Enhanced sol-gel polymerization of organoallylsilanes by solvent effect

Article abstract of DOI:10.1039/c1jm11565k

We investigated solvent effects on the acid-catalyzed deallylation of organoallylsilane precursors to identify mild sol-gel polymerization conditions. Organoallylsilanes are expected to be alternative precursors for preparation of functionalized organosil

Full text of DOI:10.1039/c1jm11565k

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PAPER
Enhanced sol–gel polymerization of organoallylsilanes by solvent effect†
ab
Yoshifumi Maegawa,ab Norihiro Mizoshita,ab Takao Tani,ab Toyoshi Shimadabc and Shinji Inagaki
*
Received 13th April 2011, Accepted 8th July 2011
DOI: 10.1039/c1jm11565k
We investigated solvent effects on the acid-catalyzed deallylation of organoallylsilane precursors to
identify mild sol–gel polymerization conditions. Organoallylsilanes are expected to be alternative
precursors for preparation of functionalized organosilica hybrids but they undergo sol–gel
polymerization with difficulty due to their low reactivity towards hydrolysis. Sol–gel polymerization of
model organoallylsilane precursors was conducted in various organic solvents and deallylation was
1
monitored by H NMR spectroscopy. The nature of the solvent was found to strongly influence the
deallylation rate and a significant correlation was observed between reaction rate and solvent basicity,
which suggests that proton activity is a key factor in enhancing the reaction rate. In particular,
acetonitrile was found to most effectively enhance the rate, and it accelerated the formation of
a spirobifluorene-bridged organosilica hybrid film from its allylsilane precursor under a mild acidic
condition. This key finding can be generally utilized for the preparation of organoallylsilane-derived
highly functionalized organosilica hybrids.
conventional organosilane precursors, unlike for general organic
Introduction
compounds.
Organosilica hybrid materials have received considerable atten-
Recently, allylsilyl groups (Si–CH2CH]CH2) have been
tion in various research areas for their potential application in
found to behave as a synthetic equivalent to conventional silyl
catalysts, adsorbents and optical devices.1–3 These materials are
groups.9–15 Allylsilyl groups can be hydrolyzed and condensed
readily prepared through sol–gel polymerization (hydrolysis and
under acidic hydrolytic conditions through the elimination of
polycondensation) of organosilane precursors with hydrolyzable
propene (deallylation) (Scheme 1).9,16–18 They have a good
silyl groups (SiRnX4ꢀn: X ¼ OR0, halide, OCOR0, NR02, etc.)4,5
A
tolerance for cross-coupling reaction conditions and silica gel
chromatography,19 and thus are expected to be an alternative
source of hydrolyzable silyl groups for synthesis of complex
organosilane precursors.19 Recently, we also reported on a new
organoallylsilane precursor containing spirobifluorene as
a bridging group, which underwent acidic sol–gel polymerization
to form a periodic mesoporous organosilica (PMO) film.20
However, allylsilyl groups are generally less reactive to hydro-
lysis than conventional silyl groups, and thus high acid concen-
trations (e.g. 0.2 M hydrochloric acid (HCl) in THF) and high
broad spectrum of functional organic groups (R) has been
incorporated into amorphous and ordered silicate frameworks
alike.1–3,6–8 Recent demands for the development of highly
functionalized organosilica hybrids necessitate the use of orga-
nosilane precursors having significant optical, chemical or elec-
trical functionalities. However, synthesis of the desired
precursors is often difficult, because the high reactivity of
conventional silyl groups causes side reactions such as poly-
merization and decomposition during the silylation reaction and
purification. In particular, a serious problem is that silica gel
chromatography cannot be utilized for purification of
ꢁ
temperatures (e.g. 60 C) are usually required for hydrolysis in
organic solvents compared to those for organoalkoxysilanes
aToyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192,
Japan. E-mail: inagaki@mosk.tytlabs.co.jp; Fax: +81-561-63-6507; Tel:
+81-561-71-7393
bCore Research for Evolutional Science and Technology (CREST), Japan
Science and Technology Agency (JST), Kawaguchi, Saitama, 332-0012,
Japan
cDepartment of Chemical Engineering, Nara National College of
Technology, 22 Yata-cho, Yamatokoriyama, Nara, 639-1080, Japan
† Electronic supplementary information (ESI) available: Observation of
sol–gel polymerization behaviors of 1a and 1b by 1H NMR
spectroscopy, identification of the generated gas during the sol–gel
polymerization of 1a and 1b, sol–gel polymerization of 1a in other
organic solvents, and the relationship between initial deallylation rate
v0 of 1a and other solvent parameters. See DOI: 10.1039/c1jm11565k
Scheme 1 Presumed deallylation mechanism of an allylsilyl group
during acid-catalyzed hydrolysis and polycondensation.
14020 | J. Mater. Chem., 2011, 21, 14020–14024
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(e.g. 0.01 M HCl at rt).14 This is likely to cause decomposition of
bridging organic groups and/or undesired Si–C bond cleavage
between the silyl group and the bridging-organic group.21,22 Thus
it is of great importance to identify mild conditions for sol–gel
polymerization of organoallylsilane precursors.
Table 1 Acidic sol–gel polymerization of model organoallylsilane
precursors 1a and 1b in various organic solventsa
b
Entry
Precursor
Solvent
v0 /M sꢀ1
Solid film formation
1
2
3
4
5
6
7
1a
MeOH
THF
2.40 ꢂ 10ꢀ5
2.14 ꢂ 10ꢀ5
0.90 ꢂ 10ꢀ5
8.64 ꢂ 10ꢀ5
4.53 ꢂ 10ꢀ4
1.23 ꢂ 10ꢀ5
3.88 ꢂ 10ꢀ5
No
No
No
In this paper, we investigate the effects of solvents on the acid-
catalyzed deallylation of organoallylsilane precursors to identify
mild conditions for sol–gel polymerization. Hydrolysis and
polycondensation of model precursors were conducted in various
organic solvents and the deallylation behaviors were monitored
DMSO
Acetone
MeCN
Acetone
MeCN
Yes (24 h)c
Yes (3 h)c
No
1b
Yes (4 h)c
1
a
by H NMR spectroscopy. The nature of the solvent strongly
Reactions of the allylsilane precursors 1a and 1b (0.10 mmol) were
carried out at 60 ꢁC for 3–48 h in the solvents (0.25 mL) in the
presence of HCl (0.2 M). The deallylation rate at the initial 0.5 h.
influenced the reaction rate and the observed solvent effect was
well explained in terms of the solvent basicity parameter (solvent
basicity: SB), which suggests that the proton activity is a key
factor in enhancing the reaction rate. The use of a solvent with
low SB was confirmed to accelerate the formation of a functional
organosilica hybrid film from its allylsilane precursor under
a mild acidic condition.
b
c
The reaction time required for the formation of a solid organosilica
film from the sol-solution.
Results and discussion
Sol–gel polymerization behavior of model organoallylsilane
1
precursors by H NMR spectroscopy
The behavior of organoallylsilanes under acid-catalyzed hydro-
lysis and polycondensation was examined in various organic
solvents containing HCl (0.2 M in the solutions). Organo-
allylsilanes 1a and 1b (Fig. 1) were used as model precursors
because both diallylethoxy- and triallyl-silyl groups have been
utilized as precursors for PMO synthesis.14,20 Water-soluble
polar solvents, methanol (MeOH), tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO), acetone and acetonitrile (MeCN)
were chosen because they were appropriate for preparing
a homogeneous solution containing the organoallylsilane, HCl
and water. Solutions of 1a and 1b were stirred at 60 ꢁC for 3–48 h
and subjected to hydrolysis and polycondensation to obtain sol
solutions. The sol solutions were cast on a glass substrate in order
to check whether a solid film was obtained (Table 1). Conversion
of the allyl groups in the organoallylsilane precursors during the
reactions was monitored by 1H NMR spectroscopy (signals from
the b-hydrogens of the allylsilyl groups, Fig. 3 and S1–S6 in the
ESI†) in deuterated solvents.
Fig. 2 Conversion of allyl groups in (a) 1a and (b) 1b with the reaction
time for the sol solutions using various deuterated solvents. Solvents are
labelled as follows: MeCN (B); acetone (O); MeOH (>); THF (,);
and DMSO (ꢂ) for 1a, MeCN (C) and acetone (:) for 1b.
1
identified as propene by H NMR spectroscopy (Fig. S7 in the
ESI†). Solid films were formed from the sol solutions after stir-
ring in MeCN and acetone for 3 h and 24 h, respectively, to allow
evaporation of the solvents (Table 1, entries 4 and 5). In MeCN,
a solid film was also obtained even at a HCl concentration
Fig. 2a plots the conversion of allyl groups in 1a in various
solvents after the addition of HCl, and Table 1 gives the deal-
lylation rates (v0) at the initial 0.5 h.23 The v0 was strongly
influenced by the solvent. The highest v0 of 4.53 ꢂ 10ꢀ4 M sꢀ1 was
obtained with MeCN, which gave complete deallylation within
3 h (Fig. 3a). The second highest v0 of 8.64 ꢂ 10ꢀ5 M sꢀ1 was
obtained with acetone, which gave 76% deallylation after 4 h
(Fig. 3b). These solutions (MeCN and acetone) released
considerable amounts of a gas during the reaction, which was
Fig. 3 Changes in the 1H NMR spectra (signals from the b-hydrogens of
the allylsilyl groups) of the sol solutions of 1a in (a) MeCN-d3, (b)
acetone-d6 and (c) DMSO-d6 after the addition of HCl.
Fig. 1 Chemical structures of model organoallylsilane precursors 1a
and 1b.
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ten-fold lower (0.02 M) than that used for the other solvents after
30 h. These results indicate that the polycondensation reaction
was significantly promoted in MeCN. In contrast, the use of
MeOH, THF and DMSO resulted in relatively low v0 of 2.40 ꢂ
10ꢀ5, 2.14 ꢂ 10ꢀ5 and 0.90 ꢂ 10ꢀ5 M sꢀ1, respectively (Table 1,
entries 1–3). For example, more than 90% of the allyl groups
remained after 4 h in DMSO (Fig. 3c). For these solvents, the sol
solutions formed soluble gel-like films even after stirring for
120 h. On the other hand, the ethoxy group in 1a was found to be
completely hydrolyzed to ethanol within 10 min regardless of the
solvent (Fig. S1–S5 in the ESI†). This surely indicates that
hydrolysis of the ethoxysilyl group is very fast compared to that
of the allylsilyl group and deallylation determines the overall
sol–gel polymerization rate of 1a.
Fig. 2b plots the conversion of allyl groups in 1b in MeCN and
acetone. The v0 of 1b is higher for MeCN (3.88 ꢂ 10ꢀ5 M sꢀ1
)
Fig. 4 Relationship between the initial deallylation rate (v0) of 1a and (a)
solvent basicity (SB), (b) dielectric constant (DC) or (c) acceptor number
(AN) for various solvents. Solvents are labelled as follows: MeCN (B);
acetone (O); MeOH (>); THF (,); and DMSO (ꢂ).
than acetone (1.23 ꢂ 10ꢀ5 M sꢀ1), which was a similar result to
that observed for 1a. On the other hand, the deallylation rates of
1b were smaller than those of 1a due to the lower reactivity of the
triallylsilyl group (Table 1, entries 6 and 7). However, despite the
low reactivity, the sol-solution of 1b in MeCN formed a solid film
after stirring for 4 h.
(an index of solvent polarity) (Fig. S9†). These results suggest
that the present strong solvent effect results from differences in
the proton activities in the solvents. This may also suggest that
electrophilic addition of a proton is the rate-determining step in
the deallylation reaction, which is reasonable since this step was
found to be slow in other electrophilic substitution reactions of
allylic silanes.17
Mechanism of solvent effect on sol–gel polymerization of
organoallylsilane precursors
In the following we outline the origin of the observed solvent
effect based on the underlying reaction mechanism. Hydrolysis
and polycondensation of the allylsilyl groups are considered to
proceed as follows: (i) electrophilic addition of a proton to the
allyl group forms a b-silyl cation intermediate A and (ii) subse-
quent nucleophilic attack of water to A forms a silanol species
along with the generation of propene (Scheme 1a). The silanol
species then further reacts with A (or another silanol species) to
form a siloxane bond, allowing polycondensation to progress
(Scheme 1b). According to this scheme, the deallyation reaction
can be influenced by three factors: (i) proton activity, (ii) stability
of the b-silyl cation intermediate and (iii) nucleophilicity of the
Preparation of spirobifluorene-bridged organosilica film under
a mild sol–gel polymerization condition
The solvent effect was also confirmed for highly functional spi-
robifluorene-bridged allylsilane precursor SBF-Si (Fig. 5). In
a previous report, SBF-Si underwent hydrolysis and poly-
condensation in the presence of a surfactant under a highly acidic
condition (e.g. 0.2 M HCl in THF) to yield a highly emissive
PMO film.20 Here, we carried out sol–gel polymerization of SBF-
Si in a mixed solvent of THF/MeCN (2 : 1) and in pure THF at
an acid concentration ten-fold lower (0.02 M HCl) without
a surfactant. The sol solution of THF/MeCN formed a solid film
after stirring for 3 h, whereas the sol solution of pure THF did
not form a solid film even after stirring for 24 h. Fig. 6 shows UV-
vis absorption and fluorescence emission spectra of the obtained
spirobifluorene-silica hybrid film. The film shows absorption
bands at 364 and 378 nm and exhibits strong blue emission bands
at 448 and 470 nm with a total fluorescence quantum yield of 0.50
(excited at 360 nm). These optical properties were largely similar
to those of a previously reported PMO film prepared under
a strong acidic condition.20 These results suggest that the solvent
water (or silanol species). These factors are closely related to the
24
Catalan solvent basicity (SB), dielectric constant (DC)25 and
ꢀ
Gutmann’s acceptor number (AN), respectively.26 Here, the SB is
an index for hydrogen-bond acceptor basicity and a low SB leads
to a high proton activity, accelerating electrophilic addition of
the proton to the allyl group. DC is an index for solvent polarity
and a high DC leads to high stability of the cationic intermediate
A, decreasing the activation barriers of both steps. AN is an
index for the strength of the solvent as a Lewis acid and a low AN
leads to high nucleophilicity of the oxygen atoms in water (or the
silanol species), accelerating the nucleophilic attack step. Fig. 4a–
c show the relationship between the initial deallylation rate v0 of
1a and SB, DC, and AN, respectively, for various solvents. A
correlation was apparent for v0 vs SB, but not for v0 vs DC or v0 vs
AN. The strong SB-dependence of v0 was further supported by
additional experiments using N,N-dimethylformamide and
2,2,2-trifluoroethanol as a solvent in which the polymerization
reaction of 1a proceeds in the solvents with low SB, regardless of
the values of their DC and AN (Sections 3 in the ESI†). The v0
was also correlated to Gutmann’s donor number26 (an index of
Fig.
5 Chemical structure of spirobifluorene-bridged allylsilane
N
Lewis basicity) but not to Dimroth–Reichardt’s ET value27
precursor SBF-Si.
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equipped with a calibrated integrating sphere (Hamamatsu
Photonics, C9920-02).
Synthesis
Diallyl(ethoxy)(p-tolyl)silane (1a). To a solution of triethoxy
(p-tolyl)silane (12.0 g, 47.2 mmol) in dry Et2O (20 mL) was added
dropwise a solution of allylmagnesium bromide (142 mL, 1.0 M
in Et2O, 142 mmol) at 0 ꢁC. The reaction mixture was stirred at
room temperature for 24 h and quenched with saturated NH4Cl
solution and extracted with Et2O. The combined organic layers
were dried over anhydrous MgSO4, filtered and concentrated
under reduced pressure. The crude residue was purified by
column chromatography (silica; hexanes) affording the title
compound as a colorless oil (9.94 g, 86%). dH (400 MHz; CDCl3;
Me4Si) 1.20 (3H, t, J 6.8 Hz, –OCH2CH3), 1.93 (4H, ddd, J 8.0
Hz, 1.2 Hz, 0.8 Hz, –SiCH2CH]CH2), 2.36 (3H, s, –CH3), 3.75
(2H, q, J 6.8 Hz, –OCH2CH3), 4.90 (2H, ddt, J 10.0 Hz, 1.4 Hz,
0.8 Hz, –SiCH2CH]CH2), 4.95 (2H, ddt, J 16.0 Hz, 1.4 Hz, 1.2
Hz, –SiCH2CH]CH2), 5.82 (2H, ddt, J 16.0 Hz, 10.0 Hz, 8.0
Hz, –SiCH2CH]CH2), 7.20 (2H, d, J 7.8 Hz, aromatic), 7.47
(2H, d, J 7.6 Hz, aromatic); dC (100 MHz; CDCl3; CDCl3) 18.4,
21.3, 21.5, 59.2, 114.6, 128.6, 131.4, 133.3, 134.1, 139.8; m/z (FI)
246.1430 (M+. C15H22OSi requires 246.1440).
Fig. 6 UV-vis absorption (C) and fluorescence emission (B, excited at
360 nm) spectra of the spirobifluorene-bridged organosilica film.
effect can be generally applied to the preparation of highly
functional organosilica hybrid materials from a variety of func-
tional and stable organoallylsilane precursors.
Conclusions
In summary, we identified a significant solvent effect on the acid-
catalyzed deallylation of organoallylsilane precursors and in mild
sol–gel polymerization conditions. The observed solvent effect
was well explained in terms of SB but not the DC or AN of the
solvent. These results suggest that proton activity is a key factor
in enhancing the reaction rate. In particular, MeCN was found to
most effectively enhance the rate, and accelerated sol–gel poly-
merization of SBF-Si to form a highly emissive spirobifluorene-
silica hybrid film under a mild acidic condition. This key finding
can be generally utilized for the preparation of a variety of
organoallylsilane-derived highly functionalized organosilica
hybrids.
Triallyl(p-tolyl)silane (1b). To a solution of (p-tolyl)tri-
chlorosilane (2.40 g, 10.6 mmol) in dry Et2O (30 mL) was added
dropwise a solution of allylmagnesium bromide (40 mL, 1.0 M in
ꢁ
Et2O, 40 mmol) at 0 C. The reaction mixture was refluxed for
16 h, quenched with saturated NH4Cl solution and extracted
with Et2O. The combined organic layers were dried over anhy-
drous MgSO4, filtered and concentrated under reduced pressure.
The crude residue was purified by column chromatography
(silica; hexanes) affording the title compound as a colorless oil
(2.46 g, 95%). dH (400 MHz; CDCl3; Me4Si) 1.85 (6H, ddd, J 8.2
Hz, 1.2 Hz, 0.8 Hz, –SiCH2CH]CH2), 2.35 (3H, s, –CH3), 4.88
(3H, ddt, J 10.4 Hz, 1.6 Hz, 0.8 Hz, –SiCH2CH]CH2), 4.91 (3H,
ddt, J 16.0 Hz, 1.6 Hz, 1.2 Hz, –SiCH2CH]CH2), 5.79 (3H, ddt,
J 16.0 Hz, 10.4 Hz, 8.2 Hz, –SiCH2CH]CH2), 7.18 (2H, d, J 8.0
Hz, aromatic), 7.41 (2H, d, J 8.0 Hz, aromatic); dC (100 MHz;
CDCl3; CDCl3) 19.6, 21.5, 114.2, 128.6, 131.5, 134.0, 134.3,
139.3; m/z (FI) 242.1493 (M+. C16H22Si requires 242.1491).
Experimental
Materials
All reactions were carried out under argon using standard high
vacuum and Schlenk techniques. Unless otherwise noted, all
materials, including dry solvents, were purchased from
commercial suppliers (Sigma-Aldrich, Tokyo Chemical Industry,
Wako Pure Chemical Industries and AZmax Co., Ltd) and used
without further purification. SBF-Si was prepared according to
a literature method.20
Observation of sol–gel polymerization of model organoallylsilane
1
precursors by H NMR measurements
Precursor 1a (25 mg, 0.10 mmol) was dissolved in MeOH-d4,
THF-d8, DMSO-d6, acetone-d6 or MeCN-d3 (0.25 mL), respec-
tively. Precursor 1b (25 mg, 0.10 mmol) was dissolved in acetone-
d6 or MeCN-d3 (0.25 mL), respectively. As an internal standard,
a small amount of 1,4-dichlorobenzene or naphthalene was
added to the precursor solution. To calculate the initial
concentration of the allyl group relative to the internal standard,
a reaction aliquot was measured by 1H NMR spectroscopy
before the reaction. Then 2 M HCl aqueous solution was added
(2.5–25 mL, 5.0–50 mmol, 0.02–0.2 M in reaction solvent), and the
General methods
Nuclear magnetic resonance (NMR) spectra were recorded on
1
a Jeol JNM-EXC400P spectrometer (400 MHz for H and 100
MHz for 13C). Chemical shifts are reported in d ppm referenced
1
to an internal SiMe4 standard for H NMR and chloroform-d
(d 77.0) for 13C NMR, respectively. Mass spectra were recorded
on a Waters GCT Premierꢀ mass spectrometer (FI: field ioni-
zation). Ultraviolet-visible (UV-vis) absorption spectra were
measured using a Jasco V-670 spectrometer. Fluorescence
emission spectra were obtained using a Jasco FP-6500 spec-
trometer. Fluorescence quantum yields were determined using
ꢁ
reaction mixture stirred at 60 C. The sol solution was cast on
a glass plate to determine whether a solid organosilica film could
a
photoluminescence quantum yield measurement system
be obtained. The reaction aliquot (25 mL) was also rapidly
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9 T. Shimada, K. Aoki, Y. Shinoda, T. Nakamura, N. Tokunaga,
S. Inagaki and T. Hayashi, J. Am. Chem. Soc., 2003, 125, 4688.
10 K. Aoki, T. Shimada and T. Hayashi, Tetrahedron: Asymmetry, 2004,
15, 1771.
11 Y.-R. Yeon, Y. J. Park, J.-S. Lee, J.-W. Park, S.-G. Kang and
C.-H. Jun, Angew. Chem., Int. Ed., 2008, 47, 109.
diluted with the same deuterated solvent and quickly measured
1
by H NMR spectroscopy. Conversion of the allyl and ethoxy
groups was determined by integrating the selected resonance
peaks for the allyl and ethoxy groups, respectively, relative to
that of the internal standard.
12 Y. Zhang, L. Zhao, S. S. Lee and J. Y. Ying, Adv. Synth. Catal., 2006,
348, 2027.
13 M. P. Kapoor, S. Inagaki, S. Ikeda, K. Kakiuchi, M. Suda and
T. Shimada, J. Am. Chem. Soc., 2005, 127, 8174.
Preparation of spirobifluorene-bridged organosilica film
14 M. P. Kapoor, M. Yanagi, Y. Kasama, T. Yokoyama, S. Inagaki,
T. Shimada, H. Nanbu and L. R. Juneja, J. Mater. Chem., 2006,
16, 3305.
15 Y. Wang, S. Hu and W. J. Brittain, Macromolecules, 2006, 39, 5675.
16 L. H. Sommer, L. J. Tyler and F. C. Whitmore, J. Am. Chem. Soc.,
1948, 70, 2872.
17 T. H. Chan and I. Feming, Synthesis, 1979, 761.
18 T. Morita, Y. Okamoto and H. Sakurai, Tetrahedron Lett., 1980, 21,
835.
19 Y. Maegawa, T. Nagano, T. Yabuno, H. Nakagawa and T. Shimada,
Tetrahedron, 2007, 63, 11467.
Spirobifluorene-bridged allylsilane precursor SBF-Si (10 mg,
7.5 mmol) was dissolved in THF/MeCN (2 : 1 ¼ v/v, 0.40 mL)
and then 2 M HCl aqueous solution (4.0 mL, 8.0 mmol, 0.02 M in
reaction solvꢁent) was added to the solution. The mixture was
stirred at 60 C for 3 h. The sol solution was diluted with THF
(1.20 mL), coated on a quartz glass plate by spin-coating (4000
rpm, 30 s) and dried under reduced pressure to give an organo-
silica film.
20 N. Tanaka, N. Mizoshita, Y. Maegawa, T. Tani, S. Inagaki,
Y. R. Jorapur and T. Shimada, Chem. Commun., 2011, 47, 5025.
21 C. Yoshina-Ishii, T. Asefa, N. Coombs, M. J. MacLachlan and
G. A. Ozin, Chem. Commun., 1999, 2539.
22 S. Shirai, Y. Goto, N. Mizoshita, M. Ohashi, T. Tani, T. Shimada,
S. Hyodo and S. Inagaki, J. Phys. Chem. A, 2010, 114, 6047.
23 The deallylation rate v0 was estimated from the linear interpolation of
the data, corresponding to decay of allyl group at the initial 0.5 h.
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