Selective dimerization of terminal acetylenes in the presence of PEPPSI precatalysts and relative chloro- and hydroxo-bridged N-heterocyclic carbene palladium dimers

Article abstract of DOI:10.1016/j.jorganchem.2017.12.041

Highly regio- and stereoselective dimerization of terminal acetylenes occurs in the presence of [PdCl₂(IPr)(3-chloropyridine)], other members of the family of PEPPSI precatalysts and the structurally related N-heterocyclic carbene palladium dim

Full text of DOI:10.1016/j.jorganchem.2017.12.041

Journal of Organometallic Chemistry 856 (2018) 63e69
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Selective dimerization of terminal acetylenes in the presence of
PEPPSI precatalysts and relative chloro- and hydroxo-bridged N-
heterocyclic carbene palladium dimers
Sylwia Ostrowska, Natalia Szymaszek, Cezary Pietraszuk*
ꢀ
ꢀ
Adam Mickiewicz University in Poznan, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly regio- and stereoselective dimerization of terminal acetylenes occurs in the presence of
[PdCl2(IPr)(3-chloropyridine)], other members of the family of PEPPSI precatalysts and the structurally
Received 6 October 2017
Received in revised form
12 December 2017
Accepted 31 December 2017
Available online 3 January 2018
related N-heterocyclic carbene palladium dimers, i.e. [{Pd(
(IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). The reaction leads to exclusive formation of E-
isomer of head to head dimerization product. [{Pd( eOH)Cl(IPr)}2] catalyzes dimerization of a broad
meCl)Cl(IPr)}2] and [{Pd(meOH)Cl(IPr)}2]
m
spectrum of aryl- and silylacetylenes at room temperature and is able to act in base-free conditions.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Acetylene
Dimerization
Palladium
N-heterocyclic carbene complexes
Palladium
1. Introduction
and selectivity. Palladium based catalytic systems have been re-
ported to enable efficient synthesis of both head to head [1,8i,11]
1 The catalytic dimerization of terminal acetylenes is an
attractive method for the preparation of disubstituted 1,3-enynes
(Scheme 1) [1]. Conjugated enynes are of great importance in
organic synthesis [2], medicinal chemistry [3] and material sciences
[4]. In general, catalytic alkyne dimerization produces a mixture of
regio- or stereoisomeric enyne products (Scheme 1) [1]. In some
systems formation of disubstituted buta-1,2,3-trienes has been also
observed [1a,5].
A variety of catalytic systems using various transition metals [4],
main group elements [6] and lanthanides [7] have been reported
over the past few decades. However, there is a limited number of
systems enabling nearly exclusive formation (selectivity exceeding
99%) of single isomers I [4c,8], II [7d-f,9] or III [10] (Scheme 1).
Therefore, the search for new active systems permitting fully se-
lective formation of single isomers for a wide range of acetylenes at
the highest possible catalytic activity is highly desirable.
and head to tail dimers [1,12]. Pd-catalyzed head to head dimer-
ization of terminal alkynes usually leads to mixtures of stereoiso-
meric 1,4-disubstituted enynes (I and II, Scheme 1).
The use of N-heterocyclic carbene complexes contributed to
further progress in catalysis of the reaction. In 2001 Herrmann
reported
that
diiodo(triphenylphosphine){1,3-di[(R)-1-
phenylethyl]imidazol-2-ylidene}palladium(II) are able to catalyze
dimerization of phenylacetylene [13]. Nolan has used Pd(OAc)2 in
combination with different imidazolium chlorides in dimerization
of high range of terminal aryl- and aliphatic terminal alkynes and
shown the dependence of selectivity on steric bulk of NHC pre-
cursors used. In the catalytic system Pd(OAc)2/imidazolium chlo-
ride/Cs2CO3, significant selectivity toward isomer trans of head to
head dimerization product was observed when precursors of steric
IMes (IMes ¼ 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) or
IPr (IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) li-
gands were used [14]. The catalytic system Pd(OAc)2/SIMes∙HCl/
Cs2CO3 permits E-selective head to head polyaddition of 2,7-
diethynyl-9,9-dioctylfluorene [4c]. Gevorgyan has proposed high-
ly regio- and stereoselective palladium-catalyzed head-to-head
dimerization reactions of a high variety of terminal acetylenes. The
use of palladium complex [Pd(IPr)2] in the presence of electron-rich
Within the catalytic systems based on transition metals,
numerous palladium complexes have been found to promote
dimerization of terminal acetylenes with relatively high activity
* Corresponding author.
E-mail address: pietrasz@amu.edu.pl (C. Pietraszuk).
https://doi.org/10.1016/j.jorganchem.2017.12.041
0022-328X/© 2018 Elsevier B.V. All rights reserved.

64
S. Ostrowska et al. / Journal of Organometallic Chemistry 856 (2018) 63e69
Scheme 2. Dimerization of 4-tolylacetylene.
Scheme 1. Dimerization of terminal alkynes.
which allows the reaction to be studied over a wide temperature
range, while the presence of methyl groups permits a convenient
analysis of the reaction mixture by 1H NMR spectroscopy. Treat-
ment of 4-tolylacetylene with 5 mol% of PEPPSI-IPr (1) at 75 ꢀC for
24 h does not lead to any transformation. The addition of the one
equivalent of KOtBu (with respect to the catalyst) led only to a trace
conversion. In the presence of 4 equiv. of KOtBu an immediate
change in color from yellow to orange and then brown was
observed and the GC analysis of the reaction mixture performed
after one hour of the reaction course indicated complete conversion
of the reagent and formation of a single product, which was af-
terwards identified by 1H NMR spectroscopy as (E)-1,4-bis(4-
methylphenyl)-but-1-en-3-yne (Scheme 2).
A similar efficient course of the reaction was also observed in the
presence of an excess of other bases. Optimization experiments
permitted specification of conditions of effective and selective
course of the reaction (Table 1).
Complete conversion was observed when the reaction was
performed in toluene at 60 ꢀC for 1 h by using 1 mol% of complex 1
and the presence of four equivalents of KOtBu relative to the cata-
lyst. From among all bases tested the highest yields were achieved
in the presence of KOtBu and Cs2CO3. Efficient conversion requires
the presence of a fourfold molar excess of the base in relation to
complex 1. The reaction must be performed in an inert atmosphere
as otherwise competitive oxidative coupling of acetylenes is
observed. It has been found that the use of non-dried toluene
and bulky phosphine TDMPP (TDMPP ¼ P[(2,6-OMe)2C6H3]3)
permitted dimerization of terminal acetylenes leading to exclusive
formation of E-isomers of head to head dimers. This reaction has
been demonstrated via PDF calculations to proceed via hydro-
palladation pathway [8e]. Analogous selectivity has been observed
by using a number of N-heterocyclic carbene palladium complexes
[15]. Recently, Nechaev has shown that cinnamyl palladium chlo-
ride complex [PdCl(cinnamyl)(SIPr)] [where SIPr ¼ 1,3-bis(2,6-
diisopropylphenyl)-(4,5-dihydroimidazol-2-ylidene)], used in the
presence of KOH as a base, permitted efficient, regio- and stereo-
selective dimerization of terminal arylalkynes under mild condi-
tions [8c].
In 2005, Organ described a structurally defined group of
[PdCl2(NHC)(pyridine)] complexes, the so-called PEPPSI pre-
catalysts (Pyridine-Enhanced Precatalyst Preparation Stabilization
and Initiation) [16]. These precatalysts exhibit attractive catalytic
properties in
a number of coupling processes [17]. Despite
numerous literature reports regarding the activity of palladium
complexes of the PEPPSI type in a number of C-C bond forming
reactions, there are no reports on their use in the dimerization of
terminal acetylenes.
Herein we report on the high catalytic activity of a series of
PEPPSI precatalysts (1e6, Fig. 1) and relative dimeric chloro- and
hydroxo-bridged complexes (7 and 8, Fig. 1) in homo-dimerization
of aryl- and silylacetylenes. We provide convenient procedures of
efficient, regio- and stereoselective synthesis of E-1,4-disubstituted
but-1-en-3-ynes.
promotes the course of the reaction. The addition of 10 mL of water
to dry and degassed toluene (2 mL) enabled practically quantitative
yields at the loading of complex 1 reduced down to 0.5 mol%
(Table 1, entry 14). Because in the presence of water, KOtBu un-
dergoes hydrolysis, another test another test was carried out, in
which KOH was used instead of KOtBu. Also in this case, the
quantitative yield of the product was observed (entry 15). The ef-
ficiency of the process in the presence of water can be explained by
the improved solubility of KOH in the reaction medium. Perfor-
mance of the reaction in such solvents as hexane or THF enables
high conversion, but the reaction leads to mixtures of isomers
2. Results and discussion
As a model reaction, we have chosen homo-dimerization of 4-
tolylacetylene, a reagent widely tested in the study of this reac-
tion, which makes it possible to compare the results obtained with
literature data. The reactant has a relatively high boiling point,
Fig. 1. PEPPSI precatalysts and related dimeric complexes tested in dimerization of terminal acetylenes.

S. Ostrowska et al. / Journal of Organometallic Chemistry 856 (2018) 63e69
65
Table 1
Dimerization of 4-tolylacetylene in the presence of PEPPSI-IPr (1). Optimization of the reaction conditions.
Entry
Solvent/T [ꢀC]
Cat.1 [mol%]
Base ([Pd]/[base])
Time [h]
Conv. [%]a
E/Z/gemb
1
2
3
4
5
6
7
8
toluene/75
toluene/75
toluene/75
toluene/75
toluene/75
toluene/75
toluene/75
toluene/75
toluene/75
toluene/60
toluene/45
toluene/60
toluene/60
toluene/60
toluene/60
toluene/60
hexane/60
THF/60
5
5
5
5
5
5
5
5
5
5
5
1
0.5
0.5
0.5
0.1
0.5
0.5
0.5
0.5
0.25
0.5
e
24
24
24
1
0
<5
55
93
>99
99
80
52
82
-
E
E
E
E
E
E
KOtBu (1/1)
KOtBu (1/2)
KOtBu (1/3)
KOtBu (1/4)
Cs2CO3 (1/4)
K2CO3 (1/4)
AgOAc (1/4)
KOH (1/4)
KOtBu (1/4)
KOtBu (1/4)
KOtBu (1/4)
KOtBu (1/4)
KOtBu (1/4)
KOH (1/4)
KOtBu (1/10)
KOtBu (1/4)
KOtBu (1/4)
KOH (1/4)
KOH (1/4)
KOH (1/4)
KOH (1/4)
1
24
24
24
6
1
1
1
6
1
1
24
6
6
1
1
74/0/26
E
E
E
9
10
11
12
13
14
15
16
17
18
19
20
21
22
>99
>80
>99
92
E
96/0/4
E
E
95/0/5
92/0/8
78/0/12
E
E
E
E
>99c
>99c
70
89
94
EtOH/60
EtOH/60
EtOH/60
EtOH/22
93
>99c
57
24
24
54
Reaction conditions: solvent (2 mL), argon.
a
Determined by GC analysis.
b
Determined by 1H NMR spectroscopy of the crude reaction mixture.
c
10 mL H2O was added to reaction mixture.
(Table 1, entries 17 and 18). High conversion and selective course of
the reaction was observed by using ethanol as a solvent and KOH as
a base (entry 20). In this case, the efficient progress of the reaction
was observed at 60 ꢀC, at catalyst loading of 0.5 mol% and in the
presence of four equivalents of the base (relative to the catalyst). In
the presence of silver acetate as a base, a decrease in selectivity and
the formation of a mixture of regioisomers is observed (entry 8).
Such phenomenon has been previously described and explained by
Gevorgyan and Ananikov [15]. Lowering the concentration of
complex 1 in the reactions performed in toluene in the presence of
KOtBu, results in a slight decrease in the selectivity of the reaction.
The formation of small amounts of head to tail dimer under the
conditions of reduced reaction rates can be the result of the
competitive reaction pathway, in which OtBuꢁ plays the similar role
to that described by Gevorgyan and Ananikov for carboxylate anion
[15]. In the optimized reaction conditions, the activity of complex 1
was compared with that of selected members of the family of
PEPPSI complexes as well as structurally related chloride (7) [18]
and hydroxide dimers (8) [19] (Fig. 1). Results are collected in
Table 2.
Under the conditions used, no catalytic activity of complexes 5
and 6 was observed. For complex 4 and chloride dimer 7 high
conversions were observed (95%) but the reactions proceeded to
form a mixture of isomers (Table 2, entries 4 and 7). In order to
precisely identify the difference in activities of complexes 1, 7 and
8, the course of dimerization over time was investigated. The re-
action profiles obtained are shown in Fig. 2 and indicate that
complex 8 enables complete conversion already after 15 min of
the reaction. In the presence of complexes 1 and 7 full conversion
was observed after approx. 1 h of the reaction. In the initial stage
of the reaction, dimer 7 exhibited higher activity than complex 1.
The high activity of complex 8 prompted us to look for optimal
reaction conditions in the presence of this catalyst. Results are
collected in Table 3.
Complex 8 was found to exhibit activity in base-free conditions,
however, high catalyst loading (7 mol%) was required to get quan-
titative conversion. The use of four equivalents of the base relative
to 8 ([Pd]/[KOH] ¼ 1/4) allowed efficient progress of the reaction at
a catalyst loading down to 0.25 mol%. The use of ethanol as a sol-
vent and excess of KOH allows the reaction to proceed at room
temperature in the presence of 0.25 mol% of catalyst.
Optimization tests (Tables 1 and 3) permitted specification of
the conditions for efficient progress of the reaction in the presence
of catalysts 1 and 8. In selected optimized reaction conditions
dimerization of a series of arylacetylenes was tested to determine
the versatility of the method. The results are shown in Fig. 3.
For all the arylacetylenes tested, nearly quantitative yields of
products were obtained. The slight differences in catalytic activity
of both palladium complexes tested were observed for more ste-
rically congested acetylenes. In the dimerization of 1-
In the presence of catalysts 1e3 and 8, high acetylene conver-
sion and exclusive formation of E-isomer of head to head dimer
(10a) was observed (Scheme 2).
Table 2
Dimerization of 4-tolylacetylene. Comparison of different palladium catalyst (1e8).
Entry
Cat.
Time [h]
Conv. [%]a
E/Z/gemb
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
1
1
1
24
24
1
>99
90
94
95
0
0
94
>99
100/0/0
100/0/0
100/0/0
80/3/4
e
ethynylnaphthalene,
9-ethynylphenanthrene
and
4-
ethynylbiphenyl catalyzed by 1, efficient conversion required
extension of the reaction time. 1H NMR analyzes of the reaction
mixtures made after completion of the reaction showed in each
case the exclusive formation of head to head dimers with E ge-
ometry around the double bond. Only in the case of phenyl-
acetylene and 9-ethynylphenanthrene, formation of up to 3% of the
Z-isomer was observed.
e
96/4/0
100/0/0
1
Reaction condition: toluene, 60 ꢀC, [Pd] (1 mol %), [Pd]/[KOtBu] ¼ 1/4, argon.
a
Determined by GC analysis.
b
Determined by GC/MS and 1H NMR spectroscopy of the crude reaction mixture.

66
S. Ostrowska et al. / Journal of Organometallic Chemistry 856 (2018) 63e69
Table 3
Dimerization of 4-tolylacetylene in the presence of [{Pd(m-OH)Cl(IPr)}2] (8). Optimization of the reaction conditions.
Entry
Solvent/T [ꢀC]
Cat. 8 [mol %]
[Cat.]/[KOH]
T [h]
Conv. [%]a
E/Z/gemb
1
toluene/60
toluene/60
toluene/60
toluene/60
toluene/60
toluene/60
toluene/60
toluene/60
EtOH/60
7
0.5
1
2
5
e
24
24
24
24
24
24
1
1
1
1
24
2
72
e
100/0/0
e
2c
3c
4c
5c
6c
7c
8c
9
e
e
<5
35
93
>99
>99
>99
95
>99
91
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
100/0/0
e
e
7
-
0.5
0.25
0.25
0.25
0.1
0.25
1/4
1/4
1/4
1/4
1/4
1/4
10c
11c
12c
EtOH/60
EtOH/60
EtOH/22
>99
a
Determined by GC analysis.
Determined by 1H NMR spectroscopy on the crude reaction mixture.
10 mL H2O was added to reaction mixture.
b
c
Further determination of the scope of the reaction revealed the
reactivity of terminal silylacetylenes. The catalytic tests performed
showed completely regio- and stereoselective course of the
reaction (Scheme 3). Table 4 summarizes the results of optimiza-
tion tests for dimerization of triethylsilylacetylene.
Efficient course of silylacetylene dimerization requires higher
catalyst concentrations (up to 2 mol%) and longer reaction times. A
drop in selectivity and formation of a mixture of stereoisomers was
observed when the reaction was run at temperatures exceeding
100 ꢀC. Effect of the addition of small amounts of water was found
to improve the reaction efficiency (compare entries 6 and 7, 8 and
9) as it was observed for dimerization of arylacetylenes. The use of
ethanol as a solvent and KOH as a base allows the reaction to
proceed at room temperature.
Under optimized reaction conditions, high yield and completely
selective course of reaction were observed (Fig. 4). However, sily-
lacetylenes 9k and 9l did not undergo efficient conversion at room
temperature.
From the point of view of the reaction mechanism, alkyne
dimerization catalyzed by palladium systems proceeds through the
SiR3
[Pd]
R3Si
9j-l
R3Si
base
Fig. 2. Reaction profiles. Dimerization of 4-tolylacetylene in the presence of complexes
1, 7 and 8. Reaction conditions: toluene 2 mL, 4-tolylacetylene 0.017 mmol, 60 ꢀC, [Pd]
(1 mol%), [Pd]/[KOtBu] ¼ 1/4, argon.
10j-l
t
SiR3 = SiEt3 (j), SiMe2 Bu (k), SiMe3 (l)
Scheme 3. Dimerization of silylacetylenes in the presence of complexes 1 and 8.
Fig. 3. Dimerization of arylacetylenes in the presence of complexes 1 and 8. Reaction conditions: A toluene, cat. 1 (1 mol%), [Pd]/[KOtBu] ¼ 1/4, 60 ꢀC, 1 h, argon; B toluene, cat. 8
a)
b)
c)
d)
(1 mol%), [Pd]/[KOtBu] ¼ 1/4, 60 ꢀC, 1 h, argon; C EtOH (99.8%), cat. 8 (1 mol%), [Pd]/[KOH] ¼ 1/4, 22 ꢀC, argon; 6 h; 24 h; 3 h; 60 ꢀC.

S. Ostrowska et al. / Journal of Organometallic Chemistry 856 (2018) 63e69
67
Table 4
Dimerization of triethylsilylacetylene in the presence of complexes 1 and 8. Opti-
mization of the reaction conditions.
Entry Solvent
Base
T [ꢀC] Cat. (mol %) t [h] Conv. [%]a E/Zb
1
2
3
4
5
6
7
8
toluene KOtBu 60
toluene KOtBu 60
toluene KOtBu 60
1 (1)
1 (1)
1 (2)
1 (2)
1 (2)
8 (0.25)
8 (0.25)
8 (1)
8 (1)
8 (2)
8 (1)
8 (1)
8 (2)
8 (2)
1
24
2
24
24
24
24
24
24
2
19
75
98
95
E
E
E
E
E
E
E
E
EtOH
EtOH
KOH
KOH
60
22
60
60
60
60
60
100
110
60
22
10
toluene KOH
toluene KOH
toluene KOH
toluene KOH
toluene KOH
toluene KOH
toluene KOH
9
40c
52
9
63c
>99c
92c
75c
>99
>99
E
E
E
10
11
12
13
14
24
6
2
80/20
E
E
EtOH
EtOH
KOH
KOH
24
Reaction condition: [Pd]:[base] ¼ 1:4, argon.
a
Determined by GC analysis.
b
Determined by 1H NMR spectroscopy of the crude reaction mixture.
c
10 mL of degassed water was added to the reaction mixture.
Si
Si
Si
Si
Si
Si
10j: A 98%; B 99%; C 99% 10k: A 92%; B 98%
10l: Ab 92%; Bb 98%
Fig. 4. Dimerization of silylacetylenes in the presence of complexes catalysts 1 and 8.
Isolated yields of products. Reaction conditions: A cat 1 (2 mol%), toluene, 60 ꢀC, [Pd]/
[KOtBu] ¼ 1/4, 2 h, argon; B cat. 8 (2 mol%), toluene, 60 ꢀC, [Pd]/[KOH] ¼ 1/4; 2 h, argon;
a)
b)
C cat 8 (2 mol%), EtOH, 22 ꢀC, 24 h; 24 h; Reaction performed in a closed system.
Scheme 4. Proposed mechanism of dimerization of terminal acetylenes in the pres-
ence of complex 1.
formation of an s-alkynyl/p-alkyne intermediate which undergoes
ligand). Ananikov and Gevorgyan have demonstrated for anionic
palladium(II) NHC complex that the formation of head to head
dimer is kinetically preferred over the head to tail one [15].
Moreover, the calculated molecular structure of transition state of
carbometallation indicates that the preferred formation of the E-
isomer [15] results from the absence of steric interactions between
the phenyl ring of the dimer and the isopropyl groups of the 2,6-
diisopropylphenyl substituent in the IPr ligand. Such steric
crowding would be expected in the case of the formation of Z-
isomer. Selective formation of E-isomer of head to head dimer in
the presence of palladium complexes containing IPr ligands (or
NHC ligands with analogous steric properties) and strongly basic
hydroxides reported by Nechaev [8c] and observed in our systems
are in good agreement with the DFT calculation results reported by
Ananikov and Gevorgyan [15].
subsequent carbopalladation or hydropalladation. For the N-het-
erocyclic palladium complexes, Ananikov and Gevorgyan have
shown by means of DFT calculations a clear kinetic preference for
an alkyne insertion into the Pd-H bond over the Pd-C bond for both
aryl and alkyl acetylenes [8e,15]. In the systems containing basic
carboxylate anion hydropalladation may be deactivated due to the
reaction of the base with hydride ligand. In such systems, the re-
action proceeds via carbometallation pathway [15].
The catalytic systems described in this work contain Pd(II)
complexes in the presence of small excess (relative to the catalyst)
of tert-butoxide or hydroxide base. In such a system deactivation of
the hydropalladation pathway is to be expected. Indeed, all at-
tempts to detect palladium hydride species in a system containing
catalyst 1 or 8, KOtBu (4 equiv.) and fivefold excess of p-tolylace-
tylene in C6D6 using 1H NMR spectroscopy have failed.
According to the proposed mechanism (Scheme 4) migratory
insertion is followed by electrophilic substitution of sigma bonded
enyne in complex B with another acetylene molecule with the
formation of complex C. In the presence of a strongly donating NHC
ligand an increased preference for electrophilic substitution should
We proposed that the formation of
s-alkynyl palladium(II)
complexes (A, Scheme 4) from chloride palladium(II) complexes of
PEPPSI type, in the presence of KOH or KOtBu, proceeds as a result
of electrophilic substitution and is preceded by the formation of
palladium hydroxide or tert-butoxide complexes (Scheme 4). Our
previous studies confirmed the formation of hydroxide complexes
in a system containing PEPPSI-IPr and an excess of KOH [19].
Monitoring of the reaction of complex 8 with 5 equiv. of tolylace-
tylene (C6D6 at 60 ꢀC) shows minor (9%) decrease in the intensity of
the signal of hydroxide ligand and formation of water. Formation of
be expected. Substitution of
p-bonded enyne with another acety-
lene molecule liberates the product and regenerates complex A.
The results presented in this work confirm the high potential of
N-heterocyclic carbene palladium complexes as catalysts for the
dimerization of terminal acetylenes reported by Nolan, Ananikov
and Gevorgyan, Nechaev and others [e.g. Refs. [8b,e,14,15]]. By
evidencing the utility of commercially available, cost competitive,
highly air and moisture stable, user friendly one component PEPPSI
precatalysts in efficient and selective dimerization of acetylenes,
we have increased the range of available catalysts of the process
and the number of effective procedures. The procedures described
allow selective syntheses of E-1,4-disubstituted but-1-en-3-ynes
already at room temperature, using small catalyst loadings.
p
-acetylene complex from 1 proceeds via substitution of pyridine
ligand. 1H NMR monitoring of the substitution process shows the
presence of only minor amount of free pyridine, which suggests
that the substitution of pyridine by acetylene is an equilibrium
process.
Once the palladium(II)
4) is formed -coordinated acetylene undergoes migratory inser-
tion into the palladium carbon bond (in the absence of hydride
s-alkynyl/p-alkyne complex (A, Scheme
p

68
S. Ostrowska et al. / Journal of Organometallic Chemistry 856 (2018) 63e69
Complex 8 allows successful dimerization in base-free conditions.
For the first time the possibility of selective dimerization of silyla-
cetylenes in the presence of N-heterocyclic palladium complexes
has been shown.
Then palladium complex (2 mol% in relation to acetylene) and
KOtBu ([Pd]/[KOtBu] ¼ 1/4) was added. The reaction mixture was
heated at 60 ꢀC for 24 h. Then the solvent was evaporated under
reduced pressure and the residue was purified by column chro-
matography (silica gel 60, hexane to hexane/CH2Cl2 ¼ 4/1). Evapo-
ration of the solvent gave an analytically pure product.
3. Conclusions
Convenient and highly efficient procedure for synthesis of E-1,4-
disubstituted but-1-en-3-ynes via dimerization of terminal aryl-
and silylacetylenes in the presence of PEPPSI precatalyst and
Acknowledgment
The research was co-financed by the National Science Centre
(Poland), project UMO-2013/11/N/ST5/01612.
structurally related palladium dimers [{Pd(meCl)Cl(IPr)}2] and
[{Pd( eOH)Cl(IPr)}2] was proposed.
m
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