Solvent-free hydrosilylation of terminal alkynes by reaction with a nonclassical ruthenium hydride pincer complex

Article abstract of DOI:10.1002/ejic.201403016

Upon the simple addition of substrates, the ruthenium pincer complex [Ru(^tBuPNP)(H₂)(H)₂] [1; ^tBuPNP = 2,6-bis(di-tert-butylphosphinomethyl) pyridine] is an active and selective catalyst system for the hydrosilylation of terminal alkyl alkynes under mild, solvent-free conditions. The reactivity of this system for other functionalized terminal alkynes was also investigated, and we observed competing catalytic cycles that produce both alkyne dimers and dehydrogenative silylation products. Kinetic measurements for the hydrosilylation of 1-octyne show that the catalyst has an initial turnover frequency of 121 h^-1 at room temperature. The stoichiometric reaction between 1 and H₂SiPh₂ yields [Ru(^tBuPNP)(H)₂(H₂SiPh₂)], which undergoes Si-H bond activation to yield the catalytically active species [Ru(^tBuPNP)-(HSiPh₂)(H)]. The reaction of 1 with phenylacetylene yielded [Ru(^tBuPNP)(H)₂(HC≡CPh)] and [Ru(^tBuPNP)(H)(C≡CPh)-(HC≡CPh)], and we propose that the latter is the active species in the dimerization reaction.

Full text of DOI:10.1002/ejic.201403016

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DOI:10.1002/ejic.201403016
Solvent-Free Hydrosilylation of Terminal Alkynes by
Reaction with a Nonclassical Ruthenium Hydride Pincer
Complex
Christopher Conifer,[a] Chidambaram Gunanathan,[b]
Torsten Rinesch,[a] Markus Hölscher,*[a] and Walter Leitner*[a]
Keywords: C–H activation / Hydrosilylation / Dehydrogenative silylation / Nonclassical hydrides / Alkynes / Ruthenium
Upon the simple addition of substrates, the ruthenium pincer
complex [Ru(tBuPNP)(H2)(H)2] [1; tBuPNP = 2,6-bis(di-tert-
butylphosphinomethyl)pyridine] is an active and selective
catalyst system for the hydrosilylation of terminal alkyl alk-
ynes under mild, solvent-free conditions. The reactivity of
this system for other functionalized terminal alkynes was also
investigated, and we observed competing catalytic cycles
that produce both alkyne dimers and dehydrogenative
silylation of 1-octyne show that the catalyst has an initial
turnover frequency of 121 h–1 at room temperature. The
stoichiometric reaction between
1 and H2SiPh2 yields
[Ru(tBuPNP)(H)2(H2SiPh2)], which undergoes Si–H bond acti-
vation to yield the catalytically active species [Ru(tBuPNP)-
(HSiPh2)(H)]. The reaction of 1 with phenylacetylene yielded
[Ru(tBuPNP)(H)2(HCϵCPh)] and [Ru(tBuPNP)(H)(CϵCPh)-
(HCϵCPh)], and we propose that the latter is the active spe-
silylation products. Kinetic measurements for the hydro- cies in the dimerization reaction.
between the complex and the alkylsilane moiety can lead
Introduction
to the formation of the thermodynamically disfavoured Z
The hydrosilylation of terminal alkynes offers an impor-
tant route to alkenylsilanes, which have applications in
organic synthesis and silicone polymer production. The
production of organosilicon compounds, in general, is im-
portant for the photoresistor, semiconductor, adhesive and
binder industries.[1] Therefore, the hydrosilylation of a range
of unsaturated bonds such as carbonyl groups has also been
investigated.[2] Catalytic hydrosilylation reactions have been
performed with a variety of silanes and metal catalysts for
both internal and terminal alkynes,[3] but the reactions gen-
erally suffer from low selectivities; therefore, selective syn-
thetic protocols that make use of homogeneous catalysts
under mild conditions are desirable.[2d]
The hydrosilylation of terminal alkynes can give different
isomeric products, namely, the α isomer, the two β stereo-
isomers (Z and E) and also the dehydrogenative silylation
product (Scheme 1). Selectivity for the different isomers can
be achieved by variation of the catalytic system. A mecha-
nism proposed by Crabtree and Ojima also includes an ex-
planation of how isomerization to reduce steric interactions
isomer.[4]
Scheme 1. Reactions of terminal alkynes to yield hydrosilylation,
dimerization and dehydrogenative coupling products.
Under hydrosilylation reaction conditions, it is also
possible to have competing catalytic cycles that produce de-
hydrogenative silylation products[2a,4a,5] and oligomers of
terminal alkynes (Scheme 1).[6] For example, Field and co-
workers observed simultaneous hydrosilylation and dimer-
ization for a variety of terminal alkynes with Co, Ru and
Ir catalysts.[6j] Alkyne dimerization reactions to enynes can
potentially occur by different mechanisms via vinylidene
and alkyne intermediates and, as in the hydrosilylation reac-
tion, it is possible to obtain the E, Z and α isomers
(Scheme 1). For example, the formation of vinylidene com-
plexes by the reactions of terminal alkynes with several
different metal centres, including a ruthenium PNP pincer
complex,[7] are well known.[6m,7,8] The development of
efficient, selective catalytic systems for these competing
reactions is also of high interest, particularly the
dehydrogenative silylation reaction[2a,4a,5,9] and de-
hydrogenative coupling reactions in general.[9c]
[a] Institut für Technische Chemie und Makromolekulare Chemie,
RWTH Aachen University,
52074 Aachen, Germany
E-mail: leitner@itmc.rwth-aachen.de
hoelscher@itmc.rwth-aachen.de
www.itmc.rwth-aachen.de
[b] School of Chemical Sciences, National Institute of Sciences,
Education & Research (NISER),
Bhubaneswar 751005, India
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403016.
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Our group has an ongoing interest in the reactivity of
Increasing the steric demand of the substrate through the
the nonclassical hydride complex [Ru(tBuPNP)(H2)(H)2] [1; use of cyclopentyl and cyclohexyl moieties led to an inter-
tBuPNP
= 2,6-bis(di-tert-butylphosphinomethyl)pyridine; esting switch in selectivity. The selectivity towards the Z
Figure 1]. Complex 1 is an effective precatalyst in aromatic hydrosilylation product with cyclopentylacetylene is 56%;
H–D exchange reactions,[10] the borylation of aromatic sub- however, the conversion to the dehydrogenative silylation
strates such as toluene, anisole and benzene,[11] nitrile product is 41%. With the cyclohexyl substrate, the steric
hydrogenation[12] and alkyne borylation.[13] In this work, we demand appears to have a greater impact on the selectivity,
report the reactivity of 1 from a fundamental point of view and the dehydrogenative silylation product accounts for
in the hydrosilylation of a range of alkynes. We also investi- 89% of the conversion. Perhaps surprisingly, 3-meth-
gate side reactions that produce both dehydrogenative sil- ylhexyne was completely inactive despite its structural simi-
ylation products and alkyne oligomers. Furthermore, stoi- larity to cyclohexylacetylene.
chiometric reactions between 1 and substrates are used to
elucidate the catalytically active species.
The reactions of aromatic alkynes were attempted next;
however, the reaction between phenylacetylene, H2SiPh2
and 1 always gave complex mixtures owing to simultaneous
hydrosilylation and alkyne dimerization. The reactions were
repeated without the silane under the same conditions, and
only the dimer product 1,4-diphenyl-1-buten-3-yne was ob-
tained with 34% conversion and selectivity for the E prod-
uct of 76%. The dimerization of octyne showed an even
lower reaction rate compared to that of phenylacetylene
with 8% conversion and similar selectivity. The low rate of
octyne dimerization relative to that of phenylacetylene can
help explain why alkyl alkynes cleanly form the hydro-
silylation products and phenylacetylene reacts to give both
hydrosilylation and enyne products. The hydrosilylation of
other aromatic substrates such as 4Ј-phenoxyphenylacetyl-
ene always gave similar mixtures of hydrosilylation and
dimer products to those observed with phenylacetylene.
As high conversions and selectivities were achieved with
terminal alkyl alkynes, substrates with a methylene linker
between a carbon–carbon triple bond and an oxygen- or
nitrogen-containing functional group were reacted under
hydrosilylation conditions. However, methyl propynoate (I)
and N-methylpropyn-1-amine (II) did not react nor did 1-
ethynylcyclohexene (III) at temperatures up to 120 °C.
These substrates probably bind through the oxygen atom,
nitrogen atom or double bond and deactivate the catalyst,
as preliminary NMR spectroscopy experiments in the
absence of silane showed. Therefore, the reaction was
attempted with the weakly coordinating ether propynyloxy-
benzene (IV), and a low conversion of 14% was observed
at 90 °C; this suggests that substrate chelation or coordina-
tion is indeed the problem. The reactions were repeated in
tetrahydrofuran (THF) at 80 °C; zero conversion was ob-
tained for methyl propynoate, and a relatively poor conver-
sion of 6% was observed for N-methylpropyn-1-amine with
95% selectivity for the Z isomer.
Figure 1. Nonclassical hydride pincer complex [Ru(tBuPNP)(H2)-
(H)2] (1).
Results and Discussion
The focus of this work is ruthenium-catalyzed hydro-
silylation with 1 as the catalyst precursor, and our initial
investigations include the hydrosilylation of simple terminal
alkyl alkynes. The reactions of 1-octyne or cycloprop-
ylacetylene with H2SiPh2 in benzene or toluene solutions of
1 at room temperature give the Z hydrosilylation products
in over 90% yield and selectivity (Table 1). For a variety of
simple terminal alkyl alkynes, very similar results could be
achieved under solvent-free conditions when the catalyst
loading was reduced from 1.3 to 0.2% and the reactions
were heated to 50 °C. Cyclopropylacetylene gave the best
results of 97% conversion and a selectivity of 95% towards
the (Z)-vinylsilane.
Table 1. Products from the hydrosilylation of alkyl alkynes.
[a] Reaction conditions: 0.2 mol-% [Ru] (1), alkyne (1.078 mmol),
H2SiPh2 (1.078 mmol), 16 h, 50 °C. [b] Reaction heated at 90 °C.
1
[c] Characterized by H NMR spectroscopy and GC–MS.
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The hydrosilylation reactions of terminal alkyl alkynes product mixtures as observed by 31P NMR spectroscopy.
with 1 result in very good conversions and selectivities, The 31P NMR spectrum of the reaction mixture during
which compare favourably to those of some commercially catalysis is similarly complex and no single species can be
available hydrosilylation catalysts.[3g,14] Therefore, a kinetic characterized.
experiment was performed with 1-octyne at room tempera-
ture to determine the turnover frequency. Over the first
45 min, there is a clear induction period, which is presum-
ably caused by the formation of the catalytically active spe-
cies. The average turnover frequency over the first 4 h ex-
cluding the induction period was calculated to be 121 h–1,
which is again competitive with those of the commercially
available systems (see Figure S37, Supporting Informa-
tion).
To investigate the cause of the induction period observed
in the kinetic experiment and to better elucidate the cat-
alytically active species, a series of stoichiometric reactions
were performed between 1, H2SiPh2 and phenylacetylene.
Complex 1 and H2SiPh2 react within a few minutes in [D8]-
toluene or [D6]benzene to yield a 1:1 (Ru–Si) complex,
which can be identified by 31P NMR spectroscopy by a
single peak at δ = 93 ppm. During the course of the reaction
between 1 and H2SiPh2, the solution turns from green to
yellow, and the colour change is accompanied by the
evolution of a gas, presumably hydrogen. Initial attempts
to isolate [Ru(tBuPNP)(H)2(H2SiPh2)] (2) by removal of the
solvent under vacuum resulted in the decomposition of the
complex. The precipitation of the complex from solution
with pentane yielded an off-white solid, which was analyzed
by mass spectrometry and IR spectroscopy.
1
The H NMR spectrum has two hydride signals at δ =
–3.5 and –7.3 ppm in a 1:2 ratio and a triplet at δ = 6.7 ppm
with an integral of one, which corresponds to a silicon-
bound hydrogen atom coupled to two protons. We
tentatively assign the spectrum to 2, in which the two
1
Figure 2. H NMR spectrum of 2 in [D6]benzene.
hydride ligands are shared between the ruthenium and sili-
con centres, and one “free” hydride ligand remains bound
to the metal centre (Figure 2). This structure fits well with
On the basis of the data for 2, we can propose an entry
the 1H NMR spectroscopic data and also a very similar point into the hydrosilylation catalytic cycle. In the first
crystallographically determined product from the stoichio- step, H2SiPh2 reacts with 1 to eliminate dihydrogen and
metric addition of pinacolborane to 1.[13] A signal for 2 at yield 2 (Scheme 2). The cleavage of a Si–H bond and the
m/z = 682 corresponding to [M]+ (48%) is observed in the subsequent elimination of a second equivalent of H2 yields
EI-MS spectrum, but it is not the major signal in a cluster a ruthenium–silane complex with a vacant coordination site
of peaks with a characteristic Ru isotope pattern centred at for alkyne coordination. The catalytic cycle is anticipated
m/z = 679, which corresponds to the hydride-free ion [M – to proceed via ruthenium vinylidene intermediates as ob-
3H]+. Analysis of the IR spectrum shows that there is no served in the analogous borylation system, and the Z-selec-
characteristic signal associated with a H2 ligand and, there- tivity observed in both systems stems from the interaction
fore, provides more evidence for dihydrogen substitution of the R group of the terminal vinyl intermediate with the
with H SiPh . The Si–H bond absorption is observed at ν sterically demanding phosphine tBu groups (Scheme 2,
˜
2
2
= 2020 cm–1, and that of the hydride ligands is observed at bottom).[13,17] The conversion of [Ru(tBuPNP)(H)(HSi-
ν = 1717 cm–1. The complex is relatively unstable even in Ph2)(HCϵCR)] to a vinylidene is followed by the migration
˜
the solid state and can only be stored for short periods at of the silyl ligand. The dehydrogenative silylation products
–20 °C; therefore, no microanalytical data could be can be obtained when β-hydride elimination is faster than
collected. The relative instability of 2 could be attributed to elimination of the vinylsilane, and this appears to be related
the cleavage of the Si–H bond over time, which is a key step to the steric demands of the substrate. Crabtree and co-
in any hydrosilylation catalytic cycle (Scheme 2) and a likely workers proposed that an interaction between the R group
cause for the 45 min induction period observed in the and the silyl moiety in the vinyl intermediate orientates the
kinetic experiment.[15] The stoichiometric addition of β-hydride towards the metal centre and, thus, facilitates the
terminal alkynes to 2 results in the formation of complex elimination reaction.[4a] Another possibility is that the steric
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Figure 3. Stacked 31P NMR spectra showing stoichiometric reac-
tions between 1 and phenylacetylene in [D8]toluene to yield alkyne
complexes 3 and 4.
Scheme 2. Top: proposed catalytic cycle for the hydrosilylation of
terminal alkynes starting from complex 2. Bottom: higher energy
E vinyl intermediate.
The addition of 10 equiv. of phenylacetylene to 1 yields
a single product with a peak at δ = 70 ppm (Figure 3),
which can also be isolated by precipitation from solution
with pentane. The product was analyzed by mass spectrom-
etry as well as IR and multinuclear NMR spectroscopy. The
mass spectrometry data shows the parent ion peak at m/z
= 700, which corresponds to the dialkyne complex
[Ru(tBuPNP)(H)(CϵCPh)(HCϵCPh)] (4). The IR spectrum
demands of vinyl intermediates with relatively large R
group slow coordination of H2SiPh2 to the metal centre and
allow the β-hydride elimination reaction to dominate over
vinylsilane formation (Scheme 2).
shows signals at ν = 2075 and 2047 cm–1 for the carbon–
Stoichiometric reactions were also conducted with phen-
ylacetylene and 1 to determine if the active species in the
dimerization reaction could be observed and isolated. To
separate solutions of 1 in [D8]toluene, phenylacetylene (1, 2
and 10 equiv.) was added, and the reactions were monitored
by 31P NMR spectroscopy (Figure 3). The addition of
1 equiv. of phenylacetylene results in the formation of a new
peak in the 31P NMR spectrum at δ = 82 ppm in addition
to a signal at δ = 109 ppm, which corresponds to 1, in an
approximate 5:1 ratio.
˜
carbon triple bonds and a hydride signal at ν = 1586 cm–1;
˜
these values are very similar to those reported by Gusev
When phenylacetylene (1.5 equiv.) is added to 1, there
are initially signals at δ = 82, 70 and 64 ppm in a 96:2:2
ratio in the 31P NMR spectrum. After 2 h, the only signal
observed is at δ = 82 ppm, and the complex can be isolated
from solution by precipitation for further spectroscopic
analysis. In the 1H NMR spectrum, the aromatic peaks
integrate to eight protons, which suggests that a reaction
occurs with 1 equiv. of phenylacetylene to displace the di-
hydrogen ligand and yield [Ru(tBuPNP)(H)2(HCϵCPh)] (3).
Mass spectrometry and 13C NMR and IR spectroscopic
analysis provide further evidence for the identity of 3. As
no dihydrogen bands are observed in the IR spectrum, the
reaction of one of the ruthenium hydride bonds with phen-
ylacetylene to yield [Ru(tBuPNP)(H2)(H)(CϵCPh)] can be
ruled out.
1
Figure 4. H NMR spectra of 4 in [D8]THF at 298 and 233 K; the
rapid exchange of the inequivalent phenyl groups at room tempera-
ture is demonstrated.
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Scheme 3. Proposed catalytic cycle for the dimerization of phenylacetylene.
and co-workers for the complex [Ru(tBuPCP)- the C–C triple bond, a significant decrease in activity is
(H)(CO)(CϵCPh)], which gives peaks at ν = 2065 cm–1 for observed for weakly coordinating groups, and no conver-
˜
the CϵC bond and 1583 cm–1 for the Ru–H bond.[16]
sion is observed for strongly coordinating groups; this
The hydride and alkyne proton signals in 3 and 4 are not suggests that the active site is poisoned. Phenylacetylene
observed in the 1H NMR spectrum. Presumably, the signals undergoes simultaneous hydrosilylation and dimerization to
are very broad owing to rapid exchange. The spectrum of 4 yield enynes with selectivity for the E product. Stoichio-
in particular has very broad signals, including those of the metric reactions between precatalyst 1, H2SiPh2 and phen-
phenyl groups, which are equivalent on the NMR timescale ylacetylene were performed, and the products were charac-
at room temperature. The cooling of 4 to 233 K results in terized. The reaction of 1 with H2SiPh2 results in the substi-
the clear resolution of the two phenyl signals; however, the tution of the dihydrogen ligand to yield the new ruthenium
alkyne proton and hydride signals are still not observed silane complex 2 (Figure 2). Presumably, this complex is in-
(Figure 4). The vinylidene tautomers of 3 and 4 cannot be volved in the catalytic cycle and undergoes Si–H bond acti-
completely ruled out as possible products; however, the vation to yield a ruthenium–silane bond. The reactions of
characteristic 13C NMR, 1H NMR and IR vinylidene spec- 1 with varying equivalents of phenylacetylene ultimately
troscopic signals are not observed.[6m,7,8]
yield the dialkyne complex 4 (Figure 3), which is also ob-
31P NMR spectroscopic measurements during the cata- served by 31P NMR spectroscopy during the catalytic di-
lytic dimerization of phenylacetylene show that 4 is the only merization reaction. The dimerization of terminal aryl alk-
observable species during the reaction. Similar alkyne com- ynes is much faster than that of terminal alkyl alkynes. This
plexes and their vinylidene tautomers have been described explains why a mixture of dimer and hydrosilylation prod-
as intermediates in the oligomerization of terminal alkynes ucts is observed from the hydrosilylation of terminal aryl
by the mechanism shown in Scheme 3, and this mechanism alkynes but only hydrosilylation products are observed with
fits very well with the catalytic cycle proposed for the hydro- terminal alkyl alkynes. An interesting observation was the
silylation and borylation systems.[6m,8g,13]
relatively unusual conversion to dehydrogenative silylation
products when sterically demanding alkynes were used.
This selectivity could be of interest for the production of
alkyne synthons for coupling reactions. These results in
Conclusions
We report a new catalyst for the hydrosilylation of combination with work previously reported by our group
terminal alkynes. We have shown 1 to be a Z-selective pre- show that 1 is a versatile precatalyst for the conversion of
catalyst for the hydrosilylation of terminal alkyl alkynes un- terminal alkynes to
a
range of useful products
der mild conditions with low catalyst loading. The reaction (Scheme 4).[13] Further investigations with sterically de-
between 1-octyne and H2SiPh2 proceeds with an initial manding silanes to obtain only dehydrogenative silylation
turnover frequency of 121 h–1 at room temperature. When products and also the catalytic synthesis of enynes are cur-
coordinating functionalities are introduced at or close to rently under investigation.
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minal alkynes to yield borylation,[13] hydrosilylation, dimerization
and dehydrogenative coupling products.
[3]
Experimental Section
General Remarks: All moisture- and oxygen-sensitive compounds
were prepared by using standard high vacuum line, Schlenk and
cannula techniques. A standard argon-filled glovebox was used for
any subsequent manipulation and storage of these compounds.
Complex 1 was prepared as described previously (see Supporting
Information). All other reagents were commercially sourced, dried
and deoxygenated before use.
General Reaction Protocol for Acetylene Hydrosilylation Reactions:
Complex 1 (1 mg, 2 μmol) was dissolved in H2SiPh2 (200 mg,
1.1 mmol) to give a yellow solution. Once hydrogen evolution had
ceased, the acetylene (1.1 mmol) was added. The reaction mixture
was then heated for 16 h. The product distribution was analyzed
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Hydrosilylation
C. Conifer, C. Gunanathan, T. Rinesch,
M. Hölscher,* W. Leitner* ................ 1–8
Solvent-Free Hydrosilylation of Terminal
Alkynes by Reaction with a Nonclassical
Ruthenium Hydride Pincer Complex
Keywords: C–H activation / Hydrosilyl-
ation / Dehydrogenative silylation / Non-
classical hydrides / Alkynes / Ruthenium
A selective catalyst system for the hydro-
silylation of alkyl alkynes under mild,
solvent-free conditions has been developed
with the ruthenium pincer complex
[Ru(tBuPNP)(H2)(H)2] [tBuPNP = 2,6-bis-
(di-tert-butylphosphinomethyl)pyridine].
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim