Unexpected Selectivity in Cyclotetrasiloxane Formation by the Hydrolytic Condensation Reaction of Trichloro(phenyl)silane

Article abstract of DOI:10.1002/ejic.201600454

The hydrolytic condensation reaction of PhSiCl₃in aqueous solution forming cyclotetrasiloxane [C₆H₅SiO(OH)]₄was studied in detail. The reaction, originally reported by J. F. Brown, was believed to provide only one cyclotetrasiloxane diastereomer, cis,cis,cis-isomer 1. However, our detailed study clearly showed not only the formation of 1 (55.5 % yield) but also the formation of two other diastereomers, the cis,cis,trans isomer (11.9 % yield) and the cis,trans,cis isomer (4.3 % yield), without the formation of the last conceivable trans,trans,trans isomer (<0.1 % yield). A possible mechanism for the selective formation of the three diastereomers was proposed, in which it was assumed that the cyclotetrasiloxanes were formed by cyclization exclusively through the meso diastereomer of the linear tetrasiloxanes but not through the dl diastereomer. This selective cyclization was reasonably explained by considering the stable conformations of the linear tetrasiloxanes in a highly polar aqueous medium.

Full text of DOI:10.1002/ejic.201600454

DOI: 10.1002/ejic.201600454
Communication
Siloxanes
Unexpected Selectivity in Cyclotetrasiloxane Formation by the
Hydrolytic Condensation Reaction of Trichloro(phenyl)silane
Fujio Yagihashi,*[a,b] Masayasu Igarashi,[a] Yumiko Nakajima,[a] Kazuhiko Sato,[a]
Yoshiyuki Yumoto,[b] Chinami Matsui,[b] and Shigeru Shimada*[a]
Abstract: The hydrolytic condensation reaction of PhSiCl3 in isomer (<0.1 % yield). A possible mechanism for the selective
aqueous solution forming cyclotetrasiloxane [C6H5SiO(OH)]4
was studied in detail. The reaction, originally reported by J. F.
Brown, was believed to provide only one cyclotetrasiloxane dia-
stereomer, cis,cis,cis-isomer 1. However, our detailed study
clearly showed not only the formation of 1 (55.5 % yield) but
also the formation of two other diastereomers, the cis,cis,trans
isomer (11.9 % yield) and the cis,trans,cis isomer (4.3 % yield),
without the formation of the last conceivable trans,trans,trans
formation of the three diastereomers was proposed, in which
it was assumed that the cyclotetrasiloxanes were formed by
cyclization exclusively through the meso diastereomer of the
linear tetrasiloxanes but not through the DL diastereomer. This
selective cyclization was reasonably explained by considering
the stable conformations of the linear tetrasiloxanes in a highly
polar aqueous medium.
Introduction
Tetrahydroxy-1,3,5,7-tetraphenylcyclotetrasiloxanes exist as four
stereoisomers, that is, compounds 1–4, as shown in Figure 1.
Brown assigned the structure of the cyclotetrasiloxane he ob-
tained as cis,cis,cis diastereomer 1 on the basis of derivatization
experiments by reaction with Me2SiCl2 and (ClMe2Si)2O. Al-
though the structural assignment of one of the derivatized
products was wrong,[4] the stereochemistry of the cyclotetrasi-
loxane product was later proved to be correct.[5] There is no
description on the possible formation of cyclotetrasiloxane dia-
stereomers 2–4 in this original report.
Phenyl-substituted silsesquioxane materials have been explored
since the pioneering work of Brown and co-workers[1] because
of their structural diversity[2] and interest in clarifying their
structures and the mechanisms by which they are formed. Such
materials are widely used in industry, and their importance is
increasing owing to their usefulness as incorporated ingredients
in organosiloxane polymers for a wide range of applications
such as sealing materials for light-emitting diodes and organic
electroluminescence diodes and as antireflective layers for ex-
treme lithography in forefront semiconductor production.[3] Or-
ganosiloxane polymers are generally produced by hydrolytic
condensation of organochlorosilanes or organoalkoxysilanes,
which usually provides uncontrolled random structures. To im-
prove their properties, structural control of such polymers is
inevitable, and for that purpose, understanding the details of
the condensation reaction including precise structural under-
standing of the intermediate oligomer species is important. De-
spite the long history of research on organosiloxane poly-
mers,[2] even the initial stage of the condensation reaction of
silsesquioxane materials is not well understood.
Brown reported the formation of cyclotetrasiloxane 1 by hy-
drolytic condensation of PhSiCl3 in aqueous medium.[1d] 1,3,5,7-
Figure 1. Structures of cyclotetrasiloxane diastereomers 1–4.
Although some reports suggested the formation of other cy-
clotetrasiloxane diastereomers from PhSiCl3 (5),[6] the reaction
conditions and product distribution were not specified, and
therefore, there are no reports that clearly describe the forma-
tion of cyclotetrasiloxane diastereomers other than 1 under
Brown's conditions. Considering the highly attractive structural
features and widespread applicability of these cyclotetrasilox-
anes, it is very regrettable that the details of the hydrolytic con-
densation reaction have not yet been clarified and that produc-
[a] Interdisciplinary Research Center for Catalytic Chemistry, National Institute
of Advanced Industrial Science and Technology (AIST),
Tsukuba, Ibaraki 305-8565 Japan
E-mail: f-yagihashi@aist.go.jp
s-shimada@aist.go.jp
http://irc3.aist.go.jp/english/team/organosilicon-chemistry/
[b] Shin-Etsu Chemical Co. Ltd.,
6-1, Ohtemachi 2, Chiyoda, Tokyo 100-0004 Japan
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600454.
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Communication
tion of 1 as the sole product of the reaction still seems to be
widely accepted half a century after Brown's first report.[5,7]
As a part of our research on organosiloxane polymers, we
recently studied the acid-catalyzed condensation reaction of
phenylsilanetriol[8] (6) in organic solvents, in which we unex-
pectedly found the formation of cyclotrisiloxane 11 as a major
product and succeeded in its isolation and structural characteri-
zation.[9] On the other hand, as mentioned above, cyclotetra-
siloxane 1 was obtained by hydrolytic condensation of 5 in
aqueous medium. In the latter reaction, 6, the same compound
as that in the abovementioned study, was expected as an inter-
mediate of the condensation reaction. On this basis, we care-
fully reinvestigated the hydrolytic condensation of 5 under
Brown's conditions to determine unambiguously the structures
of the products and to clarify the factors that determine the
reaction products. In this report, we describe the unexpected
formation of three diastereomers, 1, 2 and 3, out of four possi-
ble cyclotetrasiloxanes and provide a possible explanation for
the selective formation of these three diastereomers.
Figure 2. HPLC charts of (a) a mixture of four diastereomers obtained by the
acid-catalyzed isomerization reaction, (b) the aqueous phase, (c) after CS2
treatment, (d) the CS2 solution part, (e) the crystalline product obtained after
recrystallization from diethyl ether, and (f) the diethyl ether solution part
after recrystallization.
Results and Discussion
Silanol-containing compounds are generally considered to be
unstable because of their tendency to undergo self-condensa-
tion, and there are not many examples of their chromato-
graphic analysis.[6a,9,10] However, as described by Klement'ev[10]
and in our previous report,[9] reverse-phase HPLC is a very con-
venient and reliable method that can be used to analyze silses-
quioxane-forming condensation reactions, especially for mix-
tures of oligomers bearing silanol groups.
The condensation reaction of 5 in aqueous acetone (Brown's
conditions) was carefully analyzed by HPLC. A solid suspension
started to form after about 1 h, and the resinous suspension
observed after a sufficient reaction period (one to several days)
was collected. Successive treatment with CS2, drying under vac-
uum, and recrystallization from acetone afforded almost-pure
1. Figure 2 shows chromatograms for the solid and solution
parts of each step, and a possible reaction pathway for the
hydrolytic condensation of 5 is shown in Scheme 1.
It is clearly shown that a crystalline product separated from
the original reaction mixture (Figure 2, a) and that obtained
after CS2 treatment (Figure 2, c) consisted not only of cis,cis,cis
Scheme 1. A possible reaction pathway for the formation of silsesquioxane
oligomers.
isomer 1 (tR = 5.23 min) but also of diastereomers 2 (tR
=
4.57 min) and 3 (tR = 4.27 min). The structural assignment of
the observed peaks will be discussed later, but it is unquestion- trisiloxane 8 (tR = 2.87 min) by comparison with the chromato-
able that no significant amount of fourth diastereomer 4 was grams for the mixture of the acid-catalyzed condensation reac-
included (less than 1:100 relative to 3).[11] This observation tion of 6 in organic solvents described in our previous report.[9]
clearly suggested that the hydrolytic condensation reaction of Very small peaks corresponding to 2 at tR = 4.59 min and 1 at
5 afforded not a single diastereomer but a mixture of cyclotet- tR = 5.27 min were also observed, which showed that the solu-
rasiloxane diastereomers. Treatment with CS2 increased the rel- bility of the cyclotetrasiloxanes in aqueous solution was very
ative concentration of 1 in the solid mixture, and as a result, low.
further purification by recrystallization became successful and
afforded almost-pure 1 (96.5 % purity, Figure 2, e) in 15.6 %
yield based on 5.
Although there are reports on the structural assignment of
diastereomers 1–4,[6,10] isolation and structural assignment of
1–4 were done by us to exclude ambiguity. A solution consist-
The four peaks observed in the aqueous solution (Figure 2, ing of all four diastereomers was obtained by the acid-catalyzed
b) were easily assigned to acetone (tR = 0.32 min), phenylsil- isomerization [methanesulfonic acid (MSA) in THF] of the crys-
anetriol (6, tR = 0.34 min), disiloxane 7 (tR = 1.37 min), and talline product after CS2 treatment (Figure 2, c). Diastereomers
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1–4 were all isolated as pure crystals by silica-gel column chro-
matography with hexane/ethyl acetate mixture as the eluting
solvents (eluted in the order 4, 3, 2, and 1) and subsequent
crystallization. Figure S1 (Supporting Information) shows HPLC
chromatograms of the mixture after isomerization and each iso-
lated pure isomer. Among the four diastereomers, the structures
of 3 and 4 were unambiguously determined by single-crystal
X-ray analysis. Figures S2 and S3 show the molecular structure
of 3 and its arrangement in the crystal through hydrogen
bonds, respectively.[12] X-ray structure analysis of 4 was re-
ported by Kawakami,[6a] and our results were almost identical.
Further structural assignment of 1–4 was based on LC–MS (ESI-
1
TOF) (Figures S5 and S6) and H NMR, 13C NMR, and 29Si NMR
spectroscopy (Figures S23–S37). Each isomer was further char-
acterized by thermogravimetric analysis differential thermal
analysis (TGA-DTA), differential scanning calorimetry (DSC), IR
spectroscopy, and Raman spectroscopy (Figures S7–S22). Our
assignments are consistent with those in previous reports.[6,10]
Scheme 2. Cyclization of linear tetrasiloxanes, meso-isomer 9 and DL-isomer
10, to form cyclotetrasiloxane diastereomers.
On the basis of the above observations, the presence of only
a negligible amount of isomer 4 (less than 0.1 % yield) in the
condensation reaction products became clear, and it was con-
firmed by repeated experiments. As a result, it was concluded
that the solid products consisted only of diastereomers 1, 2,
and 3, and they were considered as the kinetic products of the
original condensation reaction but not the equilibrium prod-
ucts of the acid-catalyzed isomerization reaction, because the
latter results in the formation of all four diastereomers (see be-
low). A possible explanation for the selective formation is de-
scribed below.
cult to cyclize. We suppose that 10 is converted into 9 by inver-
sion of configuration of one of the central two silicon atoms by
acid catalysis and then the latter further cyclizes to the cyclotet-
rasiloxanes. The high yields of formation of cyclotetrasiloxane
diastereomers 1–3 (72 %) through only diastereomer 9 can rea-
sonably be interpreted by considering such isomerization. A
long reaction period for the completion of the condensation
(more than 20 h) seems likely to suggest that the formation of
linear tetrasiloxanes 9 and 10 is slow relative to the rate of
cyclization of 9, because no detectable amounts of 9 and/or 10
were observed by HPLC analysis of the reaction mixture.
It is quite reasonable to consider that the cyclotetrasiloxane
framework is produced through the intramolecular condensa-
tion reaction of two terminal silanol groups of the linear tetrasi-
loxanes, meso-isomer 9 and/or DL-isomer 10, as shown in
Scheme 2. If it is assumed that no configurational inversion
occurs at the central two silicon atoms during the cyclization
process, meso-isomer 9 can produce only three diastereomers
1, 2, and 3 but not 4, because the products must contain at
least one cis-1,3-dihydroxy-1,3-diphenyldisiloxane unit in the
molecule. On the other hand, DL-diastereomer 10 can produce
only 2, 3, and 4 but not 1, because the products must contain
at least one trans-1,3-dihydroxy-1,3-diphenydisiloxane unit in
the molecule. Consequently, the selective formation of 1, 2, and
3 without 4 can be explained by assuming that the cyclotetra-
siloxanes are produced through the exclusive cyclization of
meso-isomer 9.
For further confirmation of our hypothesis, additional experi-
ments were performed to exclude other possibilities. One of
the other possible pathways for the selective formation of dia-
stereomers 1–3 without 4 is selective crystallization of the three
diastereomers from the reaction mixture containing all four dia-
stereomers and rapid equilibration of remaining 4 in the solu-
tion under the reaction conditions. To examine this possibility,
crystallization of an equilibrated mixture consisting of all four
diastereomers (1/2/3/4 = 10.8:48.4:22.3:16.7 %) was tested by
pouring an acetone solution of the mixture into an aqueous
solution containing the same concentration of hydrochloric acid
as the condensation reaction mixture. HPLC analysis of the pre-
cipitated solid (64 % recovery) clearly showed the presence of
all diastereomers 1–4 (1/2/3/4 = 21.6:49.6:17.2:9.4 %) and not
only three. Therefore, the possibility of selective crystallization
of 1–3 and rapid equilibration of 4 was excluded.
Such a selective cyclization reaction producing 1–3 can be
explained by considering the differences in the stable confor-
mations of linear tetrasiloxanes 9 and 10 in a strongly hydro-
philic aqueous medium [acidic water/acetone (>20:1) mixture].
Under such conditions, the linear tetrasiloxanes are expected to
prefer to adopt conformations having hydrophilic hydroxy
groups and hydrophobic phenyl groups located on opposite
sides of the molecular plane. As shown in Scheme 2, meso-
diastereomer 9 would prefer “U-shape” conformation 9-U,
The product distribution of diastereomers 1, 2, and 3 in the
original solid mixture (65.5, 14.1, and 5.1 %, respectively) can
be explained qualitatively by considering the stabilities of con-
formers 9a, 9b, and 9c (Scheme 3). As discussed above, con-
formers having a greater number of hydroxy (or equally phenyl)
groups on one side of the molecular plane are considered to
be more stable in hydrophilic aqueous media. Diastereomer 1
will be formed through “U-shaped” conformer 9a, which is con-
sidered to be the most stable one among the three conformers
which is easy to cyclize by condensation between the terminal by the arrangement of four hydrophilic hydroxy groups on one
PhSi(OH)2 moieties. On the other hand, DL-diastereomer 10 side and all four hydrophobic phenyl groups on the opposite
would prefer “zigzag-shape” conformation 10-Z, which is diffi- side of the molecular plane. Diastereomer 2 will be formed
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through moderately stable conformer 9b having three hydroxy
groups on one side of the molecular plane, and diastereomer
3 will be formed through least-stable conformer 9c having two
hydroxy groups on one side of the molecular plane.
Scheme 4. A proposed mechanism for the stereochemical isomerization reac-
tion of the cyclotetrasiloxane diastereomers.
propose a SN2-Si[15] mechanism for this interconversion, which
includes protonation of a hydroxy group and backside attack
of a water molecule to the silicon atom with inversion of config-
uration through transition state 13, as shown in Scheme 4.
The acid-catalyzed isomerization reaction reached equilib-
rium after 20 h. According to the relative ratio of the compo-
nents, the relative free-energy differences (ΔG) were calculated,
as shown in Table 1. It is clearly shown that cis,cis,trans-isomer
2 is the most stable isomer and that the cis,cis,cis isomer is the
least stable one.
Scheme 3. A plausible cyclization process forming cyclotetrasiloxane dia-
stereomers 1, 2, and 3 from conformers 9a, 9b, and 9c of meso-diastereomer
9.
Stereochemical interconversion of the cyclotetrasiloxanes
was examined by the reaction of 1 in THF with MSA as a cata-
lyst. Figure 3 shows the time-dependent changes in the relative
concentrations of the diastereomers, as monitored by HPLC. The
curves for the changes in the relative abundances in the dia-
stereomers are not far different from those reported by Klem-
ent'ev[10] for the similar reaction of 1 in acetone with Me3SiCl
as a catalyst. One of the distinctive differences is the presence
of induction periods for the formation of 3 and 4. This strongly
suggests stepwise conversion of the diastereomers, as shown
in Scheme 4. Diastereomer 2 is produced by inversion of config-
uration of one of the four silicon atoms in 1. Diastereomers 3
and 4 are successively formed by further inversion of configura-
tion of the silicon atoms in 2 at the adjacent and diagonal
positions, respectively, of the initially inverted silicon atom.
Klement'ev suggested that the isomerization proceeds directly
from 1 to 3 and 4 with dissociation of the cyclotetrasiloxane
ring in addition to the stepwise manner. However, our results
suggest the isomerization proceeds, at least mainly, without dis-
sociation of the cyclotetrasiloxane ring.
Table 1. Relative composition of diastereomers 1–4 in the equilibrium mixture
determined by HPLC and their free-energy differences.
Diastereomer
Relative composition
ΔG [kJ mol–1
]
1
2
3
4
0.142
1.000
0.518
0.404
4.8
0.0
1.6
2.2
Conclusions
The condensation reaction of 5 in aqueous medium under
Brown's conditions was revealed to provide three cyclotetra-
siloxane diastereomers, that is, 1–3, without the formation of a
possible forth diastereomer, that is, 4 (<0.1 %), and not only 1,
as was believed since the original report.[1d] This was explained
reasonably by assuming a cyclization process to form the cyclo-
tetrasiloxanes exclusively from linear tetrasiloxane meso-dia-
stereomer 9, which was considered to adopt a U-shaped confor-
mation in hydrophilic aqueous media and to be more suitable
for cyclization than DL-diastereomer 10. The distribution of dia-
stereomers 1–3 in the original product mixture was also reason-
ably explained by considering the stability of the conformers of
9 leading to the cyclotetrasiloxanes. We believe this investiga-
tion provides a firm and reliable basic understanding of the
initial stage of the silsesquioxane condensation reaction proc-
ess, including the stereochemistry of the intermediates. These
findings should contribute not only to the progress of basic
siloxane chemistry but also to the industrial production of silox-
ane materials having superior properties.
Acknowledgments
This work was supported by the Ministry of Economy, Trade
and Industry, Japan (METI) (Development of Innovative Catalytic
Processes for Organosilicon Functional Materials project, grant
Figure 3. Time-dependent changes in the relative concentrations of the dia-
stereomers (blue: 1, red: 2, green: 3, purple: 4) in the acid-catalyzed isomeri-
zation reaction of 1 (5.3 mg) with 1000 ppm MSA in THF (1.0 mL) at 5 °C.
Considering these observations and detailed studies about to K. S.) and by the New Energy and Industrial Technology De-
the reaction stereochemistry on silicon atoms by Corriu[13] and velopment Organization (NEDO). The authors thank Dr. Y.-K.
the involvement of hypervalent species by Bassindale,[14] we Choe for helpful discussions.
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Keywords: Cyclization · Silicon · Cyclotetrasiloxane ·
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Received: April 19, 2016
Published Online: ■
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Siloxanes
F. Yagihashi,* M. Igarashi,
Y. Nakajima, K. Sato, Y. Yumoto,
C. Matsui, S. Shimada* ..................... 1–6
Unexpected Selectivity in Cyclotet-
rasiloxane Formation by the Hydro-
lytic Condensation Reaction of Tri-
chloro(phenyl)silane
Selective cyclization: Hydrolytic con- conceivable ones. This phenomenon
densation of PhSiCl3 in aqueous me- can reasonably be explained by exclu-
dium affords only three cyclic tetra- sive cyclization through the meso-lin-
siloxane diastereomers out of four ear-tetrasiloxane.
DOI: 10.1002/ejic.201600454
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