Hydrosilylation of alkynes catalysed by platinum on titania

Article abstract of DOI:10.1016/j.jorganchem.2010.09.068

The heterogeneous hydrosilylation of alkynes catalysed by platinum on titania is reported. A variety of hydrosilanes react with both terminal and internal alkynes to furnish the corresponding vinyl silanes in high yields and short reaction times as well as in a regio- and stereoselective manner. The catalyst can be easily recovered and reused in several consecutive cycles.

Full text of DOI:10.1016/j.jorganchem.2010.09.068

Journal of Organometallic Chemistry 696 (2011) 368e372
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Hydrosilylation of alkynes catalysed by platinum on titania
,
b
a
b
Francisco Alonso a , Robison Buitrago , Yanina Moglie , Javier Ruiz-Martínez ,
*
Antonio Sepúlveda-Escribano b , Miguel Yus
,
a
*
a Departamento de Química Orgánica, Facultad de Ciencias, and Instituto de Síntesis Orgánica (ISO), Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
b Departamento de Química Inorgánica, Facultad de Ciencias, and Instituto Universitario de Materiales (IUMA), Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The heterogeneous hydrosilylation of alkynes catalysed by platinum on titania is reported. A variety of
hydrosilanes react with both terminal and internal alkynes to furnish the corresponding vinyl silanes in
high yields and short reaction times as well as in a regio- and stereoselective manner. The catalyst can be
easily recovered and reused in several consecutive cycles.
Received 2 August 2010
Received in revised form
29 September 2010
Accepted 29 September 2010
Available online 8 October 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Hydrosilylation
Alkynes
Platinum
Titania
Heterogeneous catalysis
1. Introduction
(e.g. with platinumephosphine complexes) while the heteroge-
neous version of this reaction has been somewhat neglected.
Vinyl silanes are very versatile organosilicon compounds with
manifold applications in organic synthesis [1]. There is a general
upsurge of interest in these organometalloids due to their partic-
ular non-toxicity, high chemical stability and low molecular weight
when compared with other organometallic reagents. In fact, the
transition-metal catalysed cross-coupling reaction with vinyl and
aryl halides is one of the major applications of this type of
compounds [2], which are advantageous with respect to the orga-
notin, organoboron, and organozinc counterparts. The hydro-
silylation of alkynes represents the most straightforward and
convenient route to the preparation of vinyl silanes since proceeds
with 100% atom efficiency [3e7]. A general difficulty encountered
in the transition-metal catalysed addition of heteroatomehydrogen
bonds across the carbonecarbon triple bond [8], and in the
hydrosilylation in particular [9], is the control of the regio- and
stereochemistry of the process. The final outcome of the reaction
depends on the catalyst, the alkyne, and the silane employed.
Among the different catalysts, platinum catalysts are usually
preferred over other metals [10,11] because of their selectivity and
tolerance toward a wide range of functionalities. Most of the
hydrosilylations are carried out under homogeneous conditions [12]
Heterogeneous catalysts offer several advantages over the homo-
geneous counterparts, such as easy recovery, easy recycling, and
enhanced stability [13,14]. To the best of our knowledge, only three
reports describe the heterogeneous platinum-catalysed hydro-
silylation of alkynes, involving platinum on carbon [15], platinum on
silica [16], and unsupported platinum oxide [17]. Although high
yields and good selectivities were generally achieved, an inert
atmosphere was mandatory and the catalysts could not be reused.
Due to our ongoing interest on supported platinum catalysts and
their application to organic transformations [18,19], we want to
present herein the selective hydrosilylation of alkynes catalysed by
a reusable platinum-on-titania catalyst.
2. Results and discussion
The platinum catalysts were prepared by the impregnation
method (see the Experimental section) and characterised by XRD
and XPS. XRD analysis of the Pt/TiO2 and Pt/CeO2 catalysts did not
provide any concluding information except that relative to the
support diffraction peaks, due to the low metal loading and its high
dispersion. The XPS spectra in the Pt 4f region is similar for both
catalysts. Two broad bands appear which correspond to the Pt4f7/2
and Pt4f5/2 levels. The Pt4f7/2 peak was deconvoluted into two
components, placed at 72.2 and 74.2 eV for the Pt/TiO2 catalystand at
72.6 and 73.9 eV for the Pt/CeO2 catalyst. The accurate assignment of
* Corresponding authors.
E-mail address: falonso@ua.es (F. Alonso).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.068

F. Alonso et al. / Journal of Organometallic Chemistry 696 (2011) 368e372
369
R23Si
R1
SiR2
the peaks is noteasy, as the bindingenergies of the core electrons are
Pt/TiO2 (0.25 mol%)
70 ºC, 0.5-1 h
3
R23SiH
+
R1
+
affected not only by the metal oxidation state but also by the ligand
atoms to which they are coordinated. For instance, the binding
energy of the Pt4f7/2 level for Pt (IV) coordinated to six chlorine
atoms (as in the H2PtCl6 precursor compound) is about 75.3 eV
(for K2PtCl6), but it decreases to 74.8 eV for PtO2 and to 74.2 eV for
Pt(OH)4. The Pt4f7/2 level for PtCl2 is about 73.4 eV, whereas that for
Pt(OH)2 is about 72.4 eV. Nonetheless, it can be concluded that
platinum is in oxidised states in both calcined catalysts.
R1
β-trans isomer
α isomer
Scheme 1. Optimised reaction conditions for the platinum-catalysed hydrosilylation of
alkynes.
phenylacetylene with triphenylsilane was very similar to that with
triethylsilane as regards both yield and selectivity (entry 2). Tri-
methoxysilane, however, gave lower selectivity irrespective of the
reaction temperature, albeit the reactions were fast and high
yielding in both cases (entry 3). The presence of electron-neutral or
donating substituents at the 4-position of the arylacetylene gave
The hydrosilylation of phenylacetylene with triethylsilane was
selected as the model reaction in order to optimise the conditions.
Two different temperatures, solvents, and supports were used as
parameters with the results being shown in Table 1. The solventless
reaction in the presence of the Pt/TiO2 catalyst gave much better
conversion and selectivity (entry 3) in a shorter reaction time than
the reactions performed in THF or water (entries 1 and 2, respec-
tively). The reactions proved to be slower when ceria was used as
the support for platinum. Interestingly, in this case the regiose-
lectivity of the reaction was reversed in THF and water (entries 4
and 5, respectively) with a low conversion, while the solventless
process was quantitative but non selective (entry 6). Focussed on
the Pt/TiO2 catalyst, we observed that the reaction could be carried
out at room temperature, although a longer reaction time was
required with a concomitant loss of selectivity (entry 7). The
reaction at room temperature in water failed to give the starting
materials (entry 8). Near quantitative isolated yield and the highest
better b-selectivity than that with electron-withdrawing substitu-
ents (entries 4e6). In the latter case, not only the selectivity but also
the yields were slightly lower independently of the reaction
temperature (entry 6). The terminal aliphatic alkyne oct-1-yne was
subjected to catalytic hydrosilylation with both triethylsilane and
triphenylsilane giving very similar results (entries 7 and 8). A
decrease in the selectivity was observed in the addition of trie-
thylsilane to functionalised terminal alkynes either at 70 ꢀC or room
temperature (entries 9e11). Nevertheless, high yields of the cor-
responding products were obtained either for the a,b-unsaturated
alkyne ethyl propiolate and propargylic tertiary alcohol 1-ethy-
nylcyclohexanol (entries 9 and 10, respectively). We must point out
that the reaction of trimethylsilylacetylene with triphenylsilane was
selectivity toward the b-isomer were obtained by decreasing the
amount of catalyst up to 50 mg (entry 9). To the best of our
knowledge, this represents the highest selectivity and yield ach-
ieved for the hydrosilylation of phenylacetylene with triethylsilane
under heterogeneous conditions. A further decrease in the catalyst
amount, however, was shown to be detrimental for the selectivity
(entry 11). Finally, a blank experiment in the absence of the catalyst
gave no products but the starting materials, thus demonstrating the
crucial role of platinum in these hydrosilylation reactions.
The optimised reaction conditions (Table 1, entry 9) were
extended to a variety of alkynes and hydrosilanes (Scheme 1 and
Table 2). Some control experiments at room temperature have been
also included for comparison purposes. We studied first the
hydrosilylation of terminal aryl acetylenes with three different
silanes (entries 1e6). The outcome of the hydrosilylation of
much more selective toward the
thylsilane, very probably due to steric reasons (entries 11 and 12).
Nonetheless, the regioselectivity of the latter ( -trans/ 62:38)
greatly differs from that reported with Pd/C, where the -cis isomer
was formed instead of the isomer ( -trans/ -cis 74:26). An
b-isomer than that with trie-
b
a
b
a
b
b
interesting non-symmetric 1,2-disilylated ethylene was synthe-
sized in the former case (entry 12). The hydrosilylation of
symmetrically-substituted internal alkynes proved to be highly
efficient furnishing the expected (E)-alkenylsilanes in near quan-
titative yield and short reaction time (entries 13e15). Unfortu-
nately, the reaction of prop-1-yn-1-ylbenzene with triethylsilane
was shown to be non-regioselective under the standard conditions
(entry 16).
Table 1
Optimisation of the reaction conditions for the heterogeneous platinum-catalysed hydrosilylation of phenylacetylene with triethylsilane.a
Et3Si
Ph
SiEt3
Pt/support
conditions
Et3SiH
+
Ph
+
Ph
Entry
Catalyst
Solvent
T (ꢀC)
t (h)
Ratio
b/
ab
Conversion (%)c
1
2
3
4
5
6
7
8
Pt/TiO2
Pt/TiO2
Pt/TiO2
Pt/CeO2
Pt/CeO2
Pt/CeO2
Pt/TiO2
Pt/TiO2
THF
H2O
none
THF
H2O
none
none
H2O
none
none
none
70
70
70
70
70
70
r.t.
r.t
6
6
1
18
18
6
6
24
1
1
1
52:48
52:48
94:6
26:74
26:74
50:50
79:21
e
42
84
100 (96)
39
50
100 (95)
96
e
d
9
10
11
Pt/TiO2
Pt/TiO2
none
70
70
70
92:8
74:26
e
100 (97)
100 (96)
e
e
a
Phenylacetylene (1 mmol) and triethylsilane (1 mmol) in the solvent (2 mL) with 100 mg of the supported catalyst (1 wt.%), unless otherwise stated.
Regioselectivity determined by GLC.
Determined by GLC. Isolated yield in parenthesis.
50 mg of catalyst were used (1 wt.% Pt).
10 mg of catalyst were used (1 wt.% Pt).
b
c
d
e

370
F. Alonso et al. / Journal of Organometallic Chemistry 696 (2011) 368e372
Table 2
Hydrosilylation of alkynes catalysed by Pt/TiO2.a
Alkyne
Ph
Silane
Et3SiH
T (ꢀC)
t (h)
Product
Ratio
b
/
ab
Yield (%)c
97 (94)
1
2
3
70 (r.t.)
1 (6)
92:8 (79:21)
Et3Si
Ph
SiEt3
+
Ph
Ph
Ph3SiH
70
1
92:8
96
Ph
Ph
Ph3Si
Ph
SiPh3
+
+
(MeO)3SiH
70 (r.t.)
0.5 (1)
68:32 (66:34)
90 (91)
(MeO)3Si
Ph
Si(OMe)3
Ph
SiEt3
SiEt3
+
4
5
Et3SiH
Et3SiH
70
70
1
1
83:17
85:15
93
95
SiEt3
SiEt3
SiEt3
+
MeO
F3C
MeO
F3C
MeO
SiEt3
+
6
7
Et3SiH
Et3SiH
70 (r.t.)
1 (1)
60:40 (62:38)
89 (87)
F3C
SiEt3
SiEt3
+
+
70
1
84:16
89
5
5
5
5
SiPh3
SiPh3
8
9
Ph3SiH
Et3SiH
70
1
86:14
87
5
5
70 (r.t.)
1 (1)
52:48 (56:44)
92 (89)
EtO2C
HO
Et3Si
SiEt3
SiEt3
+
EtO2C
HO
EtO2C
10
Et3SiH
70 (r.t.)
1 (4)
63:37 (66:34)
93 (88)
Et3Si
HO
+
11
12
Et3SiH
Ph3SiH
70 (r.t.)
1 (1)
62:38 (58:42)
88 (82)
Me3Si
Me3Si
Et3Si
SiEt3
SiPh3
+
Me3Si
Me3Si
Me3Si
70
1
94:6
92
Ph3Si
Me3Si
+

F. Alonso et al. / Journal of Organometallic Chemistry 696 (2011) 368e372
371
Table 2 (continued )
Alkyne
Silane
Et3SiH
T (ꢀC)
t (h)
Product
SiEt3
Ratio
b/
ab
Yield (%)c
13
14
15
16
70
1
e
99
98
98
90
Ph
Ph
Ph
Bun
Bun
Ph
Et3SiH
Ph3SiH
Et3SiH
70
70
70
1
1
1
e
Bun
Bun
Ph
Bun
Bun
Me
SiEt3
Bun
e
SiPh3
Bun
50:50
SiEt3
Me
Et3Si
Ph
+
Ph
Me
a
Alkyne (1 mmol), silane (1 mmol), and Pt/TiO2 (50 mg, 1 wt.% Pt, 0.25 mol%). Data in parenthesis correspond to reactions performed at r.t.
Regioselectivity determined by GLC.
Isolated yield.
b
c
It is worthwhile mentioning that all reactions at 70 ꢀC were
accomplished in ꢁ1 h and that the -cis isomer was not detected in
3. Experimental
b
any hydrosilylation of terminal alkynes. In addition, the catalyst
could be easily recovered by filtration and reused without any pre-
treatment. Quantitative conversion was recorded in the hydro-
silylation of dec-5-yne with triethylsilane along four consecutive
runs, with a progressive decrease of activity that was more
pronounced in the fifth cycle (Table 3). This behaviour could be
rationalised in terms of catalyst deactivation, albeit a deleterious
effect due to partial loss of catalyst during sampling, because of the
slurry character of the reaction mixture, cannot be ruled out. At any
rate, this represents, to the best or our knowledge, the first
heterogeneous platinum-catalysed hydrosilylation of alkynes in
which the catalyst is reused without any pre-treatment.
3.1. General methods
The BET surface area of catalysts and supports was determined
by nitrogen adsorption at ꢂ196 ꢀC with a Coulter Omnisorp 610
system. Before measurements, the samples were dried at 110 ꢀC for
12 h and out-gassed at 250 ꢀC under vacuum. X-ray powder
diffraction patterns were recorded on a JSO Debye-flex 2002
system, ꢂfr1om Seifert, fitted with a Cu cathode and a Ni filter, using
a 2ꢀmin scanning rate. X-ray photoelectron spectra (XPS) were
acquired with a VG-Microtech Multilab 3000 spectrometer equip-
ped with
a hemispherical electron analyser and a Mg Ka
(h ¼ 1253.6 eV, 1 eV ¼ 1.602 ꢃ 10e19 J) 300-W X-ray source. All the
starting alkynes and hydrosilanes were commercially available of
the best grade (Aldrich, Acros, Alfa Aesar) and were used without
further purification. NMR spectra were recorded on a Bruker
In conclusion, platinum on titania has been demonstrated to
catalyse the hydrosilylation of terminal and internal alkynes,
bearing aryl, alkyl, as well as different functional groups, with three
different hydrosilanes to give the corresponding alkenylsilanes in
high yields and short reaction times. Reactions are performed in the
absence of solvent, proceeding with exclusive syn addition of the
Avance 300 spectrometer (300 MHz for 1H NMR and 75 MHz for 13
C
NMR); chemical shifts are given in (d) parts per million and
coupling constants (J) in Hertz. Mass spectra (EI) were obtained at
70 eV on an Agilent 5973 spectrometer (GCeMS); fragment ions in
m/z with relative intensities (%) in parenthesis. HRMS analyses
were carried out on a Finnigan MAT95S spectrometer. Thin-layer
chromatography was carried out on TLC plastic sheets with silica
gel 60 F254 (Merck). The purity of volatile compounds and the
chromatographic analyses (GLC) were determined with a Hewlett
Packard HP-6890 instrument equipped with a flame ionisation
SieH bond across the carbonecarbon triple bond and a b/a regio-
selectivity of up to 94:6. In contrast to previously reported
heterogeneous platinum catalysts, reactions proceed under air,
even at room temperature, and the catalyst can be easily recovered
and reused in several cycles. Further research to expand the
substrate scope as well as on the hydrosilylation of alkynes with
other supported catalysts is under way.
detector and a 30 m capillary column (0.32 mm diameter, 0.25 mm
film thickness), using nitrogen (2 mL/min) as carrier gas,
Tinjector ¼ 270 ꢀC, Tcolumn ¼ 60 ꢀC (3 min) and 60e270 ꢀC (15 ꢀC/
min); retention times (tr) are given in min.
Table 3
Reutilisation of the Pt/TiO2 catalyst in the hydrosilylation of dec-5-yne with
triethylsilane.
SiEt3
Pt/TiO2 (0.25 mol%)
Bun
Bun
+
Et3SiH
3.2. Preparation of the catalysts
Bun
Bun
70 ºC
Degussa P25 TiO2 (60% anatase, 40% rutile) with a surface area of
50 m2 gꢂ1 (N2, ꢂ196 ꢀC, BET method) was used as support, after
being calcined in air at 500 ꢀC for 5 h. The Pt/TiO2 catalyst was
prepared by the impregnation method with an aqueous solution of
H2PtCl6$6H2O (Aldrich) of the appropriate concentration to achieve
a Pt content of 1 wt.%. The slurry (10 ml gꢂ1 of support) was stirred
Run
1
2
3
4
5
t (h)
1
1
3
7
24
62
Conversion (%)a
98
98
97
98
a
Determined by GLC.

372
F. Alonso et al. / Journal of Organometallic Chemistry 696 (2011) 368e372
for 12 h and then the excess of solvent was removed by heating at
90 ꢀC under vacuum in a rotary evaporator. Finally, the catalyst was
dried at 110 ꢀC for 24 h and calcined under a flow of synthetic air at
400 ꢀC for 4 h, with a heating rate of 5 ꢀC/min [20,21].
(%) 398 (Mþ, 12%), 320 (22), 260 (25), 259 (100), 183 (26), 181 (17).
HRMS (EI): m/z calcd. for C28H34Si 398.2430, found 398.2443.
Acknowledgements
The CeO2 support was prepared by the homogeneous precipi-
tation method. An aqueous solution containing Ce(NO3)3$6H2O
(Aldrich, 99%) and urea was gently heated under stirring. Urea
decomposition produced a yellow precipitate that was filtered,
washed with water, dried at 110 ꢀC overnight, and finally calcined at
This work was generously supported by the Spanish Ministerio
de Ciencia e Innovación (MICINN; grant no. CTQ2007-65218, Con-
solider Ingenio 2010-CSD2007-00006) and the Generalitat
Valenciana (GV; PROMETEO/2009/039 and PROMETEO/2009/002).
Y.M. thanks the Vicerrectorado de Investigación, Desarrollo e
Innovación of the University of Alicante for a grant. R.B. acknowl-
edges the University of Alicante, CAM and Union Fenosa for his
grant (UF2007-X9159987F).
450 ꢀC for 2 h. It had a BET surface are of 70 m2 ꢂ1. The Pt/CeO2
g
catalyst was prepared by impregnation of the support with an
acetone solution of [Pt(NH3)4](NO3)2 (Aldrich), which allowed to
load a 1 wt.% Pt. Excess of solvent was removed as described above
and the sample was dried and calcined under the same conditions
as for the Pt/TiO2 catalyst.
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2959, 2935, 2874, 1610, 1467, 1418, 1377, 1231, 1006, 713 cmꢂ1. 1H
NMR (300 MHz, CDCl3):
d 0.52e0.64 (m, 6H), 0.86e0.98 (m, 15H),
1.22e1.41 (m, 8H), 2.03e2.17 (m, 4H), 5.68 (t, 1H, J ¼ 6.9 Hz). 13C
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d
3.8 (3ꢃCH2), 8.0 (3ꢃCH3), 14.6, 14.7
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3890e3895.
[27] C.S. Kraihanzel, M.L. Losee, J. Organomet. Chem 10 (1967) 427e437.
Yellow oil, 98% yield, Rf 0.66 (hexane), tr 20.06 min IR (neat):
3058, 2948, 2923, 2858, 1610, 1427, 1104, 696 cmꢂ1 1H NMR
.
(300 MHz, CDCl3):
d
0.67, 0.89 (2t, 6H, J ¼ 6.7 Hz), 1.00e1.15 (m, 4H),
1.29e1.42 (m, 4H), 2.15e2.28 (m, 4H), 5.97 (t, 1H, J ¼ 6.9 Hz),
7.23e7.42 (m, 9H), 7.51e7.60 (m, 6H). 13C NMR (75 MHz, CDCl3):
d
13.4, 13.7 (2ꢃCH3), 22.3, 22.6, 28.5, 29.7, 31.4, 31.9 (6ꢃCH2), 127.3,
128.9, 136.1 (15ꢃCH), 134.7, 135.1 (2ꢃC), 147.1 (CHCH2). MS (EI): m/z