Building blocks for oligomeric siloxanes –selective chlorination of hydrido-siloxanes

Article abstract of DOI:10.1016/j.jorganchem.2018.08.022

A new method was developed to achieve highly selective monochlorination of α,ω-dihydridosiloxanes, ((H)Me₂SiO-(SiMe₂O)_n-SiMe₂(H), n = 0–2; (H)R₂Si–O-SiR₂(H); R = Me, ⁱPr, Ph; 3,5-dihydridooctamethyltetrasiloxane, Me₃SiO-(Si(H)MeO)₂–SiMe₃) with trichloroisocyanuric acid (TCCA). The dependence of the selectivity of the monochlorination on the siloxane chain length is also discussed.

Full text of DOI:10.1016/j.jorganchem.2018.08.022

Accepted Manuscript
Building blocks for oligomeric siloxanes –selective chlorination of hydrido-siloxanes
Thomas Hafner, Ana Torvisco, Frank Uhlig
PII:
S0022-328X(18)30465-0
10.1016/j.jorganchem.2018.08.022
JOM 20549
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 8 June 2018
Revised Date: 6 August 2018
Accepted Date: 24 August 2018
Please cite this article as: T. Hafner, A. Torvisco, F. Uhlig, Building blocks for oligomeric siloxanes –
selective chlorination of hydrido-siloxanes, Journal of Organometallic Chemistry (2018), doi: 10.1016/
j.jorganchem.2018.08.022.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Building Blocks for Oligomeric Siloxanes –
Selective Chlorination of Hydrido-Siloxanes
Thomas Hafner, Ana Torviscoa, Frank Uhlig
Graz, Austria
aCorresponding author
Institute for Inorganic Chemistry
Stremayrgasse 9/IV
8010 Graz, Austria
Telephone: +43-316-873 32122; Fax: +43-316-873 32102
Email: ana.torviscogomez@tugraz.at
Keywords
hydridosiloxanes, trichloroisocyanuric acid, TCCA, selective monochlorination
1. Abstract
A new method was developed to achieve highly selective monochlorination of
α,ω-dihydridosiloxanes, ((H)Me2SiO-(SiMe2O)n-SiMe2(H), n = 0 - 2; (H)R2Si-O-
SiR2(H); R = Me, iPr, Ph; 3,5-dihydridooctamethyltetrasiloxane, Me3SiO-(Si(H)MeO)2-
SiMe3) with trichloroisocyanuric acid (TCCA). The dependence of the selectivity of
the monochlorination on the siloxane chain length is also discussed.
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2. Introduction
The conversion of the Si-H into Si-Cl bonds can be achieved by various
methods and has been well established over the years using hydrochloric acid,
chlorine gas, acetyl chloride, HCl in combination with PdCl2 as catalyst [1], CuCl2 [2],
CCl4 [3], PCl3 [4] or PCl5 [5]. Further advances deal with the dehalogenation of
hexachlorocyclohexane catalyzed by rhodium complexes [6] or iron catalyzed (FeCl3)
based chlorination with acetyl chloride of Si-H bonds [7]. An overview of Si-H
chlorination is given by Kunai and Oshita in 2003 [2].
Only a few examples of partial chlorination of compounds containing more
than one Si-H bond are reported mainly due to the limited selectivity of the known
methods. For example, tin tetrachloride has been used for chlorination in Bu2O where
>SiH2 can be converted into >Si(H)Cl in approx. 60% yield [8]. Also, CuCl2 was used
for selective chlorination of R2SiH2 with Et2O or THF as solvents, with optional
addition of CuI as catalyst [9]. Disadvantages of such methods are often low yields,
due to difficult workup, and therefore product isolation.
Trichloroisocyanuric acid (TCCA) [10] was reported for a full chlorination of the
Si-H functions in siloxane hydrides [11]. First usage of TCCA for a selective
chlorination of siloxanes containing more than one Si-H bond were performed by our
group [12] and afterwards applied also for mono- [13] and disilanes [14]. Recently, a
report using TCCA on the selective chlorination of Naph2SiH2 towards Naph2Si(H)Cl
was published by Diemoz, Wilson and Franz [15]. The objective of this work was to
develop and investigate this new chlorination method and provide a general synthetic
method of using TCCA as a reagent to achieve a highly selective monochlorination in
oligomeric siloxanes containing more than one Si-H unit.
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3. Results
In a first step, the possibility of a selective monochlorination by TCCA on α,ω-
dihydridosiloxanes with varying chain lengths ((H)Me2SiO-(SiMe2O)n-SiMe2(H) (n = 0
(1a), n = 1 (2a), n = 2 (3a)) was investigated (Scheme 1).
Scheme 1: Selective monochlorination of the linear α-ω dihydridosiloxanes 1-3a
In order to achieve selective monochlorination, a ratio of 1 mol siloxanes to 1/3
mol TCCA was used. It could be clearly shown that the selectivity of the reaction
depends strongly on the reaction conditions used. During chlorination, TCCA
exchanges its chlorine atoms with the hydrogens from the educt, converting TCCA to
TCCH and yields up to 85% of desired monochlorinated products (1-3b). In some
cases, the desired product is accompanied by amounts of remaining starting
materials (1-3a) and the dichloro-substituted byproducts (1-3c). Reactions were
carried out in THF well below -20 °C. Otherwise, th e well-known chlorination of THF
[11] takes place.
Early attempts for selective monochlorination of dihydridotetramethyldisiloxane
((H)Me2Si-O-SiMe2(H), (1a)) only yielded to a conversion of about 65-70% towards
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the monochlorinated product (1b) [12]. This method involved the slow addition of
TCCA to a cooled mixture (below -20 °C) of THF and 1a in a concentration of about
20 ml THF for 5.6 mmol siloxane under intense stirring and inert conditions. TCCA
was added in small amounts. Additional TCCA quantities were added only, when the
previous ones were already in solution (Chlorination Method 1, see Experimental
Section). However, this method still allowed for a local stoichiometric excess of TCCA
promoting the formation larger amounts of the dichlorotetramethyldisiloxane (1c).
In order to circumvent the formation of the dichloro-substituted species 1c,
TCCA was first dissolved in THF at -80 °C and the s olution added dropwise to a
mixture of THF and 1a also at -80 °C under intense stirring (Chlorinatio n Method 2,
see Experimental Section). In this respect a higher conversion rate of 1a up to ~85%
into the monochlorinated species (1b) can be observed. This procedure was applied
also towards all siloxanes reported in this work.
The conversion of the α,ω-dihydridosiloxanes ((H)Me2SiO-(SiMe2O)n-
SiMe2(H); n = 0 (1a), n = 1 (2a), n = 2 (3a)) into the monochloro-substituted species
are dependent on the chain length between the Si-H units (Figure 1). As mentioned
above for dihydridotetramethyldisiloxane (1a), a conversion ratio to the desired
monochlorinated species (1b) of 85% was observed. For the dihydrido-
hexamethyltrisiloxane (2a), a decreased selectivity towards the desired mono-
chlorinated species (2b) of 65% was observed in conjunction with increased 13% of
2a and 22% of the dichloro-substituted species 2c. In dihydridooctamethyl-
tetrasiloxane (3a, ((H)Me2SiO-(SiMe2O)2-SiMe2(H)) which displays the largest
distance between the Si-H positions, the reduced selectivity is much more
pronounced. The conversion ratio to the monochlorinated species (3b) is even further
reduced to only 52%, with 35% remaining educt 3a and 13% dichlorinated species
3c.
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Figure 1: Conversion ratios of the methyl substituted α,ω-dihydridosiloxanes 1-4a
The influence of the chain length between the Si-H units and the selectivity
can be emphasized by the monochlorination of 3,5-dihydridooctamethyltetrasiloxane
(Me3SiO-(Si(H)MeO)2-SiMe3, 4a) which has the same chain length as 3a but
corresponding Si-H positions as compound 1a, (Scheme 3). The conversion to the
monochloro-substituted species (4b) is with a conversion ratio of 82% in a similar
range like for 1b (85%). Therefore, it can be stated that conversion ratios are not
dependent on siloxane chain length, rather on the distances between the Si-H
positions in the siloxane chain.
In a second step, the influence of the substituents attached to a series of
i
dihydridosiloxanes ((H)R2Si-O-SiR2(H); R = Me (1a), R = Pr (5a), R = Ph (6a)) was
i
also investigated (Figure 2). While monochlorination conversion ratios of the Pr
substituted dihydridosiloxane (5a) (86%) compares well to compounds 1a and 4a, the
phenyl substituted dihydridosiloxane (6a) shows a decreased monochlorination
conversion ratio of 65% probably due to the more electron withdrawing effect of the
phenyl groups.
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Si
Si
X
O
Y
Figure 2: Conversion ratios in the 1,2-dihydridotetraorganodisiloxanes 1a, 5a and 6a
Hydrido- as well as chloro-substituted siloxanes such as compounds 1-6 are
often causing difficulties in GC measurements due to their sensitivity towards column
1
13
materials. Therefore, H-NMR, C-NMR, and 29Si-NMR measurements were also
used for determining the yields of the reported reactions. Detailed description on
product separation and characterization are described in the Supplementary Material.
The results were not only compared to the literature known chemical shifts [16] [17]
[18] but also a complete chlorination of all Si-H bonds via TCCA was performed to
verify the corresponding shifts of the dichloro-substituted species (Supplementary
Information). In any cases combinations of spectroscopical methods were used to
distinguish between the different products.
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4. Conclusion
It was shown that trichloroisocyanuric acid (TCCA) is a highly useful reagent in
performing selective chlorinations of Si-H bonds in dihydridosiloxanes. While the
monoselective chlorination of the α,ω-dihydridosiloxanes ((H)Me2SiO-(SiMe2O)n-
SiMe2(H)) is selective for n = 0 and 1, for n = 2, a substantial loss in selectivity is
observed leading to a statistically expectable product mixture only. In addition, the
i
influence of different substituents at the silicon atoms (Me, Pr, Ph) on selective
monochlorination of 1,3-dihydridodisiloxanes was examined. Through the use of this
novel selective chlorinating method, novel mono-functionalized building blocks for the
synthesis of short chain siloxanes could be synthesized. The use of those building
blocks will be discussed in an upcoming publication.
5. Experimental – General Remarks
All reactions were moisture and air sensitive and were carried out under inert-
atmosphere using Schlenk-techniques unless otherwise stated. Different procedures
where applied, depending if the target compound has to be selectively or fully
chlorinated. Also, the workup is dependent on the volatility of the target compound.
All products are colorless oils with different viscosity.
13
29
1H (300.22 MHz), C {1H} (75.50 MHz) and Si {1H} (59.6 MHz) NMR spectra were
recorded on a Mercury 300 MHz spectrometer from Varian at 25 °C. Chemical shifts
are given in parts per million (ppm) relative to TMS (δ= 0 ppm) regarding 13C and 1H
and relative to SiMe4 in the case of 29Si. Coupling constants (J) are reported in Hertz
(Hz). For more details see supplementary material.
Two different selective chlorination methods were applied:
Chlorination method 1 for selective monochlorination
Chlorination where Si-H bonds had to be chlorinated selectively were performed
analog to Chlorination Method 1, but TCCA was used with (1/3 eq. TCCA / 1 eq.
siloxane). A flask was filled with educt and THF and cooled down to -20°C. (20 ml
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THF/g educt). TCCA was added in small portions, under intense stirring. New TCCA
portions were added only, if the last portions already went into solution. After TCCA
addition was completed, stirring was continued for 15 minutes, afterwards, the
cooling bath was removed and the solution was allowed to come room temperature.
Chlorination method 2 for selective chlorination
1
TCCA was used in the following equivalents: /3 eq. TCCA / 1 eq. siloxane. A three-
neck flask was filled with THF (100 ml THF/g educt) and cooled down to -80 °C under
intense stirring. TCCA was added in small portions until everything was dissolved. A
second flask was filled with the educt/THF mixture (50 ml/g educt) and also cooled
down to -80 °C. The receiving flask was kept under constant stirring. At this point, the
THF/TCCA solution was transferred slowly (drop wise) into the educt/THF flask with a
syringe while maintaining the temperature at -80 °C . The reaction vessel was kept
under stirring until room temperature was reached.
Example for selective chlorination – chlorotetramethyldisiloxane (1b)
Starting materials: 46.1 g 1a (0.34 mol); 26.6 g (0.11 mol) TCCA. Chlorination
procedure 2 was used. The conversion rate determination was done by 29Si-NMR
before distillation. Removal of unreacted TCCA and TCCH was achieved by use of a
condensation bridge to transfer the THF and product mixtures into a liquid N2 cooled
flask under moderate heating (water bath with 60 °C ) to avoid decomposition
processes. THF removal was achieved by standard distillation and the remaining
product mixtures were separated by fractional distillation. A filling material column
was used for distillation. After distillation, 41 g (0.24 mol, 71%) of
chlorotetramethyldisiloxane 1b as colorless liquid was isolated. Boiling Point: 103-
105 °C (see also literature [17])
3
1H-NMR (C6D6) δ: 0.145 (d, 6 H, -Si(CH3)2H), JH-H = 2.72 Hz; 0.27 (s, 6H, -
Si(CH3)2Cl); 4.92 (h, 1H, SiH) 3JH-H = 2.72 Hz, 1JSi-H = 208 Hz; 13C-NMR (C6D6) δ: 0.5
1
1
(s, 2C, -Si(CH3)2H), JSi-C = 59.65 Hz; 3.8 (s, 2C, -Si(CH3)2Cl), JSi-C = 71.58 Hz;
29SiDEPT-NMR (C6D6) δ -2.9 (s, SiH); 6.0 (s, -SiCl).
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6. Acknowledgements
The authors are grateful to the Graz University of Technology and the NAWI Graz
project for support.
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Highlights
1. High yield selective chlorination of oligomeric siloxanes containing more than one Si-H unit.
2. Selectivity is depending on the chain length between the Si-H units.
3. Selectivity is depending on the organic substituents at the silicon atoms.
4. Easy to use, upscaling possible, TCCA is inexpensive and non-toxic.
5. Workup and chlorination procedures are depending on the desired target compounds.