Regio-and stereoselective homodimerization of monosubstituted acetylenes in the presence of the second generation grubbs catalyst

Article abstract of DOI:10.1007/s10562-013-1134-z

Silyl-and phenylacetylenes undergo efficient homodimerization in the presence of a second generation Grubbs catalyst. The reaction permits fully regio-and stereoselective synthesis of disubstituted 1,3-enynes. The other commonly used ruthenium-based olefi

Full text of DOI:10.1007/s10562-013-1134-z

Catal Lett (2014) 144:413–418
DOI 10.1007/s10562-013-1134-z
Regio- and Stereoselective Homodimerization of Monosubstituted
Acetylenes in the Presence of the Second Generation Grubbs
Catalyst
•
Beata Powała Cezary Pietraszuk
Received: 17 August 2013 / Accepted: 7 October 2013 / Published online: 23 October 2013
Ó The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract Silyl- and phenylacetylenes undergo effi-
cient homodimerization in the presence of a second
generation Grubbs catalyst. The reaction permits fully
regio- and stereoselective synthesis of disubstituted
1,3-enynes. The other commonly used ruthenium-based
olefin metathesis catalysts remain inactive in the
reaction.
and the f-block metals [37–39]. However, a highly selective
synthesis of conjugated enynes by dimerization is still a
challenging process. Depending on the type of catalyst and
the nature of substituent at the triple bond, the reaction may
produce different products, or a mixture thereof (Eq. 1).
R
R
R
Keywords Homogeneous catalysis Á Acetylenes Á
Dimerization Á Ruthenium Á Alkylidene complexes
R
cat.
+
+
+
R
R
R
R
R
A
B
C
D
1
1 Introduction
Conjugated 1,3-enynes are important building blocks
widely used in synthetic organic chemistry. This class of
molecules has been investigated for their antimicrobial
activity [1] and more recently for their photophysical
properties [2–4]. Moreover, conjugated 1,3-enyne skeleton
has been found in natural products [5, 6]. Homodimeriza-
tion of monosubstituted acetylenes is a convenient method
for the preparation of 1,3-enynes allowing their synthesis in
an atom economical manner. Many examples of effective
run of the process in the presence of transition metal com-
plexes have been described. Catalytic activity in the process
has been reported to be shown by complexes of such metals
as palladium [4, 7–13], rhodium [14–19], ruthenium [20–
29], nickel [30], iridium [31–33], osmium [34, 35], iron [36]
From among a number of ruthenium-based catalytic
systems used in alkyne dimerization, the application of
ruthenium-based olefin metathesis catalysts has also been
mentioned. The thermolysed first generation Grubbs cata-
lyst [RuCl2(PCy3)2(=CHPh)] (1) exhibits activity in
dimerization of phenylacetylene giving mixture of isomers
[40] or preference for the formation of E-isomer of head to
head dimer [41]. Moreover, complex 1 catalyzes the
dimerization of arylethynes to the corresponding 1,4-
substituted 1,3-enynes with pronounced Z-selectivity in
aqueous environment in the presence of sodium dodecyl
sulphate [42]. Formation of a product of phenylacetylene
homodimerization was observed in the study of hydrosi-
lylation of phenylacetylene with trisubstituted silanes in the
presence of a second generation Grubbs catalyst [43].
Moreover, Ozawa has reported highly Z-selective dimer-
ization of arylacetylenes with silylacetylenes catalyzed by
ruthenium vinylidene precursors [44].
B. Powała Á C. Pietraszuk (&)
´
Faculty of Chemistry, Adam Mickiewicz University in Poznan,
We now report on a highly efficient, regio- and stereo-
selective dimerization of terminal alkynes in the presence of
´
Umultowska 89b, 61-614 Poznan, Poland
e-mail: pietrasz@amu.edu.pl
123

414
B. Powała, C. Pietraszuk
commercially available ruthenium alkylidene complex—the
second generation Grubbs catalysts [RuCl2(H2IMes)(PCy3)
(=CHPh)] (2).
Table 1 Homodimerization of silylacetylenes in the presence of the
second generation Grubbs catalyst as compared with other Ru
catalysts
Silylacetylene Cat. Temp [°C] Conversion [%] Yield of 4 [%]
SiR3=
2 Results and Discussion
SiMe2t-Bu
SiMe2t-Bu
SiMe2t-Bu
SiMe2t-Bu
SiMe2t-Bu
SiMe2t-Bu
Si(i-Pr)3
1
2
6
7
8
9
2
2
2
80
0
0
80
89
\5
\5
\5
0
85 (80)
Addition of a second generation Grubbs catalyst (2) to a
benzene solution of tert-butyldimethylsilylacetylene at
80 °C resulted in an immediate change in colour of the
solution from red to green. Monitoring of the reaction
mixture by GC demonstrated gradual formation of a single
product, which was identified by GC–MS as a product of
acetylene dimerization (Eq. 1). The product was isolated by
liquid chromatography and subjected to analysis by NMR
spectroscopy. NMR spectrum revealed formation of Z-iso-
mer of head to head dimer (4) (Eq. 2). A similar reaction
course was also observed for other silylacetylenes (Eq. 2).
80
0
80
0
80
0
80
110a
0
77
84
88
75 (69)
83 (70)
85c (80)
SiEt3
80
80b
SiMe3
Reaction conditions: [Ru] 5 mol% in relation to alkyne, benzene,
80 °C, 24 h
a
Toluene
b
Closed system
R
c
Total yield, mixture of isomers [4d]:[5] = 1:3.3
[Ru]
R
C6H6, 80°C
R
2
3
4
In order to determine the scope of the reaction, a number
of other monosubstituted acetylenes were tested. It was
found that in the presence of ruthenium alkylidene complex
2 selective dimerization was also observed when phenyl-
acetylene and its derivatives substituted in position 4 were
used as reaction substrates. Catalytic tests performed for
selected substituted phenylacetylenes showed complete
regio- and stereoselective course of the reaction. For each
arylacetylene tested the exclusive formation of isomer 11
was observed (Eq. 4 ; Table 2).
R = SiMe2 Bu (a), SiEt3 (b), SiiPr3 (c)
t
High yields and complete regio- and stereoselectivity
was observed for triethylsilylacetylene and triiso-propyl-
silylacetylene (Table 1). From among the silylacetylenes
tested, trimethylsilylacetylene was the only one for which
non-selective reaction occurred. In this case, the reaction
leads to a mixture of head to head (4) and head to tail (5)
dimers (Eq. 3).
SiMe3
Me3Si
[Ru]
Me3Si
+
SiMe3
3
C6H6, 80°C
SiMe3
3d
4d
5
For each reaction, the resulting dimer was isolated by
liquid chromatography and spectroscopically characterized.
Identification of the product structures was made on the basis
of 1H NMR and MS spectra and by comparison of the spectra
recorded with those reported in literature. For dimerization of
trimethylsilylacetylene a mixture of isomers 4d and 5 was
isolated and analyzed by spectroscopic methods. Structure of
product 5 was confirmed by 1H and 13C NMR and DEPT (see
‘‘Experimental Section’’). Interestingly, the reaction was
observed only in the presence of catalyst 2. The other com-
monly used ruthenium alkylidene complexes (1, 6–9 in
Fig. 1) remained inactive in the reaction.
R
[Ru]
R
C6H6, 80°C
R
4
10
11
R= C6H5 (a), C6H4CH3-4 (b), C6H4CF3-4 (c)
Interestingly, dimerization of less sterically crowded
1-(prop-2-ynyl) benzene (12) in the presence of catalyst 2
leads to fully selective formation of head to tail dimer (13)
(Eq. 5). Product 13 was isolated and its structure was
proposed on the basis of spectroscopic characterization (1H
123

Regio- and Stereoselective Homodimerization of Monosubstituted Acetylenes
415
Fig. 1 Olefin metathesis
catalysts tested in dimerization
of silyl- and arylacetylenes
and 13C NMR, DEPT and MS). The presence of a geminal
methylene group at the double bond was confirmed on the
basis of DEPT sequence, which shows one CH2 carbon in
the olefinic region (d = 121.3 ppm).
Table 2 Homodimerization of phenylacetylenes in the presence of
the second generation Grubbs catalyst as compared with other Ru
catalysts
RC:CH
R=
Cat. Conv. of 10 [%] Yield (isolated) of 11 [%]
Ph
C6H5
C6H5
1
2
\5
65a
95
0
65a
Ph
2 (5 mol%)
5
C6H6, 80°C
Ph
95 (90)
75a
the only product
13
12
C6H5
6
75a
isolated yield = 70%
85
80
C6H5
7
8
9
2
0
0
When hexyne or tert-butylacetylene was heated in the
presence of catalyst 2, both in the open system (under
argon) and in the sealed glass vials (benzene, 80 °C, 24 h)
no dimerization was observed.
C6H5
\5
0
98a
100
78a
80
0
C6H5
0
96a
C6H4CH3-4
98 (80)
75a
There are two general mechanisms proposed for acety-
lene dimerization in the presence of ruthenium complexes
(Scheme 1).
C6H4CF3-4
2
77 (71)
Bianchini mechanism (Scheme 1 path a) involves a
formation of vinylidene complexes and migratory insertion
of acetylide ligand to a-carbon of a ruthenium vinylidene
intermediate [45–47]. The other mechanism, explaining
formation of head to tail dimers involves direct insertion of
the acetylene molecule coordinated to ruthenium into
Ru–C bond in acetylide complex (Scheme 1, paths b and
c). Protonation of enynyl species by the acidic proton of the
terminal alkyne, produces enynes and regenerates the
active catalyst. The insertion mechanism has been origi-
nally proposed by Trost for palladium complexes [8].
However, it has also been postulated for ruthenium com-
plexes [48].
Reactions conditions: benzene, 80 °C, 24 h
5 h
a
decomposition of alkylidene complexes of ruthenium in the
presence of monosubstituted acetylenes. The reaction pathways
leading to formation of real catalyst are not always known. For
vinylidene precursors of the type [RuCl2(Pi-Pr3)2(=C=CHPh)]
a direct transformation to catalytically active r-acetylide
complex [RuCl(C:CPh)(Pi-Pr3)2] via abstraction of HCl in
the presence of a base has been demonstrated [44, 50]. On the
other hand, Verpoort has proposed the in situ formation of an
active acetylide complex by interaction of thermolysed first
generation Grubbs catalyst with phenylacetylene [40].
Although there are some examples in literature describing
the activity of ruthenium-based olefin metathesis catalyst in
non-metathesis transformations of acetylenes [49], the systems
containing ruthenium alkylidene complexes and terminal
alkynes have not yet been thoroughly investigated. In partic-
ular, there is no literature data on the mechanism of
Our preliminary study of the equimolar reactions of
catalyst 2 with phenylacetylene or tert-butyldimethylsily-
lacetylene or phenylacetylene performed in an NMR tube
and monitored by 1H NMR (benzene, 80 °C) revealed
gradual decomposition of Grubbs catalyst (2). However, no
formation of r-acetylide complex was observed.
123

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B. Powała, C. Pietraszuk
3.2 Representative Synthesis (4a)
Schlenk vessel (20 mL) was charged under argon with
10 mL of dry benzene and 187 lL (1.00 9 10-3 mol) tert-
butyldimethylsilylacetylene. Then, the solution was heated
to 80 °C and the second generation Grubbs’ catalyst
0.042 g (4.95 9 10-5 mol) was added. The reaction was
carried out at the boiling point of benzene (80 °C) for 24 h.
After completion of the reaction the resulting mixture was
concentrated by evaporation of the solvent and the residue
was purified using column chromatography (silica MN
60/hexane). (E)-1,4-Bis(tert-butyl-dimethylsilyl)but-1-en-
3-yn (colorless oil) was obtained with 80 % of isolated
yield.
Scheme 1 Mechanisms of acetylene homodimerization in the pre-
sence of ruthenium complexes
3.3 Spectroscopic Characterization
3.3.1 (E)-1,4-bis(tert-butyldimethylsilyl)but-1-en-3-yne
Although at present a discussion on the mechanisms of
dimerisation in the systems studied would be premature,
the formation of head to tail dimers (5 and 13) in the
reaction conditions (for certain systems of reagents) justi-
fies the expectation that the reaction occurs according to
the insertion mechanism.
(4a) [51]
1H NMR (C6D6; d (ppm)): 0.18 (s, 6H); 0.30 (s, 6H); 0.96
(s, 9H); 1.03 (s, 9H); 6.00 (d, J = 15.4 Hz, 1H); 6.34 (d,
J = 15.4 Hz, 1H); 13C NMR (C6D6; d (ppm)): -5.16;
-4.51; 16.92; 17.38; 26.35; 26.60; 96.39; 106.72; 126.36;
143.12; MS m/z, (related intensity): 53(13), 59(25), 67(12),
73(56), 77(13), 125(13), 149(12), 167(100), 168(18),
169(29), 195(13), 223(29, M?).
In conclusion, we have demonstrated efficient homodi-
merization of monosubstituted silyl or arylacetylenes tak-
ing place in the presence of the second generation Grubbs
catalyst and leading (with some exceptions) to regio- and
stereoselective formation of Z-1,4-disubstituted 1,3-
enynes.
3.3.2 (E)-1,4-bis(triethylsilyl)but-1-en-3-yne (4b) [52]
1H NMR (C6D6; d (ppm)): 0.66 (q, J = 7.9 Hz, 6H,
–CH2CH3); 0.83 (q, J = 7.9 Hz, 6H, –CH2CH3); 1.06
(t, J = 5.9 Hz, 9H, –CH2CH3); 1.08 (t, J = 5.9 Hz, 9H,
–CH2CH3); 5.96 (d, J = 15.3 Hz, 1H); 6.37 (d,
J = 15.3 Hz, 1H); 13C NMR (C6D6; d (ppm)): 4.1; 4.7; 7.7;
7.8; 96.2; 106.9; 126.5; 142.5; MS m/z (related intensity):
56(17), 57(22), 59(100), 69(46), 83(46), 91(65), 97(26),
113(25), 120(36), 164(14), 195(18), 196(16), 223(52),
280(67, M?).
3 Experimental Section
3.1 Materials and Methods
Unless mentioned otherwise, all operations were per-
formed by using standard Schlenk techniques. 1H- and 13
C
NMR spectra were recorded on a Varian 400 operating at
402.6 and 101.2 MHz, respectively. 31P NMR spectra
were recorded on a Mercury 300 operating at 121.5 MHz.
GC analyses were carried out on a Varian CP-3800
(column: Rtx-5 30 m I.D. 0.53 mm) equipped with TCD.
Chemicals were obtained from the following sources:
Acetylenes, ruthenium alkylidene complexes, dichloro-
methane, benzene-d6, decane, dodecane and n-hexane
were obtained from Aldrich. All solvents were dried prior
to use over CaH2 and stored under argon. CH2Cl2 was
additionally passed through a column with alumina and
after that it was degassed by repeated freeze–pump–thaw
cycles.
3.3.3 (E)-1,4-bis(triiso-propylsilyl)but-1-en-3-yne (4c)
[51]
1H NMR (C6D6; d (ppm)): 0.94–1.04 (m, 42H, (CH3)2CH);
5.88 (d, J = 15.8 Hz, 1H); 6.47 (d, J = 15.8 Hz, 1H); 13C
NMR (C6D6; d (ppm)): 12.2; 12.4; 19.2; 19.6; 95.4; 108.3;
127.9; 141.0; MS m/z (related intensity): 54(36), 59(100),
60(15), 72(13), 83(24), 95(12), 109(12), 127(12), 128(18),
131(19), 139(19), 163(13), 237(21), 238(19), 240(15),
270(46), 280(52), 281(30), 282(12), 364(25, M?).
123

Regio- and Stereoselective Homodimerization of Monosubstituted Acetylenes
417
7.08–7.96 (m, 10H, Ph); 13C NMR (CDCl3; d (ppm)): 25.4;
43.8; 83.0; 88.0; 121.3; 126.3; 127.8; 128.4, 129.1; 131.1;
138.6; DEPT (CDCl3; d (ppm)): 121.3 (=CH2); MS m/z
(related intensity): 50(19), 51(21), 63(25), 65(38), 77(13),
78(10), 89(18), 91(72), 115(80), 116(37), 117(16), 128(38),
129(11), 139(21), 141(100), 142(11), 152(10), 153(25),
154(15), 202(23), 215(28), 216(22), 217(62), 218(13),
231(24), 232(10, M?).
3.3.4 Mixture of (E)-1,4-bis(trimethylsilyl)but-1-en-3-yne
(4d) and 2,4-bis(trimethylsilyl)but-1-en-3-yne (5)
[53, 54]
4d: 1H NMR (C6D6; d (ppm)): 0.10 (s, 18H); 6.07 (d,
J = 15.1 Hz, 1H); 6.17 (d, J = 15.1 Hz, 1H); 13C NMR
(C6D6; d (ppm)): 146.1; 124.4; 105.0; 98.5; -0.4; -1.2;
MS m/z (related intensity): 73(15), 181(100), 182(18),
183(10); 5: 1H NMR (CDCl3;d (ppm)): 0.07 (s, 18H); 5.61
(d, J = 3.4 Hz, 1H); 6.03 (d, J = 3.4 Hz, 1H); 13C NMR
(CDCl3;d (ppm)): 134.8; 124.8; 105.0; 98.6; 1.2, -0.4;
DEPT (CDCl3; d (ppm)): 105.0 (= CH2); MS m/z (related
intensity): 45(11), 73(35), 108(12), 155(52), 181(100).
Acknowledgments Financial support from the National Science
Centre (Poland), (Project No. 2011/01/N/ST5/02042) is gratefully
acknowledged.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
3.3.5 (E)-1,4-diphenylbut-1-en-3-yne (11a) [55]
1H NMR (CDCl3; d (ppm)): 7.35–7.45 (m, 6H); 7.51–7.56
(m, 2H); 7.96–7.99 (m, 2H); 5.96 (d, J = 11.9 Hz, 1H);
6.75 (d, J = 11.9 Hz, 1H); 13C NMR (CDCl3; d (ppm)):
88.2; 95.8; 107.3, 123.4; 128.1; 128.3; 128.4; 128.5; 128.7;
131.4; 136.5; 138.8; MS m/z (related intensity): 51(91),
101(100), 202(31), 203(67), 204(59, M?).
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