Silicon(II) Cation Cp*Si:+ X-: A New Class of Efficient Catalysts in Organosilicon Chemistry

Article abstract of DOI:10.1021/acs.oprd.9b00260

The catalytic activity of the pentamethylcyclopentadienylsilicon(II) cation Cp*Si:⁺ was investigated. It was shown that Cp*Si:⁺ efficiently catalyzes reactions of technical relevance in organosilicon chemistry: Cp*Si:⁺ proved to be a very efficient nonmetallic catalyst for the hydrosilylation of olefins at low catalyst amounts of <0.01 mol % and for the Piers-Rubinsztajn reaction in order to make controlled silicone topologies. The thermal induction of hydrosilylation which is important for the manufacturing of silicone rubber can be achieved by small amounts of alkoxysilanes.

Full text of DOI:10.1021/acs.oprd.9b00260

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Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Silicon(II) Cation Cp*Si:+ X−: A New Class of Efficient Catalysts in
Organosilicon Chemistry
Elke Fritz-Langhals*
WACKER Chemie AG, Consortium, Zielstattstraße 20-22, D-81379 Munich, Germany
S
* Supporting Information
ABSTRACT: The catalytic activity of the pentamethylcyclopentadienylsilicon(II) cation Cp*Si:+ was investigated. It was
shown that Cp*Si:+ efficiently catalyzes reactions of technical relevance in organosilicon chemistry: Cp*Si:+ proved to be a very
efficient nonmetallic catalyst for the hydrosilylation of olefins at low catalyst amounts of <0.01 mol % and for the Piers−
Rubinsztajn reaction in order to make controlled silicone topologies. The thermal induction of hydrosilylation which is
important for the manufacturing of silicone rubber can be achieved by small amounts of alkoxysilanes.
KEYWORDS: hydrosilylation, cationic silicon(II) compounds, homogeneous catalysis, Piers−Rubinsztajn reaction, polysiloxanes,
copolymers
yl, Figure 1),12,14 however, represents a unique cationic
silyliumylidene structure because the cationic silicon center is
INTRODUCTION
■
Silyliumylidene cations RSi:+ (1, Figure 1) are of increasing
only stabilized by a Cp* residue and therefore the positively
interest in chemistry. Two vacant orbitals and a lone pair of
charged silicon center has free coordination sites which was
proved by crystal structure analysis.14 Cp*Si:+ is stable in
combination with an inert non-nucleophilic anion, usually the
−
perfluorinated tetraarylboranate anion, B(ArF)4 (ArF
=
−
C6F5 ). The catalytic potential of this interesting cation is
almost unknown, whereas irreversible reactions of Cp*Si:+
with nucleophilic reagents have been reported.12 It was found
that the formation of cyclic ethers from oligoethyleneglycol
ethers can be catalyzed by 5−10 mol % of 1a in a slow reaction
at room temperature (rt) (Scheme 1),15 but to date this
remains the only example of using a silyliumylidene cation as a
catalyst.
Figure 1. Left: Electronic structure of silyliumylidene cations 1;
−
Right: structure of Jutzi’s Cp*Si:+ B(ArF)4 , with ArF = C6F5 and Cp*
= pentamethylcyclopentadienyl (1a).
electrons formally combine the characters of both strongly
electrophilic silylium cations1 R3Si+ and nucleophilic silylenes
R2Si:.2
Scheme 1. Formation of Cyclic Ethers Catalyzed by 1a15
The reactivity of silylenes R2Si: was investigated in detail in
the past 20 years2 and interesting reactivities and potential
applications, for example, as ligands in transition metal
catalysis,3 have been found. The catalytic potential of silylenes,
however, seems to be low due to their pronounced tendency to
undergo irreversible addition and insertion reactions leading to
tetravalent Si(IV) compounds.4 Silylium cations R3Si+,1 on the
other hand, are very strong Lewis acids and some catalytic
activities have been demonstrated in hydrodefluorinations5 and
Diels−Alder reactions,6 and furthermore in Friedel−Crafts
reactions.7 Reactivity studies of silyliumylidene cations RSi:+,
however, are still at their very beginning. Several stable
silyliumylidene cations8−12 have been synthesized in the last 14
years by applying principles of thermodynamic and kinetic
stabilization originally applied on silylenes.12,13 Silyliumylidene
cations bearing sterically demanding donor ligands for
stabilization have, for instance, been synthesized by Driess,8
Filippou,9 So,10 and Sasamori.11 Until now, a catalytic activity
of these compounds has not been found. Jutzi’s Cp*Si:+
RESULTS AND DISCUSSION
■
−
Synthesis of Cp*Si:+ B(ArF)4 (1a). 1a was originally
prepared from decamethylsilicocene16,17 2 in a comparatively
complicated synthesis by a selective protonation reaction using
the pentamethylcyclopentenyl cation 3a at −78 °C (Scheme
2).14 This strategy, however, affords the preparation of 3a
separately in a two-step procedure at low temperatures.
Recently, Filippou and coworkers17 reported that the selective
protonation can be simplified using H+(OEt2)2 B(ArF)4 (3b,
−
Scheme 2). 1a was isolated in pure form by crystallization from
fluorobenzene/pentane at −30 °C.
Received: June 9, 2019
B(ArF)4 (1a, ArF = C6F5, Cp* = pentamethylcyclopentadien-
−
© XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.9b00260
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Scheme 2. Synthesis of 1a.14,17,18
This corresponds to a TON in the range of 80 000. In another
experiment with 0.01 mol % of 1a, we were able demonstrate
that the product mixture still contains the active catalyst by
repeated addition of a mixture of the starting materials which
react to full conversion (see Experimental Section). Therefore,
1a may be recovered and used in further hydrosilylation
reactions. The stability and high activity make the catalyst 1a
especially interesting for technical applications.
As can be seen from Table 1, olefins with internal double
bonds can also be hydrosilylated but are less reactive than
terminal olefins and need higher reaction temperatures and
more catalysts (Table 1, entries 20 and 21). The hydro-
silylation of double bonds attached to silicon, however,
proceeds slowly (Table 1, entry 22). In this case, a competing
redistribution reaction of the Si bound H and the Si bound
vinyl group was detected, and, as a consequence, a mixture of
coupling products is formed, see Scheme 4.
A novel synthetic strategy with hydride abstraction from the
methyl group of one Cp* residue using the tritylium salt 3c18
at rt (Scheme 2) is even more attractive under technical
aspects because 3c is already produced in batches of more than
100 kg. The tetramethyl fulvene formed in the reaction can be
removed by simple crystallization from dichloromethane/n-
decane which gives the product 1a in good yields of 89%. The
improved availability under technical aspects makes 1a an
attractive candidate for catalysis in large-scale processes.
Herein, we report about the catalytic potential of 1a in
technical organosilicon chemistry.
The Cp*Si:+ catalyzed reaction between alkynes and
hydrosilanes gives a mixture of hydrosilylation products and
dehydrogenative silylation products (Scheme 5, eqs 1 and 2,
Table 1, entries 23 and 24), and therefore is of limited practical
use. Interestingly, Luo, Hou, and coworkers30 found that
B(ArF)3 is completely inactive as a catalyst for the hydro-
silylation of carbon−carbon triple bonds, but dehydrogenative
silylation can be achieved by using large amounts of B(ArF)3
(10 mol %) in a 1:1 mixture with DABCO.
HYDROSILYLATION OF UNSATURATED
CARBON−CARBON BONDS
■
Hydrosilylation of carbon−carbon double bonds and triple
bonds19 using silicon hydrides is an important approach to
prepare organosilicon compounds (Scheme 3). The reaction is
widely applied in the industry to produce various silanes and
siloxanes.
Si bound alkynes can be hydrosilylated with 1a as was shown
for the reaction between trimethylsilyl acetylene and triethyl
silane (Table 1, entry 25). Dehydrogenative silylation products
were only observed in traces (1%). As in the case of Si vinyl
groups, the addition reaction is accompanied by a redis-
tribution process of Si H and Si alkyne (Scheme 5, eq 3).
It is assumed that the hydrosilylation catalyzed by 1a follows
a cationic mechanism similar to the mechanism suggested by
Gevorgyan25 and Oestreich31 for B(C6F5)3 as outlined in
Scheme 6.
Scheme 3. Hydrosilylation of Unsaturated Carbon−Carbon
Bonds
In analogy to Oestreich31 and Piers32 in the first step the
silicon hydrogen bond is activated by Cp*Si:+ forming a
hydrogen-bridged complex.
1
Evidence for Si−H activation is provided by the H NMR
spectrum of triethylsilane, Figure 2, where the Si−H signal
shows coalescence after the addition of 0.10 mol % 1a.
A cationic mechanism is in accordance with the fact that the
electronically deficient methyl dichlorosilane does not react,
whereas dimethyl chlorosilane still forms a hydrosilylation
product (Table 1, entries 14 and 15).
The reaction requires catalysis, and noble metal complexes,
for example, of platinum, are the most efficient catalysts.19,20 In
most cases, products according to the anti-Markovnikov rule
are formed.19 The high and volatile price of platinum,
environmental issues associated with mining, and the
persistence of platinum, however, have inspired the develop-
ment of alternative catalysts.21−24 Complexes of earth-
abundant first-row transition metal complexes of iron, cobalt,
nickel,21−23 and manganese24 have been developed. However,
catalysts free of transition metals are the most attractive target.
Tris(pentafluorophenyl)borane, B(ArF)3, proved to be a highly
active nonmetal catalyst for the hydrosilylation of carbon−
carbon double bonds,25−27 but catalyst amounts of 1−5 mol %
are necessary to obtain full conversion due to deactivation
processes of the catalyst during hydrosilylation.28
The new hydrosilylation procedure requires chlorinated
solvents such as dichloromethane or 1,2-dichloroethane to
proceed at a high reaction rate. Hydrosilylations are much
slower in benzene (entry 13) and under solvent-free conditions
(entries 8 and 9). This can be attributed to a better ion pair
separation in dipolar solvents.
Next, we studied the influence of different anions of Cp*Si:+
on the catalytic activity. The combinations of Cp*Si:+ with
perfluorinated aluminates Cp*Si:+ Al[OCH(CF3)2]4 (1b)
−
and Cp*Si+Al[OC(CF3)3]4− (1c) exhibited a very low catalytic
activity (Table 1, entries 6 and 7) though they were soluble in
the reaction mixture. Then, in order to improve ion pair
separation even in a less polar reaction medium, we combined
Cp*Si:+ with the more bulky and more lipophilic B[C6F4-(4-
Here, we could demonstrate that Cp*Si:+ efficiently
catalyzes the hydrosilylation of olefins (Table 1).29
Terminal olefins (Table 1, entries 1−19) give the anti-
Markovnikov adducts. As can be seen from Table 1 full
conversion can be achieved with as low as 0.0013 mol % of 1a,
which was demonstrated by the hydrosilylation of α-
methylstyrene with pentamethyldisiloxane (Table 1, entry 3).
−
−
SiMe2t-Bu)]4 anion. Cp*Si:+ B[C6F4-(4-SiMe2t-Bu)]4 (1d)
was more active under neat conditions (entries 8 and 9) thus
principally verifying this concept.
B
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Table 1. Hydrosilylations of Alkenes and Alkynes Catalyzed by 1a to 1d
a
b
entry
alkene/alkyne
sil(ox)ane
catalyst (mol %)
solvent
T (°C)
t (h)/conversion (%)
yield (%, method)
c
1
2
3
4
5
6
7
8
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
α-methylstyrene
n-C4H9−CHCH2
n-C4H9−CHCH2
n-C4H9−CHCH2
n-C4H9−CHCH2
cyclo-hexene
TMS -O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
TMS-O-SiMe2-H
Et3Si−H
1a (0.012)
1a (0.012)
1a (0.0013)
1a (0.012)
1a (0.004)
1b (0.50)
1c (0.50)
1a (0.012)
1d (0.0096)
1a (0.1)
1a (0.06)
1a (0.20)
1a (0.20)
1a (0.10)
1a (0.10)
1a (0.09)
1a (0.050)
1a (0.20)
1a (0.10)
1a (0.26)
1a (0.20)
1a (0.48)
1a (0.040)
1a (0.10)
CD2Cl2
CD2Cl2
CD2Cl2
DCE
DCE
CD2Cl2
CD2Cl2
neat
25
50
25
80
80
25
25
25
25
50
50
25
25
25
25
25
25
50
25
50
50
50
25
50
2/50, 25/100
2/95
24/90, 48/98
0.16/100
0.16/33, 1/100
72/3
72/14
2/4, 20/25, 120/32
2.5/18, 20/35, 72/51
28/100
4/100
24/98
>97 (NMR)
>97 (NMR)
>97 (NMR)
>97 (NMR)
>97 (NMR)
>97 (NMR)
>97 (NMR)
>90 (NMR)
>98 (NMR)
>98 (NMR)
>98 (NMR)
>98 (GC)
d
e
e
9
neat
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
CD2Cl2
CD2Cl2
CD2Cl2
benzene-d6
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
PhMe2Si−H
TMS-O-SiMeH-O-TMS
TMS-O-SiMeH-O-TMS
Me2ClSi-H
e
5/2, 23/25
21/100
>90 (NMR)
90 (GC)
0 (NMR)
98 (NMR)
81 (GC)
>90 (GC)
94 (NMR)
96 (GC)
97 (GC)
MeCl2Si-H
48/0
0.8/100
24/100
6/100
f
TMS-O-SiMe2-H
TMS-O-SiMeH-O-TMS
Me2ClSi-H
PhMe2Si-H
Et3Si-H
Et3Si-H
Et3Si-H
Et3Si-H
Et3Si-H
g
g
g
1.5/100
4/100
11/99
50/30
18/70
5/30
norbornene
h
i
k
Me3Si−CHCH2
50 , 40 , 20 (GC)
l
m
n
Ph−CCH
34 , 41 , 20 (GC)
o
p
n-C4H9−CCH
27 , 37 (GC)
q
r
s
Me3Si−CCH
Et3Si-H
1a (0.10)
50
40/44
43 , 30 , 7 (GC)
a
b
c
d
e
f
1
Determined by H NMR. Anti-Markovnikov product. Trimethylsilyl. 1,2-Dichloroethane. Referring to conversion. 4.5 equiv of n-C4H9−
g
h
CHCH2 were used. 4.5 equiv of n-C4H9−CHCH2 were used, excess hexene was removed in vacuo for analysis. Me3Si−CH2−CH2−SiEt3.
i
k
l
m
n
o
Me3Si−CH2−CH2−SiMe3. Et3Si−CH2−CH2−SiEt3. Ph−CHCH−SiEt3. Ph−CC−SiEt3. Ph−CH2−CH(SiEt3)2. n-C4H9−CHCH−
p
q
r
s
SiEt3. n-C4H9−CC−SiEt3. Me3Si−CHCH−SiEt3. Me3Si−CHCH−SiMe3. Et3Si−CHCH−SiEt3.
Scheme 4. Redistribution Reaction and Hydrosilylation of a
Vinylsilane (see Table 1, Entry 22, for Product
Distribution)
Scheme 6. Mechanism Suggested for the Hydrosilylation
Catalyzed by 1a
Scheme 5. Different Reactivity of Alkynes toward
Triethylsilane (for Conditions and Product Distributions
see Table 1)
Therefore, the preparation and handling of the catalysts 1 have
to be performed in an inert Ar atmosphere. The crosslinking
experiment, however, can be performed successfully under
open-air conditions. It seems that in this case the catalyst is
sufficiently protected by the silicone matrix.
Furthermore, we prepared a new compound Cp*Si:+
−
Crosslinking of hydridosiloxanes and vinyl siloxanes
(Scheme 7), which is the basis of the technical silicone
elastomer syntheses, could be demonstrated using 1d (0.27%
w/w) at elevated temperatures, but it still needs some
dichloromethane to proceed.
The new catalysts 1 are sensitive to air and moisture. After
19 h under open-air conditions at rt 44% of a 20 mg sample of
crystalline 1a had decomposed (1H NMR measurement).
HB(C6F5)3 (1e) by reaction of decamethylsilicocene with
B(ArF)3.33 Interestingly, 1e is less active than 1a as a catalyst in
hydrosilylations by more than two orders of magnitude though
it is much better soluble in the reaction medium than 1a. This
can be rationalized by a comparison of the crystal structures of
1a and 1e. The crystal structure of 1e (Figure 3) revealed that
the ion pair of 1e is packed much more tightly than in 1a.14
The distance between the silicon center and the boron center
C
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Figure 3. Diamond plot of the molecular structure of 1e in a single
crystal lattice at T = 123.00(10) K. Thermal ellipsoids are set to 30%
probability.
Up to now, B(ArF)3 is the only catalyst reported for the PR
reaction which is mainly attributed both to its high
electrophilicity and to its good solubility in a hydrophobic
environment.36 Now, we found that Cp*Si:+ catalyzes the PR
reaction very efficiently with ∼0.1 mol % in a minimum
amount of solvent without significant catalyst deactivation.37
Scheme 8. Piers−Rubinsztajn (PR) Reaction
1
Figure 2. Time-dependent H NMR spectrum of triethylsilane (32%
w/w solution in CD2Cl2) with 0.10 mol % of 1a, T = 25 °C,
coalescence of the Si−H signal (a) 6, (b) 12, (c) 18, and (d) 24 h
after the addition of 1a. The chemical shift of the NMR signal remains
constant.
Scheme 7. Schematic Metal-Free Elastomer Synthesis Using
1d as a Catalyst
Scheme 9. Products of the PR reaction Catalyzed by 1e.
Bonds Marked in Bold Red are Formed During the
Reaction
is much smaller in 1e (3.64 Å) than in 1a (5.39 Å)14 which
suggests that the solvent-induced separation of the intimate ion
pair of 1e is even more difficult in dichloromethane because it
is more densely packed than 1a. The distance between the
boron bound hydrogen and silicon is 2.79 Å, thus ruling out
Si−H hydrogen bridging.
PIERS−RUBINSZTAJN (PR) REACTION
■
Piers34 and Rubinsztajn35 found that B(ArF)3 catalyzes the
reaction between silyl ethers and SiH to give siloxanes and the
corresponding alkanes (Scheme 8). This process is called the
PR reaction.36
Scheme 9 and Table 2 shows the products we obtained. First,
to test the performance of the reaction, compound 4 was made
on a preparative scale by reaction of diethoxydiphenylsilane
with pentamethyldisiloxane and isolated in 86% yield (Table 2,
entry 1). Chlorosubstituted siloxanes, for example, 5 (Table 2,
entry 4) can efficiently be prepared and used as building
blocks, for example, to prepare siloxane copolymers. It is
reported38 that chlorosiloxanes with a definite structure can
Whereas the classical acid or base catalyzed condensation
processes to make siloxanes are often accompanied by
uncontrolled equilibration reactions, the PR reaction allows
for the synthesis of well-defined siloxane architectures.
D
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Table 2. Piers−Rubinsztajn (PR) Reaction of Ethoxysilanes and Hydrido Sil(ox)anes Catalyzed by 1a, 1d, and 1e
a
b
c
d
entry
alkoxysilane
hydrido sil(ox)ane
catalyst (mol % )
CH2Cl2 (% )
T (°C)
t (h)/conversion (%)
product (Mw, Mn, D )
e
f
g
h
1
2
Ph2Si(OEt)2
Ph2Si(OEt)2
TMS -O-SiMe2-H
1e (0.014)
1a (0.11)
5
5
40
5
40
5
10
10
10
60
55
55
55
55
60
60
60
60
4/>95
4/0
2/>95
4/0
2/>95
2/>95
2/>95
2/>95
4
f
TMS-O-SiMe2-H
4
f
3
Ph2Si(OEt)2
TMS-O-SiMe2-H
1d (0.11)
4
5
f
i
4
5
6
7
Ph2Si(OEt)2
Ph2Si(OEt)2
Ph2Si(OEt)2
Me3SiOEt
Me2ClSi-H
1,4-(SiMe2H)benzene
H-SiMe2-O-SiMe2-H
TMS-O-D9-DH24-TMS
1e (0.13)
1e (0.11)
1e (0.10)
1e (0.033)
6 (18 600, 5040, 3.69)
7 (8100, 4300, 1.9)
8
f
j
1/>95
a
b
c
d
e
f
Based on the amount of Si−H. (w/w). Residual Si−H was determined in the 1H NMR spectrum. Dispersity D = Mw/Mn. Trimethylsilyl. Si−
g
h
H/Si−OEt = 1:1. GC: 80 area % 4, 10 area % 1-ethoxy-1,1-diphenyl-3,3,5,5,5-pentasiloxane). Preparative example, see Experimental Section,
i
j
yield 86%. GC: 72 area % 5, 10 area % 1-chloro-1,1-dimethyl-3-ethoxy-3,3-diphenyl-trisiloxane). D = Me2SiO, DH = MeHSiO, statistical
distribution of Si−H, Me3SiOEt was used in 10% excess (Si−H/Si−OEt = 1:1.1).
also be prepared very selectively by stepwise reactions of
silanols with dichlorosilanes. This transformation, however,
affords a base to remove the HCl in order to prevent the self-
condensation of silanols and therefore might be of limited
technical use.
Scheme 10. Reversible Inhibition of the Hydrosilylation by
Means of Alkoxysilanes
Most interesting on technical aspects, however, is to prepare
siloxane copolymers directly by a combination of bifunctional
alkoxysilanes with bifunctional hydrido sil(ox)anes (Table 2,
compounds 6,7, entries 5 and 6). Furthermore, it is possible to
prepare branched silicones, for example, compound 8 (Table 2,
entry 7).
Interestingly, compound 1e is highly active in the PR
reaction, even with less than 10% of dichloromethane (Table 2,
entry 1), whereas 1a and 1d showed no catalytic activity under
these conditions, but the PR reaction took place with 40%
dichloromethane solvent (Table 2, entries 2 and 3). This is
quite different from the hydrosilylation reaction, where 1a is by
far more active than 1e, which was attributed to an inefficient
separation of the intimate ion pair in 1e. We assume that the
alkoxysilane coordinates at the cationic silicon center similar to
the Cp*Si:+ coordination of ethers reported by Jutzi,15 and this
coordination facilitates the separation of the intimate ion pair.
In a further experiment, we could show that small amounts
of an alkoxysilane (e.g. ∼2% w/w 1-ethoxy pentamethyldisilox-
ane or ethoxy trimethylsilane) completely suppress the
hydrosilylation of α-methylstyrene at rt for more than a
week. This is a further indication that the catalytically active
cationic silicon center is masked by the alkoxysilane.
Interestingly, inhibition by alkoxysilanes fails with the
B(C6F5)3 catalyst.
temperature dependence. The catalyst becomes active again
and the hydrosilylation proceeds at a regular reaction rate.
CONCLUSIONS
■
New catalysts based on a cationic silicon(II) center, Cp*Si:+,
have been developed and their potential in technically relevant
processes of organosilicon chemistry was demonstrated.
Hydrosilylation still proceeds at catalyst amounts of 0.0013
mol % (TON ≈ 80 000) which is in the range of platinum
catalysts. Separation of the catalysts’ intimate ion pair is
considered important for activity and can be enhanced by the
introduction of lipophilic counter anions which allows for
syntheses without polar solvents. Si bound vinyl groups are less
reactive than C bound vinyl groups. Nevertheless, silicone
elastomer curing was achieved by Cp*Si:+ catalyzed hydro-
silylation of vinyl substituted siloxanes. Furthermore, Cp*Si:+
efficiently catalyzes the PR reaction between the Si−OR and
Si−H groups and gives access to well-defined siloxane
structures. Cp*Si:+ is a good alternative to the well-known
B(ArF)3 catalyst as it shows no deactivation during the
reaction. Finally, the Cp*Si:+ catalyzed hydrosilylation can be
reversibly blocked by low amounts of an alkoxysilane offering a
way to temperature-induced hydrosilylation.
The use of reversible inhibitors is a well-established
technology in the platinum-catalyzed silicone rubber manu-
facturing in order to guarantee a certain pot life of the
hydrosilylation mixture, which means that the mixture of all
components remains stable at ambient temperature. Curing
can be performed on demand by an increase of temperature.
For the platinum-catalyzed hydrosilylation, small amounts of
alkynes are often used which block the hydrosilylation reaction
by coordination to the metal center.39
The present work gives insights into the reactivity of cationic
silicon(II) compounds and their catalytic behavior and
therefore may also pave the way towards further development.
In the Cp*Si:+ catalyzed hydrosilylation, alkoxysilanes play
the role of reversible inhibitors,40 see Scheme 10. At ambient
temperatures the PR reaction which consumes the alkoxysilane
is very slow (see Experimental Section for details). At higher
temperatures (>60 °C), however, the alkoxysilane is quickly
consumed by the PR reaction because it has a pronounced
EXPERIMENTAL SECTION
Analytical Methods. All H NMR spectra were measured
by using a Bruker AVANCE 500 spectrometer. GC: Agilent
Technologies, HP 5 column (polydimethylsiloxane with 5%
■
1
E
DOI: 10.1021/acs.oprd.9b00260
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Organic Process Research & Development
Article
NMR δ (CD2Cl2): −399.3. 1c: Yield 76%, 1H NMR δ
(CD2Cl2): 2.26 (s, Cp*). 19F NMR δ (CD2Cl2): −75.7 (s,
CF3). 29Si NMR δ (CD2Cl2): −398.9. 1d: Yield 92%. 1H NMR
δ (CD2Cl2): 0.35 (s, 24H, 4 SiMe2), 0.91 (s, 36H, 4-tert-butyl),
2.18 (s, 15H, Cp*). 19F NMR δ (CD2Cl2): −132.3 (mc, 8F),
−130.6 (mc, 8F). 29Si NMR δ (CD2Cl2): −398.9 (s, Cp*Si),
5.58 [s, Si(Me2tert-butyl)].
Table 3. Hydrosilylation of α-Methylstyrene with
Pentamethyldisiloxane at 25 °C with 0.1 mol % 1a and 2.0
a
mol % of an Alkoxysilane as Inhibitor
t (h)
ethoxy group
hydrosilylation product
1a Cp*
b
0.3
1
4
24
48
1.96
1.95
1.96
1.67
1.56
1.45
1.00
0.04
n.d.
0.1
0.083
0.081
0.083
0.083
0.083
0.084
0.091
0.071
0.071
0.082
0.067
Synthesis of 1e. A solution of B(ArF)3 (172 mg, 335
mmol) in 1 mL of dichloromethane was added dropwise to a
solution of 100 mg (335 mmol) of decamethylsilicocene in 1
mL of dichloromethane at rt in an argon atmosphere. The
color of the solution turned dark orange indicating the
formation of tetramethylfulvene. n-Decane was added, the
precipitate was washed twice with n-decane and dried in vacuo.
0.15
0.28
0.45
0.52
0.78
1.13
1.27
95
72
120
168
192
216
240
b
n.d.
n.d.
n.d.
1
Yield: 155 mg (64%), colorless solid. H NMR δ (CD2Cl2):
100
−
2.10 (s, 15H, Cp*), 3.48 [q, broad, 1H, H−B(C6F5)3 ]. 19F
ethoxy groups
NMR δ (CD2Cl2): −167.3 (mc, 8F, 3,5-F), −163.6 (mc, 4F, 4-
F), −133.7 (mc, 8F, 2,6-F). 29Si NMR δ (CD2Cl2): −399.9.
Hydrosilylation of α-Methylstyrene with Pentame-
thyldisiloxane in CD2Cl2Standard Procedure. Under an
argon atmosphere, a solution of 1a (16.9 mg, 2.00 μmol) in
1.76 g of dichloromethane was prepared. A mixture of α-
methylstyrene (2.37 g, 20.0 mmol) and pentamethyldisiloxane
(2.97 g, 20.0 mmol) in 1.77 g of dichloromethane was added at
c
t (h)
EtO-TMS
EtO-PDMS
hydrosilylation product
1a Cp*
0.3
1
4
24
48
1.48
1.44
1.35
0.83
0.50
n.d.
n.d.
n.d.
n.d.
n.d.
0.16
0.21
0.33
0.60
0.93
0.30
0.11
n.d.
0.11
0.10
0.17
0.40
0.79
1.04
1.70
2.10
93
0.085
0.078
0.080
0.077
0.081
0.081
0.075
0.080
0.088
0.090
72
1
120
168
192
216
20 °C. The colorless solution was stirred at rt overnight. H
NMR (CD2Cl2) indicated a complete conversion into Ph−
CH(CH3)−CH2−SiMe2O−SiMe3.22 1H NMR δ (CD2Cl2):
0.00 (2s, 2 CH3), 0.12 (s, TMS), 1.01 (mc, CH2), 1.33 (d, J =
7 Hz, CH3), 2.98 (mc, CH), 7.15−7.34 (5 aromat. H); 29Si
NMR δ (CD2Cl2): 6.58 (s), 7.32 (s). GC−MS m/z: 266 (5,
M+), 251 (19, M+ − CH3), 209 (12), 147 [100,
n.d.
n.d.
98
a
Figures are given in mol %; top: ethoxy pentamethyldisiloxane was
used as the inhibitor, bottom: ethoxy trimethylsilane was used as the
b
c
inhibitor. Not detected. Because of SiH/SiOEt exchange reaction.
+
+
(Me2SiOTMS)+], 133 (45, Ph−C4H7 ), 105 (13, C8H9 ).
Repeated Addition of Educts. A further mixture of α-
methylstyrene (2.37 g, 20.0 mmol) and pentamethyldisiloxane
(2.97 g, 20.0 mmol) in 1.77 g of dichloromethane was then
added to the product mixture at 20 °C. After 1 h, the
hydrosilylation reaction was complete (1H NMR) indicating
that the catalyst was still active in the product mixture. The
same procedure was repeated twice; for each repetition
complete conversion was reached after 1 h. The catalyst was
deactivated by the addition of pyridine (∼20 mg of a ∼10%
solution in dichloromethane) and the mixture was fractionally
distilled. Yield of the isolated product: 14 g (66%), bp 56 °C/
0.3 mbar.
phenyl groups); length, 30 m; internal diameter, 0.25 mm; film
thickness, 0.25 μm; injector temp., 290 °C; detector FID, 320
°C; carrier gas, He. GC−MS: Agilent Technologies; GC unit,
6890N; column, Agilent HP-5MS UI; injector temperature,
250 °C; MS unit, 5975C; EI, 70 eV. Relative molecular masses
of silicone copolymers were measured by GPC using an
Agilent MesoPore and OligoPore column; length, 300 mm;
internal diameter, 7.5 mm; particle size, 3 and 6 μm
respectively, at 35 °C; eluent, toluene; RI Detektor; PDMS
calibration.
Materials. (C6H5)3C+B(ArF)4 (3c) and B(ArF)3 were
−
commercially available and were used without further
purification. (C6H5)3C+Al[OCH(CF3)2]4 and (C6H5)3C+Al-
−
The following products were prepared according to the
standard procedure (for further details see Table 1) and
identified directly in the product solution.
41
−
[OC(CF3)3]4
were provided by Prof. Dr. I. Krossing,
University of Freiburg/Germany. Decamethylsilicocene (2)
was synthesized according to the literature.17 (C6H5)3C+
25
1
Ph−CH(CH3)−CH2−SiEt3 (entry 10) H NMR δ
(CD2Cl2): 0.83 (mc, 3 Si−CH2−CH3), 1.29 (t, J = 8 Hz, 3
Si−CH2−CH3), 1.36 (mc, −CH2−), 1.67 (d, J = 7 Hz, CH3−
CH), 3.27 (mc, CH3−CH), 7.45−7.51 (m, 1 aromat. H),
7.54−7.63 (m, 4 aromat. H). 29Si NMR δ (CD2Cl2): 6.18,
GC−MS m/z: 234, 219 (∼0.1, M+ − CH3), 205 (77, M+ −
−
B[C6F4−SiMe2t-Bu]4 was synthesized according to the
literature.42
Synthesis of 1a−d. A solution of 0.336 mmol of the
tritylium salt in 3.5 mL of dichloromethane was mixed with a
solution of 102 mg (0.342 mmol) of decamethylsilicocene (2)
in 3.5 mL of dichloromethane at rt in an argon atmosphere. n-
Decane was added dropwise to the homogeneous solution
until the product precipitates as a solid. The mother liquor was
decanted off and the solid was washed 3 times with n-hexane
and dried in vacuo. 1a: Yield 89%, off-white crystalline solid.
1H NMR δ (CD2Cl2): 2.24 (s, Cp*). 29Si NMR δ (CD2Cl2):
−398.8. 19F NMR δ (CD2Cl2): −167.5 (mc, 8F, 3,5-F),
−163.6 (mc, 4F, 4-F), −133.0 (mc, 8F, 2,6-F). 1b: Yield 62%.
1H NMR δ (CD2Cl2): 2.25 (s, 15H, Cp*), 4.52 [mc, 4H,
−CH(CF3)2]. 19F NMR δ (CD2Cl2): −77.2 (s, CF3). 29Si
+
C2H5), 163 (100, C6H5SiEt2 ), 135 (55, 163 −C2H4).
43
1
Ph−CH(CH3)−CH2−SiMe2Ph (entry 11)  H NMR δ
(CD2Cl2): δ 0.71 (s, Si−CH3), 0.76 (s, Si−CH3), 1.79 (mc,
2H, CH2), 1.83 (d, J = 7 Hz, CH3), 3.46 (mc, J = 7.5 Hz, CH−
CH3), 7.6−8.1 (10 aromat. H). 29Si NMR δ (CD2Cl2): −3.87.
GC−MS m/z: 254 (∼0.1, M+), 239 (8, M+ − CH3), 197 (16),
+
176 (47), 135 (100, PhSiMe2 ).
21c
1
Ph−CH(CH3)−CH2−SiMe(OTMS)2
NMR δ (CD2Cl2): 0.09 (s, SiMe), 0.31 (s, TMS) 0.32 (s,
TMS), 1.12 (mc, Si−CH2), 1.49 (d, J = 6.5 Hz, CH3), 3.15
(entry 12) H
F
DOI: 10.1021/acs.oprd.9b00260
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
[mc, −CH(CH3)], 7.30−7.34 (m, 1 aromat. H), 7.39−7.46
(m, 4 aromat. H). 29Si NMR δ (CD2Cl2): −22.8, 7.13, 7.26.
GC−MS m/z: 340 (5, M+), 325 (12, M+ − CH3), 221{100,
[SiMe(TMS)2]+}, 207 (70), 73 (58).
(30). n-C4H9−CC−SiEt3GC−MS m/z: 196 (1, M+), 167
(100, M+ − C2H5), 139 (95, 169 −C2H4), 111 (30).
Products from the reaction of Me3Si−CCH with Et3Si−H
(entry 25): Me3Si−CHCH−SiEt3GC−MS m/z: 214 (3,
M+), 199 (4, M+ − CH3+), 185 (86, M+ − C2H5), 157 (43, 185
Ph−CH(CH3)−CH2−SiMe2Cl43 (entry 14) H NMR δ
1
+
(CD2Cl2): 0.67 (2s, 2 CH3), 1.45 (mc, CH2), 1.50 (d, J = 7
Hz, CH3), 3.21 (mc, CH), 7.1−7.6 (aromat. H), GC−MS m/
z: 212 (5, M+), 197 (12, M+ − CH3), 93 [100, (SiMe2Cl)+].
−C2H4), 115 (7, SiEt3 ), 101 (56), 87 (100, Me3SiCH2 ).
Me3Si−CHCH−SiMe3GC−MS m/z: 172 (11, M+), 157
(33, M+ − CH3), 99 (4, M+ − SiMe3), 73 (100, SiMe3 ).
+
C5H11-CH2-SiMe2-O-TMS44 (entry 16) H NMR δ
Et3Si−CH2−CH2−SiEt3GC−MS m/z: 256 (1, M+), 227
1
(56, M+ − C2H5), 199 (32, 227 −C2H4), 115 (100, SiEt3 ), 87
+
(CD2Cl2): 0.11 (s, SiMe2), 0.13 (s, TMS), 0.54 (mc, Si−
CH2), 0.90 (mc, CH3), 1.2−1.4 (4 CH2). 29Si NMR δ
(CD2Cl2): 7.00, 7.64. GC−MS m/z: 232 (0.1, M+), 217 (15,
M+ − CH3), 159 (2, M+ − TMS), 147 (100, TMS-O-SiMe2),
133 (50).
(54).
Crosslinking Experiment. Catalyst component A: a
solution of 10.0 mg of 1d in 600 mg of CH2Cl2 was mixed
with 1.54 g of α,ω vinyl terminated polydimethylsiloxane (see
Scheme 9) in a speed mixer. Crosslinking component B: 1.97 g
of the α,ω vinyl terminated polydimethylsiloxane was mixed
with a polysiloxane having lateral Si−H functions (see Scheme
9) in a speed mixer. A and B were mixed in a speed mixer and
heated to 130 °C for 1 h. Most of the CH2Cl2 evaporates first,
thenat higher temperaturescrosslinking takes place. A
colorless and clear elastomer was formed.
1
C5H11−CH2−SiMe(TMS)2 (entry 17) H NMR δ
(CD2Cl2): 0.035 (s, 3H, SiMe), 0.13 (s, 18H, 2 TMSO),
0.50 (mc, 2H, Si−CH2), 0.92 (mc, 3H, CH3), 1.2−1.4 (m, 8H,
4 CH2). GC−MS m/z: 306 (∼0.1, M+), 291 (30, M+ − CH3),
221 {100, [SiMe(TMS)2]+}, 207 (57).
1
C5H11−CH2−SiMe2Cl (entry 18) H NMR δ (CD2Cl2):
0.50 (s, 6H, SiMe2), 0.94 (mc, 2H, Si−CH2), 1.03 (mc, 3H,
CH3), 1.2−1.5 (m, 8H, 4 CH2). GC−MS m/z: 178 (2, M+),
163 (36, M+ − CH3), 121 (18), 93 (100, Me2SiCl+).
PR ReactionPreparative Example. Diethoxydiphenyl-
silane (5.66 g, 20.8 mmol) and pentamethyldisiloxane (6.29 g,
42.4 mmol) were mixed at rt in an argon atmosphere, a
solution of 2.9 mg (4.3 μmol, 0.02 mol %) of 1e in 0.9 g of
dichloromethane was slowly added under stirring. An
exothermic reaction set in, the temperature reached a
maximum of 100 °C and was accompanied by the evolution
of gas (ethane). The temperature reached rt again after about 1
h, GC-analysis of the reaction mixture yielded 84 area %
1,1,1,3,3,7,7,9,9,9-decamethyl-5,5-diphenyl-pentasiloxane47 (4)
and 8.2 area % 1-ethoxy-1,1-diphenyl-3,3,5,5,5-pentasiloxane.
The mixture (12.8 g) was fractionally distilled in vacuo using a
short column filled with glass spirals to give 3.7 g of a mixture
of 4 (66 GC-area %) and 1-ethoxy-1,1-diphenyl-3,3,5,5,5-
pentasiloxane (33 GC-area %) with bp 54−62 °C/0.02 mbar
and 6.7 g of 4 (99 GC area %) with bp 91 °C/0.03 mbar. Yield
C5H11−CH2−SiMe2Ph43 (entry 19) H NMR δ
1
(CD2Cl2): 0.27 (s, 6H, 2 Si−CH3), 0.77 (mc, 2H, 2 Si−
CH2), 0.89 (mc, 3H, CH2−CH3), 1.24−1.37 (m, 10H), 7.33−
7.38 (m, 3 aromat. H), 7.50−7.55 (m, 2 aromat. H). 29Si NMR
δ (CD2Cl2): −3.10.
cyclo-C6H11−SiEt3 (entry 20)45,46 H NMR δ (CD2Cl2):
1
0.61 (q, J = 8 Hz, 6H, Si−CH2−CH3), 0.82 (mc, 1H, CH−
SiEt3), 1.02 (t, J = 8 Hz, 9H, Si−CH2−CH3), 1.29−1.38 (m,
5H, cyclohexyl), 1.71−1.86 (m, 5H, cyclohexyl). 29Si NMR δ
(CD2Cl2): 5.64. GC−MS m/z: 198 (9, M+), 169 (41, M+ −
C2H5), 141 (4, 169 −C2H4), 115 (53, Et3Si+), 87 (100, Et3Si+
−C2H4).
exo-2-Norbornyl-SiEt3 (entry 21)45,46 H NMR δ
1
(CD2Cl2): 0.56 (q, J = 8 Hz, 6H, Si−CH2−CH3), 0.72 (t, J
= 8.5 Hz, 1H), 1,00 (t, J = 8 Hz, 9H, Si−CH2−CH3), 1.14−
1.33 (m, 4H), 1.38−1.64 (m, 4H), 2.23 (m, 1H), 2.27 (m,
1H). 13C NMR δ (CD2Cl2): 0.90 (Si−CH2−CH3), 5.67 (Si−
CH2−CH3), 24.4 (Si−CH), 27.0, 31.0, 32.5, 35.1, 36.1, 36.2
(ring carbons of norbornyl). 29Si NMR δ (CD2Cl2): 5.63 (s,
SiEt3).
1
86%. H NMR δ (CD2Cl2): 0.16 (s, 2 TMS), 0.21 (s, 2
OSiMe2), 7.4−7.5 (m, 3 aromat. H), 7.7−7.8 (m, 2 aromat.
H). 13C NMR δ (CD2Cl2): 0.49 (2 OSiMe2O), 0.90 (2 TMS),
127.0, 129.2, 133.7, 135.8 (2 Phenyl-Si). 29Si NMR δ
(CD2Cl2): −48.2 (SiPh2), −20.2 (2 TMS-SiMe2), 7.74 (2
Me3Si).
Products from the reaction of Me3Si−CHCH2 with
Et3Si−H (entry 22): Et3Si−CHCH2GC−MS m/z: 142
(∼1, M+), 113 (69, M+ − C2H5), 85 (100, 113 −C2H4).
Me3Si−CH2−CH2−SiEt3GC−MS m/z: 216 (0.1, M+), 201
(6, M+ − CH3), 187 (100, M+ − C2H5), 159 (43, 187 −C2H4),
115 (16, Et3Si+), 99 (46), 87 (97). Me3Si−CH2−CH2−
SiMe3GC−MS m/z: 174 (2, M+), 159 (42, M+ − CH3), 85
(43), 73 (100, Me3Si+). Et3Si−CH2−CH2−SiEt3GC−MS
m/z: 258 (7, M+), 229 (31, M+ − C2H5), 201 (14, 229
−C2H4), 115 (100, Et3Si+), 87 (63).
Products from the reaction of Ph−CCH with Et3Si−H
(entry 23): Ph−CHCH−SiEt3GC−MS m/z: 218 (2,
M+), 189 (100, M+ − C2H5), 161 (59), 131 (67), 105 (33), 59
(30). Ph−CC−SiEt3GC−MS m/z: 216 (8, M+), 187
(100, M+ − C2H5), 159 (95), 131 (99), 105 (30). Ph−CH2−
CH(SiEt3)2GC−MS m/z: 334 (<1, M+), 305 (45, M+ −
C2H5), 189 (100), 161 (20), 115 (48), 87 (48).
PR ReactionTypical Procedure. Alkoxysilane (1.0
mmol) and hydrido sil(ox)ane (1.0 mmol) were mixed
stoichiometrically (Si−OEt/SiH = 1:1), 30 mg of a 3.6 ×
10−5 mmol/mg solution of the catalyst 1e in dichloromethane
(1.1 μmol, 0.11 mol % 1e) was added to the mixture at rt in an
inert atmosphere (Ar), and the mixture was heated (for
individual starting materials, catalyst amounts, reaction times,
temperatures, and yields see individual products). 1,5-Dichloro-
1,1,5,5-tetramethyl-3,3-diphenyl-trisiloxane48 (5) H NMR δ
1
(CD2Cl2): 0.55 (s, 12H, 2 SiMe2), 7.35−7.55 (m, 3 aromat.
35
29
H), 7.66−7.75 (m, 2 aromat. H). Siloxane copolymer 6  Si
NMR δ (CD2Cl2): −46.2 (OSiPh2O), 0.1 (Me2SiPh); GPC:
Mw = 18 600, Mn = 5040, Mw/Mn = 3.69. Siloxane copolymer
48
29
7  Si NMR δ (CD2Cl2): −48.1 (OSiPh2O), −20.1
(Me2SiPh); GPC: Mw = 8100, Mn = 4300, Mw/Mn = 1.9.
29
Branched polysiloxane 8 Si NMR δ (CD2Cl2):
−69.1(−67) (MeSiO3), −21.9 (Me2SiO2), 7.2 (SiMe3).
Inhibition of Hydrosilylation by Alkoxysilanes. Under
an argon atmosphere, a solution of 1a (1.7 mg, 2.0 μmol) in
755 mg of d2-dichloromethane and ethoxy trimethylsilane (5.2
Products from the reaction of n-C4H9−CCH with Et3Si−
H (entry 24): n-C4H9−CHCH−SiEt3GC−MS m/z: 198
(1, M+), 169 (100, M+ − C2H5), 141 (95, 169 −C2H4), 113
G
DOI: 10.1021/acs.oprd.9b00260
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Article
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hydrosilylation product was detected by H NMR spectrosco-
py. Then the mixture was heated to 50 °C for 1 h. The
hydrosilylation product was detected quantitatively. The
experiment was repeated without ethoxy trimethylsilane.
After 2 h at 25 °C the hydrosilylation product was detected
quantitatively.
Determination of Pot Life. Under an argon atmosphere, a
solution of 1.7 mg (2.0 μmol) of 1a in 540 mg of d2-
dichloromethane and 7.8 mg (40 μmol, 2 mol %) of ethoxy
1,1,2,2,2-pentamethyldisiloxane were added to a mixture of
239 mg (2.02 mmol) of α-methylstyrene and 300 mg (2.02
mmol) of 1,1,2,2,2-pentamethyldisiloxane with stirring. The
slow disappearance of the ethoxy group of the alkoxysilane was
1
monitored by H NMR spectroscopy, see Table 3 (top), the
aromatic signals were taken as a reference. Hydrosilylation
starts after 8 days when the alkoxysilane was completely used
up.
The experiment was repeated with 1a (1.8 mg, 2.1 μmol)
and ethoxy trimethylsilane (4.8 mg, 40 μmol, 2 mol %). The
slow disappearance of the ethoxy group of the alkoxysilane was
1
monitored by H NMR spectroscopy, see Table 3 (bottom),
and the aromatic signals were taken as reference. Ethox-
ypentamethyldisiloxane was formed as an intermediate (see
Table 3, bottom) by the H/OEt exchange reaction.49
Hydrosilylation started after ∼7 days when the alkoxysilanes
are completely used up.
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Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. Synthesis and
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.9b00260.
(14) Jutzi, P.; Mix, A.; Rummel, B.; Schoeller, W. W.; Neumann, B.;
Stammler, H.-G. The (Me5C5)Si+ Cation: A Stable Derivative of HSi+.
Science 2004, 305, 849−851.
■
S
́
(15) Leszczynska, K.; Mix, A.; Berger, R. J. F.; Rummel, B.;
N e u m a n n , B . ; S t a m m l e r , H . - G . ; J u t z i , P . T h e
Pentamethylcyclopentadienylsilicon(II) Cation as a Catalyst for the
Specific Degradation of Oligo(ethyleneglycol) Diethers. Angew.
Chem., Int. Ed. 2011, 50, 6843−6846.
NMR spectra, X-ray crystal structure data of 1e (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: elke.fritz-langhals@wacker.com.
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(16) Kuhler, T.; Jutzi, P. Decamethylsilicocene: Synthesis, Structure,
̈
Bonding and Chemistry. Adv. Organomet. Chem. 2003, 49, 1−34.
(17) Ghana, P.; Arz, M. I.; Schnakenburg, G.; Straßmann, M.;
Filippou, A. C. Metal−Silicon Triple Bonds: Access to [Si(η5-
C5Me5)]+ from SiX2(NHC) and its Conversion to the Silylidyne
Complex [TpMe(CO)2MoSi(η3-C5Me5)] (TpMe = κ3-N,N′,N″-
hydridotris(3,5-dimethyl-1-pyrazolyl)borate). Organometallics 2018,
37, 772−780.
ORCID
Elke Fritz-Langhals: 0000-0002-1015-3276
Notes
The author declares no competing financial interest.
(18) Fritz-Langhals, E. WACKER Chemie AG; Method for
Producing Cationic Silicon(II) Compounds. WO2,018,188,749 A1,
2018.
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Pergamon Press: Oxford, 1992. (b) Matisons, J. Hydrosilylation; In
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ACKNOWLEDGMENTS
I thank Prof. Dr. I. Krossing, University of Freiburg, Germany,
for providing the tritylium aluminates.
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