A new catalytic route for the activation of sp-hybridized carbon-hydrogen bonds

Article abstract of DOI:10.1002/anie.200603582

(Chemical Equation Presented) Vinyl-substituted silicon compounds react selectively with terminal alkynes in the presence of complexes containing [Ru]-H and [Ru]-Si bonds to form functionalized silylethynes (see picture). This reaction opens up a new catalytic route for the preparation of a class of potent organosilicon reagents for organic synthesis.

Full text of DOI:10.1002/anie.200603582

Communications
À
À
Rh, Ir, and Co) containing an M H and M Si bond initially,
or one generated in situ, catalyze this process.[1,2]
)
The mechanism for SCcatalyzed by ruthenium complexes
elucidated by Wakatsuki[3] and by us[4] involves insertion of a
À
vinylsilane into the M H bond and b-Si transfer to the metal,
À
with elimination of ethylene, to generate M Si species, then
À
insertion of the alkene into the M Si bond and b-H transfer to
the metal, with elimination of a substituted vinylsilane as
product, and regeneration of the catalyst.
The SCprocess under optimum conditions has become an
excellent synthetic tool for the regio- and stereoselective
synthesis of functionalized vinylsilicon compounds such as
(E)-N-(silyl)vinylcarbazole,[5a] amides,[5b] 1-silyl-1-(boryl)-
ethenes,[5c] functionalized cyclosiloxanes, cyclosilazanes,[5d]
and silsesquioxanes[5e] as well as macromolecular organo-
silicon compounds containing (E)-1,2-bis(silyl)fragments.[6]
These compounds can be used as synthetic reagents for
organic synthesis and as materials precursors and are difficult
to prepare by other TM-catalyzed reactions such as cross-
metathesis. The trans-silylation reaction has recently been
extended to other metalloids such as boron[7] and germa-
nium.[8]
C–H Activation
DOI: 10.1002/anie.200603582
A New Catalytic Route for the Activation of
sp-Hybridized Carbon–Hydrogen Bonds**
= À
The catalytic addition of the C H bond in aromatic
ketones, esters, and amines to vinylsilanes has been reported
by Murai et al. to yield compounds of the type
ArCH2CH2SiR3.[9a,b] These authors have also reported a
[Ru3(CO)12]-catalyzed coupling (silylation) of 3-acetylthio-
phene with trimethylvinylsilane that gives 3-acetyl-2-(trime-
thylsilyl)thiophene (64%, toluene, 20 h, 1158C),[9c] and Park
et al. have reported a rhodium-catalyzed silylation of benzyl
alcohol with trimethylvinylsilane that yields a siloxy deriva-
tive (93%, toluene, 2 h, 2008C).[9d] Ethylene was produced in
both reactions as a secondary product. In view of all the
results cited above it is worth emphasizing that the vinyl-
silicon compounds function as a silylating agent and a
hydrogen acceptor.
Bogdan Marciniec,* Beata Dudziec, and
Ireneusz Kownacki
In the last two decades we have developed a new type of
transition-metal (TM)-catalyzed reaction of vinyl-substituted
organosilicon compounds with olefins, known as a silylative
coupling (SC, or trans-silylation; Scheme 1), that is comple-
Herein we present a new catalytic reaction that involves a
coupling of terminal alkynes with vinylsilanes of the general
=
formula H2C CHSiR3 (where SiR3 is SiMe2Ph, Si(OEt)3,
Scheme 1. Silylative coupling of olefins with vinylsilanes.
SiMe(OSiMe3)2, and SiMe2(OSiMe3) as well as divinyltetra-
methyldisiloxane and divinyltetramethyldisilazane), which,
mentary to metathesis. This reaction occurs by cleavage of the by analogy, can be considered a silylative coupling of alkynes
= À
= À
C H bond of an olefin and the C Si bond of a vinylsilane,
in contrast to cross-metathesis, which uses the same substrates
and also gives the same products (except the gem-isomer), by
and which proceeds in the presence of complexes containing
À
À
[Ru] H and/or [Ru] Si bonds, such as [RuHCl(CO)(PCy3)2]
(I, C y= cyclohexyl), [RuHCl(CO)(iPr3)2] (II), [RuH(CO)-
(MeCN)2(PCy3)2][BF4] (III), [Ru(SiMe3)Cl(CO)(PPh3)2]
(IV),[10] and [RuHCl(CO)(PPh3)3] (V), and leads to the
evolution of ethylene and formation of the silyl-substituted
derivatives (Scheme 2). Interestingly, the hexacoordinate
complex V appears to be inactive in this reaction.
=
cleavage of the C Cbond. Various TM complexes (i.e. Ru,
[*] Prof. Dr. B. Marciniec, B. Dudziec, Dr. I. Kownacki
Department of Organometallic Chemistry
Faculty of Chemistry, Adam Mickiewicz University
Grunwaldzka 6, 60-780 Poznan (Poland)
Fax: (+48)618-291-508
Substituted alkynylsilanes are commonly used as alkyny-
lating agents in the synthesis of organic and natural prod-
E-mail: Bogdan.Marciniec@amu.edu.pl
[**] Financial support from the Ministry of Science and Higher
Education (Poland) (grants 3T09A 145 26 and PBZ-KBN 118/T09/
17) is gratefully acknowledged.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 2. Coupling of terminal alkynes with vinylsilanes.
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ucts[11] as well as precursors of optoelectronic materials.[12]
They can be prepared by classical stoichiometric routes
from organometallic reagents and, more recently, by
metal-complex-catalyzed silylation of terminal alkynes
with aminosilanes[13a] and chlorosilanes,[13b] dehydrogen-
ative silylation with hydrosilanes,[13c] or other transforma-
tions of silyl-substituted alkynes.[13d] These reactions have
been reviewed recently.[14]
Table 1: Silylative coupling of terminal alkynes with vinylsilanes.
R
R’3
Cat.
Cat./alkyne/
Conversion [%] Yield [%]
(TOF [hÀ1])
(Temp. [8C]) vinylsilane
tBu[a]
Me2Ph
I (100)
I (110)
10À2:1:5[b]
100
93 (21)
100
88[d]
10À2:1:3.15[c]
The use of ruthenium catalysts I–IV for silylative cross-
coupling of the selected alkynes, that is, alkyl, cycloalkyl,
and silyl ethynes, gives, under the optimum conditions, the
respective alkynylsilane, alkynylvinyltetramethyldisilox-
ane, and alkynylvinyltetramethyldisilazane as the main or
exclusive product (see Tables 1 and 2), in some cases
accompanied by the products of homo-coupling of the
vinylsilicon compounds used in excess. The turnover
frequency (TOF) parameters are of the order of 16–
27 hÀ1 (Table 1).
All the products were detected by GCand GC-MS
methods and identified by 1H, 13C, and 29Si NMR
spectroscopy. The reaction was examined in the presence
of catalysts I–V in an open system in toluene, or without
solvent, under a gentle stream of argon, although when
volatile 3,3-dimethyl-1-butyne was used the reaction was
conducted in a closed system. The equimolar reaction of
vinylsilanes with alkynes yields some enynes as a result of
the dimerization of alkynes, therefore an excess of vinyl-
silane (two- to fivefold) is necessary to prevent the
formation of these enynes. Silylation of 1-ethynyl-1-
(trimethylsiloxy)cyclohexane with divinylsiloxane and
divinylsilazane, even with a 6- or 10-fold excess of vinyl-
silicon compound, yields exclusively the monoalkynylvi-
nyldisiloxane and -disilazane with high yield and selectivity
(Table 2), accompanied by the homocoupling product of
the vinylsilicon substrates. It is noteworthy that, under the
conditions examined, the silylation of phenylacetylene
with vinylsilane does not occur (only traces).
II (100)
10À2:1:2[c]
93
90[e]
SiEt3
SiEt3
SiEt3
SiEt3
I (120)
I (100)
I (120)
IV (120)
IV (120)
I (120)
I (100)
10À2:1:2[c]
10À2:1:4[b]
100 (27)
43
94[d]
43
63
5”10À3:1:2[c] 63
10À2:1:2[c]
35
35
SiEt3
2À”210À2:1:5[f] 67
67[e]
70[d]
92
Si(tBu)Me2
10 :1:2.3[f]
10À2:1:10[g]
70
92
tBu[a]
(OEt)3
I (110)
10À2:1:4[c]
10À2:1:5[h]
10À2:1:4[b]
100 (16)
100
92[d]
III (120)
100
II (110)
50
50
SiEt3
I (120)
10À2:1:3.5[c] 90
74[d,e]
Me(OSiMe3)2 I (120)
Me2(OSiMe3) I (120)
10À2:1:4[c]
10À2:1:4[c]
100
100
91[d,e]
90[d,e]
[a] Closed reaction vessel. [b] CH2Cl2 (0.5m), closed reaction vessel. [c] Toluene
(0.5m). [d] Yield of isolated product. [e] Accompanied by traces of vinyl-substituted
silane homo-coupling product. Initial TOF measured after 0.5 h of reaction and
expressed in moles of alkyne per mole of Ru per hour. Reaction time: 24 h. Yields were
determined by GCanalysis. [f] Toluene (0.6 m). [g] Toluene (0.35m). [h] Without
solvent.
To explain the mechanism of this new alkyne trans-
formation a series of experiments involving the equimolar
reactions of [Ru(SiMe3)Cl(CO)(PPh3)2] (IV) with phenyl-
acetylene and silylacetylene were carried out to yield the
Table 2: Silylative coupling of terminal alkynes with divinyldisiloxane and
-disilazane.
1
respective main insertion products (identified by H NMR
spectroscopy and GC-MS) according to Scheme 3. The
spectra taken at various temperatures between À20 and
+ 308Cunexpectedly show an important difference in the
reaction of IV with the given acetylenes. Thus, while the
reaction with phenylacetylene occurs immediately, even at
À208C, to form the vinylenephenylruthenium complex VI
R
X
Cat./alkyne/divinylsilicon
compound
Conversion [%] Yield [%]
SiEt3
O
O
NH
NH
2”10À2:1:10[a]
10À2:1:2[c]
100
52
45
64
68
100[b]
52[d]
45[d]
64[b]
68[b]
10À2:1:2[c]
10À2:1:6[c]
NH 2”10À2:1:10[a]
À
and no [Ru] H is detected (see Figure 1), the same process
with silylacetylene at low temperatures allows the formation
O
O
O
NH
10À2:1:5[e]
2”10À2:1:10[a]
10À2:1:2[c]
95
100
94
97
100
95[b]
100[b]
94[d]
À
of a [Ru] H complex to be detected, as confirmed by the
appearance of a doublet of triplets at d = À6.63 ppm (JH,P
=
105.2, JH,P = 24.3 Hz), which at room temperature yields the
vinylenesilylruthenium analogue VII (see Figure 2).
10À2:1:6[c]
97[d]
NH 2”10À2:1:10[a]
87[d,f]
À
The [Ru] H complex is apparently produced in both
[a] Toluene (0.3m). [b] Accompanied by divinylsilicon compound homo-
coupling products. [c] Toluene (0.6m). [d] Accompanied by traces of
divinylsilicon compound homo-coupling products. [e] Toluene (0.5m).
[f] Yield of isolated product. Reaction time: 24 h. Yields were determined
by GCanalysis. Cat.: [RuHCl(CO)(PCy 3)2] (I); temperature: 1208C.
experiments at elevated temperatures (similar to the insertion
of olefins into a Ru Si bond[3,4]). This complex is active (in the
À
absence of vinylsilane) for the hydrogenation of acetylenes as
well as for transformations of ruthenium complexes into
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Communications
Scheme 3. Equimolar reactions of IV with acetylenes.
À
absence of vinylsilicon compounds) the [Ru] H complex
formed reacts with acetylenes to give dimers and hydro-
genated products as well as vinyleneruthenium complexes,
Figure 1. Temperature dependence of the 1H NMR spectra of the
reaction of IV with phenylacetylene. The boxed signals correspond to
those of the vinyl hydrogen atoms shown in italics.
clusters. Acetylene dimerization and co-dimerization with
styrene is also observed as a result of the well-known catalysis
of these reactions by ruthenium complexes.[15]
In separate experiments, we monitored the equimolar
reaction of the hexacoordinate complex V with silylacetylene
and phenylacetylene at room temperature. These reactions
lead to the formation of vinylene complexes VI and VII, both
of which were isolated. The evidence for their vinyleneruthe-
nium structure comes from the 1H NMR spectra of the
isolated complexes, in which two doublets of triplets for the
vinylene signals appear at d = 8.93 (JH,H = 13.3, JH,P = 2.3 Hz)
and 6.09 ppm (JH,H = 13.2, JH,P = 2.0 Hz; phenylvinylene) and
d = 8.84 (JH,H = 13.0, JH,P = 1.7 Hz) and 5.70 ppm (JH,H = 13.0,
J
H,P = 2.1 Hz; triethylsilylvinylene). The NMR spectroscopic
data for both isolated vinylene complexes are identical to
those reported in references [16,17]. The above experiments
À
Figure 2. Temperature dependence of the 1H NMR spectra of the
reaction of IV with HCꢀCSiEt3. The boxed signals correspond to those
of the vinyl hydrogen atoms shown in italics.
provide evidence that the insertion of acetylene into the Ru
Si bond occurs in a similar manner to the insertion of
alkenes.[3,4b] Under stoichiometric conditions (i.e. in the
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See Supporting Information for full experimental procedures and
whereas under the conditions of catalysis (80–1208C) vinyl-
silane, particularly if added in excess, reacts preferentially
with the ruthenium hydride complex according to a well-
the 1H, 13C, and 29Si NMR spectra, the GC-MS traces, and the
elemental analysis data of the products.
À
documented process to yield a [Ru] Si complex (see, for
example, references [3,4]) and ethylene (see Scheme 4),
Received: September 1, 2006
À
Keywords: alkynes · C H activation · homogeneous catalysis ·
ruthenium · silanes
.
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silicon compounds.
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À
À
which explains the catalytic activity of the [Ru] H/[Ru] Si
systems in the SCof alkynes. However, in the case of
phenylacetylene, the 1H NMR spectra of the equimolar
À
experiments indicate that its reaction with the [Ru] H
complex to form the vinylene complex occurs much faster
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ied.
The results of the above experiments allow us to propose a
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subsequent b-H elimination to give silyl-substituted ethyne
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(Scheme 4). This general mechanism is demonstrated by the
stoichiometric study of the insertion of a vinylsilicon com-
pound into the ruthenium hydride bond and the above-
À
described insertion of acetylene into the [Ru] Si bond.
In conclusion, we have shown that the general reaction
reported herein opens up a new catalytic route for the
À
activation of ꢀ C H bonds and, in combination with the well-
= À
known activation of C Si bonds, is an efficient method for
the selective synthesis of a variety of molecular compounds
with an acetylene functionality. The silylalkynyl derivatives
synthesized in this manner could play a very important role as
organometallic reagents in organic synthesis.[11a]
Experimental Section
General procedure: All experiments were performed under dry and
oxygen-free argon using standard Schlenk techniques for the
organometallic synthesis.
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