Hydrosilylation of internal alkynes catalyzed by tris- Imidazolium salt-Stabilized palladium nanoparticles

Article abstract of DOI:10.1002/adsc.201300641

Palladium nanoparticles stabilized with tris-imidazolium tetrafluoroborates catalyze the stereoselective hydrosilylation of internal alkynes in a dry inert atmosphere to give (E)-vinylsilanes in excellent yields. In the presence of controlled amounts of water a transfer hydrogenation reaction takes place with the formation of (Z)-alkenes or the corresponding alkanes.

Full text of DOI:10.1002/adsc.201300641

FULL PAPERS
DOI: 10.1002/adsc.201300641
Hydrosilylation of Internal Alkynes Catalyzed by Tris-
Imidazolium Salt-Stabilized Palladium Nanoparticles
Marc Planellas,a Wusheng Guo,a Francisco Alonso,b Miguel Yus,b
Alexandr Shafir,a,* Roser Pleixats,a,* and Teodor Parellaa,c
a
Departament de Quꢀmica, Universitat Autꢁnoma de Barcelona, Cerdanyola del Vallꢂs, 08193 Barcelona, Spain
Fax : (+34)-93-581-2477; phone: (+34)-93-581-2067; e-mail: Alexandr.Shafir@uab.cat or Roser.Pleixats@uab.cat
Departamento de Quꢀmica Orgꢃnica, Facultad de Ciencias and Instituto de Sꢀntesis Orgꢃnica (ISO), Universidad de
b
Alicante, Apdo. 99, 03080 Alicante, Spain
Servei de Ressonꢄncia Magnꢂtica Nuclear, Universitat Autꢁnoma de Barcelona, Cerdanyola del Vallꢂs, 08193-Barcelona,
c
Spain
Received: July 22, 2013; Revised: October 4, 2013; Published online: January 13, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300641.
Abstract: Palladium nanoparticles stabilized with place with the formation of (Z)-alkenes or the corre-
tris-imidazolium tetrafluoroborates catalyze the ste- sponding alkanes.
reoselective hydrosilylation of internal alkynes in
a dry inert atmosphere to give (E)-vinylsilanes in ex- Keywords: alkynes; hydrosilylation; imidazolium
cellent yields. In the presence of controlled amounts salts; nanoparticles; palladium; transfer hydrogena-
of water a transfer hydrogenation reaction takes tion
Introduction
Terminal acetylenes tend to be far more reactive in
this process, although certain systems, including
The transition metal-catalyzed hydrosilylation of al- Trostꢅs ruthenium catalyst,[6g] do perform well with
kynes represents the most straightforward and con- the more challenging internal alkynes.
À
venient route for the preparation of vinylsilanes. This
Our own work on C C bond formation catalyzed
transformation proceeds with 100% atom efficiency,[1] by Pd nanoparticles led us to explore the preparation
and the resulting organosilicon reagents are versatile of vinylsilanes using the same class of catalyst. We
building blocks in a number of synthetic processes. found only few reports dealing with the addition of si-
Some examples of reactions involving vinylsilanes in- lanes to acetylenic compounds catalyzed by metal
clude protodesilylation[2] to produce the correspond- nanoparticles. In one instance, the regioselective hy-
ing alkene, the Hiyama cross-coupling[3] with vinyl drosilylation of terminal alkynes was recently ach-
and aryl halides, as well as the Tamao–Fleming oxida- ieved[7] with supported rhodium,[7a] rhodium-platinu-
tion[4] to generate the carbonyl derivatives. In general, m[7a] and gold nanoparticles;[7b] in addition, the hydro-
vinylsilanes exhibit reactivity similar to that of certain silylation of internal alkynes and diynes has been de-
organometallic vinyl derivatives, but may offer advan- scribed with platinum deposited on titania[8a,b] and on
tages in terms of cost, low molecular weight, low tox- magnetite.[8c] Although palladium nanoparticles (Pdnp)
icity, functionality tolerance and high chemical stabili- have found widespread applications as catalysts,
ty. Platinum-based catalysis is the most usual choice mainly in hydrogenation, oxidation and cross-coupling
for the addition of silanes to unsaturated C C reactions,[9] little attention has been paid to their use
À
bonds,[5] most famously chloroplatinic acid (Speierꢅs in the hydrosilylation reactions of unsaturated hydro-
catalyst),[5a,b] and olefin-stabilized Karstedtꢅs cata- carbons. Thus, while there are few examples describ-
lyst.[5c,d] More recently, ruthenium complexes, despite ing their use in the hydrosilylation of enals, enones,[10]
being generally less reactive, have also become popu- and styrene,[11] to the best of our knowledge they
lar in alkyne hydrosilylation due to their high levels have never been reported as catalysts in the hydrosi-
of stereoselectivity.[6] Thus, the regio- and stereochem- lylation of alkynes. In fact, even discrete Pd com-
istry of the metal-catalyzed alkyne hydrosilylation is plexes are quite uncommon in this process, with some
known to be controlled by the nature of the catalyst. examples appearing in the past years.[12]
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Marc Planellas et al.
Scheme 1. Preparation of Pdnp via hydrogenation of PdACHTNUTRGNEUNG(dba)2.
Recently, our group reported[13] a family of palladi- tion and subsequent displacement of a ligand from
um nanoparticles (Pdnp) stabilized by tris-imidazolium a zerovalent organometallic precursor. In our case,
[15]
salts that were active as catalysts in Suzuki–Miyaura a THF solution of PdACHTNUTRGNEUNG(dba)2 was stirred overnight
cross couplings. We describe herein our results on the under 3 atm of hydrogen in the presence of the corre-
hitherto unreported hydrosilylation of internal al- sponding stabilizer (1a or 1b, Pd:L=1:1), affording
kynes catalyzed by this family of palladium catalysts. palladium nanoparticles 1a-Pd and 1b-Pd with
We also report a competitive process of transfer hy- a mean diameter of 2.9–4.2 nm in good yields.
drogenation of alkynes achieved by replacing the neat
silane with a silane/water mixture.
Hydrosilylation of diphenylacetylene with Et3SiH
was chosen as the model substrate combination, and
both 1a-Pd and 1b-Pd were tested as catalyst under
a range of conditions (Table 1).
Results and Discussion
After an extensive testing, it was found that neat
silane (4 equiv.) was the best reaction medium. Thus,
Full details on catalyst preparation are reported in treatment of the model symmetrical alkyne with neat
our earlier publication (Scheme 1).[13] Briefly, the tris- triethylsilane in the presence of 5 mol% of the iodide
imidazolium-stabilized nanoparticles were prepared salt 1a-Pd at 908C under an inert atmosphere for 19 h
following the organometallic approach developed by gave complete conversion of the alkyne with good se-
Chaudret and co-workers,[14] consisting in the reduc- lectivity for the syn addition product (E)-2 (entry 1,
Table 1. Optimization of the reaction conditions for the hydrosilylation of diphenylacetylene with triethylsilane under Pdnp
catalysis.[a]
Entry Catalyst mol% Pd Equiv. Silane[b] Temperature [8C] Conversion [%][c] Yield of (E)-2 [%][c] Ratio (E)-2:(Z)-2:3
1
2
3
4
5
6
1a-Pd
1a-Pd
1a-Pd
1b-Pd
1b-Pd
1b-Pd
1b-Pd
1b-Pd
1b-Pd
1b-Pd
1b-Pd
–
5
1
0.5
5
1
0.5
0.25
0.5
0.5
0.5
0.5
–
4
4
4
4
4
4
4
4
4
2
1
4
90
90
90
90
90
90
90
90
25
90
90
90
100
72
67
100
100
100
100
100
30
95
67
64
99
97
97
98
56
18
66
1
93:7:0
67:21:12
75:17:8
100:0:0
100:0:0
100:0:0
100:0:0
51:0:49
100:0:0
96:0:4
–
7
8[d]
9
10
11[e]
12
79
4
0
0
–
[a]
Performed in closed vessels (45 mL) in a multireactor, with 0.5 mmol of alkyne under dry N2 atmosphere.
Used as solvent.
% Yield/conversion determined by GC (n-C11H24 as internal standard).
Reaction under air atmosphere.
THF was added (0.25 mL).
[b]
[c]
[d]
[e]
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Hydrosilylation of Internal Alkynes
Table 2. Hydrosilylation of symmetric alkynes by 1b-Pdnp.[a]
Entry
1
R1
Ph
R2, R3
Et, Et
Product
Yield (%)[b]
97
2
3
Et, Et
Et, Et
97
99
4
5
6
Bu
Ph
Ph
Ph
Et, Et
60[c]
99
OEt,OEt
Ph, Ph
Ph, H
86
7[d]
78
[a]
In closed vessels (45 mL) with 1 mmol alkyne, 4 mmol triethylsilane under dry nitrogen.
Isolated yield. The conversion was complete in all entries.
Lower yield due to evaporation.
[b]
[c]
[d]
1 mmol of alkyne and 2 mmol of silane, reaction time=170 h.
Table 1). However, incomplete conversions and lower Pd
selectivities were achieved upon decreasing the cata- were achieved in terms of conversion and selectivity.
lyst loading, with the stereoisomeric vinylsilane (Z)-2 Having established nanoparticles 1b-Pd as the best
ACHTUNGTREN(UNNG PPh3)4, were tested as catalysts and poorer results
(anti addition) and (Z)-stilbene (3) formed as by- catalyst, the optimized conditions (0.5 mol% Pd,
products (entries 2 and 3, Table 1). To our delight, 4 equiv. of silane, 908C) were then applied to the hy-
analogous experiments with the tetrafluoroborate cat- drosilylation of some symmetrical internal alkynes
alyst 1b-Pd gave full conversion and complete selec- with triethylsilane (entries 1–4, Table 2). This catalyst
tivity for the vinylsilane (E)-2 with catalyst loading proved to be highly efficient and selective, furnishing
down to 0.25 mol% Pd (entries 4–7, Table 1). When the corresponding (E)-alkenylsilanes (entries 1–3,
the reaction with 0.5 mol% of 1b-Pd took place under Table 2) in excellent yields, with the carbonyl groups
air, the chemoselectivity dropped to about 50% in of (E)-4 unaltered under the reaction conditions. For
favor of the semihydrogenation product 3 (entry 8, the non-aromatic substrate (entry 4), the reduced
Table 1). These experiments established 1b-Pd as the yield was attributed to some loss in the isolation pro-
best catalyst for the hydrosilylation, and showed that cess due to higher volatility. For the reactions involv-
an inert atmosphere was crucial for achieving high ing volatile silanes, the work-up consisted in filtering
chemoselectivity. A lower yield of (E)-2 was obtained the reaction mixture through a short plug of silica gel
at 258C (entry 9, Table 1). Using less silane resulted with hexane as eluent and evaporating the solvent
in incomplete conversion and the formation of traces and excess reagent under an air flow, to afford a prod-
of 3 (entry 10, Table 1). An experiment with a stoi- uct that did not require any further purification.
chiometric amount of silane in THF as solvent was
Next, the hydrosilylation of diphenylacetylene with
unsuccessful (entry 11, Table 1). Control experiments other silanes was examined under the same condi-
showed that the reaction did not occur in the absence tions. The reactions with triethoxysilane and triphe-
of catalyst (entry 12). For comparison, other palladi- nylsilane (entries 5 and 6, Table 2) proceded with high
um sources, such as PdACHTNUGTRNNEUG(dba)2, PdACHUTNGTRENN(GUN OAc)2 and stereoselectivity, affording the corresponding (E)-vi-
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Table 3. Hydrosilylation of differentiated unsymmetric alkynes.[a]
Entry
1
Yield [%][b]
85
Products and % selectivity[c]
2
95
99
3
[a]
Reactions performed in closed vessels (45 mL) with 0.72 mmol alkyne, 2.88 mmol triethylsilane under N2 for 15–22 h.
Isolated yield of the mixture of vinylsilanes; complete conversion in all entries.
Determined by GC.
[b]
[c]
nylsilanes 7 and 8 in good isolated yields. In the case propiolate, although the syn addition and a regioselec-
of the diphenylsilane (Ph2SiH2, entry 7), 2 mmol of tivity were nevertheless predominant [product (E)-12,
this reagent were used for 1 mmol of alkyne and entry 2, Table 3]. However, the use of triphenylsilane
a longer reaction time was required for the full con- with the latter alkyne substrate led to a lower regiose-
sumption of the starting alkyne. Under these condi- lectivity, giving a 55:45 mixture of the two syn re-
tions, the monoaddition product (E)-9 was isolated in gioisomers (E)-14 and (E)-15 (entry 3, Table 3). In
78% yield (entry 7, Table 2). Once again, in all cases substrates lacking such a directing group (Table 4),
the only product detected arose from the syn addition differentiation could be observed for an alkyne bear-
À
of the Si H bond across the carbon-carbon triple ing one alkyl and one aryl substituents. Thus, selective
bond, as determined by the selective NOE experi- syn addition with high regioselectivity took place in
ments.
the reaction of triethylsilane with prop-1-yn-1-ylben-
Next, the nanocatalyst 1b-Pd was used in the hy- zene, giving an 88:12 mixture of (E)-16 and (E)-17
drosilylation of asymmetrical internal alkynes (entry 1, Table 4). In general, unsymmetrical diarylal-
(Table 3). In this case, four different vinylsilanes kynes gave mixtures of the two syn regioisomeric vi-
could be envisaged, with two stereoisomers (syn and nylsilanes (entries 3–7, Table 4), although the selectiv-
anti addition) possible for each of the two regioiso- ity could be improved by introducing 1-naphthyl or 3-
meric forms. Although the expected syn preference quinolyl substituents (entries 6 and 7). However, in
might reduce this number to two, a reasonable degree entry 2, a 76:24 mixture of the two stereoisomers (E)-
of regioselectivity would only be possible for sub- 18 and (Z)-18 was obtained, and only a trace amount
strates with the two sides clearly differentiated.
of the syn regioisomeric compound (E)-19 was detect-
1
Indeed, the reaction between ethyl 2-butynoate and ed by H NMR (see the Supporting Information). The
triethylsilane was highly regio- and stereoselective, high regioselectivity can be attributed to the electron-
with the major product (E)-10 arising from the syn withdrawing effect of the acetyl group. In addition,
addition that placed the triethylsilyl moiety gem to we were able to prepare enyne (E)-30 in 95% isolated
the electron-withdrawing ester group; only minor yield from the syn monohydrosilylation of the corre-
amounts of the anti addition compound (Z)-10 and sponding symmetric diyne (entry 8, Table 4).
traces of the regioisomer (E)-11 were observed by
Interestingly, terminal alkynes, such as phenylacety-
1H NMR (entry 1, Table 3). A similar a-directing lene, proved unreactive under the reaction conditions.
effect of the electron-withdrawing groups in the addi- We traced this lack of reactivity to a poisoning effect
tion of silanes to internal triple bonds has been previ- exerted by this substrate on the catalyst. In fact, addi-
ously reported by other groups.[12b,16] The selectivity tion of phenylacetylene was found to halt the other-
decreased somewhat in the case of the ethyl 3-phenyl- wise efficient hydrosilylation of diphenylacetylene.
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Hydrosilylation of Internal Alkynes
Table 4. Hydrosilylation of unsymmetric alkynes.[a]
ative (E)/(Z) configuration of the corresponding tri-
substituted double bonds. This assignment was further
confirmed by measuring the long-range proton-
carbon and proton-silicon coupling constants using
the selHSQMBC method[17] and determining the
through-space NOE effects mainly on the olefinic
proton. For instance, in the case of the (E)-14 isomer,
a large coupling of 15.4 Hz was measured between
the olefinic proton and the carbonyl carbon, confirm-
ing their relative trans disposition. On the other hand,
a large value of 11.1 Hz was measured between the
olefinic proton and the ipso aromatic carbon resonat-
ing at 140.9 ppm in (E)-15, also confirming the trans
relationship between phenyl group and the olefinic
Entry Products
Ratio[b] Yield
[%][c]
1
2
88:12
76:24
33:67
42:58
96
98[d]
1
proton. The three-bond H,29Si coupling constant was
also used for unambiguous determination of double
bond configuration. As a general trend, values around
7–8 Hz were measured when the olefinic proton and
the silicon atom are in a relative cis configuration,
whereas larger values around 11 Hz were measured in
trans dispositions. The regioselectivity of the process
was studied by using chemical shift assignments and
NOE contacts obtained from HMBC and NOESY
spectra, respectively. The deshielding effects observed
for the chemical shifts of olefinic carbons in the b posi-
tion with respect to the ester group in isomers of en-
tries 1–3 of Table 3, allow us a tentative determination
of the a or b position of the silane group. For in-
3[e]
90
4
99
5
46:54
94:6
99
96
À
stance, whereas the olefinic carbon (=C H) resonates
at 147.5 ppm in (E)-14, this carbon appears at higher
field (133.1 ppm) in the corresponding regioisomer
(E)-15. In other isomers from entries 1–7 of Table 4,
chemical shift differences of olefinic carbons are not
so evident and the concerted use of HMBC and NOE
data was successfully applied to elucidate the struc-
ture of each regioisomer (see the Supporting Informa-
tion).
6
7
75:25
–
99
95
As mentioned above, the failure to employ an inert
atmosphere (dry N2) led to a decrease in the selectivi-
8
ty, giving
a vinylsilane/alkene mixture (entry 8,
Table 1). After some experimentation, we found that
this was a general trend and that the formation of the
semihydrogenated products was due to the moisture
present in the air atmosphere. We thought we could
take advantage of this observation and to perform the
selective transfer hydrogenation of alkynes by the ad-
dition of controlled amounts of water to the alkyne-
silane reaction mixture
[a]
Performed in closed vessels (45 mL) with 0.72 mmol alkyne,
2.88 mmol Et3SiH and 0.5 mol% of Pd under N2.
Determined by GC.
Isolated yield of the mixture of vinylsilanes. Complete con-
version in all entries.
[b]
[c]
[d]
[e]
Trace amount of regioisomeric syn addition product (E)-19
1
was detected by GC and H NMR.
Reaction time=40 h.
The preliminary experiments with added water con-
firmed this assumption (vide infra). Presumably, dihy-
Although no further experiments were conducted, it drogen is formed in situ from the reaction of the
is possible that the formation of Pd s-alkynyl species silane with water, which would also give rise to the
is responsible for catalyst deactivation.
corresponding silanol. Indeed, silanol and disiloxane
Structural characterization of all compounds includ- (from the co-condensation of silanol) were the main
ed two-dimensional NMR techniques (COSY, HSQC, side products in the hydrosilylation reactions per-
HMBC and NOESY experiments). The stereoselectiv- formed under an air atmosphere. The related hydro-
ity of the reaction was studied by determining the rel- gen production by the hydrolytic or alcoholytic oxida-
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Table 5. Hydrogenation of internal alkynes using triethylsilane/water under Pdnp catalysis.[a]
Entry
1
R1, R2
Ph, Ph
Alkyne:H2O[b]
1:1
Products
Ratio[c]
83:17
Conversion (yield [%])[d]
100
2
3
Ph, Ph
Ph, Ph
1:3
1:4
–
–
100
100 (94)
4
5
6
Ph, CO2Et
Ph, CO2Et
Ph, CO2Et
3-quinolyl, Ph
1:3
1:4
1:8
1:8
–
100
29:71
6:94
–
100
100 (62)
100 (78)
7
[a]
Reaction conditions: 1 (or 0.5) mmol alkyne, 4 (or 2) mmol triethylsilane, 0.005 (or 0.025) mmol Pd (and the correspond-
ing amount of water) in a closed vessel (45 mL) under dry nitrogen atmosphere at 908C.
Mmol alkyne/mmol water.
Ratio of products determined by GC.
GC conversion. Isolated yield in parentheses.
[b]
[c]
[d]
tion of silanes is a hot topic that has received consid- genation was highly dependent on the nature of the
erable attention in the recent literature.[18] Thus, the alkyne substrate. Thus, in the case of the diphenylace-
reaction has been reported to take place in the pres- tylene, the addition of 1 mmol of water per mmol of
ence of catalysts such as metal NPs (silver,[18a] alkyne gave a mixture of vinylsilane (E)-2 and cis-stil-
gold,[18b–e] Pd,[18f,g] Ni[18h]) as well as transition metal bene 3, with the hydrosilylation still predominating
complexes (rhenium,[18i] iridium,[18j,k] ruthenium,[18l–o] (entry 1, Table 5). Increasing the amount of water to
and zinc[18p]). Hydrogenation of unsaturated carbon- 3 mmol water/mmol alkyne led to the suppression of
carbon bonds with the system silane-water under pal- the formation of the silylated product, but afforded
ladium(II) acetate catalysis had also been previously a mixture of alkene 3 and 1,2-diphenylethane, 31
described.[19] Oxidative cycloaddition of 1,1,3,3-tetra- (entry 2, Table 5). Finally, a quantitative yield of the
methyldisiloxane to terminal alkynes[20] and oxidative same fully hydrogenated alkane (31) was isolated
hydrolysis of 1,2-disilanes,[21] both under catalysis by when a 4:1 water/alkyne ratio was employed (entry 3,
Au/TiO2 NP, also proceed with concomitant evolution Table 5). In contrast, for ethyl phenylpropynoate the
of hydrogen gas. Selective semireduction of alkynes addition of three equivalents of water still produced
has also been recently accomplished with silane/alco- an almost exclusive formation of the hydrosilylated
hol under copper catalysis.[22] Although Pd/C-induced product (E)-12. Selective hydrogenation in this case
catalytic transfer hydrogenation of several types of required eight mmol of water/mmol of alkyne to give
substrates with triethylsilane has also been report- the corresponding cis-alkene 32, whereas with four
ed,[23] no discussion about the origin of the hydrogen equivalents of water a mixture of both products was
atoms was included.
observed (entries 4–6, Table 5). Finally, while we were
Our preliminary results for the transfer hydrogena- unable to selectively semihydrogenate the 3-(phenyle-
tion of internal alkynes with triethylsilane/water cata- thynyl)quinoline, the fully hydrogenated compound
lyzed by 1b-Pd are summarized in Table 5. The 33 was isolated when using 8 mmol of water per
amount of water required for the semi to full hydro- mmol of alkyne (entry 7, Table 5).
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Hydrosilylation of Internal Alkynes
putative hydrogenation rates v2 or v3 (or both), dimin-
ished with respect to v1. A partial selectivity towards
the hydrogenated product could be achieved by fur-
ther increasing the amount of D2O (entries 3 and 4,
Table 6). Thus, for a molar ratio alkyne:D2O of 1:15,
a mixture of the vinylsilane (E)-12, the cis-alkene 32
and alkane 34 was achieved (entry 3), whereas with
a ratio of 1:20 the alkane 34 was the major product
Scheme 2. Competitive hydrosilylation and transfer hydroge-
nation of internal alkynes.
1
isolated (Table 6, entry 4). The H NMR spectrum of
the fully reduced compound 34 obtained with D2O
The hypothesis of formation of semi-hydrogenated (see the Supporting Information) showed two methyl-
products through protodesilylation of the intermedi- ene resonances at 2.6 and 2.9 ppm, but with an inte-
ate vinylsilanes was discarded as compound (E)-2 was grated intensity of 1H each, instead of the 2H expect-
recovered unaffected after being subjected to the con- ed for the fully protio species. This observation indi-
ditions of entry 2 of Table 5. This indicates that the cates that: a) about 50% of the newly incorporated H
hydrosilylation (rate v1) and transfer hydrogenation atoms proceed from water; and b) the deuterium in-
processes (rates v2 and v3) occur in a competing fash- corporation takes place indiscriminately at both ends
ion, with the relative rates (and thus the outcome) de- of the unsaturated bond. This was confirmed by re-
2
pendent on the amount of water added (Scheme 2).
cording the corresponding H NMR spectrum (see the
The origin of each of the two hydrogen atoms of Supporting Information).
the in situ formed dihydrogen molecule[24] was probed
One possibility for the statistical 50% deuterium in-
using D2O. For this purpose, the reaction correspond- corporation into 34 is the hydrogenation with two HD
ing to entry 6 of Table 5 was repeated with D2O molecules to give exactly 2D/molecule. However,
(Table 6, entries 1 and 2). Under these conditions, the bulk 34 could also simply consist of a mixture of iso-
use of D2O caused the reaction to proceed more topomers arising from indiscriminate hydrogenation
slowly and to give a mixture with the vinylsilane as with a fully (or partially) scrambled HH/HD/DD mix-
the major product. Since the rate of the hydrosilyla- ture. Focusing on the initial addition of dihydrogen to
tion v1 would be unaffected by the change, this out- the alkyne, the reaction with the pure HD would give
come indicates a strong kinetic isotope effect in the two monodeuterated regioisomers (Scheme 3, A). On
Table 6. The effect of using D2O in the hydrosilylation/hydrogenation manifold with 1b-Pd.[a,b]
Entry
1[e]
Water (equiv.)[c]
H2O (8)
Products
Ratio
6:94
Conversion (yield [%])[d]
100 (62)
2
3
D2O (8)
D2O (15)
D2O (20)
>95:5
43:45:12
10:90
75[f]
100
4
100 (80)
[a]
Conditions are the same as in Table 5.
Product ratio determined by GC.
This refers to mmol water per mmol alkyne.
GC conversion. Isolated yield in parentheses.
Entry taken from Table 5.
[b]
[c]
[d]
[e]
[f]
Approximate value.
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Marc Planellas et al.
integral intensities. Based on the signal pattern and
integration (Figure 1, see also the Supporting Infor-
mation), the product was confirmed as a statistical
mixture of isomers (a, b and c, 25% each, Scheme 3,
C), with the “invisible” dideuterated isotopomer d ac-
counting for the remaining 25%. This was confirmed
by recording the corresponding 2H NMR spectrum
(see the Supporting Information).
Conclusions
We have developed an efficient and stereoselective
syn addition of silanes to internal alkynes, providing
(E)-vinylsilanes in excellent yields, by the use of pal-
ladium nanoparticles stabilized by tris-imidazolium
tetrafluoroborates as catalyst under a dry inert atmos-
phere. Fair to good regioselectivities have been ach-
ieved in the case of asymmetric alkynes. To the best
of our knowledge, this is the first report of palladium
Scheme 3. Products of the transfer hydrogenation of internal
asymmetric alkynes using D2O. (A): without HD scram-
bling; (B): with HD scrambling; (C): the four trans olefins
obtained as side-products for the case of ethyl phenylpropio-
late.
the other hand, hydrogenation with a scrambled iso- nanoparticles involved in hydrosilylation reactions of
tope mixture would yield four isotopomers internal alkynes, substrates that are much less reactive
(Scheme 3, B).
than the terminal acetylenes and which have received
Here, our analysis was aided by the fact that some less attention in the literature. The addition of con-
of the semihydrogenated cis alkene 32-d1 (minor trolled amounts of water to the silylation mixture pro-
product, entry 4, Table 6) had partially isomerized to motes the oxidative hydrolysis of silanes with the con-
its trans isomer during work-up, and that the olefinic comitant formation of dihydrogen, leading to a com-
1H NMR resonances (two H in trans) for this new petitive process of transfer hydrogenation of the al-
species were well discernible from the rest of the sig- kynes to the (Z)-alkenes or the corresponding alkanes
nals in the mixture containing 34-d2 as major product. depending on the amount of water and the nature of
Characteristically, each olefinic resonance appeared the acetylenic substrates.
as a combination of a triplet with a small JH,D (2.4 Hz)
nestled within a large J doublet (16 Hz) for the trans
H,H coupling (see Figure 1). Thus, the two large dou- Experimental Section
blets correspond to the H,H product, while each of
the triplets (coupling to D, S=1) belongs respectively Hydrosilylation of Diphenylacetylene with Triethyl-
to each of the two H,D isotopomers; the presence of silane by Pdnp (Entry 1, Table 2); Typical Procedure
the fully deuterated form is deduced from the missing
Diphenylacetylene (178 mg, 1 mmol) and catalyst 1b-Pd
(0.5 mol% Pd) were weighed into a screw-top sealable tube.
The system was subjected to three evacuate-refill cycles
with dry nitrogen. Triethylsilane (640 mL, 1=0.73 gmLÀ1,
4 mmol) was added under nitrogen atmosphere. The reac-
tion was left under stirring at 908C until total conversion of
the alkyne (GC monitoring). The mixture was filtered
through a plug of silica-gel eluting with hexane and the sol-
vent was removed under air to afford the (E)-(1,2-diphenyl-
vinyl)triethylsilane, (E)-2, as
a yellowish liquid; yield:
1
287 mg (97%). H NMR (400 MHz, CDCl3): d=7.30 (t, J=
7.2 Hz, 2H, Ph-H), 7.20 (t, J=7.2 Hz, 1H, Ph-H), 7.12–7.07
(m, 3H, Ph-H), 7.03–7.01 (m, 1H, Ph-H), 7.00–6.94 (m, 3H,
Ph-H), 6.78 (s, 1H, SiC=CH), 0.97 (t, J=7.8 Hz, 9H,
-CH2CH3), 0.65 (q, J=8 Hz, 6H, -CH2CH3); 13C NMR
(101 MHz, CDCl3): d=144.2, 143.3, 138.9, 137.6, 129.7,
128.7, 128.0, 127.4, 127.1, 125.7, 7.5, 2.9; MS: m/z=294.1
(M+).
1
Figure 1. A fragment of the H NMR spectrum of trans-32-
d1 (minor component; major component not shown for clari-
ty).
186
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ꢆ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 179 – 188

FULL PAPERS
Hydrosilylation of Internal Alkynes
2011, 696, 368–372; b) F. Alonso, R. Buitrago, Y.
Supporting Information
Moglie, A. Sepffllveda-Escribano, M. Yus, Organometal-
lics 2012, 31, 2336–2342; c) R. Cano, M. Yus, D. J.
Ramꢇn, ACS Catal. 2012, 2, 1070–1078.
The Supporting Information contains details of the experi-
mental procedures and characterization of compounds along
1
with copies of all H and 13C NMR spectra.
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(Projects CTQ2007-65218, CTQ2009-07881, CTQ2012-32436
and CTQ2011-24151), Consolider Ingenio 2010 (Project
CSD2007-00006), Generalitat Valenciana (PROMETEO/
2009/039), Generalitat de Catalunya (project SGR2009-1441)
and FEDER. A. Shafir was supported through a Ramꢀn y
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