Water-soluble N-heterocyclic carbene platinum(0) complexes: Recyclable catalysts for the hydrosilylation of alkynes in water at room temperature

Article abstract of DOI:10.1021/om300148q

The synthesis and characterization of new water-soluble platinum(0) complexes bearing sulfonated N-heterocyclic carbene (NHC) and divinyltetramethylsiloxane (dvtms) ligands are described. These complexes, of the general formula (NHC)Pt(dvtms), are active

Full text of DOI:10.1021/om300148q

Article
pubs.acs.org/Organometallics
Water-Soluble N-Heterocyclic Carbene Platinum(0) Complexes:
Recyclable Catalysts for the Hydrosilylation of Alkynes in Water at
Room Temperature
Gustavo F. Silbestri, Juan Carlos Flores,* and Ernesto de Jesus*
́
́ ́ ́
Departamento de Química Inorganica, Campus Universitario, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain
ABSTRACT: The synthesis and characterization of new water-
soluble platinum(0) complexes bearing sulfonated N-heterocyclic
carbene (NHC) and divinyltetramethylsiloxane (dvtms) ligands are
described. These complexes, of the general formula (NHC)Pt(dvtms),
are active and recyclable catalysts for the hydrosilylation of phenyl-
acetylene and other alkynes at room temperature in water. Our findings
indicate that the NHC−Pt(0) bonds are reasonably stable under these
catalytic conditions, although hydrolysis is observed at temperatures
above 80 °C in pure water.
co-workers in 2001.15 Subsequent papers have mainly dealt
with ruthenium-catalyzed olefin metathesis,16−22 palladium-
catalyzed Suzuki−Miyaura cross-coupling reactions,23−29 or,
more recently, the hydration of terminal alkynes with gold
complexes.30,31 In addition to these metals, synthetic efforts
have mainly been directed toward the synthesis of water-soluble
NHC Ag complexes, especially for use as NHC transfer
agents23,32,33 or for medical applications,34,35 although several
examples have been reported containing other metals.36−38
Alkenylsilanes are especially valuable intermediates in modern
organic chemistry,39,40 especially for palladium-catalyzed reac-
tions,41,42 such as the vinylation of aryl halides, recently adapted
by our group and others to aqueous conditions.43−45 Methods
for the preparation of alkenylsilanes by hydrosilylation of alkynes
in aqueous media would, therefore, be highly desirable.46−50 In
this respect, although the Pt Karstedt complex ([Pt2(dvtms)3],
dvtms = 1,3-divinyl-1,1,3,3-tetramethyldisiloxane)51,52 and the
Speier system (H2PtCl6/iPrOH)53,54 are widely used in industry,
they form colloidal Pt particles during the course of the reaction.
INTRODUCTION
■
Catalytic reactions in water involving metal complexes and
organic substrates have been growing in importance over the
past few years.1−4 Indeed, a large number of water-soluble
ligands have been designed for this purpose by attaching
neutral or, more often, ionic hydrophilic groups (sulfonate,
carbonate, ammonium, etc.) to traditional hydrophobic ligands,
such as phosphanes.5 In the past decade, N-heterocyclic
carbenes (NHCs) have contributed to significant advances in
ruthenium-mediated olefin metathesis, palladium-promoted
cross-coupling, and other transition-metal-catalyzed reac-
tions.6−8 The relative stability of NHC−metal bonds toward
hydrolysis has promoted the use of NHC complexes in metal-
catalyzed organic syntheses performed in water. Unmodified
hydrophobic NHC ligands are often used for this purpose when
ionic species are involved in the catalytic process or when water
solubility is achieved by attaching another ligand to the metal
center.9−12 However, the attachment of hydrophilic substitu-
ents directly to the NHC ligands is more convenient to avoid
leaching of the bond between the water-soluble ligand and the
metal. Retention of the catalyst in the aqueous phase during
and after the catalytic cycle by means of the hydrophilic groups
is a key issue for catalyst recovery in biphasic processes.
Herrmann and co-workers published the first examples of
complexes with water-soluble NHC ligands, in which a butyl-4-
sulfonate group was bonded to one of the NHC nitrogen
atoms, in a patent filed in 1995.13 The same group sub-
sequently reported the stability of rhodium(I) complexes with
hydroxyalkyl- or carboxylate-functionalized NHC ligands in
water.14 Sulfonates, carbonates, ammonium groups, sugar moieties,
or polyethers are among the most common solubilizing groups
employed for this purpose. Applications in aqueous-phase catalysis
have appeared more recently since the report of a ruthenium
́
Marko and co-workers have demonstrated that NHC platinum(0)
complexes avoid the formation of platinum colloids55 and catalyze
the hydrosilylation of alkynes with remarkable efficiency and
selectivity.56,57 Moreover, the regioselectivity of these catalysts
is proposed to be controlled by steric crowding when bulky aryl
substituents are present at the nitrogen atoms of the NHC ligand.
Herein, we report the first examples of water-soluble NHC
platinum(0) complexes and their use as catalysts for the hydro-
silylation of terminal alkynes. These catalysts are active at room
temperature and highly recyclable, a property that can be linked
to the stability of the platinum−carbene bond in the aqueous
reaction conditions.
Received: February 23, 2012
Published: March 30, 2012
̈
catalyst for the synthesis of 2,3-dimethylfuran by Ozdemir and
© 2012 American Chemical Society
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complexes are completely dissolved into the aqueous phase in
mixtures of this solvent with diethyl ether or toluene.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The water-soluble
platinum(0)−carbene complexes 3 and 4 were prepared in
high yield in THF or DMSO from the commercial Karstedt
complex and the corresponding imidazolium compounds 123
and 232,58 using sodium tert-butoxide as the deprotonating
agent (Scheme 1). A similar procedure was previously used by
Hydrosilylation of Terminal Alkynes in Water. The
NHC complexes 3 and 4 were initially tested as catalysts in the
hydrosilylation of phenylacetylene with triethylsilane in water.
A first run of experiments showed that catalyst 4 was able to
attain complete conversions at room temperature after 6 h of
reaction even with Pt loadings as low as 0.05 mol % (Table 1).
Scheme 1. Synthesis of Water-Soluble Pt(0) N-Heterocyclic
Carbene Complexes 3 and 4
Table 1. Hydrosilylation of Phenyacetylene with
Triethylsilane Catalyzed by Complex 4 in Water
a
b
b
entry
Pt (mol %)
time (h)
β(E)/α
conversion (%)
1
2
4
5
6
0.5
1
3
6
6
6
93:7
91
100
100
100
100
0.5
92:8
0.25
0.1
91:9
90:10
87:13
0.05
a
In a typical experiment, 1 mmol of alkyne, 1.1 mmol of triethylsilane,
and the Pt catalyst were stirred for the specified period of time at 30 °C
in 3 mL of water. The reaction mixture was extracted with diethyl
b
1
ether and the solvent removed in vacuo. Determined by H NMR
spectroscopy. Formation of the β(Z) isomer was not observed.
The activity shown by this complex is comparable to that of the
best Pt phosphane-based catalysts in hydrocarbon solvents for
the same reaction.62 The regioselectivity (β-(E) with respect to
1
α isomers) was determined by H NMR spectroscopy and
́
Marko and co-workers to prepare related NHC platinum(0)
found to be comparable to that found with [Pt(PCy)3] for the
same substrate, but lower than that reported for other
phosphane or NHC complexes.56,57,62 The more encumbered
complex 3 was, as expected, less active (cf. entries 1 and 2 in
Table 2) and showed poor selectivity. The efficiency of the
complexes.59,60 Purification of these products was straightfor-
ward due to their insolubility in acetone. They are very stable in
air and can be stored for prolonged periods of time when pro-
tected from light. Complexes 3 and 4 can also be obtained by
carbene transmetalation61 from the appropriate NHC silver
complex, prepared according to reported procedures,32 to the
Karstedt platinum(0) complex, although the solids thus
obtained are contaminated by significant amounts of byproduct.
Table 2. Comparison of Catalysts for the Hydrosilylation of
a
Phenyacetylene with Triethylsilane in Water
1
These complexes were fully characterized by H, 13C, and
195Pt NMR spectroscopy, elemental analysis, and mass
1
spectrometry. H NMR spectroscopy and elemental analyses
showed their tendency to trap solvent molecules upon crys-
tallization (see the Experimental Section for details). The
electrospray ionization mass spectra (ESI-TOF) obtained in
methanol showed the molecular anion corresponding to the
loss of a sodium(1+) ion for both complexes. The platinum-195
chemical shift at ca. −5340 ppm is similar to that found for
related NHC platinum(0) complexes.59 Coordination of the
NHC ligand to the platinum atom was confirmed by the ob-
servation of 195Pt satellites in the resonances assigned to the
proton and carbon-13 nuclei at the 4- and 5-positions of the
imidazole ring (4J(1H−195Pt) = 10 Hz, 3J(13C−195Pt) = 40 Hz).
The 195Pt satellites for the carbene carbon at ca. 180 ppm could
not be detected with accuracy due to the low intensity of the
observed resonance.
b
b
c
entry
Pt catalyst
β(E)/α
conversion (%)
yield (%)
1
2
3
4
4
3
90:10
60:40
85:15
74:26
100
71
97
67
89
60
Pt2(dvtms)3
>90
64
(NH4)2[PtCl6]
a
See Table 1 for general reaction conditions. Reaction time: 6 h. Pt
b
c
loading: 0.1 mol %. Determined by 1H NMR spectroscopy. Isolated
yield (mixture of isomers).
commercial Karstedt catalyst in terms of activity or selectivity is
intermediate between those shown by 3 and 4 (Table 2, entry 3),
whereas the ammonium salt of the hexachloridoplatinate(IV)
anion showed lower activity (Table 2, entry 4).
All our attempts to use tris(ethoxy)silane instead of tri-
ethylsilane failed to attain observable conversions. It is well
known that the silicon−hydrogen bonds in alkoxysilanes are
generally less reactive than those in alkylsilanes. However, in
this case, the complete failure of the tris(ethoxy)silane to react
The partition coefficients for the new platinum complexes in
mixtures of water and common organic solvents could not be
determined accurately due to the absence of characteristic
absorptions in their UV−vis spectra. Qualitative information
obtained by 1H NMR spectroscopy showed that both
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is due to hydrolysis of the silicon−alkoxide bonds, which gives
rise to the formation of polysiloxane species that are scarcely
soluble in water. In fact, the platinum(0) catalysts reported here
are quite active when the same reaction is performed in neat
tris(ethoxy)silane as the solvent (Table 3).
formation of such bonds is sometimes possible in this solvent.30,63
However, the literature gives scarce information on the long-term
hydrolytic durability of metal−carbene bonds, especially at high
temperatures or under catalytic conditions. Catalysts 3 and 4 are
stable in D2O at room temperature for at least 1 month and at 70 °C
for at least 3 days. However, partial hydrolysis (ca. 50%) was
observed when the solutions were heated at 80 °C for 24 h.
A similar decomposition temperature in D2O was reported for the
complex [RuCl(p-cymene)(PTA)(NHC)]+, which bears a hydro-
phobic 1-butyl-3-methylimidazol-2-ylidene NHC ligand.64
Although the stability of these complexes during the catalytic
experiments at room temperature is debatable, two findings
support their durability under these conditions. First, the forma-
tion of colloidal Pt particles was never observed by TEM micro-
scopy when samples of the aqueous solutions were analyzed at the
end of the reaction under the conditions specified in entries 1 and
2 of Table 2. Second, catalysts 3 and 4, which differ only in terms
of the carbene moiety, afforded very different regioselectivities
with substrates such as phenylacetylene, and these regioselectivities
were maintained in recycling experiments, as described below.
Catalyst Recycling. In an ideal situation, a hydrophilic
catalyst can be recovered from the organic product stream by
simple phase separation at the end of the reaction. We, therefore,
reasoned that high recyclability, in addition to practical advantages,
should serve as experimental support for the proposed stability of
the metal−carbene bonds in these platinum(0) complexes during
the catalytic cycle. Recovery experiments were performed using tri-
methylsilylacetylene as the substrate and 0.01 or 0.5 mol %
loadings of the NHC−Pt complex (3 or 4) or the Karstedt
catalyst. Each recycling experiment was repeated until the
conversion dropped below 95%; the number of cycles that
attained this conversion can be found in Table 5, together with
the reaction conditions. The Karstedt and hexachloridoplatinate-
(IV) complexes suffered from significant activity losses after the
initial cycles, whereas the NHC catalysts attained high levels
of recyclability. The percentage of platinum retained in the
aqueous solution at the end of the recycling experiments was
Table 3. Hydrosilylation of Phenylacetylene in Neat
a
Tris(ethoxy)silane
b
b
entry
Pt catalyst
β(E)/α
conversion (%)
1
2
4
3
73:27
70:30
80
77
a
See Table 1 for general reaction conditions. Reaction time: 16 h. Pt
b
1
loading: 0.01 mol %. Determined by H NMR spectroscopy.
The scope of these reaction conditions was subsequently
studied with a selection of terminal alkynes (Table 4). An
interesting observation is that the reactivity differences between
3 and 4 are especially noticeable with alkynes bearing aryl
substituents (entries 7−12). Both catalysts were inactive in the
hydrosilylation of propargyl alcohol and propiolic acid under
these conditions. The selectivities were found to depend on
both the catalyst and the substrate, with both catalysts affording
similar regioselectivities in most cases. Interestingly, the regio-
selectivity was drastically modified upon replacing a phenyl by a
naphthyl group (cf. Table 2, entry 1, and Table 4, entry 12).
The tremendous popularity of N-heterocyclic carbenes
(NHCs) as ligands for transition metals is based on the en-
hanced stability imparted by their strong σ-donating properties,
which is associated with high steric hindrance. Here, we raise
the question of the stability of the metal−carbene bonds in
aqueous solution during the catalytic process. The studies reported
to date, and summarized above, have shown that metal−NHC
complexes are stable when dissolved in water and even that the
a b c d
, , ,
Table 4. Hydrosilylation of Terminal Alkynes in Water
a
b
c
d
1
See Table 1 for general conditions. Reaction time: 6 h. Determined by H NMR spectroscopy. Isolated yield (mixture of isomers). Reaction
time: 24 h.
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Plus spectrometer. Chemical shifts (δ, parts per million) are quoted
relative to SiMe4 (1H, 13C) and H2PtCl6 (195Pt) and were measured by
internal referencing to the 13C or residual 1H resonances of the
deuterated solvents, or by the substitution method in the 195Pt case.
Coupling constants (J) are given in hertz. The Analytical Services of
the Universidad de Alcala performed the C, H, S, and N analyses using
́
a Heraeus CHN-O-Rapid microanalyzer, and the ESI mass spectra
using an Automass Multi, ThermoQuest spectrometer. The SIDI
Table 5. Recycling Experiments Using
Trimethylsilylacetylene as Substrate
a
b
c
catalyst
Pt (mol %)
no. of cycles
β(E)/α
d
3
4
0.5
>8
55:45 to 60:40
56:44 to 65:35
57:43 to 59:41
56:44 to 60:40
58:42 to 59:41
56:44
d
0.5
>9
Karstedt
0.5
2
4
4
1
1
3
0.01
0.01
0.01
0.01
́
laboratories of the Universidad Autonoma de Madrid performed
4
TXRF analysis using an 8030 C spectrometer (FEI Company, Munich,
Germany), equipped with two X-ray fine focus lines, Mo and W
anodes, and a Si(Li) detector with an active area of 80 mm2 and a
resolution of 148 eV at 5.895 keV (Mn KR). STEM images were
obtained by the Microscopy Centre “Luis Bru” of the Universidad
Complutense de Madrid using a JEOL 2000FX microscope operating
at an accelerating voltage of 200 kV. Samples were prepared by placing
two drops of the aqueous solutions on a holey-carbon-coated grid and
allowing the solvent to evaporate in air.
Karstedt
(NH4)2[PtCl6]
56:44
a
See Table 1 for general conditions. Substrate: trimethylsilylacetylene.
Reaction time: 6 h. After every cycle, the products were extracted with
diethyl ether and a new load of substrate was added to the aqueous
b
solution. Number of cycles in which conversions of >95% are
c
1
attained. Isolated yields were always >87%. Determined by H NMR
d
spectroscopy. 100% conversions were obtained in all the cycles
Synthesis of the Imidazolium Salts. Compounds 123 and 232,58
were prepared according to reported procedures. The yield in the
preparation of 1 was increased from the previously reported 70% to
98% by replacing the final chromatographic purification by
precipitation of the product from DMSO solution with acetone.
Water solubility at 25 °C: 160 (1) and 72.5 g/L (2).
performed.
high (TRXF analysis showed that the final Pt concentrations
were 60−86% of the initial concentrations), thus showing the
efficiency of the NHC ligands in trapping the platinum(0)
species in this phase.
Synthesis of [1,3-Bis(2,6-diisopropyl-4-sodiumsulfo-
natophenyl)imidazol-2-ylidene](1,1,3,3-tetramethyl-1,3-
divinyldisiloxane)platinum(0) (3). A commercial solution of
Karstedt’s catalyst (0.1 M in poly(dimethylsiloxane), 1.5 mL, 0.15 mmol)
and the imidazolium salt 1 (0.085 g, 0.15 mmol) were dissolved in DMSO
(3 mL) under argon at 0 °C. Sodium tert-butoxide (0.020 g, 0.21 mmol)
was then added to the above solution and the reaction mixture stirred for
24 h at room temperature. The mixture was filtered through a pad of
kieselguhr, acetone (6 mL) was added to the resulting solution, and the
mixture was stirred overnight. Subsequent filtration of the reaction
mixture afforded complex 3 as a pale yellow solid (0.142 g, 96%), which
was washed with acetone (3 × 5 mL) and dried under vacuum. Complex
3 is soluble in water, methanol, tert-butanol, and DMSO; partially soluble
in isopropanol; and insoluble in tetrahydrofuran, diethyl ether, and
acetone. 1H NMR (500 MHz, DMSO-d6): δ 7.73 (s with 195Pt satellites,
When phenylacetylene was used as the substrate, only two
cycles attained conversions above 95%, even when Pt loadings
of 0.5 mol % were employed. Recycling of the catalyst can
alternatively be performed using a temperature-dependent or
thermomorphic multicomponent solvent (TMS) system, which
consists of a mixture of a polar and a nonpolar solvent together
with a third component of intermediate polarity. The mixture
forms two phases at low temperature but becomes homoge-
neous at high temperature.65,66 The catalytic reaction is then
run homogeneously at high temperature, with phase separation
being performed at low temperature. The hydrosilylation of
phenylacetylene with thiethylsilane was performed in a water/
toluene/DMF TMS system at 80 °C in the presence of 0.1 mol %
of catalyst 3 and monitored by gas chromatography (see the
Experimental Section for details). The reaction was complete in
9 h and afforded a mixture of β(E) (86%), β(Z) (4%), and α
(10%) isomers. The catalyst could be reused in four cycles, during
which the catalytic activity and selectivity remained practically
constant, thus meaning that recyclability was enhanced in com-
parison with experiments performed in pure water.
3
4JPt−H = 10, 1H, HIm), 7.41 (s, 2H, Ar), 2.88 (h, JH−H = 7.0, 2H,
3
2
−CHMe2), 1.63 (d with 195Pt satellites, JH−H = 12.0, JPt−H = 50, 1H,
3
vinyl), 1.42 (d, JH−H = 13.5, 1H, vinyl), 1.21 (dd, 1H, vinyl), 1.16 (d,
3JH−H = 7.0, 6H, CHMe2), 1.09 (d, 3JHH = 7.0, 6H, CHMe2), 0.00 (s, 3H,
SiMe2), −0.84 (s, 3H, SiMe2). 1H NMR (500 MHz, D2O): δ 7.57 (s, 1H,
3
HIm), 7.55 (s, 2H, Ar), 2.87 (h, JH−H = 7.0, 2H, −CHMe2), 1.70 (d,
3JH−H = 11.5, 1H, vinyl), 1.49 (d, 3JH−H = 13.5, 1H, vinyl), 1.29 (dd, 1H,
3
3
vinyl), 1.17 (d, JH−H = 7.0, 6H, CHMe2), 1.05 (d, JHH = 7.0, 6H,
CHMe2), 0.00 (s, 3H, SiMe2), −0.84 (s, 3H, SiMe2). 13C NMR (75 MHz,
DMSO-d6): δ 181.8 (Pt−C2 im), 148.4 (C Ar), 144.0 (CH Ar), 135.9 (C
Ar), 124.7 (3JPt−C = 41.4, C1 Ar), 120.1 (3JPt−C = 39.6, C4,5 im), 40.7
(SiCHCH2), 33.9 (1JPt−C= 121.3, SiCHCH2), 27.5 (CHMe2), 24.9
(CHMe2), 21.6 (CHMe2), 1.2 (SiMe2), −2.6 (SiMe2). 195Pt NMR (107
MHz, DMSO-d6): δ −5332. ESI-MS (negative ion, MeOH) m/z: 950.23
[M − Na]−, 928.25 [MH − 2Na]−, 463.6 [M − 2Na]2−. Anal. Calcd for
C39H64N2Na2O9S4Si2Pt (3·2DMSO): C, 41.44; H, 5.71; N, 2.48. Found:
C, 40.98; H, 5.68; N, 2.60.
CONCLUSIONS
■
In summary, we have shown that water-soluble platinum(0)
complexes containing sulfonated N-heterocyclic carbene
ligands are active and recyclable catalysts that can activate the
hydrosilylation of some terminal alkynes at room temperature
in water. Our results support the proposal that NHC−Pt(0)
bonds have a reasonable stability under these catalytic con-
ditions, although they are hydrolyzed at high temperatures in
pure water. Further work underway in our laboratories is focused
on the reactivity of these water-soluble NHC platinum(0)
complexes.
Synthesis of [1-Mesityl-3-(3-sodiumsulfonatopropyl)-
imidazol-2-ylidene](1,1,3,3-tetramethyl-1,3-divinyldisiloxane)-
platinum(0) (4). Complex 4 was obtained as a pale yellow solid
(0.095 g, 98%) by a similar procedure to that described for complex 3,
starting from a commercial solution of Karstedt’s catalyst (0.1 M in
poly(dimethylsiloxane), 1.5 mL, 0.15 mmol), the imidazolium salt 2
(0.046 g, 0.15 mmol), and sodium tert-butoxide (0.020 g, 0.21 mmol).
Complex 4 is soluble in water, methanol, tert-butanol, isopropanol, and
DMSO and insoluble in tetrahydrofuran, diethyl ether, and acetone.
EXPERIMENTAL SECTION
■
General Procedures. All operations were performed under an
argon atmosphere using standard Schlenk techniques. Deionized water
(type II quality) was obtained using a Millipore Elix 10 UV Water
Purification System. Organic solvents were dried and distilled under
argon and degassed prior to use. Unless otherwise stated, reagents
were obtained from commercial sources and used as received. 1H, 13C,
and 195Pt NMR spectra were recorded with a Varian Unity 300 or 500
3
4
1H NMR (500 MHz, DMSO-d6): δ 7.69 (d, JH−H = 2.0, JPt−H = 10,
4
1H, HIm), 7.35 (d, JPt−H = 10, 1H, HIm), 6.84 (s, 2H, Ar), 4.02 (t,
3JH−H = 7.0, 2H, NCH2), 2.30 (t, 3JH−H = 7.5, 2H, CH2S), 2.16 (s, 3H,
p-MeAr), 1.97 (q, 2H, CH2CH2CH2), 1.96 (s, 6H, o-MeAr), 2.05 (m,
2H, vinyl), 1.63 (d, JH−H = 11.5, 2H, vinyl), 1.48 (t with 195Pt
3
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3
2
satellites, JH−H = 12.5, JPt−H = 42.1, 2H, vinyl), 0.09 (s, 6H, SiMe2),
−0.26 (s, 6H, SiMe2). 13C NMR (75 MHz, DMSO-d6): δ 179.8 (Pt−
C2 im), 137.1 (C Ar), 136.1 (4JC−Pt = 12.3, C1 Ar), 134.1 (C Ar), 127.8
(CH Ar), 123.0 (3JPt−C = 40.1, CH im), 121.5 (3JPt−C = 40.1, CH im),
47.9 (NCH2), 47.7 (SCH2), 32.7 (1JPt−C = 117.9, SiCHCH2), 30.2
(SiCHCH2), 26.0 (CH2), 20.0 (p-MeAr), 17.1 (o-MeAr), 1.1
(SiMe2), −2.9 (SiMe2). 195Pt NMR (107 MHz, DMSO-d6): δ −5352.
ESI-MS (negative ion, MeOH) m/z: 688.21 [M − Na]−. Anal. Calcd
for C25H43N2S2O5NaSi2Pt (4·DMSO): C, 38.01; H, 5.49; N, 3.55.
Found: C, 38.49; H, 5.63; N, 3.47.
(12) Turkmen, H.; Can, R.; C
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Arıcı, C. J. Organomet. Chem. 2001, 633, 27−32.
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General Procedure for the Hydrosilylation of Alkynes in
Water. The alkyne (1.0 mmol) and a slight excess of triethylsilane
(1.1 mmol) were added to a solution of the corresponding Pt catalyst
(0.1−5 μmol; see Tables 1−3 for mol % Pt) in 3 mL of water
previously warmed to 30 °C. The mixture was stirred magnetically for
3 h, then extracted with diethyl ether, and the solvent removed under
vacuum. Yields and selectivities were obtained by analysis of the
diethyl ether solutions by GC chromatography (naphthalene internal
standard) and complemented by analysis of the final crude product by
1H NMR spectroscopy. The aqueous phase was reused for several
cycles as specified in the Results and Discussion section.
Hydrosilylation in a Thermomorphic System. The alkyne (1.0
mmol), a slight excess of triethylsilane (1.1 mmol), and the
[(NHC)Pt(dvtms)] catalyst 3 (1 μmol) were dissolved in 4 mL of a
thermomorphic solvent mixture of water/toluene/dimethylformamide
(0.5/2.1/1.4, respectively). The biphasic reaction mixture was saturated
with argon and stirred magnetically for 9 h at 80 °C, the temperature at
which it became homogeneous. After this time, the reaction was rapidly
cooled to 0 °C and the organic phase separated and analyzed as specified
above.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ernesto.dejesus@uah.es (E.d.J.), juanc.flores@uah.es
(30) Almas
2010, 29, 2484−2490.
(31) Czegeni, C. E.; Papp, G.; Katho,
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́
́
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Spanish Ministerio de Ciencia
e Innovacion
24096). G.F.S. is grateful to the Ministerio de Educacion
Postdoctoral Fellowship.
́
(projects CTQ2008-02918/BQU and CTQ2011-
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