An effective hydrosilylation of alkynes in supercritical CO2 – A green approach to alkenyl silanes

Article abstract of DOI:10.1016/j.jcat.2017.10.005

Hydrosilylation of a wide group of alkynes (terminal and internal) with four structurally different silanes has been for the first time performed in supercritical CO₂ (scCO₂). The results clearly showed the advantages as well as the

Full text of DOI:10.1016/j.jcat.2017.10.005

Journal of Catalysis 356 (2017) 206–213
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
An effective hydrosilylation of alkynes in supercritical CO – A green
2
approach to alkenyl silanes
Kinga Stefanowska , Adrian Franczyk , Jakub Szyling a,b, Katarzyna Salamon a,b, Bogdan Marciniec a,b
Jedrzej Walkowiak
a
a,b
a
,
a,
⇑
Centre for Advanced Technologies, Adam Mickiewicz University in Poznan, Umultowska 89c, 61-614 Poznan, Poland
Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89b, 61-614 Poznan, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrosilylation of a wide group of alkynes (terminal and internal) with four structurally different silanes
has been for the first time performed in supercritical CO (scCO ). The results clearly showed the advan-
tages as well as the limitations of using of scCO as a reaction and extraction medium for hydrosilylation
Received 13 July 2017
Revised 29 September 2017
Accepted 5 October 2017
2
2
2
of numerous alkynes with different functionality and volatility. Procedures for the synthesis and isolation
of over forty silyl ethenes were described, among which more than twenty for the first time. Obtained
1
13
29
products were fully characterized by H, C, Si NMR, GC–MS, and EA. Moreover, by X-ray crystallogra-
Keywords:
Hydrosilylation
Alkyne functionalization
phy, the molecular structures of (E)-3-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)-2,5-dimethylhex-3-ene-
2,5-diol (3n) and (E)-triethyl(2-(triphenylsilyl)vinyl)silane (12a) have been determined for the first time.
Ó 2017 Elsevier Inc. All rights reserved.
Supercritical CO
2
Homogeneous catalysis
Platinum
Green process
1
. Introduction
Transition-metal catalyzed hydrosilylation of alkynes is one of
alytic systems containing the precious or nonprecious metals
which often can be applied multiple times, ensure easy product
separation, and can be performed in nontoxic and nonvolatile sol-
vents such as ionic liquids (IL) [15–30].
the most straightforward and effective ways for the synthesis of
alkenyl-functionalized silanes [1–8]. The process occurs with
Recently, our research interest has been directed to the use of
1
00% atom efficiency and depending on the type of a catalyst
scCO
of carbon-carbon triple bond in alkynes. The characteristic features
of scCO include low critical parameters (73.8 bar, 31.2 °C, 0.456 g/
mL) low price, non-flammability, wide availability due to its natu-
ral abundance, tunable density and solvent-like properties
(through temperature and pressure variations) [31–36]. All of
these make it a perfect medium allowing the recovery of catalyst,
easy product separation, and significantly reduce cost of the isola-
tion. Moreover, silicon-containing and unsaturated organic com-
2
as a reaction and extraction medium for the hydrosilylation
and reaction conditions, it gives the opportunity for the selective
formation of all theoretically possible isomers of SiAH addition
to both terminal and internal carbon-carbon triple bonds (C„C).
The obtained alkenyl silanes, due to their versatility, low cost, lack
of toxicity, wide functionality, and high chemical stability, are con-
sidered as an important class of organosilicon building blocks for
the synthesis of molecules, polymers, and natural products. For
example, they can be used in palladium or rhodium-catalyzed
cross-coupling reactions and in the Tamao-Fleming oxidation pro-
viding carbonyl derivatives [9–14].
Although hydrosilylation of alkenes and alkynes with silanes or
siloxanes has been widely studied, new strategies, which can
increase the process effectiveness, decrease its costs and make it
more environmentally friendly are continuously searched for. In
our group, we have tried to overcome these challenges by applying
the procedures based on the use of highly active and selective cat-
2
pounds are generally soluble in scCO
even small organosilicon substituents such as SiMe
ligands, significantly increase the solubility of transition metal
complexes in scCO [37–39]. The effect of silyl groups is the same
as perfluorinated ones, and therefore organosilicon compounds are
predominantly well soluble in nonpolar scCO . This property gives
2
. It has been reported that
3
attached to
2
2
the opportunity for the hydrosilylation of a wide spectrum of
reagents as well as the extraction of the resulting hydrosilylation
products.
Despite many advantages of synthetic methods based on scCO
the hydrosilylation of carbon-carbon unsaturated bonds in this
2
,
⇑
Corresponding author.
E-mail address: jedrzej.walkowiak@amu.edu.pl (J. Walkowiak).
https://doi.org/10.1016/j.jcat.2017.10.005
021-9517/Ó 2017 Elsevier Inc. All rights reserved.
0

K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
207
medium has been poorly explored. Up to date, only seven reports
on the hydrosilylation of carbon-carbon double bond (C@C) have
been published. Among them, just two concern the synthesis of
molecular compounds by the hydrosilylation of perfluoro, alkyl
or aryl olefins with dimethoxymethyl-, triethoxy- and triethylsi-
silanes were 1,1,1,3,5,5,5-heptamethyltrisiloxane (1a), triethoxysi-
lane (1b), triethylsilane (1c), and triphenylsilane (1d). Both groups
of reagents were composed of compounds with different molecular
weight (M ) and physical properties such as boiling (b.p.) and
w
melting (m.p.) points or solubility, which are the factors having a
significant influence on the effectiveness of the process performed
3 3 2 3 2
lane, in the presence of RhCl(PR ) or RuCl (PR ) (R = phenyl, or
perfluoroarene) [40,41]. The remaining papers refer to the synthe-
sis of macromolecular systems, obtained by the hydrosilylation of
perfluoro-1-hexene, bifunctional olefins or oligo(vinylsiloxanes)
2
in scCO and the extraction of resulting products with this med-
ium. Selected physical properties of all tested herein alkynes and
silanes are presented in Supporting Information (Table S1).
with poly(methylhydrosiloxanes) or modified silica, mostly in the
2 3
The commercially available Karstedt’s catalyst (Pt (dvs) )
0
presence of Karstedt’s catalyst (Pt
2
(dvs)
3
) or 2,2 -azobis(2-methyl
[47,48], well-known for its high activity in hydrosilylation of both
terminal and internal carbon-carbon multiple bonds, was used. So
far, it has been mainly applied for the modification of alkenes with
a wide spectrum of functionalities (also with steric hindrance), in a
wide range of temperatures, in the air or inert atmosphere [49–51].
Other catalytic systems could be considered, however, due to the
propionitrile) (AIBN) as radical initiator [42–46]. Moreover, there
are no reports on the hydrosilylation of C„C bond in scCO at all.
2
In view of the above-mentioned information, we decided to
describe for the first time the synthesis of alkenyl silanes per-
formed in scCO , afforded by hydrosilylation of a wide range of ter-
2
minal and internal alkynes with different types of silanes, in the
presence of Karstedt’s catalyst.
2
lack of information concerning the alkyne hydrosilylation in scCO ,
the Karstedt’s catalyst seems to be the most reasonable choice for
the first tests of the reactivity of structurally different alkynes with
silanes performed in this medium. The presence of tetramethyl-
divinylsiloxanes as ligands coordinated to the Pt atom will have
also the positive impact on the solubility of the catalyst in com-
The main purposes of our studies were: (i) to examine if the
tested reagents can form homogeneous solutions in scCO
vations through the sapphire windows of the reactor), (ii) to deter-
mine if the chosen reagents can react in the presence of Pt (dvs)
and how the scCO affects the catalyst activity and selectivity, (iii)
finally to establish which products of hydrosilylation can be
extracted with scCO in high yields and purity (Fig. 1).
Such studies will clearly show for which set of reagents and
hydrosilylation products the use of scCO as a reaction and extrac-
2
(obser-
2
3
,
2
2
pressed CO . Although the Karstedt’s catalyst has been widely used
in alkene hydrosilylation, we have found only a few examples of its
use in hydrosilylation of alkynes [52–61]. There is no work con-
cerning detailed studies on the hydrosilylation of structurally dif-
2
2
ferent alkynes with various silanes, catalyzed by Pt
performed in conventional media.
2 3
(dvs) ,
tion medium can be successful, and for which, other media should
be used.
Since the Karstedt’s catalyst is one of the most effective and
widespread catalysts among all used, in our opinion, more complex
catalytic systems should be applied only in the processes in which
Karstedt’s one will prove to be not selective or inactive. Moreover,
our choice of reagents and catalyst which are stable and easily
available should be considered as an advantage of presented herein
synthetic and isolation methods since it makes the process more
versatile, practical and meeting the green chemistry requirements.
2
. Results and discussion
In the studies, seventeen structurally different alkynes were
reacted with four silanes in scCO or toluene (or THF) under differ-
2
ent reaction conditions. A wide range of internal and terminal alky-
nes, as well as organosilicon compounds with different functional
groups, were applied to check if the physicochemical properties
and the type of groups attached to the silicon atom or to the Csp
atom in alkyne, affects the possibilities to carry out the reaction
2.1. Hydrosilylation of alkynes with 1,1,1,3,5,5,5-heptamethyl-
trisiloxane (1a)
in scCO
metrically disubstituted alkyl as well as aryl alkynes. Some of them
possessed the functionalities like hydroxyl, -OSiMe , -OCH , bromo
or boronic acid pinacol ester. On the other hand, among tested
2
. The set of alkynes contained terminal and internal sym-
At the first step of the study, the hydrosilylation of alkynes with
1,1,1,3,5,5,5-heptamethyltrisiloxane (1a) was performed. Com-
3
3
pound 1a is a liquid with a molecular weight M
w
equal 222.5
g/mol, b.p. at 142 °C and it is considered as a molecular model of
2
Fig. 1. Hydrosilylation of the terminal and internal alkynes with 1a-1d performed in scCO .

2
08
K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
poly(siloxanes). The addition of such a fragment to the alkyne
molecules should improve their solubility in scCO and should
allow the extraction of resulting products even if the reacting alky-
nes were characterized by limited solubility in scCO
Before testing the hydrosilylation process in scCO
2
soluble in scCO under applied reaction conditions. Therefore, the
2
contact between the reagents and the catalyst was not limited by
diffusion and its activity in all experiments was very high.
It was revealed that in the hydrosilylation of terminal alkynes
2
.
2
, detailed
2
2a-e the complete conversions in scCO were reached in a time
optimization of the reaction conditions in toluene was performed
to establish the output parameters for processes under high-
pressure conditions. The experiments in toluene were carried out
similar to that estimated for toluene. The solvent impact on the
selectivity was observed for the reaction of 1a with trialkylethynyl-
silanes (2a and b) and 1-heptyne (2c).
ꢀ4
in the presence of Pt
2
(dvs)
3
(10 mol of Pt), at 100 °C, under air
The hydrosilylation of triethylethynylsilane (2a) with 1a per-
atmosphere without any purification of acquired chemicals.
2
formed in scCO led to the almost exclusive formation of isomer
1
CO2
The progress of the reaction was determined by GC–MS and H
NMR. When the complete conversion of reagents was confirmed,
3a (92% ). On the other hand, the reaction in toluene resulted in
a mixture containing a higher amount of isomer 4a (3a/4a =
1
13
tol.
the post-reaction mixtures were analyzed by GC–MS and H, C,
75/25 ). Similar results were obtained for hydrosilylation of triiso-
2
9
Si NMR and the selectivity of the SiAH addition to the triple bond
was determined. Then the phase behavior tests of alkynes in the
presence of silanes in scCO were performed and the hydrosilyla-
propylethynylsilane (2b) and 1-heptyne (2c), however, in these
cases, the process occurred to be less selective. The ratios of obtained
tol.
CO2
2
isomers amounted 3b/4b = 60/40 and 82/18
lation of 2b, and 3c/4c = 72/28
for the hydrosily-
and 60/40 for 2c. Then the con-
ditions for the extraction of the hydrosilylation products with scCO
were determined. The best results of isolationwere obtained at 40 °C
and the pressure of CO of 190 bar. The extraction was highly effec-
CO2
tol.
tion process according to the procedure described above was
repeated. Most of the reagents were soluble under pressures above
2
1
2
20 bar of CO and at 100 °C – the best temperature previously
chosen for the hydrosilylation in toluene. A comparison of the
2
results obtained in both media is presented in Table 1.
tive and only 30 min were necessary to get products with a high
yield of isolation. The extracted products were collected in the cold
traps to avoid the possibility of removal of volatile compounds.
Subsequently, the terminal aryl alkynes 2e-i were tested. It was
The conversion and selectivity determined for toluene and
scCO
2
are given with the superscript ‘‘CO2” and ‘‘ ”, respectively.
tol.
If obtained results were comparable for both media, we labeled
CO2,tol.
them with one superscript ‘‘
”.
found that 4-bromophenylacetylene (2e) was well soluble in scCO
2
It is worth to emphasize that the Karstedt’s catalyst, due to its
chemical structure (the presence of siloxane ligands) was very well
and the products of its hydrosilylation (3e and 4e) could be
obtained in the process at 100 °C and 185 bar, in 6 h. Also, the
Table 1
Hydrosilylation of alkynes 2a-q with 1,1,1,3,5,5,5-heptamethyltrisiloxane (1a) in the presence of Karstedt’s catalyst.
0
Ent
Alkyne 2
R
t [h]
Conv. of Si-H [%]
Selectivity [%]
R@H
3/4/5
1
2
3
a
b
c
Si(C
Si(i-C
n-C
C(CH
2
H
5
)
3
2
2
2
99CO2,tol.
99CO2,tol.
99CO2,tol.
92/8/0CO2
tol.
7
5/25/0
CO2
3
H
7
)
3
82/18/0
tol.
60/40/0
CO2
5
H
11
72/28/0
tol.
60/40/0
CO2,tol.
CO2,tol.
CO2,tol.
4
5
6
d
e
f
3
)
2
OSi(CH
Br-4
(BO
3
)
3
2
6
24
99
82/18/0
52/48/0
-
52/48/0
54/46/0
-
CO2,tol.
C
C
6
H
H
4
99
CO2
6
4
2
C
2
(CH
3
)
4
)-4
0
9
tol.
tol.
tol.
9
tol.
7
8
g
g
4-biphenyl
4-biphenyl
24
24
23
a
CO2
0
9
0
9
0
9
tol.
tol.
tol.
tol.
9
59/41/0
-
CO2
9
h
i
1-naphthyl
6
tol.
9
22/78/0
-
CO2
1
0
9-phenanthryl
24
9tol.
13/87/0
0
R@R
3=4/5
100/0
99CO2,tol.
complex mixture
CO2,tol.
11
12
13
14
15
16
j
k
l
m
n
o
n-C
3
H
OH
OCH
7
2
24
2
6
24
6
CO2,tol.,THF
CH
CH
CH
2
CO2,tol.
CO2,tol.
2
3
99
100/0
CO2,tol.
CO2,tol.
THF
2
OSi(CH
OH
)
3 3
99
95/5
THF
C(CH
)
3 2
99
100/0
100/0
100/0
100/0
100/0
-
a
CO2
CO2
tol.
C
6
C
6
C
6
C
6
H
H
H
H
5
5
4
4
99
tol.
7
80
4
1
1
1
7
8
9
ob
pa
qa
24
24
72
CO2
tol.
CO2
tol.
4
0
9
0
9
4
CO2
Br-4
(BO
tol.
tol.
tol.
3
100/0
-
100/0
CO2
2 2 3 4
C (CH ) )-4
9tol.
Reaction conditions: CO2[1a]:[2]:[Pt] = 1:1:10ꢀ4, n1a/VCO2 = 0.058 mmol/mL, 100 °C, 120–190 bar; [1a]:[2]:[Pt] = 1:1:10 , n1a/Vtol. = 0.058 mmol/mL, 100 °C; THF[1a]:[2]:
tol.
ꢀ4
ꢀ
4
a
ꢀ3
b
ꢀ3
1
[
Pt] = 1:1:10 , n1a/VTHF = 0.058 mmol/mL, 60 °C; [1a]:[2]:[Pt] = 1:1:10 , 100 °C; [1a]:[2]:[Pt] = 1:1:10 , 60 °C. Conversion was measured by H NMR and GC–MS.
Selectivity was determined on the basis of H NMR. An efficiency of product extraction with scCO was in the range of 95–99% (40 °C, 120 bar, 30–40 min).
2
1

K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
209
extraction of the 3-4e mixture with scCO
time of extraction was the same as that applied for alkyl
derivatives.
On the other hand, phenylacetylene with boronic acid pinacol
ester (2f) as well as 4-ethynylbiphenyl (2g), 1-ethynylnaphthalene
2
was highly efficient. The
It should be underlined that lower levels of conversion were
observed for the experiments performed in toluene. They reached
74 and 44% at 100 and 60 °C, respectively (Table 1, entry 16
and 17).
2
The extraction of 3o with scCO was performed according to the
(
2h) and 9-ethynylphenanthrene (2i) were not soluble in scCO
2
.
same procedure as described for terminal alkynes, however, longer
time was needed to obtain a similar yield.
Despite this fact, hydrosilylations of these alkynes were performed.
It was checked if the reagents react at the interface of the two phases
2
The application of scCO as a solvent and extractant is especially
and if the products can be extracted with scCO
conversion of these alkynes was observed in scCO
2
. Unfortunately, no
under all tested
valuable when the total conversion of reagents is observed and one
isomer is exclusively formed. Extraction of the silylated product in
2
conditions. Mixtures of isomers 3 and 4f-i were obtained in toluene.
The reactions of 1a with internal, symmetrically disubstituted
alkynes (2j-q) showed that alkynes 2j and 2 l-m can be dissolved
2
CO stream allows obtaining the pure product, and therefore other
costs and time-consuming purification steps are not required.
The phase behavior tests of bis(4-bromophenyl)acetylene (2p),
as well as its analog with boron pinacol ester groups (2q), indicated
that their homogeneous solutions cannot be obtained in the range
of 40–100 °C and the pressures of 120–190 bar. In contrast to 1,2-
diphenylacetylene, 2p and 2q did not melt during the hydrosilyla-
tion process. Through the sapphire windows of the reactor, the par-
ticles of the solid were observed, irrespectively of the conditions
used. The high yields of products 3p-q were achieved only as a
result of the hydrosilylation performed in toluene.
and reacted in scCO
,5-dimethyl-3-hexyne-2,5-diol (2n) occurred to be insoluble in
toluene as well as in scCO at 40–100 °C, and at different pressures
of CO . The reaction performed in THF led to the exclusive forma-
2
. The compounds 2-butyne-1,4-diol (2k) and
2
2
2
THF
tion of product 3n (3n/4n = 100/0 ). Its structure was confirmed
by X-ray crystallography (Fig. 2). As a result of the reaction of 2 k
with 1a in the presence of Karstedt’s catalyst, a complex mixture
of unidentified products was obtained.
At the next stage of the research, 1,2-diphenylacetylene (2o)
In the next step of the study, Pt-contamination in selected, iso-
lated products (3a, j, o) was measured by ICP-MS. The amount of Pt
was 0.08–0.14 ppm. The presence of small amounts of metal in the
extracts was caused by the very good solubility of Karstedt’s cata-
ꢀ3
was examined at 100 °C in the presence of 10 mol of Pt. The
increase in the catalyst concentration was due to the steric hin-
drance in the neighborhood of the triple bond which significantly
affected the rate of the process. The analysis of post-reaction mix-
lysts in scCO
experiments cannot be compared with other catalytic systems
(used in hydrosilylation of alkenes performed in scCO ) since the
2
. The values of Pt-contamination obtained in our
CO2
tures indicated that after 6 h, product 3o was formed with 99%
yield. However, throughout the whole experiment, droplets sus-
pended in the solution were observed. This suggested that 1,2-
diphenylacetylene, as well as the product of its hydrosilylation,
2
authors did not include them in the published reports [40–46].
Presently, we are working on the catalytic systems based on Pt
and Rh complexes which will be characterized by better selectivity
in hydrosilylation of terminal alkynes with non-steric silanes and
which could be easily isolated from the products and then reused
in subsequent catalytic cycles. However, having in mind the results
obtained in traditional media, such attempts will probably result in
the systems characterized by lower activity than that observed for
Karstedt’s catalyst.
were insoluble in scCO
formed at 60 °C and the same pressure (190 bar) of CO
these conditions, the density of CO was higher (d = 0.71 g/cm
compared to d = 0.45 g/cm ) and 2o was completely dissolved. Its
hydrosilylation proceeded in the homogeneous system for 24 h.
Since at a lower temperature, the rate of the reaction decreased,
after the appointed time, the conversion of reagents reached 80%.
2
at 100 °C. The same experiment was per-
2
. Under
3
2
3
It should be also mentioned that from among all products syn-
thesized in this part, 3a, j, l, n, p and q were selectively obtained
and characterized for the first time.
Moreover, the hydrosilylation of 2b, d, g, h, i with silane 1a was
described for the first time, however, it led to the mixtures of
isomers.
2.2. Hydrosilylation of alkynes with triethoxy- (1b) and triethylsilane
(1c)
Silanes 1b and 1c are liquids with the b.p. at 134–135 °C and
07–108 °C, respectively. Despite being structurally different from
silane 1a, they were found to be well soluble in scCO and toluene
1
2
under all tested conditions. Their reactivity in hydrosilylation of
alkynes was tested using the same procedure as described for 1a.
The results are presented in Tables 2 and 3.
The selectivity of the process was found to be affected by the
type of solvent for hydrosilylation of terminal alkynes with 1b. As
established for 1a, higher selectivity was obtained for the reactions
with trialkylsilanes (2a-b) and 1-heptyne (2c), performed in scCO
2
.
On the other hand, the reaction of 4-bromophenylacetylene (2e)
with 1b performed in scCO proved to be less selective compared
2
to that in toluene.
Also, in the reaction of 1c with of 1-heptyne (2c), the influence
of scCO on the selectivity was found. As a result, product 9c was
obtained in high predominance (9c/10c = 91/9). If toluene was
used, the ratio of isomers was 65/35.
The examination of silane 1b and c in the hydrosilylation of
internal alkynes 2j, l, m, o indicated that the processes performed
Fig. 2. The molecular structure of compound 3n. Displacement ellipsoids are shown
at the 30% probability level. Selected geometric parameters: C3AC4 1.343(3) Å,
Si2AC3 1.8845(17) Å, C2AC3 1.535(2) Å, C4AC5 1.518(2) Å, C4AH4C 0.9500 Å,
C2AO1 1.447(2) Å, O1AH1O 0.9600 Å; Si2AC3AC4 114.44(13), C2AC3AC4 125.97
2
(
16)°, C3AC4AC5 134.14(16)°; parameters for hydrogen bonds: O1AH1ꢁ ꢁ ꢁO2 1.69
Å, O1AH1ꢁ ꢁ ꢁO2 155.1°.

2
10
K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
Table 2
Hydrosilylation of alkynes 2 with triethoxysilane (1b) in the presence of Karstedt’s catalyst.
0
Ent
Alkyne 2
R
t [h]
Conv. of SiAH [%]
Selectivity [%]
R@H
6/7/8
CO2
1
2
3
a
b
c
Si(C
Si(i-C
n-C
C(CH
2
H
5
)
3
2
2
2
99CO2,tol.
99CO2,tol.
99CO2,tol.
98/2/0
tol.
93/7/0
CO2
tol.
3
H
7
)
3
100/0/0
0/10/0
9
CO2
tol.
5
H
11
90/10/0
7/13/0
8
CO2,tol.
CO2,tol
CO2
4
5
d
e
3
)
2
OSi(CH
3
)
3
6
6
99
99
94/6/0
CO2,tol
C
C
C
H
6 4
H
6 4
H
6 4
Br-4
65/35/0
tol.
8
–
4/16/0
6
7
f
(BO
2
C
2
(CH
3
)
4
)-4
)-4
48
24
0CO2
tol.
tol.
5
0
9
5
79/21/0
–
fa
(BO
C
(CH
)
CO2
2
2
3
4
9tol
78/22/0
0
R@R
n-C
CH
6=7/8
CO2,tol.
CO2,tol.
CO2,tol.
CO2,tol.
CO2,tol.
CO2
8
9
j
l
3
H
2
OCH
OSi(CH )
3 3
7
2
2
6
99
99
99
100/0
100/0
100/0
3
1
1
1
0
1
2
m
CH
2
tol.
98/2
CO2
CO2,tol.
o
C
C
6
H
5
24
6
77
100/0
tol.
83
oa
99
CO2,tol.
100/0CO2,tol
6
H
5
Reaction conditions: CO2[1b]:[2]:[Pt] = 1:1:10ꢀ4, n1a/VCO2 = 0.059 mmol/mL, 100 °C, 120–190 bar; [1b]:[2]:[Pt] = 1:1:10 , n1a/Vtol. = 0.059 mmol/mL, 100 °C; THF[1b]:[2]:
tol.
ꢀ4
ꢀ
4
a
ꢀ3
1
[
Pt] = 1:1:10 , n1a/VTHF = 0.059 mmol/mL, 60 °C, [1b]:[2]:[Pt] = 1:1:10 , 100 °C; Conversion was measured by H NMR and GC–MS. Selectivity was determined on the basis
of H NMR. An efficiency of product extraction with scCO was in the range of 90–99% (40 °C, 120 bar, 30–40 min).
2
1
Table 3
Hydrosilylation of alkynes 2 with triethylsilane (1c) in the presence of Karstedt’s catalyst.
0
Ent
Alkyne 2
R
t [h]
Conv. of SiAH [%]
Selectivity [%]
R@H
9/10/11
99CO2,tol.
99CO2,tol.
64/36/0
5
91/9/0
6
CO2
1
2
a
c
Si(C
2
H
5
)
3
2
2
tol.
1/49/0
CO2
tol.
n-C
5
H
11
5/35/0
0
R@R
9=10/11
CO2,tol.
CO2,tol.
3
4
5
6
7
8
j
l
m
n
o
n-C
3
H
7
2
2
24
24
24
48
99
99
96
99
99
0
9
100/0
100/0
75/25
100/0
100/0
-
CO2,tol.
CO2,tol
CO2,tol.
THF
CH
CH
2
OCH
OSi(CH
OH
3
CO2,tol.
2
)
3 3
THF
C(CH
)
3 2
CO2,tol.
CO2,tol.
C
C
6
H
H
5
a
CO2
p
6
4
Br-4
9tol.
100/0
tol.
Reaction conditions: CO2[1b]:[2]:[Pt] = 1:1:10ꢀ4, n1a/VCO2 = 0.059 mmol/mL, 100 °C, 120–190 bar; [1b]:[2]:[Pt] = 1:1:10 , n1a/Vtol. = 0.059 mmol/mL, 100 °C; THF[1b]:[2]:
tol.
ꢀ4
ꢀ
4
a
ꢀ3
1
[
Pt] = 1:1:10 , n1a/VTHF = 0.059 mmol/mL, 60°C, [1b]:[2]:[Pt] = 1:1:10 , 100 °C. The conversion was measured by H NMR and GC–MS. Selectivity was determined on the
basis of H NMR. An efficiency of product extraction with scCO was in the range of 90–99% (40 °C, 120 bar, 30–40 min).
2
1
in scCO
2
were as effective as those carried out in toluene. As
charged with more serious limitations than the extraction with
scCO and moreover, they can lead to the decomposition of products.
From among all synthesized products, 6b, d, m were selectively
obtained and characterized for the first time. Moreover, the
hydrosilylation of 2e and 2f with silane 1b was described for the
first time, however, these reactions led to mixtures of isomers.
observed for 1a, the hydrosilylation of alkyne 2p could be per-
formed only in toluene while 2n only in THF.
2
2
The extraction of all products obtained in scCO was performed
with the same procedure as that developed for 1a. The yields of
extraction were in the range of 90 and 99%, for di- as well as trisub-
stituted ethenes.
2
It should be emphasized, that extraction with scCO is a perfect
2.3. Hydrosilylation of alkynes with triphenylsilane (1d)
method for isolation of the products containing reactive or/and
unstable fragments such as alkoxy groups. The hitherto applied
methods for isolation of such compounds were based on distillation
or less often on the filtration through silica gel. Both methods are
In the last part of the studies, triphenylsilane 1d was examined.
The appearance of this silane is different from all previously
described ones, it is a solid with a melting point at 43–45 °C. It

K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
211
Table 4
Hydrosilylation of alkynes 2 with triphenylsilane (1d) in the presence of Karstedt’s catalyst.
0
Ent
Alkyne 2
R
t [h]
Conv. of SiAH [%]
Selectivity [%]
12/13/14
R@H
CO2,tol.
CO2,tol.
1
2
3
4
5
6
a
b
c
d
e
f
Si(C
Si(i-C
n-C
C(CH
2
H
5
)
3
2
2
2
6
24
24
99
99
96
99
99
0
100/0/0
100/0/0
96/4/0
100/0/0
93/7/0
–
CO2,tol.
CO2,tol.
3
H
7
)
3
CO2,tol.
CO2,tol.
CO2,tol.
5
H
11
CO2,tol.
3
)
2
OSi(CH
3
)
3
CO2,tol
CO2,tol.
C
C
6
H
H
4
Br-4
CO2
6
4
(BO
(BO
2
C
2
(CH
(CH
3
)
4
)-4
)-4
tol.
tol.
tol.
78
78/22/0
90/10/0
7
fa
C
6
H
4
2
C
2
3
)
4
24
99
tol.
0
R@R
n-C
12=13/14
CO2,tol.
tol.
CO2,tol.
8
9
j
l
l
m
m
n
o
o
3
H
OCH
OCH
OSi(CH
OSi(CH
7
2
24
6
24
24
6
24
6
99
65
99
88
99
0
85
99
100/0
100/0
100/0
92/8
92/8
–
tol.
CH
CH
CH
CH
2
3
a
CO2,tol.
tol.
CO2,tol.
10
11
12
13
14
15
2
2
2
3
CO2,tol.
CO2,tol.
3
)
)
3
a
CO2,tol.
3
3
THF
C(CH
3
)
2
OH
CO2,tol.
CO2,tol.
CO2,tol.
C
C
6
H
H
5
100/0
100/0
a
CO2,tol.
6
5
Reaction conditions: CO2[1d]:[2]:[Pt] = 1:1:10ꢀ4, n1a/VCO2 = 0.058 mmol/mL, 100 °C, 120–190 bar; [1d]:[2]:[Pt] = 1:1:10 , n1a/Vtol. = 0.058 mmol/mL, 100 °C; THF[1d]:[2]:
tol.
ꢀ4
ꢀ
4
a
ꢀ3
1
[
Pt] = 1:1:10 , n1a/VTHF = 0.058 mmol/mL, 60 °C, [1d]:[2]:[Pt] = 1:1:10 , 100 °C. The conversion was measured by H NMR and GC–MS. Selectivity was determined on the
basis of H NMR. An efficiency of product extraction with scCO was in the range of 92–95% (40 °C, 120 bar, 60 min).
2
1
contains three phenyl bulky substituents bonded to the silicon
atom, which makes it the bulkiest reagent from among the tested
ones. Before it was used in the hydrosilylation process, its phase
1d led to the exclusive formation of one isomer as a result of
hydrosilylation of alkyl and aryl terminal alkynes (12a-e) (see
Table 4). The structure of compound 12a, obtained in scCO was
2
behavior was studied in scCO
2
under various conditions. At 100
also confirmed by X-ray crystallography (Fig. 3). This proved the
high purity of compounds resulted in the reaction and extraction
°C, it melted and its droplets were observed as a suspension in
scCO
2
. However, the hydrosilylation of terminal alkynes 2a-e
2
with scCO .
under the same conditions could be performed, and the conver-
sion of reagents was complete. Moreover, the steric hindrance of
On the other hand, a mixture of isomers was found when
phenylacetylene with boronic acid pinacol ester (2f) was tested.
The examination of silane 1d in the hydrosilylation of internal
alkynes 2j, l, m, o indicated that the processes performed in scCO
2
were as effective as carried out in toluene.
2
At 60 °C and 190 bar of scCO , silane 1d got dissolved and
homogenization of the reaction mixture was observed. The test
on the hydrosilylation carried out under these conditions, led to
high reagents conversions. However, longer reaction time and/or
higher concentration of the catalyst were needed.
All hydrosilylation products obtained in scCO
fully extracted. The isolation was performed at 40 °C and under
90 bar, for 60 min. The Pt contamination of product 12d was mea-
2
were success-
1
sured by ICP-MS. The analysis confirmed the presence of Pt at a
level of 0.06 ppm.
It is worth emphasizing that synthesized products 12a, b, d, l
and m were selectively obtained and characterized for the first
time. The hydrosilylation of 2f with silane 1a was described for
the first time, however, it led to the mixture of isomers.
3
. Conclusions
Hydrosilylation of structurally different alkynes with silanes in
scCO
group of alkynes (terminal and internal) and silanes were well sol-
uble in scCO and could be effectively transformed in hydrosilyla-
tion products in the presence of Karstedt’s catalyst.
The studies clearly showed that scCO can replace traditionally
used organic solvents and for some cases, its positive influence on
the catalytic activity and selectivity of Karstedt’s catalyst was evi-
denced. Moreover, the conditions for the extractions of the
2
has been studied for the first time. It was found that a wide
2
Fig. 3. The molecular structure of compound 12a. Displacement ellipsoids are
shown at the 30% probability level. The alternative position of the ethyl groups has
been omitted for the clarity. Selected geometric parameters: C19AC20 1.312(6) Å,
Si1AC19 1.857(5) Å, Si2AC20 1.869(5) Å, Si1AC7 1.867(4) Å, Si2AC25 1.835(4) Å;
C20AC19ASi1 129.1(4)°, C19AC20ASi2 126.6(4)°, C19ASi1AC7 106.92(19)°,
C25ASi2AC20 108.2(3)°.
2

2
12
K. Stefanowska et al. / Journal of Catalysis 356 (2017) 206–213
hydrosilylation products by scCO
high levels of yields of isolated products.
Extraction with scCO seemed to be an attractive alternative of
products isolation especially of these silyl ethenes, which contain
reactive or/and unstable functionalities. Moreover, when one iso-
mer is exclusively formed and the product is easily extracted with
2
were optimized, leading to very
2
with sapphire windows. The cell was sealed and charged with CO .
The pressure was increased to 55–60 bar. Then the reactor was
placed on the stirrer with heating function. The reactor was heated
to the required temperature (60 or 100 °C), and the pressure was
raised up to 65–70 bar. The additional CO was added to the cell
2
until a clear homogeneous mixture was visually determined
through the sapphire window (120–190 bar depending on the
2
2
CO stream, no other post-reaction procedures are required, mak-
ing this atom efficient synthesis more sustainable.
2
experiment). The reaction in scCO was performed on stirring
Despite the fact that hydrosilylation of alkynes has been so far
widely studied, in the study presented herein a wide series of
(350 rpm) for estimated reaction time. Subsequently, the reactor
was cooled down to 40 °C and resulting product was extracted
new silyl ethenes were synthesized (in toluene, THF or scCO
and characterized in detail for the first time.
The results presented in this paper make a perfect groundwork
for developing more effective catalytic systems for the hydrosilyla-
2
)
2
with CO steam (10 ml/min) and collected in the cold traps. The
extraction process was carried out for 30–60 min depending on
the experiment. The conversion of Si-H bond and the selectivity
1
13
29
of the product were determined by H, C, Si NMR and GC–MS
analysis. After the extraction process, the reactor was cleaned
using hexane and the efficiency of the process was estimated.
2
tion of alkynes performed in scCO , which will be the subject of our
forthcoming reports.
4
. Experimental
Acknowledgements
4
4
.1. General procedures
The authors acknowledge the financial support from the
National Centre for Research and Development in Poland-Lider
Programme No. LIDER/26/527/L-5/13/NCBR/2014.
.1.1. Phase behavior studies performed in scCO
At the first step, the solubility of reagents in scCO
2
2
at 100 °C and
in the pressure range of 120–190 bar was tested (the pressure was
increased at a step of 10 bar). For the compounds which solubility
was low at 100 °C, phase behavior studies at lower temperatures
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2017.10.005.
(
60 and/or 40 °C) and in the same pressure range (120–190 bar)
were performed. The solubility of the reagents and the progress
of the reaction were visually observed through the sapphire win-
dow of the reactor.
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