A practical ruthenium based catalytic system bearing a switchable selectivity between the dimerization and cyclotrimerization reactions of alkynes

Article abstract of DOI:10.1016/j.apcata.2012.05.020

In this study, a practical and inexpensive switchable catalytic system (cyclotrimerization vs. dimerization), [RuCl₂(p-cymene)] ₂/PR₃ has been developed for the catalytic dimerization of terminal alkynes. Bulky and basic phosphine derivatives, PCy₃ and P(^i-Pr)₃, were used with [RuCl₂(p-cymene)] ₂ and excess of terminal alkyne to in situ formation of vinylidenic intermediates which are active towards dimerization reactions. Effect of phosphine/ruthenium ratio has been investigated. A solvent study was carried out and toluene was found to be the most versatile solvent for both cyclotrimerization and dimerization reactions. A set of aryl and alkyl acetylenes were chosen as substrates to investigate the effect of the nature of the substrates on alkyne dimerization reactions catalyzed by [RuCl ₂(p-cymene)]₂/PR₃. In conclusion, we have shown that [RuCl₂(p-cymene)]₂/PCy₃ can be used as a practical and inexpensive catalytic system which has a switchable selectivity towards cyclotrimerization and dimerization reactions. Best results in means of regioselectivity and yield were observed by using arylacetylene derivatives in these reactions. This catalytic system emerges as an economical method for the transformation of arylacetylenes to corresponding enyne and arene derivatives in excellent yields and selectivity.

Full text of DOI:10.1016/j.apcata.2012.05.020

Applied Catalysis A: General 433–434 (2012) 214–222
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Applied Catalysis A: General
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A practical ruthenium based catalytic system bearing a switchable selectivity
between the dimerization and cyclotrimerization reactions of alkynes
∗
˙
Bengi Özgün Öztürk, Solmaz Karabulut , Yavuz Imamog˘lu
Hacettepe University, Department of Chemistry, 06800 Beytepe, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a practical and inexpensive switchable catalytic system (cyclotrimerization vs. dimeriza-
tion), [RuCl2(p-cymene)]2/PR3 has been developed for the catalytic dimerization of terminal alkynes.
Bulky and basic phosphine derivatives, PCy3 and P(i-Pr)3, were used with [RuCl2(p-cymene)]2 and excess
of terminal alkyne to in situ formation of vinylidenic intermediates which are active towards dimerization
reactions. Effect of phosphine/ruthenium ratio has been investigated. A solvent study was carried out and
toluene was found to be the most versatile solvent for both cyclotrimerization and dimerization reac-
tions. A set of aryl and alkyl acetylenes were chosen as substrates to investigate the effect of the nature of
the substrates on alkyne dimerization reactions catalyzed by [RuCl2(p-cymene)]2/PR3. In conclusion, we
have shown that [RuCl2(p-cymene)]2/PCy3 can be used as a practical and inexpensive catalytic system
which has a switchable selectivity towards cyclotrimerization and dimerization reactions. Best results
in means of regioselectivity and yield were observed by using arylacetylene derivatives in these reac-
tions. This catalytic system emerges as an economical method for the transformation of arylacetylenes
to corresponding enyne and arene derivatives in excellent yields and selectivity.
Received 24 March 2012
Received in revised form 2 May 2012
Accepted 16 May 2012
Available online 24 May 2012
Keywords:
Alkyne dimerization
Ruthenium
Enynes
Switchable catalysis
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
regio- and stereoselectivity towards one isomer. To date, several
transition metal complexes have been reported to catalyze the
Transition metal catalyzed carbon carbon bond formation reac-
tions have proven to be highly selective, and atom-economical
synthetic methods have been used to obtain novel organic inter-
mediates which cannot be obtained by other means [1]. Alkynes
undergo a great diversity of reactions in the presence of transi-
tion metal catalysts, thus making them good candidates as starting
materials for the synthesis of both organic and organometallic sub-
stances [2]. The catalytic conversion of alkynes yields important
organic intermediates like enynes, enol-esters and arene deriva-
tives [3].
Alkyne dimerization reactions are of essential importance
because the resulting enyne substructures are important build-
ing blocks for organic synthesis and are significant components
in various biologically active compounds [4]. Enynes are also
potential candidates for making conjugated and light-emitting
polymers [5]. It has been reported that enynes can also be
used for alkyne dimerization reactions form several possible iso-
dimerization reactions of alkynes in a selective manner [6]. Among
these transition metals, recent studies have focused on ruthenium
complexes because they exhibit high functional group tolerance
and a wide range of oxidation states and coordination geometries,
providing unique properties for catalysis [2a]. A wide range of very
different (mechanistically) processes are catalyzed by ruthenium
complexes using these unique properties. Each carbon carbon
bond forming reaction has a unique intermediate and mechanism.
For example, alkyne dimerization reactions are known to be cat-
alyzed by the formation of vinylidene or acetylide intermediates
[7], whereas inter- or intramolecular [2 + 2 + 2] cyclotrimerization
reactions of alkynes proceed by the formation of a metallocyclopen-
tadiene intermediate [8] or in the presence of a metal alkylidene
moiety, the reaction proceeds via cascade metathesis reactions [9].
Due to the growing interest in this field, several research groups
have focused their attention on the dimerization reactions of ter-
minal alkynes catalyzed by various ruthenium complexes [10].
Pavlik et al. reported the activity of neutral ruthenium vinyli-
dene ␴-alkynyl complexes on alkyne dimerization reactions [11].
Katayama and Ozawa [12] reported an efficient protocol to convert
[RuCl2(p-cymene)]2, in the presence of basic and bulky phosphine
ligands and terminal alkynes, to corresponding ruthenium vinyli-
mers, among them, (Z)-RCH CH
C CR, (E)-RCH CH C CR and
(gem) H2C CR CR are the most common ones. For real and
C
practical applications, one desires a process that exhibits both high
dene complexes with the general formula RuCl2{
C
C(E)R}(L)2,
∗
Corresponding author. Tel.: +90 312 297 60 82; fax: +90 312 299 21 63.
E-mail address: solmazk@hacettepe.edu.tr (S. Karabulut).
where E = H; R = Ph, and Fc, t-Bu and L = PCy3 and P(i-Pr)3. These
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.020

B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
215
vinylidene complexes were successfully applied to the homo-
and cross-dimerization reactions of terminal alkynes resulting in
conjugated enyne products with excellent yields and regioselec-
tivities [13]. Opstal and Verpoort [14] also showed that these
ruthenium moieties are versatile catalysts for alkyne dimerization
and enol-ester synthesis reactions. [RuCl2(p-cymene)]2 dimer dis-
solved in acetic acid was reported to promote the formation of
linear enynes with high E-selectivity under phosphine free con-
ditions [15a]. The same complex exhibits even higher activity, and
larger substrate scope, when added to terminal alkynes obtained
in situ from the corresponding trimethylsilyl derivatives, in a one
pot desilylation–dimerization sequence [15b]. The hexamethyl-
benzene ruthenium(I(I) dimer [{RuCl(␮-Cl)(␩6-C6Me6)}]2 was also
reported to promote the dimerization reactions of terminal alkynes
with high yields and E-selectivities in mixed acetic acid-aqueous
media [16].
Although the catalytic dimerization of alkynes has been inten-
sively studied, only a few reports in the literature have achieved an
efficient catalytic process that has a switchable selectivity between
the cyclotrimerization and dimerization reactions of alkynes.
Recently, the successful application of an organolanthanide-based,
binary metallic system with switchable selectivity from dimeriza-
tion to cyclotrimerization reactions was reported [17]. Hilt et al.
[18] investigated solvent and ligand effects on cobalt-catalyzed
cyclotrimerization and dimerization reactions. The reaction of a
CoBr2(dppe)/Mg catalyst system led to linear enyne dimerization
products with terminal alkynes, whereas the usage of a Lewis
acid instead of Mg led to cyclotrimerization products. A simi-
lar approach, which utilized a multimetallic zirconocene based
catalytic system on both the dimerization and cyclotrimerization
reactions of alkynes, was also reported [19].
phosphine were purchased from Sigma–Aldrich and used as
received. (PCy3)2RuCl2 CPhH and Ru(Cl2)(p-cymene)(PCy3)
C
were synthesized according to the literature procedure [21d].
Dichloro(p-cymene)ruthenium(I(I) dimer was purchased from
Strem and used as received. Toluene, THF and acetone were
purchased from Sigma–Aldrich. Toluene, THF was distilled under
Na/benzophenone, acetone was distilled under CaCl2 and stored
under an inert atmosphere of nitrogen.
2.2. Instrumentation
1H, 13C and 31P NMR spectra were recorded at 25 ◦C with a
Bruker GmbH 400 MHz high performance digital FT NMR spectrom-
eter using CDCl3 as solvent. Tetramethylsilane was used for the 1
H
and 13C NMR reference. H3PO4 was used for the 31P NMR refer-
ence. GC–MS analyses were performed with a Shimadzu GC–MS
QP5050A using an Optima column, 5–1.0 ␮m (50 m × 0.32 mm) and
a temperature range of 50–320 ◦C (10 ◦C/min). The carrier gas was
helium with a flow rate of 1 ml/min.
2.3. General procedure for the cyclotrimerization reactions of
terminal alkynes
A reactor was charged with [RuCl2(p-cymene)]2 (0.033 mmol,
0.020 (g) and phenylacetylene (1a, 0.825 mmol, 90 ␮l) in 1 ml
toluene and stirred at 80 ◦C under a nitrogen atmosphere. Aliquots
taken periodically from the reaction mixture were analyzed by
GC–MS. The solvent was evaporated and crude product mixture
was passed through a plug of silica gel using n-hexane to remove
by-products and catalytic impurities. The final product was isolated
as a mixture of 1,2,4-trisubstitued (2a) and 1,3,5-trisubstitued (3a)
arene isomers. The mixtures were analyzed by 1H and 13C NMR.
As previously mentioned, commercially available [RuCl2(p-
cymene)]2 is the key starting material for synthesizing ruthenium
vinylidene intermediates in
a convenient and easy fashion.
2.4. General procedure for the dimerization of terminal alkynes
Recently, our group reported that [RuCl2(p-cymene)]2 and its
homobimetallic alkylidene analogs can be used as an efficient cata-
lyst for the transformation of terminal alkynes to arene derivatives
by the intermolecular [2 + 2 + 2] cyclotrimerization reactions [20].
The [RuCl2(p-cymene)]2/PR3 catalytic system was previously used
for selective enol-ester synthesis [21a], ring opening metathesis
[21b–e] and atom transfer radical addition and polymerization
[21f–j] and microwave-assisted enol-ester synthesis reactions
[21k]. From this starting point, we envisioned that a protocol route
can be developed to obtain an in situ, switchable catalytic sys-
tem bearing high selectivity between cyclotrimerization and alkyne
dimerization reactions in the presence of [RuCl2(p-cymene)]2/PR3
(R: cyclohexyl, isopropyl) and excess alkyne. Because the selectiv-
ity of the reaction (dimerization vs. cyclotrimerization) depends
highly on the intermediate formed by the substrate and the
transition metal, the formation of metallocyclopentadiene and
vinylidene intermediates can be controlled by introducing steric
and bulky phosphine ligands to the medium. In this study, we
report a highly selective catalytic system with selectivity towards
cyclotrimerization and dimerization reactions that can be switched
in a controlled manner.
A reactor was charged with [RuCl2(p-cymene)]2 (0.033 mmol,
0.020 g), phenylacetylene (1a) (0.825 mmol, 90 ␮l) and PCy3
(0.33 mmol, 0.093 (g) in 1 ml toluene and stirred at 80 ◦C under a
nitrogen atmosphere. The solvent was evaporated and crude prod-
uct mixture was passed through a plug of silica gel using n-hexane
to remove by-products and catalytic impurities. The final product
was isolated as a mixture of 4a and 4b. The mixtures were analyzed
by 1H and 13C NMR.
2.5. General procedure for the in situ switching between
cyclotrimerization and dimerization reactions
A reactor was charged with [RuCl2(p-cymene)]2 (0.033 mmol,
0.020 (g) and phenylacetylene (1a) (0.825 mmol, 90 ␮l) in 1 ml
toluene and stirred at 80 ◦C. After 1 h, 10 mol equivalent of
PCy3(0.33 mmol, 0.093 (g) in 1 ml toluene was added to the reaction
medium and stirred at 80 ◦C under a nitrogen atmosphere. Aliquots
taken periodically from the reaction mixture were analysed by both
1H NMR, 31P NMR and GC–MS.
3. Results and discussion
2. Experimental
To obtain a ruthenium based catalytic system bearing switch-
able selectivity towards alkyne dimerization and cyclotrimeriza-
tion reactions, we proposed using [RuCl2(p-cymene)]2/PR3 (R:
Cy, iPr) as an active catalyst system. We hypothesized that
[RuCl2(p-cymene)]2 catalyzed cyclotrimerization reactions of ter-
minal alkynes could be switched to alkyne dimerization reactions
by the addition of excess phosphine derivatives, yielding vinyli-
dene species which are believed to be the reaction intermediates
2.1. Materials
All manipulations were carried out under an inert atmosphere
of nitrogen using Schlenk techniques. Phenylacetylene (1a), 4-
ethynyl-toluene (1b), 2-ethynyl-toluene (1c), 4-ethynyl-anisole
(1d), 2-ethynyl-anisole (1e), tert-butyl phenylacetylene (1(f)
1-octyne (1g), 3,3-dimethyl-1-butyne (1h) and tricyclohexyl

216
B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
Scheme 1. A representative ruthenium catalyzed cyclotrimerization reaction.
in alkyne dimerization. For this purpose, we systematically inves-
tigated the following reaction conditions. First, the optimum
reactions conditions for [RuCl2(p-cymene)]2 catalyzed alkyne
cyclotrimerization reactions were determined. Second, the effect
of varying the temperature and the phosphine/ruthenium ratio on
the selectivity of the catalytic system was investigated.
To optimize the reactions conditions, two representative exam-
ples of alkyl and aryl acetylenes, 1-octyne and phenylacetylene,
were selected as starting test materials. Alkynes were reacted in
the presence of 4 mol% [RuCl2(p-cymene)]2 in toluene (Scheme 1)
and the results are listed on Table 1. The reactions were monitored
by using GC–MS by taking aliquots from the reaction mixture. After
no improvement in yield was observed, the solvent was evaporated
and the organic products were purified by passing the reaction mix-
ture through a silica column, using hexane as eluent. The resulting
product mixture was found to be an isomeric mixture of 1, 2, 4
and 1, 3, 5 substituted benzene derivatives, as confirmed by both
GC–MS and NMR analyses.
2a; 90: 10) in the same catalytic loading. THF and acetone were
found to be the most versatile solvents for the transformations of
arylalkynes to the corresponding benzene derivatives in a selective
manner.
Next, the effect of the addition of phosphine derivatives (PCy3
and P(i-Pr)3) on the selectivity of the reaction was investigated
by varying the PR3/Ru ratio (Scheme 2). The first reactions were
carried out in THF and acetone at 65–55 ◦C, respectively. A solu-
tion of phenyl acetylene was treated with [RuCl2(p-cymene)]2 and
PCy3 in a ratio of PCy3/Ru: 10/1 in THF at 65 ◦C, and the reaction
was analyzed periodically by GC–MS. During the early stage of the
reactions (where the conversion of 1a was 10%), the predominant
reaction product was 1,2,4-triphenyl benzene (2a; 9%) with traces
of 1,3,5-triphenylbenzene (3a; 1%). After 1 h, a peak in the forma-
tion of the dimerization products was observed (4a and 5a; 45%)
and increased regularly until the 1a was totally consumed after
6 h. At this stage, GC–MS analysis revealed that the reaction mix-
ture contained dimerization product of phenyl acetylene (88%; 4a:
5a; 95: 5) and the cyclotrimerization products of phenyl acetylene
(12%; 2a: 3a; 90: 10). The reactions carried out with acetone gave
similar results and was completed in 7 h. Subsequent GC–MS analy-
sis indicated the formation of cyclotrimerization and dimerization
products in 14% (2a: 3a; 87: 13) and 86% (4a: 5a; 93: 7) yields,
respectively.
Although the observed yields are quite high in the reactions
using THF and acetone, the formation of cyclotrimerization prod-
ucts (2a + 3a; % 12 for THF and % 14 for aceton(e) is a serious
drawbackwhen consideringthatthe goal ofthis studywas to design
a catalytic system that had switchable selectivity. A possible expla-
nation for this trend may be that the effect of the temperature on
the formation rate of vinylidene complexes. As indicated above,
the reactions were carried out at 65 ◦C in THF, 55 ◦C in acetone.
Formation of vinylidene complexes requires high temperature and
among these conditions (in the case of THF and acetone), formation
rate of vinylidene species may not be sufficient enough to suppress
cyclotrimerization reactions. Another possible explanation for this
trend may be the faster dissolution of [RuCl2(p-cymene)]2 dimer
in both acetone and THF compared to toluene. The faster dissolu-
tion of THF and acetone may allow the cyclotrimerization process
to start more rapidly. Subsequently, toluene was employed as the
reaction solvent at 80 ◦C in the presence of 4% [RuCl2(p-cymene)]
with phenyl acetylene and PCy3 in a ratio of (PCy3: Ru; 10: 1). After
8 h, phenyl acetylene was totally consumed, and the formation of
Alkynes were reacted at 25 ◦C, 50 ◦C, 80 ◦C and 100 ◦C in toluene.
The reaction proceeded poorly at 25 ◦C, yielding the correspond-
ing 1, 2, 4 arene derivative as the major isomer at a 20% (2a: 3a;
89: 11) yield for phenyl acetylene. This yield value was 5% (2g:
3g; 71: 28) for 1-octyne. Significant improvements in yield values
were observed by increasing the reaction temperature to 50–80 ◦C.
A maximum yield of 99% (2a: 3a; 90: 10) was observed at 80 ◦C, and
increasing the temperature up to 100 ◦C gave the same result. No
significant change in isomeric product distribution was observed
by varying the temperature in the range of 25–100 ◦C.
After the optimum reaction temperature was determined, a sol-
vent study was carried out to determine the best solvent for the
reaction in means of the regioselectivity and yield. THF, acetone and
toluene were employed as reaction solvents. The reactions were
carried out in the presence of 4% Ru at the reflux temperatures of
acetone and THF, and for toluene the reaction was carried out at
80 ◦C.
Among these solvents, the polar coordinating solvents THF and
acetone gave the best yields, regioselectivities, and reaction times.
The reaction of phenyl acetylene in the presence of 4% [RuCl2(p-
cymene)] was complete within 2 h at 65 ◦C in THF (yield 100%, 2a:
3a; 92: 8) and 2.5 h at 55 ◦C in acetone (yield 100%, 2a: 3a; 90:
10). The cyclotrimerization reaction of phenyl acetylene carried out
with toluene required a relatively long reaction time to complete.
At 80 ◦C, a 99% yield was observed with high regioselectivity (1a:
Scheme 2. Representative dimerization reactions of alkynes 1a–h.

B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
217
Table 1
Cyclotrimerization reactions of terminal alkynes.
Runa
Alkyne
Time
Yieldb (%)
2a–h (%)
3a–h (%)
1
2
1a
1b
5 h
8 h
99 (90)
99 (91)
90
91
10
9
Me
Me
3
4
1c
8 h
6 h
95 (87)
99 (90)
80
87
20
13
MeO
1d
OMe
5
1e
6 h
93 (85)
84
16
t-C4H9
6
7
1f
8 h
99 (90)
80 (73)
85 (80)
92
75
80
8
25
20
C6H13
t-C4H9
1g
1h
10 h
10 h
8
a
All reactions were carried out at a catalytic loading of 4% [RuCl2(p-cymene)]2 in toluene at 80 ◦C.
Yields and isomer ratios were determined by GC–MS and 1H NMR methods. Isolated yields is in parantheses.
b
93% (4a: 5a; 94: 6) dimerization products and 7% cyclotrimeriza-
tion products was observed. Among the selected solvents, toluene
gave the minimal amount of cyclotrimerization product, exhibiting
high selectivity towards alkyne dimerization reactions. Although
the reaction proceeds smoothly in both THF and acetone, toluene
seems to be the most appropriate solvent for the selective forma-
tion of dimerization products. Considering the need for a relatively
high temperature and the presence of a non-coordinating solvent
for the formation of ruthenium vinylidene species, toluene seems
to be the most suitable solvent.
The effect of the amount of phosphine derivatives on the selec-
tivity of the reaction was investigated by performing the catalytic
reaction with varying PCy3/Ru ratios (mol/mol) of 40/1; 20/1; 10/1;
5/1; 2/1; and 1/1, and the results are listed in Table 2 (Scheme 3).
At the PCy3: Ru; 1:1 ratio, a yield of 75% (2a: 3a; 93: 7) towards
the cyclotrimerization product of phenylacetylene was obtained.
At a 2:1 PCy3/Ru ratio, the reaction resulted in a mixture of both
cyclotrimerization (64%, 2a: 3a; 89: 11) and dimerization (36%,
4a: 5a; 94: 6) products. At this phosphine/ruthenium ratio, forma-
tion of monometallic Ru(Cl2)(p-cymene)(PCy3) was anticipated. To
further validate this observation, Ru(Cl2)(p-cymene)(PCy3) were
synthesized and employed as reaction catalysts under the same
reaction conditions. Similar results were obtained with Ru(Cl2)(p-
cymene)(PCy3) catalyzed reactions which give cyclotrimerization
product in 70% yield (2a: 3a; 87: 13) and dimerization product
in 30% yield (4a: 5a; 93:7). In addition, the signal at 20.5 ppm
in 31P NMR of the reaction mixture confirmed the formation of
Ru(Cl2)(p-cymene)(PCy3), which then slowly disappeared due to
the coordination of the alkyne into the metal center. At this point,
an increase in the signal corresponding to the free PCy3 ligand was
observed.
towards one reaction. The preceding experiments indicated that
the selectivity of the system strictly depends on the presence of
excess phosphine in the reaction media. The desired selectivity was
obtained by performing the reaction with a PCy3/Ru ratio of 10/1.
At this ratio, the minimum amount of cyclotrimerization products
2a and 3a were formed (7%). Under these conditions, the dimer-
ization of phenyl acetylene accounted for 93% of the product yield
(4a:5a; 94:6). No improvement in yield or selectivity was observed
by increasing the phosphine/ruthenium ratio from 20:1 to 40:1.
At the early stages of the reaction, the in situ formation
of Ru(Cl2)(p-cymene)PCy3 was observed at all varying phos-
phine/ruthenium ratios. As shown in Table 1, the RuCl2(p-cymen(e)
dimer successfully transforms phenyl acetylene into the cor-
responding arene derivatives. Detailed studies of this reaction
showed us that the cyclotrimerization product forms only at
the very beginning of the reaction, and the catalytic precursors
[RuCl2(p-cymene)]2 and Ru(Cl2)(p-cymene)PCy3 may be respon-
sible for this trend. As shown in Table 1, [RuCl2(p-cymene)]2
converts phenyl acetylene into the corresponding arene deriva-
tives without any side reactions, such as dimerization. On the
other hand, Ru(Cl2)(p-cymene)PCy3 resulted in an 88% (2a: 3a;
85: 15) cyclotrimerization and 12% (4a: 5a; 80: 20) dimerization
product; therefore, changing the starting material from [RuCl2(p-
cymene)]2 to Ru(Cl2)(p-cymene)PCy3 may minimize the amount
of cyclotrimerization product formed. From this point of view, we
foresee that a binary mixture of Ru(Cl2)(p-cymene)PCy3 and 5 mol
equivalent PCy3 may result in a more selective processes. The reac-
tion of Ru(Cl2)(p-cymene)PCy3 and 5 mol equivalent PCy3 with
phenyl acetylene in the presence of toluene at 80 ◦C resulted in
a 98% yield of dimerization product (4a: 5a; 91: 9) and a 2% yield
(2a + 3a) of cyclotrimerization product as we expected.
The phosphine/ruthenium ratio was increased to 5:1 to pro-
mote the in situ formation of diphosphine coordinated ruthenium
vinylidene species, which are predicted to react rapidly with the
excess phenyl acetylene in the reaction media. Although the for-
The next set of experiments was carried out with P(i-Pr)3 instead
of PCy3. Similar promising results were obtained in terms of both
yield and regioselectivity (Table 2). Although the catalytic system
[RuCl2(p-cymene)]2/P(i-Pr)3 catalyzes these reactions in relatively
high yields and exhibits a slightly high regioselectivity, the air and
moisture sensitivity of P(i-Pr)3 emerges as a serious disadvantage
mation of ruthenium vinylidene species was observed by both 1
H
and 31P NMR, the reaction still did not display a distinct selectivity

218
B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
Table 2
Reactions of phenylacetylene in the various PR3/Ru ratios.
Runa
PR3/Ru (mol/mol)
Time (h)
Dimerization yield (%)b (4a:5a:6a)
Cyclotrimerization yield (%)b (2a:3a)
1
2
3
4
5
6
7
8
9
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
40
20
10
5
2
1
40
20
10
5
2
1
12
12
12
12
12
12
10
10
10
10
10
10
90 (90:10:0)
93 (93:7:0)
93 (94:6:0)
45 (95:5:0)
36 (94:6:0)
25 (84:16:0)
89 (89:8:3)
91 (93:6:1)
94 (90:5:5)
50 (93:6:1)
32 (92:8)
10 (89:11)
7 (90:10)
7 (90:10)
55 (90:10)
64 (89:11)
75 (93:7)
11 (90:10)
9 (91:9)
6 (89:11)
53 (89:11)
65 (87:13)
73(93:7)
P(i-Pr)3
P(i-Pr)3
P(i-Pr)3
P(i-Pr)3
P(i-Pr)3
P(i-Pr)3
10
11
12
20 (82:18)
a
Reactions were carried out at in the presence of [RuCl2(p-cymene)2] (0.02 g, 0.033 mmol) and in various phosphine amounts at 80 ◦C in 1 ml of toluene. Yield and
selectivities were determined by GC–MS analysis.
b
GC yield.
when considering the practical aspects of the reaction. PCy3 was
found to be much more stable to the presence of air and moisture
than P(i-Pr)3. Hence, PCy3 was used in further in situ switching
studies.
The design of this study was based on the in situ forma-
tion of ruthenium vinylidene complexes with the general formula
was formed upon reaction of para-substituted alkynes, 1b, 1d and
1f with [RuCl2(p-cymene)]2 dimer and PCy3 for 4 h gave relatively
high yields and selectivity in the dimerization reactions of pheny-
lacetylene and in all cases, no traces of cross-dimerization product
was observed. The catalytic system bearing ortho-substituted
alkynes 1c and 1e as vinylidene ligands showed relatively poor yield
and selectivity. After addition of phenylacetylene, the resulting
mixtures were reacted for 16 h until no improvement in the yields
(60% 4a: 5a: 6a; 2: 75: 23) for 1c and 50% 4a: 5a: 6a; 0: 77: 23 for 1(e)
were observed. This sharp decrease in the yield of the dimerization
reactions is probably due to incomplete formation of vinylidene
intermediates due to the steric hindrance of ortho-subtituents in
terminal alkynes which makes the coordination of terminal alkyne
into the metal center more difficult. RuCl2(p-cymene)PCy3, which
is formed by subsequent addition of PCy3 and could not be reacted
with ortho-substitutedalkynetoform vinylidene intermediate may
responsible for the selectivity lost. These results indicated that the
longer induction periods of the first reaction mixture or an excess
of the terminal alkyne is required for the complete formation of
vinylidene intermediate which is responsible for the dimerization
of terminal alkynes. Similar results were obtained with the experi-
ments carried out with alkylacetylenes 1g and 1h. Expected results
were observed with 1g which is not a suitable alkyne for the forma-
tion of vinylidene intermediates due to lack of bulky group in the
molecular structure. The catalytic system conducted with 1g gave
40% (4a: 5a: 6a; 45: 50: 5) yield. Tertbutylacetylene, 1h, a bulky
alkyne gave relatively high results 75% (4a: 5a: 6a; 2: 87: 11).
Once the optimum reaction conditions were determined, a
series of aryl and alkyl acetylene derivatives were chosen and tested
under the specified reaction conditions; the results are listed in
Table 5. The most remarkable point in the listed results is the
high tendency of the aryl acetylenes to form the most selective
dimerization products regardless of the substituent in both the
meta- and para- positions. This trend can be explained by the
relative stability of the ruthenium complexes bearing bulky and
aromatic vinylidene ligands, in this case substituted aryl-alkynes.
The substrates, 1a–f, can be successfully transformed into the
L2RuCl2
ify this design, (PCy3)2RuCl2
C
CPhH to obtain selective dimerization products. To ver-
CPhH was synthesized according
C
to the literature and reacted with phenyl acetylene at a catalytic
loading of 4% Ru in toluene at 80 ◦C. After 10 h, phenylacetylene
was totally converted into 3a and 4a with a 100% yield (3a: 4a; 96:
4).
Next, the influence of the terminal alkyne on reaction yields and
selectivities was investigated by reacting the [RuCl2(p-cymene)]2
dimer with 4.2 mole equivalent of PCy3 and 2.2 mol equivalent
of phenylacetylene for varying time periods before introduction
of excess phenylacetylene into the reaction medium at 80 ◦C in
toluene (Table 3). The formation of enyne product is strictly
depends on the formation of vinylidene intermediate. As it can be
seen in Table 3, the competition of dimerization vs. cyclotrimeriza-
tion reactions is prominent at runs 1–3. The dimerization product
ratio was steadily increased by the relatively longer reaction peri-
ods of the first reaction mixture which consists of only 1 mol
equivalent of [RuCl2(p-cymene)]2 dimer, 4.2 mole equivalent of
PCy3 and 2.2 mol equivalent of phenylacetylene. The optimum
reaction time of the first reaction mixture was found to be 4 h before
addition of phenylacetylene. 95% yield (4a: 5a: 6a; 93: 6: 1) for
dimerization product was obtained after 12 h at 80 ◦C in toluene.
Increasing the induction period did not display significant effects
on both yield and selectivity.
Subsequently, the effect of substituents on ortho and para posi-
tions of terminal alkynes on formation of vinylidene intermediates
was investigated. For this purpose, 1 mol equivalent of [RuCl2(p-
cymene)]2 dimer, 4.2 mole equivalent of PCy3 and 2.2 mol equiva-
lent of ortho or para-substituted terminal alkynes were reacted for
4 h in toluene at 80 ◦C before introduction of excess phenylacety-
lene and results were listed in Table 4. The catalytic system, which
Scheme 3. Representation of the switchable selectivity of the catalytic system in the presence of PR3.

B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
219
Table 3
Dimerization of phenylacetylene.
Runa
Addition time of excess alkyneb (h)
Reaction time (h)c
Dimerization yield (4a:5a:6a)d
Cyclotrimerization yield (2a:3a)d
1
2
3
4
5
6
0.5
1
2
4
6
12
12
12
12
12
12
50 (80:12:8)
70 (85:10:5)
83 (93:6:1)
95 (93:6:1)
95 (94:4:2)
93 (93:6:1)
40 (90:10)
30 (88:12)
17 (90:10)
5 (89:11)
5 (91:9)
12
7 (88:12)
a
[RuCl2(p-cymene)]2 dimer (0.02 g, 0.033 mmol), PCy3(0.041 g, 0.0132 mmol) and 2.2 mol equivalent of phenylacetylene (0.072 mmol, 8 ␮l) were reacted before introduc-
tion of excess phenylacetylene (0.825 mmol, 90 ␮l) into the reaction medium at 80 ◦C in toluene (1 ml).
b
Represents the time when the excess phenylacetylene was added.
Represents the reaction time after the addition of excess alkyne to the reaction medium.
GC yield.
c
d
corresponding dimerization products in excellent yields by the
catalytic system [RuCl2(p-cymene)]2/PCy3. The reaction proceeded
smoothly with arylacetylenes bearing the following para- groups:
1b, 4-ethynyl toluene (99%; 4b: 5b: 6b; 95: 5: 0); 1c, 4-ethynyl
anisole (99%; 4d: 5d: 6d; 95: 4: 1) and 1f, 4-ethynyl tert-butyl
benzene (99% 4f: 5f: 6f; 93: 7: 0). Nevertheless, in contrast to the
para-substituted arylacetylene derivatives, ortho-substituted aryl
acetylenes lack the high regioselectivity of their para-substituted
analogs. The reaction of 1e (2-ethynyl toluen(e) had a 95% yield
(2e: 3e: 4e; 88: 12: 0), and under the same reaction conditions, 1f
(2-ethynylanisol(e) had a 93% yield (2f: 3f: 4f; 84: 16: 0).
in which the in situ synthesized ruthenium vinylidene species
reacts with aryl acetylenes to form selective dimerization products.
Selected alkyl acetylenes (i.e., 1g and 1(h) were reacted under the
same reaction conditions and the results are in Table 5. The results
were according to our expectations. 4-ethynyl-tert-butyl benzene
(1(f) exhibited a high selectivity towards the dimerization reaction
and gave a yield value of 99% (4f: 5f; 93: 7), whereas the reaction
with 1-octyne (1(g) resulted in a mixture of both cyclotrimerization
and dimerization products.
After all the reaction parameters were determined and opti-
mized for both the cyclotrimerization and dimerization reactions,
in situ discrimination between the cyclotrimerization and dimer-
ization processes were investigated; the results are shown in
Table 6. Phenylacetylene was reacted at 80 ◦C in the presence
of 4% [Ru(Cl2)(p-cymene)]2 in toluene, and the content of the
As previously indicated, sterically hindered bulky alkynes coor-
dinate more readily into the ruthenium center to form stable
ruthenium vinylidene intermediates. Our preceding experiments
suggested that this fact could be practically applied to the processes
Table 4
Effect of nature of the terminal alkyne on dimerization reactions of phenylacetylene.
Runa
Terminal alkyne used to form vinylidene intermediate
Alkyne
Reaction time (h)b
Dimerization yield (%)c (4a:5a:6a)
Me
1b
1
16
90 (5:90:5)
Me
1c
2
3
16
16
60 (2:75:23)
86 (2:93:5)
MeO
1d
OMe
1e
4
16
16
50 (0:77:23)
89 (1:91:8)
t-C4H9
1f
5
6
C6H13
1g
16
16
40 (45:50:5)
75 (2:87:11)
t-C4H9
1h
7
a
[RuCl2(p-cymene)]2 dimer (0.02 g, 0.033 mmol), PCy3(0.041 g, 0.0132 mmol) and 2.2 mol equivalent of alkyne 1b-h (0.072 mmol) were reacted before introduction of
excess phenylacetylene (0.825 mmol, 90 ␮l) into the reaction medium at 80 ◦C in toluene (1 ml).
b
Represents the time when the excess phenylacetylene was added.
GC yield.
c

220
B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
Table 5
Dimerization of terminal alkynes in the presence of [RuCl2(p-cymene)]2/PCy3 in toluene at 80 ◦C.
Runa
Alkyne
Time (h)
Yield(%)b
E(%)
Z(%)
Gem(%)
1
2
1a
1b
8 h
8 h
99 (90)
99 (91)
1
0
94
95
5
5
Me
Me
3
4
1c
8 h
8 h
95 (90)
99 (92)
0
1
88
95
12
4
MeO
1d
OMe
5
1e
8 h
93 (84)
0
0
84
16
t-C4H9
6
7
1f
8 h
8 h
99 (90)
70 (55)
93
60
7
0
C6H13
t-C4H9
1g
40
8
1h
8 h
98 (90)
0
89
11
All reactions were carried out at a catalytic loading of 4% [RuCl2(p-cymene)]2 and 40% PCy3 in toluene at 80◦C.
GC yield. Isolated yields is in parantheses.
a
b
mixture was analyzed by GC–MS. For the first run, phenylacety-
for dimerization (4a: 5a 6a; 93: 6: 1) and 20% cyclotrimerization
(2a: 3a; 88: 12) product was observed. These results indicate that
the addition of phosphine has a strong impact on the selectivity
of the reaction. Following the addition of phosphine, production of
the cyclotrimerization product drastically decreased and only 4–7%
more of the cyclotrimerization product was formed at the end of
the reaction. Runs 1–3 are in sharp contrast with each other, and
the results of these runs show that the in situ switching selectivity
can be observed at any time during the reaction by introducing an
excess of PCy3 into the reaction media.
Despite our countless efforts, we failed to reverse the selectivity
trend from the dimerization reaction towards cyclotrimerization
reaction. Addition of CuCl, a well-known phosphine scavenger, to
a mixture of phenylacetylene and [RuCl2(p-cymene)]2/PCy3 has
no significant effect on the product distribution in terms of either
selectivity or yield.
In order to gain some insight of the reaction mecha-
nism, a detailed study was carried out by 1H and 31P NMR
(Figs. 1 and 2). A reactor was charged with [Ru(Cl)2(p-cymene)]2
(0.033 mmol, 0.02 g), PCy3 (0.33 mmol, 0.093 (g) and phenylacety-
lene (0.825 mmol, 90 ␮l) in toluene and reacted at 80 ◦C. The
reaction mixture was periodically analyzed by 1H and 31P NMR in
CDCl3.
lene was allowed to react with 4% [Ru(Cl2)(p-cymene)]2 for 2 h and
before the addition of a 10 mol equivalent of PCy3. Aliquots taken
from the reaction mixture were analyzed by GC–MS. Phenylacety-
lene (1a) was converted to its cyclotrimerization product at a 55%
yield (2a: 3a; 90: 10), and no dimerization product was detected.
After the addition of a 10 mol equivalent of PCy3, a sharp increase
in the formation of the dimerization product (4a as major iso-
mer) was observed. After 8 h, the phenylacetylene (1a) was totally
consumed, and subsequent GC–MS analysis showed that the for-
mation of the cyclotrimerization product was at a 62% yield (2a:
3a; 89: 11), and the dimerization product was formed at a 38%
yield (4a: 5a: 6a; 94: 1: 5). For the second run, the mixture was
allowed to react for 1 h before introducing the 10 mol equivalent
of PCy3 to the reaction medium. Directly before the addition of
PCy3, GC–MS analysis showed that the resulting mixture consisted
of 40% cyclotrimerization (2a: 3a; 89: 11), and no dimerization
product was observed. After 8 h, the reaction mixture yielded 47%
cyclotrimerization (2a: 3a; 90: 10) product and 53% dimerization
(4a: 5a: 6a; 94: 6: 0) product. For the third run, 15% cyclotrimeriza-
tion (2a: 3a; 90: 10) product was observed before the addition of
PCy3. Following the addition of PCy3, a sharp increase in the dimer-
ization product was observed and after 8 h, a yield value of 80%
Table 6
In situ switchable selectivity of the [Ru(Cl)2(p-cymene)]2/PCy3 catalytic system between cyclotrimerization and dimerization reactions.
Beforeaddition of PCy3 yield (%)d
After addition of PCy3 yield (%)d
Runa
Timeb
Catalyst
2a + 3a
4a + 5a
Timec
2a + 3a
4a + 5a
1
2
3
2
1
0.5
[Ru(Cl)2(p-cymene)]2
[Ru(Cl)2(p-cymene)]2
[Ru(Cl)2(p-cymene)]2
55
40
15
0
0
0
8
8
8
62
47
20
38
53
80
a
Ruthenium catalyst (0.033 mmol), and phenylacetylene (0.825 mmol, 90 ␮l) was added to a reactor and heated under inert atmosphere of nitrogen in toluene at 80 ◦
C
and analyzed periodically by GC–MS.
b
Represents the addition time of PCy3 (10 mol equivalent).
Represents the time when the maximum conversion values were observed.
GC yield.
c
d

B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
221
Fig. 1. 1H NMR spectrum of the reaction mixture, taken at different time intervals.
Fig. 2. 31P NMR spectrum of the reaction mixture, taken at different time intervals.
At the beginning of the reaction, free PCy3 signal was observed at
11.5 ppm in 31PNMRand asignal dueto thetricyclohexylphosphine
oxide was appeared at 50 ppm. Only characteristic coordinated
p-cymene doublet peaks at 5.45 ppm (J = 5.1 Hz) and 5.32 ppm
(J = 5.1 Hz) and a single peak at 3.15 ppm was observed in 1H NMR
due to the acetylenic proton of phenylacetylene. After 1 h, sev-
eral phosphine peaks were observed, indicating the formation of
a complex mixture. An intense signal at 20.5 ppm in 31P NMR indi-
cated the formation of Ru(Cl2)(p-cymene)(PCy3), also unidentified
peaks were observed at 22.0, 25.2 and 34.0 ppm in 31P NMR. 1H
NMR spectrum of the reaction mixture at 2 h was also in good
agreement with the 31P NMR results. It is possible that doublet at
5.20 ppm and a quartet peak at 5.70 ppm may arise from the for-
mation of metallocyclopentadiene intermediate, which is believed
to be responsible for the formation of alkyne cyclotrimerization
product. After 2 h, phosphine peak at 20.5 ppm, which was related
with Ru(Cl2)(p-cymene)(PCy3), was drastically decreased. The new
peak at 21.2 ppm was assigned to be the ruthenium vinylidene
specie. At this point, a triple proton peak at 4.45 ppm (J = 3.6 Hz)
indicated the presence of vinylidene proton. The intensity of the
characteristic p-cymene proton peaks which appears as doublets
at 5.45 and 5.32 ppm in 1H NMR was decreased due to cleavage of
p-cymene fragment. Formation of dimerization product 4a [(Z)-1,4-
diphenyl-1-buten-3-yne] was confirmed by doublet proton peaks
at 6.70 ppm (1H, J = 12 Hz) and 5.92 ppm (1H, J = 12 Hz).
4. Conclusion
In this study, we have shown that [RuCl2(p-cymene)]2/PCy3 can
be used as a practical and inexpensive catalytic system which has a
switchable selectivity towards cyclotrimerization and dimerization
reactions. Best results in means of regioselectivity and yield were

222
B.Ö. Öztürk et al. / Applied Catalysis A: General 433–434 (2012) 214–222
observed by using arylacetylene derivatives in these reactions.
This catalytic system emerges as an economical method for the
transformation of arylacetylenes to corresponding enyne and arene
derivatives in excellent yields and selectivity. Moreover, addition
of the phosphine derivatives after a certain period of reaction time
switches the selectivity of the process from cyclotrimerization to
dimerization, as an effect of changing the nature of the catalytic
species.
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