Acid-catalyzed condensation reaction of phenylsilanetriol: Unexpected formation of cis,trans -1,3,5-trihydroxy-1,3,5-triphenylcyclotrisiloxane as the main product and its isolation

Article abstract of DOI:10.1021/om500691c

cis,trans-1,3,5-Trihydroxy-1,3,5-triphenylcyclotrisiloxane was successfully isolated and fully characterized by spectroscopic methods and single-crystal X-ray diffraction. This compound was believed to be highly unstable and only exist as a transient species in low concentration during the condensation reaction of phenylsilanetriol or its precursors. The isolated pure cyclotrisiloxane is, however, surprisingly stable even in acidic solution and partially isomerizes to the cis,cis isomer.

Full text of DOI:10.1021/om500691c

Communication
pubs.acs.org/Organometallics
Acid-Catalyzed Condensation Reaction of Phenylsilanetriol:
Unexpected Formation of cis,trans-1,3,5-Trihydroxy-1,3,5-
triphenylcyclotrisiloxane as the Main Product and Its Isolation
Fujio Yagihashi,*,†,‡ Masayasu Igarashi,† Yumiko Nakajima,† Wataru Ando,† Kazuhiko Sato,†
Yoshiyuki Yumoto,§ Chinami Matsui,|| and Shigeru Shimada*,†
†Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba, Ibaraki 305-8565, Japan
‡Shin-Etsu Chemical Co. Ltd., 6-1, Ohtemachi 2, Chiyoda, Tokyo 100-0004, Japan
§Specialty Chemicals Research Center, Shin-Etsu Chemical Co. Ltd., Kubiki, Joetsu, Niigata 942-8601, Japan
||Gunma Plant, Shin-Etsu Chemical Co. Ltd., Isobe, Annaka, Gunma 379-0915, Japan
S
* Supporting Information
ABSTRACT: cis,trans-1,3,5-Trihydroxy-1,3,5-triphenylcyclotrisi-
loxane was successfully isolated and fully characterized by
spectroscopic methods and single-crystal X-ray diffraction. This
compound was believed to be highly unstable and only exist as a
transient species in low concentration during the condensation
reaction of phenylsilanetriol or its precursors. The isolated pure
cyclotrisiloxane is, however, surprisingly stable even in acidic
solution and partially isomerizes to the cis,cis isomer.
the cyclotrisiloxanes are unexpectedly stable and can be isolated
ilicone resins containing phenylsilsesquioxane units
S((PhSiO ) ) are widely used in industry, and their without any special stabilization agents such as cage
1.5
n
compounds. Here we wish to report the isolation, structural
characterization, and properties of cyclotrisiloxanes from
phenylsilanetriol 1.
importance is increasing in a number of applications such as
sealing materials for LED lights and organic electrolumines-
cence diodes and as an antireflective layer for extreme
lithography for semiconductor production.1 The production
of silicone resins is usually perfomed by polycondensation
reaction of alkoxysilanes or chlorosilanes under hydrolytic
conditions. For the development of silicone materials having
improved properties, an understanding of the reaction details
and control of the polysiloxane structure are indispensable. In
spite of the long history of the research on phenylsilsesquioxane
materials, the initial stages of the condensation reaction have
not been very well understood.2
It has been established that cyclotetrasiloxanes as four
isomeric forms are produced on hydrolytic condensation of
trialkoxyphenylsilanes and trichlorophenylsilane.3 On the other
hand, the corresponding cyclotrisiloxanes have been believed to
be highly unstable and could be stabilized only under special
circumstances; Fujita and co-workers succeeded in stabilizing
the cis,cis-cyclotrisiloxane by using a cage that could incorporate
the cyclotrisiloxane in it.4 Several cyclotrisiloxanetriols stabi-
lized by bulky subsituents such as (2,6-dimethylphenyl)-
(trimethylsilyl)amino,5a bis(trimethylsilyl)methyl,5b and 2,4,6-
triisopropylphenyl5c groups have been isolated. In 2004, Unno
and Matsumoto revealed that a cyclotrisiloxanetriol having
relatively small substituents, isopropyl groups, could be
isolated.6,7 During the course of our research to clarify the
condensation process of phenylsilsesquioxane, we found that
We selected phenylsilanetriol 18 as the starting material
because its condensation reaction is simpler and easier to
analyze than those of trialkoxyphenylsilanes and trichloro-
(phenyl)silane, which have usually been used for such
condensation reactions. By using 1, it is possible (1) to avoid
complexity arising from partially hydrolyzed intermediates such
as alkoxyhydroxysilanes and chlorohydroxysilanes and (2) to
control condensation reacion rates by avoiding the formation of
hydrochloric acid resulting from the hydrolysis of trichloro-
(phenyl)silane. In addition, 1 is easily available by a known
method8 and has sufficient stability. Acid-catalyzed condensa-
tion reactions of compound 1 under various conditions were
studied. Scheme 1 shows the condensation pathways of
compound 1. We found that HPLC analysis is highly useful
to follow the condensation reaction.
Chromatograms obtained from the reaction of 1 in the
presence of 1000 ppm9 of methanesulfonic acid (MSA) in
tetrahydrofuran (THF) using a UV detector at 263 nm are
shown in Figure 1. Peaks observed are labeled A−L.10 Among
these peaks, A−D and H were assigned to compound 1,
disiloxane 2, linear trisiloxane 3, and cyclotrisiloxanes cis,trans-4
Received: July 2, 2014
© XXXX American Chemical Society
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Scheme 1. Condensation Pathways of Phenylsilanetriol 1
Figure 3. LC-ESI-TOF MS chromatograms of the condensation
reaction of 1 with 1000 ppm of MSA in THF: (1) chromatogram by
UV absorption at 254 nm by PDA detector; (2) mass chromatogram
for m/z 432.
Information) clearly suggested that C is the linear trisiloxane
3 (calcd for [C18H20O7Si3·NH4]+ 450.0855; found 450.0874)
and D and H are the cyclotrisiloxanes 4 (calcd for
[C18H18O6Si3·NH4]+ 432.0749; found 432.0758 and
432.0752, respectively).
To our surprise, cyclotrisiloxane D could be isolated by silica
gel column chromatography by careful tuning of the separation
conditions.11 Addition of a small amount of CF3CO2H is highly
effective for the stabilization of the cyclotrisiloxane during the
separation. So far, the isolation of cis,trans-4 has been successful
only for a sample at an early stage of reaction (∼3 h), and the
isolated yield was 9.9%. As shown in Figure 1, cis,trans-4
became the main product after a longer reaction time (45.7%
by HPLC area ratio after 42.5 h). However, attempts to isolate
cis,trans-4 by using these samples resulted in the formation of
an unidentified complex mixture of products, which have longer
retention times by HPLC analysis. We also tried to isolate
cis,cis-4 but failed, because cis,cis-4 was formed only in a low
yield and its retention time under the separation conditions was
probably similar to those of other products, as observed in the
HPLC chromatograms in Figure 1.
Figure 1. Chromatograms of the condensation reaction of 1 (initial
concentration, 5.34 × 10−2 mol/L) with 1000 ppm of MSA in THF at
5 °C. Each chromatogram was obtained at a 30 min interval except the
last one, which was obtained by injecting the sample after 42.5 h.
We obtained single crystals of cis,trans-4 from its ether
solution and succeeded in an unambiguous determination of its
structure. The crystal contained ether molecules, which are
severely disordered. Figure 4(1) shows the molecular structure
of cis,trans-4. In the crystal, this compound forms a cylindrical
arrangement along the a axis through hydrogen bonds by the
three hydroxy groups (Figure 4(2)). A similar arrangement
through hydrogen bonds is also observed for isopropyl-
substituted cyclotrisiloxanetriol.6 One of the hydrogen atoms
of the three OH groups seems to form another hydrogen bond
between the oxygen atom of the disordered ether molecule.
The average Si−O bond length of the six-membered ring is
1.633 Å, and the average O−Si−O and Si−O−Si angles are
108.4 and 131.8°, respectively. The cyclotrisiloxane six-
membered ring has good planarity, and the highest deviation
from the least-squares plane of the six atoms is 0.132(1) Å
(O2). These bonding parameters and the planarity are similar
to those of the other cyclotrisiloxanetriols.4b,5b,c,6
With this information in hand, we further analyzed the
condensation mixture by 29Si NMR spectroscopy. With a lower
concentration of MSA, 10 ppm in THF-d8, the condensation
reaction was slow and signals of monomer 1 (−53.3 ppm),8c
disiloxane 2 (−62.3 ppm),12,13 and linear trisiloxane 3 (−61.8
and −71.4 ppm)13 were clearly observed for the sample after
17.6 h at 25 °C (Figure S3 in the Supporting Information). The
29Si NMR spectrum of the condensation solution with more
concentrated MSA (1000 ppm) after 11.5 h is shown in Figure
5. New signals corresponding to cyclotrisiloxanes 4 were
Figure 2. Time-dependent composition of 1 and condensation
products 2, 3, cis,trans-4, cis,cis-4, and cyclotetrasiloxanes 6.
and cis,cis-4, respectively. Details of their identification are
shown below. Figure 2 gives a graphical presentation of the
time-dependent composition of 1−3, cis,trans-4, cis,cis-4, and
cyclotetrasiloxanes 6 in the same reaction. Unexpectedly,
considerable amounts of cyclotrisiloxanes 4, especially the
cis,trans isomer, were formed under these reaction conditions.
Figure 3 shows the chromatograms obtained by LC-ESI-
TOF MS analysis of the condensation reaction of 1 with 1000
ppm of MSA in THF at 20 °C. The mass chromatogram for m/
z 432, which corresponds to the ammonium cation adduct of
dehydrated trisiloxane ([C18H18O6Si3·NH4]+), showed three
peaks (C, D, and H). High-resolution mass analysis of these
peaks by ESI techniques (Figure S2 in the Supporting
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Figure 6. Chromatograms for 1.5 mg of cis,trans-4 (1) before the
reaction and (2) after the reaction with 1000 ppm of MSA in THF at 5
°C for 21.7 h.
Figure 4. X-ray crystallographic analysis of cis,trans-4: (1) thermal
ellipsoid plot (50% probability level) of cis,trans-4; (2) cylindrical
arrangement of cyclotrisiloxane rings through hydrogen bonds.
Selected bond lengths (Å) and angles (deg): Si1−O2 1.628(2),
Si1−O3 1.633(2), Si1−O4 1.630(3), Si2−O1 1.631(2), Si2−O3
1.633(2), Si2−O5 1.625(3), Si3−O1 1.636(2), Si3−O2 1.639(2),
Si3−O6 1.618(3); O2−Si1−O3 107.40(12), O2−Si1−O4 105.28(14),
O3−Si1−O4 110.35(14), O1−Si2−O3 107.27(12), O1−Si2−O5
108.31(13), O3−Si2−O5 109.64(13), O1−Si3−O2 106.26(12),
O1−Si3−O6 111.71(13), O2−Si3−O6 109.14(14), Si1−O3−Si2
131.37(14), Si1−O2−Si3 131.06(14), Si2−O1−Si3 133.07(15).
It became clear that, once cis,trans-4 was isolated in a pure
crystalline form, it is unexpectedly stable and does not change
for months. Furthermore, it is also stable in THF or CH3CN
solution for more than 1 week. Even with 1000 ppm of MSA in
THF solution, only a small amount of cis,trans-4 was consumed
by a ring-opening reaction to form the linear trisiloxane 3 and
an isomerization reaction to form cis,cis-4 without any other
degradations or polymerizations. Under these conditions, an
equilibration mixture was obtained at 5 °C after 21.7 h. Figure 6
shows the HPLC chromatograms before and after the
equilibration of cis,trans-4. From the relative ratio of cis,trans-
4 and cis,cis-4 (86.0/14.0),16 the free energy difference between
the two isomers was calculated to be 4.2 kJ/mol.
Cyclotrisiloxanes from phenylsilanetriol have long been
considered as short-lived kinetic products and to be rapidly
converted to thermodynamically favored tetrasiloxanes and
further condensed products. However, this investigation clearly
shows that the cyclotrisiloxanes 4 are unexpectedly stable not
only in the solid state but also even in acidic solution.
Therefore, the cyclotrisiloxanes would play an important role in
the condensation process to form polyphenylsilsesquioxanes.
We expect these findings will help to control silicone resin
structures and will lead to the improvement of silicone
materials properties such as shelf life and brittleness.
Figure 5. 29Si inverse gated 1H-decoupled NMR spectrum of the
condensation reaction mixture of 1 (42.4 mg) with 1000 ppm of MSA
in THF-d8 (0.7 mL) after 11.5 h (measurement duration 37 min) at 25
°C (−61.5 to −63.0 ppm and −69.75 to −71.25 ppm regions).
ASSOCIATED CONTENT
* Supporting Information
Text, figures, and a CIF file giving experimental details and
characterization and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
observed at −62.16, −62.31, and −62.59 ppm in addition to six
signals observed at −70 to −72 ppm assignable to six different
silicon atoms belonging to four stereoisomers of cyclo-
tetrasiloxanes 6.14
AUTHOR INFORMATION
Corresponding Authors
*E-mail for F.Y.: f-yagihashi@aist.go.jp.
*E-mail for S.S.: s-shimada@aist.go.jp.
■
It was generally observed that 29Si NMR signals of the silicon
atoms consisting of cyclotrisiloxane six-membered rings appear
at about 10 ppm lower magnetic field positions in comparison
to those of the similarly substituted silicon atoms of linear and
cyclic tetrasiloxane or larger ring siloxanes.15 In consideration of
the 29Si NMR chemical shifts of 2 and 3, we now can confirm
this general observation and unambiguously assign the larger
signal at −62.59 ppm to two identical silicon atoms of cis,trans-
4 and the smaller signal at −62.31 ppm to the remaining silicon
atom. In addition, we can conclude that the signal at −62.16
ppm belongs to the silicon atoms of cis,cis-4.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Future Pioneering Projects
“Development of Innovative Catalytic Processes for Organo-
silicon Functional Materials” (PL: K.S.) from the Minister of
Economy, Trade and Industry of Japan (METI) and the New
Energy and Industrial Technology Development Organization
(NEDO).
C
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(14) Makarova, N. N.; Petrova, I. M.; Petrovskii, P. V.; Kaznacheev,
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300.
(16) The relative ratio of isomers was obtained from the peak
intensities of the HPLC analysis. See the Supporting Information on
the reliability of these values.
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(10) Other peaks were assigned as follows: E, 6; F, unidentified; G, 6;
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(11) Synthesis and isolation of cis,trans-4: to a stirred mixture of 1
(2.0 g, 12.8 mmol) and anhydrous MgSO4 (20 g) in 40 mL of Et2O
was added 40 μL of MSA. The mixture was stirred at room
temperature for 3 h. After the mixture was filtered through Celite, a
small amount of CF3CO2H was immediately added to the filtrate to
make about a 100 ppm solution. After evaporation of the solvent to 5
mL, the mixture was subjected to chromatography over silica gel.
Elution was done using solvents (hexane/Et2O) containing 100 ppm
of CF3CO2H for the stabilization of products with gradient conditions
10% Et2O for 2 min, then up to 100% Et2O for 18 min, and further for
20 min with 100% Et2O at a rate of 20 mL/min. cis,trans-4 eluted as
the first fraction, which was evaporated to just before dryness and kept
at 5 °C to give 175 mg (1.27 mmol, 9.9% yield) of crystalline cis,trans-
4 with a purity of 98.6% as judged by HPLC. Anal. Calcd for
1
C18H18O6Si3: C, 52.15; H, 4.38. Found: C, 52.14, H, 4.41. H NMR
(600 MHz, THF-d8): δ 6.57 (1H, br s, OH), 6.62 (2H, br s, OH), 7.26
(4H, m, m-Ph), 7.33 (2H, m, p-Ph), 7.34 (2H, m, m-Ph), 7.39, 1H, m,
p-Ph), 7.68 (4H, m, o-Ph), 7.83 (2H, m, o-Ph), 13C NMR (151 MHz,
THF-d8): 128.17 (m-Ph), 128.20 (m-Ph), 130.63 (p-Ph), 130.66 (p-
Ph), 134.58 (ipso-Ph), 134.80 (ipso-Ph), 135.13 (o-Ph), 135.24 (o-Ph).
29Si NMR (119 MHz, THF-d8): −62.59, −62.31.
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D
dx.doi.org/10.1021/om500691c | Organometallics XXXX, XXX, XXX−XXX