to the yields (m- > p- > o-, 2b > 2c and
2d > 2e > 2f, Scheme 4); complete absence of the reactivity was ob-
served for (o-TolS)2. Interestingly, the pure 2h product can be obtained
from (p-HOC6H4S)2 in a yield of 73%, while the counterpart synthesis of
1,2-bis(4-hydroxyphenylthio)ethene from the same disulfide did not
Fig. 1. The structure of 1g, as determined by X-ray crystallography.
the analysis clearly indicated cis-configuration of the product (Fig. 1).
3.4. Synthesis of (Z,Z)-1,4-bis(arylthio)-1,3-dienes (2)
Performances of various aromatic disulfides in the reaction of bu-
tadiene 2 synthesis are shown in Scheme 4. The products were isolated
in moderate to high yields (28–85%). Effects of methyl, bromide,
Scheme 4. The scope of afforded 1,4-bis(arylthio)-1,3-butadienes with corresponding isolated yields (the yields determined by 1H NMR are shown in parentheses).
174

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temperature for 10 min. An aliquot of the resulting suspension was
placed into a microcapsule with an electron-beam permeable mem-
brane [45]. The presence of spherical particles about 500 nm in dia-
meter was observed in the sample. Electron beam irradiation of the
sample area with particles produced the signals in energy dispersive X-
Ray (EDX) spectrum corresponded to carbon, nickel, sulfur and minor
amount of phosphorus (Fig. 3a). Comparison with the background
spectrum of liquid sample area clearly demonstrated that the particles
were enriched with nickel and sulfur, thus most likely they represent
the nickel thiolate polymeric species. The uniform background areas of
the sample formed by solution phase contained carbon, sulfur, phos-
phorous and traces of nickel (Fig. 3b). Obtained results allow to propose
that before the catalysis nickel is accumulated in the solid phase in the
form of [Ni(SPh)2]n coordination polymer. At the first stage of [Ni
(SPh)2]n formation Ni(acac)2 reacted with Ph2S2 with generation of Ni
(SPh)2 species in solution (Scheme 5a). Most likely Ni(II) precursor at
first reduced to Ni(0) by PPh3 (this is a known process, see [46] for
example), followed by oxidative addition of SeS bond. The coordina-
tion between Ni(SPh)2 units led to the formation of particles with the
polymeric structure [Ni(SPh)2]n (Scheme 5a).
Rapid dissolution of the [Ni(SPh)2]n particles was observed under
the electron beam irradiation at relatively high beam current densities.
The complete disappearance of the particles was observed within 10 s of
irradiation (Fig. 3c, Movie S1). Gradual and uniform decrease of the
particles size without notable changes in their shape was observed
during dissolution. Thus, the local heating of the nickel thiolate parti-
cles in the presence of phosphine and disulfide solution led to the
transfer of Ni-containing species to the liquid phase which can be
considered as a direct evidence of catalyst leaching (Scheme 5b). The
dissolution of [Ni(SPh)2]n did not proceed without PPh3.
Fig. 2. The structure of 2e, as determined by X-ray crystallography.
proceed. At the same time, the usage of para-substituted bis(4-meth-
oxyphenyl) disulfide allowed obtaining the products 1g and 2i in 85%
and 66% yields, respectively. The reaction with n-butyl disulfide did not
lead to the product formation.
Molecular structure of 2e was determined by X-ray crystallography;
the study clearly indicated cis, cis-configuration of butadiene skeleton
(Fig. 2).
3.5. Mechanistic studies
Mechanistic features, which govern exclusive formation of products
1 or 2, were of much interest to investigate. Both reactions are nickel-
driven and utilize the same starting materials, which is a great practical
advantage. The main tool of switching among the products of disulfide
addition to acetylene toward a particular product is the variation in
phosphine ligand. Application of PPh3 results in the slow formation of
ethene 1, while PPhCy2 promotes the fast conversion of the reactants
into butadiene 2.
The mechanistic investigation was initially focused on the reaction
of acetylene and phenyl disulfide catalyzed by Ni(acac)2 with PPh3 in
MeCN (Scheme 2A). A ESI-MS monitoring of the model reaction was
carried out and the obtained spectrum is shown on Figure S1. The
dominant peaks correspond to the group of ions [PPh3(SPh)]+ ion (m/
z = 371.1027), [PPh3(SPh)C2H2]+ (m/z = 397.1177), [PPh3(SPh)
(C2H2)2]+ (m/z
=
423.1331) and [PPh3(SPh)(C2H2)3]+ (m/
In order to demonstrate the correlation between the processes in a
microcapsule in liquid-phase SEM and in a test-tube under the reaction
conditions, we isolated the precipitate formed in the reaction mixture
for the 1a synthesis after 3 h at 60 °C. The morphology of the isolated
solid phase was analyzed by SEM (Figure S2). It consisted of small
spherical particles about 25 nm in diameter. EDX study confirmed the
presence of nickel and sulfur in its composition and the absence of
phosphorus. Therefore, the formation of [Ni(SPh)2]n under the catalytic
conditions was confirmed independently by liquid- and solid-phase
SEM studies.
z = 449.1484). Thus, none of the nickel-containing species were de-
tected by the ESI-MS spectrum in positive ion mode. Using a number of
different substrates and ligands more prone to ionization (para-meth-
oxyphenyl disulfide, tris(para-methoxyphenyl) phosphine) led to si-
milar results.
Since nickel-containing ions were not detected in solution by ESI-
MS, we assumed that the catalyst might have heterogeneous contribu-
tion. The precipitate behavior in the liquid phase was studied by liquid-
phase scanning electron microscopy (SEM) (Fig. 3). All components of
the reaction, except acetylene, were mixed and stirred at room
Fig. 3. SEM-EDX study of the nickel thiolate morphology in the liquid medium and its dissolution process. EDX spectra collected from the sample area with round-
shaped particles (a) and from uniform sample area (b). Snapshots taken from the SEM video of particles dissolution under electron beam irradiation (c).
175

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Scheme 5. Evolution of sulfur-containing
nickel species: formation of coordination
polymer from Ni(acac)2 precursor (a) and
leaching of mononuclear nickel species from
[Ni(SPh)2
] in the presence of excess of phos-
n
phine ligand L = PPh3 (b). The following steps
are indicated: i – reduction of Ni(acac)2 by
phosphine ligand and reaction with Ph2S2 to
form Ni(SPh)2 species; ii - coordination of SPh
groups in μ2-mode; iii - formation of polymeric
species via coordination; iv – ligand-induced
reversible leaching process.
The comparison of two types of the precipitate showed a significant
difference in the sizes of the particles. For the evaluation of the size
effect, [Ni(SPh)2]n was synthesized under the conditions identical to the
liquid-phase SEM experiment. The analysis of its morphology and
chemical composition by SEM/EDX showed the formation of spherical
nickel thiophenolate particles with the size of 600–700 nm (Figure S3).
Upon utilization of the obtained [Ni(SPh)2]n as a catalyst in the model
reaction, the yield of the desired product 1a reached 20% after 3 h
which was similar to the Ni(acac)2 system (Table 1, entry 6). SEM
analysis of the solid [Ni(SPh)2]n after the reaction revealed a slight
surface modification (Figure S3), what could be indicative of the only
near-surface layer participation in catalysis. It should be noted that the
PPh3 addition was also required for the reaction of acetylene with
phenyl disulfide catalyzed by [Ni(SPh)2]n.
To establish the role of leaching in the catalytic transformation, we
carried out a hot centrifugation test of the reaction mixture (Fig. 4). The
synthesis of 1a from acetylene and phenyl disulfide catalyzed by the Ni
(acac)2/PPh3 system in acetonitrile was chosen as a model reaction.
After 3 h, the Ph2S2 conversion reached 18%, and the solution was se-
parated from the precipitate. The catalytic activity of the solid was
evaluated in the model reaction that shown high conversion of Ph2S2
into 1a (78%).
18% to 55%. Notably, the formation of a thin layer of brown solid on
the test-tube inner walls was observed. Therefore, the solution was
removed and the test-tube was washed twice with acetonitrile (a solid
layer remained on the walls after washing). Afterwards a new portion of
the reagents and ligand were added. Subsequent heating of the reaction
mixture resulted in formation of alkene 1a (30%) and diene 2a (3%).
The leaching test and liquid-phase SEM demonstrate that catalyti-
cally active species can be transferred from the solid phase to solution
during the leaching process, which is probably initiated by phosphine
ligands (Scheme 5b). According to the experiments, it can be proposed
that the reaction proceeds in the solution (rather than on the surface of
the precipitate) and transfer of metal-containing species between the
solid precipitate and solution is an important feature of the studied
system. It should be mentioned that leaching is most likely a reversible
process and the ratio between solid polymer and soluble species may be
affected by reaction conditions (excess of ligands, solvent, temperature,
etc.).
To get a deeper understanding of the leaching process reversibility
and of the role of thiolates in the catalytic transformation, we in-
troduced two types of SAr groups into the reaction: Sp-BrC6H4 in the
form of nickel thiolate and Sp-ClC6H4 in the form of disulfide. The
transfer of groups with different halide substituent can be easily de-
tected by EDX and SEM analysis of the solid. The freshly prepared
complex [Ni(Sp-BrC6H4)2]n with a characteristic biconcave morphology
The separated solution was bubbled with a new portion of acetylene
and then was heated at 60 °C for 17 h. The yield of 1a increased from
Fig. 4. Mechanistic study of the nature of the catalytic system by using the hot centrifugation leaching test.
176

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Fig. 5. SEM-EDX study of the leaching-re-
deposition process in Ni-catalyzed disulfide
addition to acetylene. Model reaction of p-
chlorophenyl disulfide addition to acetylene in
the presence of [Ni(Sp-BrC6H4)2]n thiolate
precatalyst (a). SEM images of the initial nickel
thiolate (b) and nickel thiolate isolated after
the reaction (c) with corresponding EDX
spectra (d and e, respectively).
(Fig. 5b) was utilized as a catalyst in (p-ClC6H4S)2 addition to acetylene
(Fig. 5a). After 17 h the 54% conversion of (p-ClC6H4S)2 was reached,
and the morphology and composition of the precipitate were in-
vestigated (Fig. 5). The experiment resulted in a complete transforma-
tion of the initial biconcave [Ni(Sp-BrC6H4)2]n morphological patterns
(Fig. 5b) into agglomerated nickel thiolate nanoparticles (Fig. 5c), with
almost complete replacement of Br by Cl on the surface, what was
proved with EDX analysis (compare spectra in Fig. 5d and e). This result
supports the idea of leaching reversibility.
A plausible reaction mechanism was proposed taking into account
the experimental data and catalytic cycles reported earlier
[34,35,37,46–49]. Particularly, two possibilities of the interaction be-
tween the nickel precatalyst and substrates were considered: the reac-
tion with disulfide resulting in the generation of a nickel thiolate
complex 3 (Scheme 6) or the reaction with acetylene leading to the
formation of a metallocycle (Scheme S1). On the basis of a set of ex-
periments showing formation of nickel thiolate complexes the pre-
ference was given to the thiolate-based pathway (Scheme 6). This also
agrees with the reported earlier studies of Ni-based systems
[34,35,37,46–48].
The strength of Ni-P bond in the nickel-phosphine complex with the
more electron-donating PPhCy2 is higher than in the complex with PPh3
[48,50]; thus, the formation of solid [Ni(SAr)2]n is observed for PPh3,
but not for PPhCy2. As we mentioned above (Scheme 5) nickel thiolate
can form a coordination polymer with bridging SAr groups. Involve-
ment in homogeneous catalysis may be achieved under the PPh3 in-
fluence, resulting in the formation of soluble Ni(SAr)2L2 complex 4.
This transformation from a heterogeneous form to a homogeneous one
is reversible and depends on reaction conditions. The PPhCy2 ligand
apparently favor an alternative pathway of the direct catalyst activation
(Scheme 6). The identity of the main product is therefore pre-
determined by the nature of the ligand.
The catalytic cycle starts after the precatalyst activation, which is
followed by coordination of acetylene to nickel and its subsequent in-
sertion into the Ni-S bond (Scheme 6). These events lead to inter-
mediate 5. In the case of lower electron-donating L = PPh3 the inter-
mediate 5 is more prone to bis(arylthio)ethene 1 elimination. Whereas
for L = PPhCy2 the rate of elimination step is lower, thus the reaction
proceeds via the second alkyne insertion into Ni-C bond and results in
formation of bis(arylthio)-1,3-butadiene 2 [51]. The way of insertion of
the second acetylene molecule and the structure of intermediate 6 are
deduced from computational studies [35]. An interesting consideration
concerns life time of metal species in solution phase. In the case of
L = PPh3 ligand nickel intermediate may have a shorter life time, which
may not be sufficient for reaction with the second acetylene molecule.
While for nickel species stabilized with L = PPhCy2 ligand sufficient life
time in solution may provide better opportunities for reaction with
more than one acetylene molecule.
3.6. Synthesis of 1,4-bis(arylthio)-1,3-butadienes using CaC2
Storage and handling of acetylene under high pressure requires
compliance with special safety precautions. Chemical synthesis with the
direct involvement of gaseous C2H2 requires careful optimization of the
reaction conditions and proper selection of a catalyst; for instance, Ag,
Cu and some other contaminants should be avoided to exclude the
formation of explosive acetylenides. Here we avoided usage of excess of
gaseous acetylene and tried alternative experimental approach as well.
A method of synthesis of (Z,Z)-1,4-bis(arylthio)-1,3-butadienes,
which is proposed in the current article, utilizes extra source of acet-
ylene connected directly to a test tube (Fig. 6A). Alternative convenient
way of handling acetylene in the laboratory, is to use solid acetylene
source - CaC2, with direct release of C2H2 into reaction mixture.
The proof-of-principle for selective production of 2a with utilization
of CaC2 was studied here. It employs H-type test-tube with screw caps
(Fig. 6B). One chamber of the H-type test-tube was loaded with the
reaction mixture; another chamber contained CaC2, water and DMF. It
should be noted, that CaC2 with water were used only to provide ad-
ditional pressure of acetylene. The amount of CaC2 was chosen to fit the
same excess pressure range. The acetylene pressure is continuously
maintained by slow release of acetylene from the carbide-and-water
chamber. The reaction was carried out for 1 h at 60 °C and resulted in
products 1a:2a with 6%:69% yields.
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Scheme 6. The plausible mechanism of switchable ethene/butadiene formation.
the phenomenon of dynamic transformations of catalytic centers in
more details.
Acknowledgements
Mechanistic study in the work was supported by the Russian Science
Foundation (project 14-13-01030), synthetic part was supported by
Russian Foundation for Basic Research (project 18-33-01075).
The authors acknowledge Dr. V. Khrustalev for X-Ray study and A.
Tsedilin for ESI-MS measurements.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2018.12.018.
Fig. 6. Design of reactionware for the synthesis of 2a under acetylene pressure.
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