Copper-Promoted Formation of Vinylsiloxanes

Article abstract of DOI:10.1002/ejic.202000134

The metathesis reactions of chlorovinylsilanes with copper(I) oxide are reported. The process is catalyzed by HCl which can be in situ generated via hydrolysis of chlorosilanes by traces of water present in the system. An appearance of olefin/CuCl π-intermediates can provide a stereoselectivity of the process and alter the pathway of siloxanes formation employing Cu₂O as the source of oxygen. It was also shown that acetonitrile may be available as a solvent for construction of the systems with large preparative capabilities, including those based on the use atmospheric oxygen in siloxane synthesis.

Full text of DOI:10.1002/ejic.202000134

A Journal of
Accepted Article
Title: Copper-promoted formation of vinylsiloxanes
Authors: Marietta Białoń, Błażej Dziuk, and Volodymyr Olijnyk
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000134
Link to VoR: http://dx.doi.org/10.1002/ejic.202000134

European Journal of Inorganic Chemistry
10.1002/ejic.202000134
COMMUNICATION
Copper-promoted formation of vinylsiloxanes
[
a]
[a,b]
[a]
Marietta Białoń , Błażej Dziuk , and Volodymyr Olijnyk *
[
a]
Dr. M. Białoń, Mgr. B. Dziuk, Prof. V. Olijnyk
Faculty of Chemistry
University of Opole
Oleska 48, 45-052 Opole (Poland)
E-mail: Wlodzimierz.Olijnyk@uni.opole.pl
Mgr. B. Dziuk
[
b]
Faculty of Chemistry
Wrocław University of Science and Technology
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław (Poland)
1
13
Supporting information (including experimental procedures, FTIR, H, and C NMR spectra as well as GC-MS, XRD and thermal analysis results) can be
found under:
Abstract: The metathesis reactions of chlorovinylsilanes with
copper(I) oxide are reported. The process is catalyzed by HCl which
can be in situ generated via hydrolysis of chlorosilanes by traces of
water present in the system. An appearance of olefin/CuCl -
intermediates can provide a stereoselectivity of the process and alter
precise design of siloxane-based materials. Several types of
[12]
cross-coupling reactions of halosilanes with silanols,
[13]
[14]
silanolates,
or alkoxysilanes
have been reported, but
special emphasis must be placed on processes using an
[
15]
[16]
extraneous source of oxygen, such as ethers,
dimethylsulfoxide.
air
or
[17]
the pathway of siloxanes formation employing Cu
2
O as the source of
In summary, the development of one-pot
oxygen. It was also shown, that acetonitrile may be available as a
solvent for construction of the systems with large preparative
capabilities, including those based on the use atmospheric oxygen in
siloxane synthesis.
efficient synthesis of siloxanes has continued to intensify in the
last decade and will continue to expand in the future. In this
paper, we report an easy and convenient route to vinylsiloxanes
starting from chloroorganosilanes.
The present studies were initiated by the observation that
copper(I) oxide quickly dissolves in chlorovinylsilanes and the
liquid becomes red-orange, but the colour vanishes when the
substrates were carefully dried in a desiccator over NaOH and
Demand for higher performance and higher functionality of
siloxane compounds is increasing year by year, and there is a
need for techniques capable of precisely controlling the main
skeleton of organosilicon products. Special attention is paid to
vinyl siloxanes that are highly versatile substrates for a wide
2 4
H SO . It may therefore be assumed that, in the presence of
[1]
traces of water, the released hydrogen chloride can dramatically
alter the pathway of siloxanes formation using the copper(I)
oxide as the source of oxygen. To assess the usefulness of the
envisioned reaction design, we started our investigation with a
series of model chlorovinylsilane substrates varying in number of
both vinyl groups and chlorine atoms: chlorodimethylvinylsilane,
S1; chloromethylphenylvinylsilane, S2; chlorotrivinylsilane, S3;
dichloromethylvinylsilane, S4; trichlorovinylsilane, S5. Table 1
gives the results, based on spectroscopic (FTIR), thermal
range
of
chemical
modification
reactions
especially
[2]
copolymerization and hydrosilylation.
Poly(vinylsiloxane)
provides a flexible middle segment in the copolymers, while vinyl
side group may be used as a cross-linker or chain extender.
That is how vinylsiloxanes became the favoured materials for
[
3]
the preparation of vinyl colloidal quantum dots,
vapour
optically
[4]
[5]
deposited silicone polymer thin films, catalysts,
[
6]
transparent hybrid network polymers,
porous cross-linked
networks, inks for copper direct-write and even of dental
(
TGA/DSC ) and structural (XRD) data of five compounds: four
[7]
[8]
[18]
[19]
new phases, P3-P6,
and one previously reported, P1. The
[9]
impression polymers.
experimental procedures and analyses results are described in
There are several methods for Si–O–Si bond formation,
but most of them either are multistage with huge amount of by-
[10]
Table 1. Formation of vinylsiloxanes stabilized by complexation with CuCl.
product or require the use of specific substrates
other than
organochlorosilanes manufactured by the Rochow process. A
conventional method for the synthesis of siloxane from
chlorosilanes is hydrolysis promoting dehydrative condensation
of the formed silanol intermediates. Since the reactivity of
organochlorosilanes is much greater than that of allylsilanes or
organohydrosilanes, that is why organochlorosilanes are used
Substrate
Solvent
Product
(L1) ], P1
5CuCl·L2, P2
Yield
94%
91%
S1
S2
S3
S4

Toluene

[Cu
4
Cl
4
2
[1]
[Cu
4
4
Cl
Cl
4
(L3)
2
(toluene)
2
], P3
60%
mainly as polysiloxane precursors or as surface modifiers. In
addition, in the case of organodichloro– and
organotrichlorosilanes, spontaneous self-condensation of
Allyl
chloride
Less than
10%
[Cu
4
(S4)
4
], P4
a
Allyl
chloride
[Cu
[Cu
4
Cl
Cl
4
(L4)
4
(L5)], P6
2
], P5, and
silanols makes it difficult to control structurally well-defined
siloxane compounds. Another disadvantage of most hydrolytic
reactions is that a hydrogen chloride scavenger is required to
shift the equilibration toward the desired siloxane, and the fact
that time-consuming, laborious step of phase separation must
P4
S5


4

No reaction was observed
[
1]
detail in the Supporting Information. We found that S1-S4 can
react with copper(I) oxide at room temperature to give the olefin
be carried out. In the last decades, non-hydrolytic processes,
including sol-gel reactions,
[11]
were found to be very efficient for
1
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European Journal of Inorganic Chemistry
10.1002/ejic.202000134
COMMUNICATION
CuCl complexes
hexavinyldisiloxane, L2, dimethyldiphenyldivinyldisiloxane, L3,
trimethyltrivinylcyclotrisiloxane, L5, and
with
tetramethyldivinyldisiloxane,
L1,
(R,R and S,S), but mainly to very stable and kinetically favoured
[22]
meso-form (R,S).
However, if the condensation reaction is
Cl cores with a twofold
accompanied by the formation of Cu
4
4
tetramethyltetravinylcyclotetrasiloxane, L6. The vinyl ligands can
therefore act as a strong trap for copper(I) chloride, thus
axis, relating two homochiral monomers, the pair of enantiomers,
R,R and S,SL2, is formed as the sole products (Figure 1).
This is the phenomenon observed in the crystal structure of P4
preventing Cu
is also confirmed by the FTIR spectra of P1-P3 products as well
as by NMR data of the mixtures of Cu O/S1 and Cu O/S3 in
2
O surface passivation. The π-complex formation
2
2
toluene. The infrared spectra of the olefin complexes really show
the low-frequency shift of the νC=C stretching mode by 86106

1
1
cm indicating a strong Cu(I)(C=C) interaction. In the H NMR
spectra, three olefinic proton signals () appear as a doublet of
doublets or as a broad doublet and are slightly upfield-shifted
(
 = 0.14 to 0.32 ppm) compared to those of free
vinyldisiloxanes, confirming a moderate Cu(C=C) interaction. In
contrast, in the case of S5, the reaction does not proceed
because copper chloride is in stoichiometric excess in relation to
Scheme 1. Formation of vinylsiloxanes: copper(I) oxide forms copper(I)
chloride which, in turn, binds vinylsiloxanes shifting the equilibrium toward the
product side.
2
vinyl groups (3:1), causing Cu O surface passivation. In an effort
to confirm preparative advantages of the described synthetic
approach, TGA/DSC analysis of P1-P3 compounds was
performed. It has been shown that the complexes undergo
controlled thermal decomposition through a one-step pathway
(

P1 at 129 C, P3 at 272 C) or a two-step process (P2 at 177
C and 197 C) and release disiloxane requiring no further
purification. An additional attraction of this synthesis approach is
that copper(I) chloride acts as a stabilizer of vinylsiloxanes, a
remarkably simple strategy well known in the chemistry of
[20]
polymers.
Considering the ability of CuCl to stabilize reactive species,
a special attention was paid to the identification and structural
characterization of key elusive reaction intermediates. As
illustrated in Scheme 1, the catalytic cycle of the metathesis
2
reaction starts with Cu O as a hydrogen chloride scavenger to
form CuCl and O. The copper(I) chloride may bind to
H
2
vinylchlorosilane substrate generating a -complex as observed
in P4. The hydrolytic condensation of the active chloro-monomer
followed by the release of siloxane CuCl -complex and HCl
completes the catalytic cycle. Although the mechanism of the
condensation in the last step of the catalytic cycle is not
particularly clear, it may be described as scarce water
Scheme 2. Formation of the cyclosiloxane complexes, P5 and P6, via the
dichlorosilane intermediate P4.
[21]
hydrolysis
and obviously provides the formation of CuCl -
complexes described below. To our satisfaction, the surmised
vinylchlorosilane -intermediate, P4 (Figure S2 in the Supporting
Information), could be isolated in the reaction of S4, as shown in
Scheme 2. Because the hydrolytic condensation of
organochlorosilanes provides uncontrolled random structures,
cyclooligosiloxanes can be produced as an isomeric mixture
consisting two and four isomeric forms for L5 and L6
respectively. However, in the crystal structure of the products,
only one diastereomer (cis, trans for P5 and cis, trans, cis for
P6) was observed (Figures S3, S4 in the Supporting
Information). Such a selective cyclization reaction can be
explained by endo,endo-arrangement of the vinyl groups with
respect to the Cu
Scheme 2).
Assuming that the condensation process involves the prior
4 4
Cl fragment running from P4 to P5 and P6
(
complexation of copper(I) chloride with active chloro-monomer,
the stereoselectivity of disiloxanes formation in P3 (Figure S1 in
the Supporting Information) could also be explained. As an
inseparable mixture of stereoisomers, R and S, the commercial
substrate S3 should lead to the pair of enantiomeric disiloxanes
Figure 1. Homochiral precursor of P3 modeled in analogy to the complex P4
(a) and homochiral disiloxane in P3 (b).
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202000134
COMMUNICATION
comprising not only the homochiral enantiomers but also the
preserved twofold symmetry of the complex. Moreover, the
tetrameric Cu Cl species were also observed in the two
4 4
remaining crystal structures, P5 and P6, where they adopt
centrosymmetric chair conformation, mimicking to the
cyclosiloxane rings.
The selection of solvent is another important factor to shift
the reaction equilibrium to the right in favour of CuCl -
complexes. Toluene, for instance, has been found to be unique
and efficient in assisting the formation of P3, moreover, it was
observed as guest solvent in the porous crystalline phase P3.
Meanwhile, allyl chloride has been proven to provide
crystallization of both precursor (P4) and products (P5, P6). It
turned out that these solutions become colourful too, like the
in siloxane synthesis. We repeated the above GC-MS
experiment for the reaction of S2-S4 in acetonitrile, but used
copper powder instead of copper(I) oxide. The results were
almost equal to those shown in Table 2, and SiOSi bridges
formation was confirmed by IR spectroscopy. This process
involves one of the important reactions in copper catalysis,
which utilizes stoichiometric metallic copper(0) under oxygen to
generate Cu(II) species:
2 2 2
2Cu + O + 4HCl ⟶ 2CuCl + 2H O
Thus, the possible total reaction equations can be presented as
follows:
2R
3
SiCl + Cu + 1/2O
2
3 3 2
⟶ R Si-O-SiR + CuCl
2
solutions of Cu O in S1 and S3. We tried to explain this
n(CH =CH)(CH )SiCl + nCu + n/2O ⟶
2
3
2
2
phenomenon for systems without chromophores, and found out
that ultrasonic vibration or centrifugal spinning lead to their sol-
gel conversion. Moreover, when the reaction with S1 was
monitored by UV−Vis spectroscopy, the absorbance at 478 nm
first rose, then decreased following the growth of P1 crystals
2 3 n
[(CH =CH)(CH )Si-O-] + nCuCl2 n = 3-6
Consequently, if the reaction will be carried out in a proper
solvent, such as toluene, but in the presence of stoichiometric
amounts of acetonitrile as a coligand in addition to vinylsilanes, it
will be possible to exploit the versatility and stereoselectivity of
this synthetic approach. In recognition of this fact, we have
recently launched a new project to apply this strategy to the
(
Figure S5 in the Supporting Information). Consequently, the
observed colour may be due to a surface plasmon resonance on
copper(I) nanoparticles, which are metastable with respect to the
crystalline products.
stereoselective synthesis
trichlorovinylsilane.
of
vinylsilsesquioxanes
from
Thus, this synthetic route has both advantages and
shortcomings related to the reactivity of vinyl ligands and
solvents. Therefore, we have slightly modified the process using
acetonitrile simultaneously as solvent and as additional CuCl
trapping agent. The experimental procedures are described in
the Supporting Information. The GC-MS study (see Table 2 and
Figures S6-S10) indicated that both disiloxanes, including
asymmetric ones, and cyclosiloxanes can be obtained in this
way:
Table 2. Synthesis of vinylsiloxanes in acetonitrile.
Substrate
Product
Retention
time, min
S2
S3
S4
Hexavinyldisiloxane
5.15
Dimethyldiphenyldivinyldisiloxane
13.09
2R
3
SiCl + Cu
2
O + 2CH
3
CN ⟶ R
3
Si-O-SiR
3
+ 2[CuCl(CH
CN ⟶
3
CN)]
Trimethyltrivinylcyclotrisiloxane,
3.67
6.61
9.42
tetramethyltetravinylcyclotetrasiloxane,
pentamethylpentavinylcyclopentasiloxane,
hexamethylhexavinylcyclohexasiloxane
n(CH
2
=CH)(CH
3
)SiCl + nCu
2
2
O + 2nCH
3
[
(CH =CH)(CH
2
3
)Si-O-] + 2n[CuCl(CH
n
3
CN)]
n = 3-6
12.05
The SiOSi bridges formation was confirmed in the infrared
S1
+
+
S2
S3
Tetramethyldivinyldisiloxane
 2.10
3.15
5.16
[
a]
1,1-dimethyl-1,3,3,3-tetravinyldisiloxane,
spectra by the presence of very strong bands in the region of
-
1
hexavinyldisiloxane
1
120-1010 cm . It is clearly important to emphasize that linear
siloxanes were not identified in the sample with S4 as the
substrate. This is another evidence of the fact that the prior
complexation of copper(I) chloride with both chloro-monomer
and siloxane has a significant influence on the formation of
cyclic oligomers. Besides that, the production of cyclosiloxanes
S2
Hexavinyldisiloxane,
1-methyl-1-phenyl-1,3,3,3 tetravinyldisiloxane,
dimethyldiphenyldivinyldisiloxane
5.16
9.33
13.09
[
a]
[
a] Major product.
[23]
is also favoured by the mechanism of scarce water hydrolysis.
In summary, the metathesis condensation reaction
In this approach, the synthesis time (1-2 h) was greatly
shortened, by twenty times, compared with the most time-saving
solventless process described above. Furthermore, although the
presented here offers a simple and convenient pathway to CuCl-
stabilized or pure vinylsiloxanes and allows for optimization of
the process resulting in isomeric or diastereomeric purity. It is
established that the use of copper(I) oxide both as hydrogen
chloride trap and as oxygen source makes it possible to run the
catalytic cycle of “scarce water” hydrolysis of chlorovinylsilanes,
forming copper(I) chloride and vinylsiloxane as the final products.
The latter generate -complex, thus shifting the chemical
equilibrium and driving the catalytic cycle. The involvement of
acetonitrile in the process has opened up the possibility of using
air as oxygen source to produce vinylsiloxanes, but there is still
insufficient evidence to show that the reactions are as
stereoselective as those without acetonitrile.
[24]
formation of acetonitrile 1:1 complex of CuCl was identified by
X-ray single crystal analysis, we noticed that oxygen from the air
was greedily taken up by the reaction system, resulting in the
oxidation of Cu(I) to Cu(II):
2 2 2
4CuCl + O + 4HCl ⟶ 4CuCl + 2H O
This reaction, well known from Wacker process, is additionally
involved in the propagation of the catalytic cycle in the presence
of HCl/H O, like the one that is shown in Scheme 1, but
2
including both the copper(I) oxide and the atmospheric oxygen
as the source of oxygen. The recognition of this fact led us to
2
explore the possibility of using O as the sole source of oxygen
3
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European Journal of Inorganic Chemistry
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COMMUNICATION
Acknowledgements
The authors gratefully acknowledge the excellent technical
assistance of Dr. Dorota Wieczorek (Faculty of Chemistry, University
of Opole, Poland) in the NMR measurements reported here. We are
also grateful to Mgr. Beata Sacher–Majewska and Mgr. Marek
Grzymek (Faculty of Chemistry, University of Opole, Poland) for
helping with the TGA-DSC and FTIR analysis respectively.
Keywords: metathesis • vinylsiloxane • chlorovinylsilane •
copper(I) oxide • copper(0)
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4
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European Journal of Inorganic Chemistry
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5
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