Gem-selective cross-dimerization and cross-trimerization of alkynes with silylacetylenes promoted by a rhodium-pyridine-n-heterocyclic carbene catalyst

Article abstract of DOI:10.1002/cctc.201402327

The gem-selective cross-dimerization and -trimerization of silylacetylenes with alkynes through C-H activation using a rhodium(I)-pyridine-N-heterocyclic carbene catalyst have been developed. This reaction is applied to various aliphatic or aromatic termi

Full text of DOI:10.1002/cctc.201402327

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DOI: 10.1002/cctc.201402327
gem-Selective Cross-Dimerization and Cross-Trimerization
of Alkynes with Silylacetylenes Promoted by a Rhodium–
Pyridine–N-Heterocyclic Carbene Catalyst
Ramꢀn Azpꢁroz,[a] Laura Rubio-Pꢂrez,[a] Ricardo Castarlenas,*[a, b] Jesffls J. Pꢂrez-Torrente,[a] and
Luis A. Oro*[a, c]
The gem-selective cross-dimerization and -trimerization of sily-
lacetylenes with alkynes through CÀH activation using a rhodiu-
m(I)–pyridine–N-heterocyclic carbene catalyst have been devel-
oped. This reaction is applied to various aliphatic or aromatic
terminal alkynes, internal alkynes, and gem-1,3-disubsituted
enynes to afford the corresponding enynes and dienynes with
high regio- and stereoselectivities and in good isolated yields
(up to 91%).
Introduction
The presence of 1,3- or 1,4-dis-
ubstituted enynes as key struc-
tural units in many biologically
active molecules, polymers, or
photoactive compounds has
stimulated the interest in seek-
ing for new and simple synthetic
routes.[1] The metal-catalyzed ho-
modimerization of terminal al-
kynes to selectively synthesize E-
, Z-, and gem-enynes has been
studied in depth.[2] However, the
cross-dimerization of two al-
kynes is more limited because of
the competitive homo- and oli-
godimerization reactions. In this
context, different chemo-, regio-,
and stereoselectivities have been
achieved with Ir,[3] Ni,[4] Pd,[5]
Rh,[6] Ru,[7] Ti,[8] and Co[9] cata-
lysts, among others.[10] The cross-
Scheme 1. Possible products of the cross-coupling of silylacetylene derivatives with alkynes: a) terminal and
b) internal. Compounds prepared herein are shown in dashed boxes. R, R’, R“=alkyl, aryl.
dimerization of silylacetylenes as
a donor acetylene group with unactivated internal alkynes has
[a] R. Azpꢀroz, Dr. L. Rubio-Pꢁrez, Dr. R. Castarlenas, Prof. J. J. Pꢁrez-Torrente,
Prof. L. A. Oro
been disclosed.[4,5f,6b,c,7a,9c] In contrast, the addition of silylacety-
lenes to terminal alkynes is challenging because of the com-
petitive homodimerization reaction, particularly for aromatic
acetylenes that present comparable acidity (Scheme 1).[11] Al-
though several Ru,[7b] Rh,[6d] and Ir[3b] metal complexes selec-
tively catalyzed the head-to-head cross-dimerization and af-
forded both Z and E isomers, reports on the formation of
head-to-tail products are scarce.[5f,8a] In contrast, the selective
cross-trimerization of three alkynes by combining silylacetylene
and internal alkynes leading to 1,3-dien-5-ynes has been per-
formed with Ni[4,12] and Pd[5c] catalysts. However, the formation
of selective (2-alkynyl)-gem-1,3-dienes with a combination of
three terminal alkynes has not been reported till date.
Departamento de Quꢀmica Inorgꢂnica-ISQCH
Universidad de Zaragoza - CSIC
Facultad de Ciencias
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
E-mail: rcastar@unizar.es
oro@unizar.es
[b] Dr. R. Castarlenas
ARAID Foundation Researcher
[c] Prof. L. A. Oro
Center of Research Excellence in Refining & Petrochemicals
King Fahd University of Petroleum & Minerals
Dhahran 31261 (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402327.
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Our research group has been developing new efficient and
selective catalytic systems based on Rh–N-heterocyclic carbene
(NHC) metal complexes for CÀC and CÀheteroatom coupling
reactions.[13] Dinuclear compounds of type [Rh(m-Cl)(NHC)(h2-
olefin)]2 (1) have been revealed as valuable starting materials
for the preparation of mononuclear complexes of type RhCl-
(NHC)(h2-olefin)(L) (2) via simple bridge cleavage with a nucleo-
philic ligand.[13d–h] These derivatives demonstrated excellent
performance in catalytic alkyne hydrothiolation[13d,h] and in the
preparation of 4H-quinolizines through CÀH activation.[13e] It
has also been found that the complex [RhCl(IPr)(h2-coe)(py)]
observed that selectivity changes dramatically with the tem-
perature. At 608C, a 37% of phenylacetyl–trimethylsilylacetyle-
ne cross-dimerization product 5a and a 50% of phenylacety-
lene homodimerization derivative 7a were obtained (entry 2).
At 808C, the cross-coupled compound 5a was the major prod-
uct (81%) along with a 5% of the homodimerization product
7a (entry 3). In both cases, the formation of the trimer 8a
(ꢀ14%), resulting from the coupling of two molecules of phe-
nylacetylene with trimethylsilylacetylene, was also observed as
a byproduct. The observed temperature effect over the selec-
tivity is probably due to a modification of the overall kinetic
parameters, which indicates that at high temperature the CÀH
activation plays a pre-eminent role and thus favors cross-dime-
rization. Moreover, an excess amount of pyridine accelerates
the cross-dimerization to form 5a with high chemo- and regio-
selectivity. Notably, if dinuclear complexes 1, lacking a pyridine
ligand, were used as catalysts, no dimerization products were
formed; however, the presence of cyclotrimers was observed,
as described in our previous work.[13f]
(2a)
[IPr=1,3-bis-(2,6-diisopropylphenyl)imidazol-2-carbene,
coe=cyclooctene, and py=pyridine] promotes the selective
homodimerization of alkynes to head-to-tail enynes, in which
the chemo- and regioselectivity is controlled by the addition of
pyridine.[13f] Herein, we report an efficient chemo-, regio-, and
stereoselective Markovnikov-type cross-dimerization and -tri-
merization involving trimethylsilylacetylene and aliphatic and
aromatic terminal alkynes, internal alkynes, and gem-1,3-disub-
stituted enynes.
The unequivocal characterization of the head-to-tail cross-di-
merization product 5a was accomplished by comparison with
1
the reported H and 13C{1H} NMR data[15] and the heteronuclear
1
correlation H–13C HMBC experimental data. The more signifi-
Results and Discussion
cant part of the 2D NMR spectrum is shown in Figure 1. The
Cross-dimerization of trimethylsilylacetylene with terminal
alkynes
The catalytic system 2a+10 equiv. of pyridine[14] was tested
for the cross-dimerization of trimethylsilylacetylene (3a) with
phenylacetylene (4a) (Table 1 and Scheme 2). At 408C, the re-
action is fully selective to head-to-tail (gem) phenylacetylene
homodimerization product 7a (Table 1, entry 1). However, we
Table 1. Temperature screening for the cross-dimerization of trimethyl-
silylacetylene (3a) with phenylacetylene (4a) catalyzed by 2a+10 equiv.
of pyridine.[a]
Entry
t
[h]
T
[8C]
Conversion
[%]
Selectivity [%]
5a
6a
7a
8a
1
2
3
3
3
2
40
60
80
65
99
99
18
37
81
–
–
–
82
50
5
–
13
14
Figure 1. 1H–13C HMBC spectrum of compound 5a in C6D6.
[a] Reaction conditions: 0.5 mL of C6D6, 0.2 mmol of 3a, 0.2 mmol of 4a,
0.01 mmol of catalyst 2a+0.1 mmol of pyridine.
terminal olefinic protons (d=5.80 and 5.77 ppm) correlates
with one Csp atom (d=104.8 ppm), whereas, more significant-
ly, the other alkynyl carbon atom (d=95.9 ppm) interacts with
the protons of the trimethylsilyl group (d=0.31 ppm) and not
with the aromatic protons, discarding the phenyl–alkynyl con-
nectivity. A correlation between the geminal protons and the
ipso-phenyl quaternary carbon (d=137.3 ppm) was also ob-
served (see the Supporting Information).
We have then studied the scope of the cross-dimerization
reaction between terminal alkynes and trimethylsilylacetylene
(Table 2). In general, full alkyne conversion was attained in 1–
4 h, which gave the corresponding gem-1,3-disubstituted
enynes (5) in good isolated yields (up to 80%). Cross-coupled
E-enynes (6), homodimerization derivatives (7), and gem-dien-
Scheme 2. Catalytic transformation of alkynes with 2a+10 equiv. of pyridine
(py). R=alkyl, aryl.
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Cross-trimerization of terminal alkynes with tri-
methylsilylacetylene
Table 2. Catalytic cross-dimerization of alkynes with trimethylsilylacetylene (3a) medi-
ated by 2a+10 equiv. of pyridine.[a]
Entry Alkyne
t
Conversion
Selectivity [%]
Yield
[%]
In some cases, aromatic and aliphatic trimers 8 were
also formed as secondary products (Table 2). Most
probably, these derivatives result from the homocou-
pling reaction of the corresponding terminal alkyne
and a subsequent cross-trimerization with trimethyl-
silylacetylene. Thus, this process could be a straight-
forward way to construct p-conjugated systems. To
promote the formation of (E)-(2-alkynyl)-(1,3-disubsti-
tuted)-1,3-butadiene derivatives (8) as major prod-
ucts, we performed the reaction in two steps. In the
first step, the homodimerization reaction of the cor-
responding terminal alkyne was performed under re-
action conditions of our previously reported work
using 5 mol% of catalyst 2a and 10 equiv. of pyridine
at 408C in C6D6.[13f] In the second step, trimethylsilyl-
acetylene was added and the crude mixture heated
at 808C for 12 h to form gem-1,3-dienynes in good
isolated yields and high regioselectivity (Scheme 4).
The structure of the dienynes was confirmed by per-
forming 1H–13C HMBC and 1H–1H NOE NMR experi-
ments (see the Supporting Information). Reactions of
aromatic substituted gem-1,3-enynes 7 gave good
yields of the expected gem-dienyne products, for ex-
ample, 8a and b. Unfortunately, compound 8e could
[h] [%]
5
6
–
–
7
8
1
2
4a
4b
2
1
99
99
81
91
5
14 71 (5a)
83 (5b)
4
–
5
3
4c
2
99
85
–
15 78 (5c)
4
5
4d
4e
2
1
99
92
86
–
–
–
10
4
–
76 (5d)
0 (5e)
98[c]
6
4 f
4g
4h
4i
2
3
4
4
5
99
99
90
95
98
82
–
3
8
6
–
4
15 70 (5 f)
7
75 12
81 11
80 12
5
2
8
68 (5g)
74 (5h)
70 (5i)
8
9
10
4j
83
–
13 71 (5j)
11
12
4k
4l
3
4
99
99
78
–
22
–
–
–
60 (5k)
80 (5l)
86 14[d]
[a] Reaction conditions: 0.5 mL of C6D6, 0.2 mmol of 3a, 0.2 mmol of 4, 0.01 mmol of
catalyst 2a+0.1 mmol of pyridine at 808C; [b] Isolated yield of 5; [c] 2% of E-homo-
dimer was also observed.[d] E/gem isomer
yne trimers (8) were also formed in variable amounts.
As a general trend, aromatic alkynes react faster than
aliphatic alkynes (Table 2, entries 1–4 vs. entries 6–
12). Phenylacetylene was completely consumed after
2 h, and gem-5a was formed with 81% selectivity
(entry 1). The substituted phenylacetylenes bearing
an electron-donating group such as ÀOMe, at either
the para or the meta position, or tert-butyl demon-
strate high regioselectivities and good yields (en-
Scheme 3. Double cross-dimerization of 2 equiv. of trimethylsilylacetylene with 1 equiv.
of 1,7-dioctyne.
tries 2–4). However, the para-CF3-substituted substrate 4e
gave exclusively the homodimerization product 7e. In the case
of aliphatic alkynes, head-to-tail products were obtained as the
major product (>75%). In contrast to aromatic alkynes, E-
enynes (6) were also formed in some cases (<15%). Thus, 1-
benzylacetylene was transformed in 2 h, which gave 82% gem-
5 f (entry 6). 1-Hexyne reacted in 3 h and gave 75% gem-5g
and 12% E-6g as major products (entry 7). Terminal alkynes
bearing ÀNMe2 and ÀOMe groups were also cross-dimerized
with 3a to form 5h and i in good yields and high regioselec-
tivities (entries 8 and 9). The reaction worked equally well with
functionalized enynes such as 2-methyl-1-buten-3-yne and 1-
cyclohexenylacetylene to give gem-dienynes 5j and k, respec-
tively, in good yields and high regioselectivities (entries 10 and
11). 1,7-Octadiyne reacted with 2 equiv. of 3a and afforded the
gem-bis-enyne 5l in good yield because of a double cross-di-
merization process (entry 12 and Scheme 3).
Scheme 4. Rh-catalyzed cross-trimerization of terminal alkynes with tri-
methylsilylacetylene through the formation of 1,3-gem-enynes.
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not be obtained owing to the decomposition of dienyne 7e,
bearing a para-C6H4–CF3 substituent, under the reaction condi-
tions. The aliphatic substituted gem-1,3-enyne arising from
benzylacetylene gave 8 f, which was isolated in 71% yield. Het-
eroatom-substituted and olefin-functionalized alkynes also un-
derwent the coupling reaction towards the head-to-tail trimers
8i and j, respectively.
Mechanism of cross-dimerization and -trimerization of
terminal alkynes
A plausible mechanism for head-to-tail cross-dimerization (step
a) and head-to-tail cross-trimerization (step b) is shown in
Scheme 6. This reaction probably proceeds by a mechanism
similar to that proposed for the Rh-catalyzed dimerization of
terminal alkynes.[13f] The first step involves the substitution of
the cyclooctene ligand by the alkyne and the subsequent oxi-
dative addition generates RhIII–alkynyl–hydride intermediates.
Then, the insertion of alkyne or gem-1,3-enyne can proceed via
two pathways: carbometalation via 1,2 insertion (path I) or hy-
drometalation via 2,1 insertion (path II). Finally, reductive elimi-
nation affords the corresponding enynes or dienynes and the
coordination of a second alkyne or gem-1,3-enynes to the
metal regenerates the RhI active species. The selectivity out-
come is governed by two key facts: 1) The CÀH activation of
trimethylsilylacetylene in the first step is favored compared
with that of aromatic or aliphatic alkynes owing to its higher
acidity,[11] and 2) the bulkier trimethylsilyl group hinders the in-
sertion step for this substrate, thus, aliphatic and aromatic al-
kynes react faster. The increase in acidity of the alkyne through
incorporation of a ÀCF3 group in 1-ethynyl-4-(trifluoromethyl)-
benzene (4e) could explain the observed preferential
Cross-dimerization of trialkylsilylacetylenes with internal
alkynes
The efficiency of catalyst 2a was demonstrated for the cross-
dimerization of internal alkynes (9) with trimethylsilylacetylene
(3a) and triisopropylsilylacetylene (3b) (Scheme 5 and Table 3).
Scheme 5. Cross-dimerization of trialkylsilylacetylenes with internal alkynes.
R’, R“=alkyl, aryl.
homodimerization of this alkyne. Finally, note the
Table 3. Cross-dimerization of silylacetylenes with internal alkynes.[a]
beneficial effect of using an excess of pyridine, which
plays a crucial role in the catalytic cycle for the stabi-
lization of Rh–alkynyl–hydride intermediates.[13f]
Entry Alkyne
3
T
Conversion Molar ratio
Yield[b]
[%]
[h] [%] 10/11
1
2
3
4
5
6
9a 3a 12 71
90/10
64 (10a)
91 (10b)
83 (10c)
76 (10d)
75 (10e)
Conclusions
9b 3a
9c 3a
1
3
99
92
100/–
We have described an efficient protocol for the selec-
tive head-to-tail cross-dimerization and -trimerization
of silylacetylenes with alkynes promoted by a Rh–N-
heterocyclic carbene catalyst. This catalytic system is
an alternative for the synthesis of elaborated enynes
and dienynes, which are difficult to prepare by using
other available methods. The reaction could be ap-
plied to various terminal alkynes, symmetrical and
unsymmetrical internal alkynes, and gem-1,3-disubsti-
tuted enynes with substituents of different electronic
character.
(100/–)[c]/–
80/20
9a 3b 32 93
9b 3b 24 92
9c 3b 44 90
82/18
(40/60)[d]/– 81 (10 f)[d]
[a] Reaction conditions: 0.5 mL of C6D6, 0.2 mmol of 3a or b, 0.2 mmol of 9,
0.01 mmol of catalyst 2a+0.1 mmol of pyridine at 808C; [b] Isolated yield; [c] (E)-1-
Trimethylsilyl-3-phenylpent-3-en-1-yne derivative as the major regioisomer; [d] Isolat-
ed as a mixture of regioisomers.
Experimental Section
Both silylacetylenes reacted with 3-hexyne (9a) and diphenyla-
cetylene (9b) to give preferentially the syn-addition products
General
E-enynes 10, which were isolated in moderate to high yields.
The formation of thermodynamically more stable Z-enynes 11
resulted from the isomerization of 10, as observed at pro-
longed reaction times. Alkyne 3a was slightly more reactive
and selective than 3b (Table 3, entries 1 and 2 vs. entries 4 and
5). An unsymmetrical internal alkyne such as 1-phenyl-1-pro-
pyne (9c) reacted with 3a and gave preferentially E-1-trime-
thylsilyl-3-phenylpent-3-en-1-yne in high yield (entry 3). In con-
trast, the reaction of 9c with 3b afforded a mixture of E re-
gioisomers in a ratio of 40/60 (3-phenyl/4-phenyl; entry 6).
All reactions were performed with rigorous exclusion of air by ap-
plying Schlenk techniques. All reagents were commercially avail-
able and used as received, except for phenylacetylene, which was
distilled under Ar and stored over molecular sieves. Organic sol-
vents were dried by using standard methods and distilled under Ar
before use or obtained oxygen- and water-free with a Solvent Pu-
rification System (Innovative Technologies). The starting complexes
[Rh(m-Cl)(IPr)(h2-coe)]2 (1a)[16] and 2[13d] were prepared as described
1
previously in the literature. The H and 13C{1H} NMR spectra were
recorded with either a Bruker ARX 300 MHz or a Bruker 400 MHz
instrument. Chemical shifts for 1H and 13C{1H} NMR spectra were
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version was monitored with an NMR apparatus and
quantified by the integration of the 1H NMR signals of
the aliphatic or aromatic alkyne and the products
formed.
Isolation of gem-1,3-disubstituted enynes
General procedure:
A
solution of catalyst 2a
(0.02 mmol), pyridine (0.2 mmol), silylacetylene (3a or b,
0.4 mmol), and terminal alkyne (4, 0.4 mmol) or internal
alkyne (9, 0.4 mmol) in toluene (10 mL) was placed in
a Schlenk tube and stirred at 808C for the time indicated
in Table 1. The solution was analyzed by using GC–MS to
quantify the remaining substrate and was later concen-
trated under reduced pressure, affording a crude residue,
which was purified by using column chromatography on
silica gel (70–230 mesh) and eluted with hexane–diethyl
ether (99:1) to isolate the corresponding products.
Isolation of gem-1,3-disubstituted dienyne deriva-
tives
General procedure:
A
solution of catalyst 2a
(0.02 mmol), terminal alkyne (4, 0.4 mmol), and pyridine
(0.2 mmol) in toluene (10 mL) was placed in a Schlenk
tube and stirred at 408C. Upon conversion of the corre-
sponding alkyne into gem-1,3-enyne (7), trimethylsilyl-
acetylene (3a, 0.2 mmol) was added and the solution
was heated at 808C for 12 h. The solution was concen-
trated under reduced pressure, affording a crude residue,
which was purified by using column chromatography on
silica gel (70–230 mesh) and eluted with hexane–diethyl
ether (90:10) to isolate the corresponding dienynes (8).
NMR data
See the Supporting Information for full NMR data of all
compounds.
1-Trimethylsilyl-3-phenylbut-3-en-1-yne (5a): Isolated as
a light yellow oil; yield 71%; 1H NMR (400 MHz, C6D6,
298 K): d=7.81 (d, JHÀH =8.3 Hz, 2H; HoÀPh), 7.23 (dd,
J
HÀH =8.3, 7.4 Hz, 2H; HmÀPh), 7.18 (t, JHÀH =7.4 Hz, 1H;
Scheme 6. Plausible mechanism for the cross-dimerization and -trimerization of alkynes.
HpÀPh), 5.80 and 5.77 (both d, JHÀH =1.0 Hz, 2H; H4),
0.31 ppm (s, 9H; SiMe); 13C{1H} APT NMR plus HSQC and
HMBC (100 MHz, C6D6, 298 K): d=137.3 (s, CqÀPh), 131.1
(s, C3), 128.5 and 128.4 (both s, Cm,pÀPh), 126.2 (s, CoÀPh), 121.5 (s, C4),
104.7 (s, C2), 88.8 (s, C1), À0.3 ppm (s, SiMe); GC–MS: m/z (%): 200
[M+], 185 [M+ÀMe], 170, 145, 129, 105.
referenced to residual solvent peaks. Spectral assignments were ac-
1
complished by performing a combination of H–1H COSY, 13C-APT,
1
and H–13C HSQC and HMBC NMR experiments. The GC–MS spectra
were recorded with an Agilent 5973 mass selective detector inter-
faced to an Agilent 6890 series gas chromatograph system
equipped with an HP-5MS 5% phenyl methyl siloxane column
(30 m”250 mm with a 0.25 mm film thickness).
(E)-1-Trimethylsilyl-3-benzylidene-4-phenylpent-4-en-1-yne
(8a):
Isolated as a colorless oil; yield 65%; 1H NMR (400 MHz, C6D6,
298 K): d=7.42 (d, JHÀH =8.2 Hz, 2H; HoÀPhb), 7.15 (d, JHÀH =8.0 Hz,
2H; HoÀPha), 7.09 (s, 1H; H6), 6.93 (dd, JHÀH =8.2 Hz, 7.6, 2H; HmÀPhb),
6.89 (t, JHÀH =7.6 Hz, 1H; HpÀPhb), 6.75 (dd, JHÀH =8.0 Hz, 7.6, 2H;
HmÀPha), 6.72 (t, JHÀH =7.6 Hz, 1H; HpÀPha), 5.40 and 5.25 (both d,
J
HÀH =1.0 Hz, 2H; H5), 0.00 ppm (s, 9H; SiMe); 13C{1H} APT NMR
Catalytic cross-dimerization of silylacetylenes with terminal
or internal alkynes
plus HSQC and HMBC (100 MHz, C6D6, 298 K): d=145.6 (s, C4),
139.4 (s, C6), 138.0 (s, CqÀPhb), 136.0 (s, CqÀPha), 129.4 (s, CoÀPha), 128.7
and 128.6 (both s, Cm,pÀPhb), 128.3 and 128.2 (both s, Cm,pÀPha), 126.8
(s, CoÀPhb), 124.6 (s, C3), 116.2 (s, C5), 107.8 (s, C2), 92.0 (s, C1),
0.0 ppm (s, SiMe); HRMS (ESI): m/z: calcd for C21H22Si: 303.1564[
M++1]; found: 303.1544.
An NMR tube containing a solution of catalyst 2a (0.01 mmol,
5 mol%) in C6D6 (0.5 mL) was treated with silylacetylene (3a or b,
0.2 mmol), terminal alkyne (4, 0.2 mmol) or internal alkyne (9,
0.2 mmol), and pyridine (0.1 mmol) and heated at 808C. The con-
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(E)-1-Trimethylsilyl-3,4-diphenylbut-3-en-1-yne (10b): Isolated as
a colorless oil; yield (91%); H NMR (400 MHz, C6D6, 298 K): d=7.55
(d, JHÀH =8.1 Hz, 2H; HoÀPh), 7.30 (s, 1H; H4), 7.15–7.07 (m, 6H; HPh),
1
6.98 (d,
JHÀH =8.1 Hz, 2H; HoÀPh), 0.32 ppm (s, 9H; SiMe);
13C{1H} APT NMR plus HSQC and HMBC (100 MHz, C6D6, 298 K): d=
137.8 and 136.2 (both s, CqÀPh), 137.6 (s, C4), 129.5, 129.2, 128.6,
128.2, 127.8, and 127.7 (all s, CPh), 124.7 (s, C3), 108.4 (s, C2), 94.5 (s,
C1), À0.1 ppm (s, SiMe); HRMS (ESI): m/z: calcd for C19H20Si:
277.1407 [M++1]; found: 277.1426.
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Acknowledgements
We gratefully acknowledge financial support from the Spanish
Ministerio de Economꢀa y Competitividad (MEC/FEDER) of Spain
Project (CTQ2010-15221), the Diputaciꢃn General de Aragꢃn
(E07), the KFUPM-UNIZAR agreement, and the CONSOLIDER-IN-
GENIO 2010 program under the project MULTICAT (CSD2009-
00050). L.R.-P. thanks CONACyT, Mexico (186898 and 204033) for
a postdoctoral fellowship.
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Keywords: CÀC coupling
carbenes · rhodium
· dimerization · trimerization ·
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Received: May 12, 2014
Published online on July 25, 2014
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 2587 – 2592 2592