An efficient method for the synthesis of symmetrical disiloxanes from alkoxysilanes using Meerwein's reagent

Article abstract of DOI:10.1055/s-0031-1290668

We report here a new and efficient route to symmetrical disiloxanes from their corresponding alkoxysilanes using Meerwein's reagent as mediator and potassium carbonate as additive under mild reaction conditions in acetonitrile. Our methodology is very simple, economic, and high yielding. We have also proposed a reaction mechanism with the plausible silyloxonium intermediates. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290668

LETTER
▌1633
lAettner Efficient Method for the Synthesis of Symmetrical Disiloxanes from
Alkoxysilanes Using Meerwein’s Reagent
fficient Method for the Synthesis of Symmetrical Disiloxanes
Yogesh R. Jorapur,a,b Toyoshi Shimada*a,b
a
Department of Chemical Engineering, Nara National College of Technology, 22 Yata-cho, Yamatokoriyama, Nara 639-1080, Japan
b
Core Research for Evolutional Science and Technology (CREST), JST Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Fax +81(743)556154; E-mail: shimada@chem.nara-k.ac.jp
Received: 08.03.2012; Accepted after revision: 29.03.2012
ported the use of transition-metal catalyst to generate di-
Abstract: We report here a new and efficient route to symmetrical
siloxanes from hydridosilanes.16
disiloxanes from their corresponding alkoxysilanes using Meer-
wein’s reagent as mediator and potassium carbonate as additive un-
der mild reaction conditions in acetonitrile. Our methodology is
very simple, economic, and high yielding. We have also proposed a
reaction mechanism with the plausible silyloxonium intermediates.
In our recent report, we discovered a novel sol-gel poly-
condensation of tetraethoxysilane (TEOS) to silica using
Meerwein’s reagent (MR)17 in acetonitrile (MeCN) sol-
vent.18 We extended our work on MR and successfully
achieved various fluorosilanes from alkoxysilanes via nu-
cleophilic fluorination to the silicon centre (SN2–Si) in ex-
cellent yields (Scheme 1).19 Here, in this work we
suppressed the effect of fluoride ion by using K2CO3 as an
additive and demonstrated the synthesis of various sym-
metrical disiloxanes from their corresponding monoalk-
oxysilanes (Scheme 1). We further propose the possible
reaction mechanism of alkoxysilanes condensation. To
our knowledge this is the first report of MR-assisted alk-
oxysilanes condensation to corresponding symmetrical
disiloxanes proceeding via silyloxonium intermediate.
Key words: Meerwein’s reagent, disiloxane, alkoxysilane conden-
sation, fluorine anion, silicon
Siloxanes with Si–O–Si backbone are ubiquitous in na-
ture. They have a broad range of potential applications, in-
cluding health care products, dry-cleaning process,1a and
adhesives.1b Liquid crystalline siloxane oligomers were
investigated for their mesogenic properties.2 Water repel-
lent siloxanes act as antiperspirants.3
A
poly-
organosiloxane4a emulsion4b is used for the preparation of
cosmetic products.4c Particularly, disiloxanes act as an im-
portant class of compounds with versatile applications.
They are used in microelectronic device and sensor
fabrication5 and as liquid crystal display element.6 Anion
recognition phenomenon was observed by Kondo and
Unno in the presence of disiloxane-1,3-diol.7 Gunji and
co-workers used 1,3-diphenyldisiloxane as a building
block of silsesquioxanes.8 Narumi and Miyano reported
the use of disiloxane to prepare bridged calix[4]arenes.9
Napier used disiloxanes as cross-coupling partner in
Hiyama-type coupling reaction with aryl halides.10
Marciniec efficiently used divinyl-substituted silanes and
disiloxanes in cross-metathesis in the presence of Grubbs
catalyst.11
n-Si(OEt)4
R3Si OEt
ref. 19
R3Si F
Si O Si
n
ref. 18
R = aryl, alkyl
Meerwein's
reagent
R3Si OEt
K2CO3
this work
SiR3 O SiR3
Scheme 1 MR-Mediated symmetrical disiloxane synthesis
In our present approach, the primary concern was to over-
come the problem of unexpected nucleophilic substitution
of fluoride ion to the silicon center and completely sup-
press the formation of fluorosilanes. Consequently, we
first examined various additives that will serve as a fluo-
ride ion inhibitor (see Table S1 in the Supporting Informa-
tion). We have used (4-bromophenyl)ethoxy[di(prop-2-
enyl)]silane (1a)20 as a model substrate and examined var-
ious alkali metal carbonates like Li2CO3, Na2CO3, K2CO3,
and Cs2CO3 as additives. However, K2CO3 was found to
be the best additive, affording disiloxane 2a in high yield
without the formation of fluorosilane 3 as a side product.
The general strategy to efficiently generate disiloxanes is
via hydrosilanes. Hydrolytic oxidation of hydrosilanes in
the presence of cationic oxorhenium catalyst provides di-
siloxanes.12 Matsuo and Kawaguchi observed the forma-
tion of disiloxane from CO2 and hydrosilanes using
zirconium–borane complexes.13 Chojnowski et al. used
tris(pentafluorophenyl)borane as Lewis acid catalyst to
produce disiloxanes via coupling reaction of hydrosilanes
with alkoxysilanes.14 Lewis acids were further utilized as
catalyst in the synthesis of symmetrical disiloxanes via
aerobic oxidation of hydrosilanes.15 Schubert recently re-
Table 1 illustrates optimization of mediator for the synthe-
sis of disiloxane 2a under various reaction conditions in-
cluding equivalent of mediator, temperature and solvent.21
SYNLETT 2012, 23, 1633–1638
Advanced online publication: 25.05.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290668; Art ID: ST-2012-U0209-L
© Georg Thieme Verlag Stuttgart · New York

1634
Y. R. Jorapur, T. Shimada
LETTER
Initially, we performed a blank reaction in the absence of larly, entry 6 with <1.0 equivalent of K2CO3 is unable to
mediator in MeCN. This reaction did not provide any inhibit the naked fluoride ion attack to the silicon center
product even after 24 hours of stirring at 100 °C (entry 1). which consequently leads to undesired fluorosilane 3 in
Astonishingly, the same reaction in the presence of 80% yield. Entry 7, with two-fold excess of K2CO3 afford-
Me3OBF4 under identical conditions reached to comple- ed disiloxane 2a in 91% yield. The use of Et3OBF4 instead
tion within 1.5 hours to give exclusively disiloxane 2a in of Me3OBF4 also showed favorable results with the for-
high yield (90%, entry 2). Although entry 3 with 0.1 mation of disiloxane 2a (90%, entry 8). The change in
equivalent of Me3OBF4 provided traces of disiloxane 2a, counteranion from BF4 to PF6 did not adversely affect the
entries 4 and 5 with 0.25 equivalent and 0.5 equivalent of reaction and provided excellent yield of disiloxane 2a
mediator improved the yields of disiloxane 2a to 29% and (91%, entry 9). Reaction temperature was found to be one
54%, respectively. Comparison of entry 2 with entries 3– of the important parameters to successfully achieve disil-
6 indicates that both the mediator and additive were essen- oxane in high yields. Although the reaction carried out at
tial to achieve excellent yields of disiloxane 2a. Particu- 25 °C failed to provide disiloxane 2a, at 50 °C it reached
Table 1 Symmetrical Disiloxanes Synthesis under Various Reaction Conditionsa
Br
Br
Br
mediator, K2CO3
solvent
O
Si
Si
Si
Si
F
OEt
Br
1a
3
2a
Entry
Mediator (equiv)
Solvent
Time (h)
Yield (%)b
1a
2a
3
1
2
no mediator
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
toluene
12
1.5
91
–
–
–
Me3OBF4 (1.0)
Me3OBF4 (0.10)
Me3OBF4 (0.25)
Me3OBF4 (0.50)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Et3OBF4 (1.0)
Et3OPF6 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
ZnCl2 (0.25)
90
trace
29
54
12
91
90
91
–
–
3
12
12
12
90
66
41
–
–
4
–
5
–
6c
0.5
80
–
7d
8
1.5
1.5
2
–
–
–
9
–
–
10e
11f
12g
13
14
15
16
17h
12
12
0.17
91
–
–
89
18
6
–
–
67
80
91
8
2
2
trace
–
1,4-dioxane
t-BuOH
MeCN
–
2
–
65
–
12
12
91
–
–
MeOTf (1.0)
MeCN
–
–
a Reactions were carried out on a 1.0-mmol reaction scale of (4-bromophenyl)di(prop-2-enyl)ethoxysilane (1a), Me3OBF4 (1.0 equiv), and
K2CO3 (1.0 equiv) in MeCN (5.0 mL) at 100 °C.
b Isolated yield.
c K2CO3 (0.5 equiv) was used.
d K2CO3 (2.0 equiv) was used.
e Reaction was carried out at 25 °C.
f Reaction was carried out at 50 °C.
g Reaction was carried out at 125 °C.
h 1H NMR showed a complex mixture.
Synlett 2012, 23, 1633–1638
© Georg Thieme Verlag Stuttgart · New York

LETTER
Efficient Method for the Synthesis of Symmetrical Disiloxanes
1635
completion after 12 hours to the desired product (entries in the above solvents. The use of Lewis acid like zinc
10 and 11). The reaction at 125 °C generated fluorosilane chloride did not furnish the desired disiloxane (entry
as a major side product due to enhanced nucleophilic fluo- 16).23 Our attempt with methyl triflate as the conventional
rination (entry 12). Following our optimization studies, methylating agent failed to give disiloxane (entry 17).
we next screened different solvents to find that pure
With these optimized conditions, we proceeded to estab-
MeCN was the best-performing medium.22 Entries 13–15,
lish the scope of our protocol with various alkoxysilanes
with toluene, 1,4-dioxane, and tert-butyl alcohol provided
6%, 0%, and 65% of desired disiloxane 2a, respectively.
(Table 2).
This behavior is presumably due to poor solubility of MR
Table 2 Symmetrical Disiloxanes Synthesis in the Presence of MRa
Entry
Alkoxysilane
Disiloxane
Product yield (%)b
Method A
Method B
R
R
Si
R
R
O
Si
Si
R
OEt
R
1
85
91
88
86
1b
2b
OMe
R
R
R
Si
R
O
OEt
Si
Si
R
R
2
3
88
89
MeO
MeO
1c
2c
Me
R
R
R
Si
R
O
Si
Si
OEt
R
R
Me
Me
1d
2d
Me
Me
Si
Me
Me
Ph
Ph
Ph
O
O
O
O
Si
Si
OEt
Me
Ph
Ph
Ph
4
5
6
7
78
82
Me
1e
2e
Me
Me
Si
Ph
Ph
Ph
Si
Si
OEt/OMe
OEt/OMe
OEt/OMe
88:85
89:88
78:75
86:82
88:86
75:74
Me
1f/g
1h/i
1j/k
4
Et
Si
Et
Si
Si
Et
5
R
R
Si
Si
Si
R
6
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1633–1638

1636
Y. R. Jorapur, T. Shimada
LETTER
Table 2 Symmetrical Disiloxanes Synthesis in the Presence of MRa (continued)
Entry
Alkoxysilane
Disiloxane
Product yield (%)b
Method A
Method B
90:88
Ph
Ph
Ph
Ph
Si
O
Si
Si
OEt/OMe
Ph
8
90:89
Ph
1l/m
7
R
R
R
R
R
O
R
Si
Si
R
Si
R
R
9
72:75
72:74
OEt/OMe
1n/o
8
R
R
Si OEt
R
R
R
Si
Si
10c
65
67
R
O
EtO Si
R
R
9
1p
a All reactions were carried out on a 1.0-mmol reaction scale of ethoxy-/methoxysilane 1 using K2CO3 (1.5 mmol) and Me3OBF4 (1.0 mmol,
method A) and Et3OBF4 (1.0 mmol, method B) in MeCN (5.0 mL) at 100 °C and R = allyl.
b Isolated yield.
c Reaction was carried out at 80 °C for 4 h.
All the alkoxysilanes reacted smoothly with MR to yield equiv) was run in MeCN-d3 using NMR tube with J
the corresponding symmetrical disiloxanes with moderate Young valve whereas, reaction B with 1.0 equivalent each
to excellent yields. Diallylphenylethoxysilane subse- of ethoxytrimethylsilane (1q) and Me3OBF4 in the ab-
quently reacted with Me3OBF4 and Et3OBF4 to generate sence of K2CO3 was run in MeCN-d3 (Scheme 2). The for-
disiloxane 2b in 85% and 86% yields, respectively (entry mation of dimethyl ether and ethyl methyl ether together
1). para-Substituted phenyl alkoxysilanes 1c and 1d also with hexamethyldisiloxane (10) in reaction A as well as
furnished excellent yields of novel symmetrical disilox- that of dimethyl ether and ethyl methyl ether together with
anes 2c and 2d, respectively (entries 2 and 3). Entry 4 with fluorotrimethylsilane (11) in reaction B were observed,
ethoxy(dimethylphenyl)silane (1e) afforded disiloxane 2e respectively (see the Supporting Information). On the ba-
in moderate yield. Both ethoxy- and methoxysilanes were sis of these findings, a plausible reaction mechanism for
found to react smoothly with MR giving moderate to ex- MR-assisted disiloxane synthesis is outlined in the
cellent yields of disiloxanes (entries 5–9). Interestingly, Scheme 3. In our previous reports, we proposed diethyl-
1,2-bis[ethoxydi(prop-2-enyl)silyl]ethane (1p) success- silyl oxonium and ethylmethylsilyl oxonium as intermedi-
fully furnished a new cyclic disiloxane 9 in moderate ates for TEOS polycondensations with Et3OBF4 and
yield (entry 10). In order to propose a plausible mecha- Me3OBF4, respectively.18 On the same ground and also
nism, additionally we performed two separate reactions in from our present NMR studies, we propose silyloxonium
MeCN-d3 (Scheme 2).
intermediates 1qa and 1qb.
Me3OBF4 dissociates into methyl cation which presum-
ably alkylates the ethoxy group of ethoxytrimethylsilane
1q to form ethylmethyl silyloxonium intermediate 1qa via
path I. At a later stage, disilyl ethyloxonium 1qb will be
also generated by the nucleophilic attack of 1q toward the
silicon atom of 1qa. In the absence of K2CO3 silyloxoni-
um 1qa will get attacked by naked fluoride ion to generate
fluorotrimethylsilane 11 via path II.19 In the presence of
K2CO3, the nucleophilic attack by fluoride ion to the Si
centre is inhibited, whereas the attack by ethoxy group
with additional mole of ethoxysilane 1q will be preferred.
Independent reports by Shimada,19 Sakurai,24 Olah,25 and
Charpentier26 on the detection of silyloxonium intermedi-
B
A
Me
Me3OBF4
Me3OBF4, K2CO3
Me Si OEt
MeCN-d3, 25 °C
MeCN-d3, 80 °C
Me
1q
Me
Me
Me Si
Me
Me
Me Si
Me
11
+ MeOMe + EtOMe
F
O
Si Me
Me
10
+ MeOMe + EtOMe
Scheme 2 Additional experiments
Reaction A with 1.0 equivalent each of ethoxytrimethyl-
silane (1q) and Me3OBF4 in the presence of K2CO3 (1.5
Synlett 2012, 23, 1633–1638
© Georg Thieme Verlag Stuttgart · New York

LETTER
Efficient Method for the Synthesis of Symmetrical Disiloxanes
1637
Me3O+BF4
+
–
Me3O+BF4 + MeCN
–
K2CO3 + MeCN
Me
Me Si OEt
ref. 19
Me
1q
MeOMe
MeOMe
path II
path I
MeC N+MeBF4
–
Me
Me Si
Me
Me
Et
Et
O
Me
–
MeC N+MeBF4
O
Me Si
Me
Me
1qa
1qa
no K2CO3
Me2O:BF3
X
F–
+
F–
K2CO3
Me
SiMe3
Me Si
O
EtOMe
Et
Me
1qb
EtOMe
Me
Me Si
Me
11
Me
Me Si
Me
Me
Si Me
Me
detected by NMR
F
O
10
Scheme 3 Mechanistic proposal
Me
Me
Me
Ph
Ph
Ph
O
Me3OBF4
K2CO3
Me3OBF4
X
K2CO3
Si
Si
Si
Si
Ph
OEt
OH
Me
1f
4
12
Me
Me3OBF4
(ref. 19)
Me3OBF4
K2CO3
Ph
Si
F
X
13
Scheme 4 Additional control experiments
ates supports our claim of the proposed intermediates 1qa
and 1qb.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
Two additional control experiments were run to exclude
the possibility of other reaction pathways (Scheme 4).27 References and Notes
First, subjecting silanol 12 to the optimized conditions af-
(1) (a) Perry, R. J.; Hubbard, P. A. PCT Int. Appl WO
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Chem. 2010, 695, 1911.
forded no product. Second, the reaction of fluorosilane
1319 with Me3OBF4 (1.0 equiv) and K2CO3 (1.5 equiv) un-
der identical conditions did not afford a disiloxane 4,
showing that a disiloxane is formed via silyloxonium in-
termediates as depicted in the Scheme 3.
(3) Gavillon, R.; Simonnet, J.-T.; Aubrun, O. PCT Int Appl.
2010070138, 2010; Chem. Abstr. 2010, 153, 125361.
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In conclusion, a simple, convenient, efficient and eco-
nomical method is developed to achieve various symmet-
rical disiloxanes using well-known and commercially
available MR. Furthermore, we investigated our protocol
with additional experiments to determine intermediates
and provide insight into the reaction mechanism of con-
densation of alkoxysilanes to disiloxane. This protocol
can be considered as an environmentally friendly process
due to the absence of any toxic metal catalyst or hazardous
chemicals. The current methodology is a good alternative
to prepare symmetrical disiloxanes from the correspond-
ing alkoxysilanes in excellent yields.
(6) (a) Corrie, T. I.; Peter, A. H. Chem. Soc. Rev. 2007, 36,
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1633–1638

1638
Y. R. Jorapur, T. Shimada
LETTER
(7) Kondo, S.; Fukuda, A.; Yamamura, T.; Tanaka, R.; Unno,
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symmetrical disiloxane.
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1,3-Bis(4-bromophenyl)-1,1,3,3-tetra(prop-2-enyl)-
disiloxane (2a): colorless liquid. 1H NMR (270 MHz,
CDCl3): δ = 1.87–1.93 (m, 8 H), 4.86–4.88 (m, 4 H), 4.90–
4.94 (m, 4 H), 5.64–5.80 (m, 4 H), 7.37 (d, J = 8.4 Hz, 4 H),
7.49 (d, J = 8.4 Hz, 4 H). 13C NMR (68 MHz, CDCl3): δ =
23.1, 115.3, 124.7, 130.9, 132.6, 134.8, 135.2. HRMS
(FAB+): m/z [M – C3H5]+ calcd for C21H23Br2OSi2:
504.9654; found: 504.9651.
(11) Pietraszuk, C.; Rogalski, S.; Majchrzak, M.; Marciniec, B.
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Cypryk, M.; Kurjata, J.; Kaźmierski, K. Organometallics
2005, 24, 6077.
1,3-Diphenyl-1,1,3,3-tetra(prop-2-enyl)disiloxane (2b):
colorless liquid. 1H NMR (270 MHz, CDCl3): δ = 1.88–1.92
(m, 8 H), 4.86–4.94 (m, 8 H), 5.69–5.85 (m, 4 H), 7.32–7.40
(m, 6 H), 7.52–7.56 (m, 4 H). 13C NMR (68 MHz, CDCl3):
δ = 22.3, 114.8, 127.7, 129.7, 133.2, 133.7, 136.3. HRMS
(EI+): m/z [M]+ calcd for C24H30OSi2: 390.1835; found:
390.1839.
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(19) (a) Jorapur, Y. R.; Shimada, T. Synlett 2012, 23, 1064.
(b) For detailed experimental procedure for the synthesis of
fluorosilanes using Meerwein’s reagent, see the Supporting
Information.
1,3-Bis(4-methoxyphenyl)-1,1,3,3-tetra(prop-2-enyl)-
disiloxane (2c): colorless liquid. 1H NMR (400 MHz,
CDCl3): δ = 1.85–1.87 (m, 8 H), 3.82 (s, 6 H), 4.86–4.91 (m,
8 H), 5.73–5.80 (m, 4 H), 7.89 (d, J = 6.8 Hz, 4 H), 7.46 (d,
J = 6.8 Hz, 4 H). 13C NMR (100 MHz, CDCl3): δ = 23.4,
55.0, 113.4, 114.6, 127.3, 133.4, 135.2, 160.8. HRMS (EI+):
m/z [M]+ calcd for C26H34O3Si2: 450.2046; found: 450.2049.
1,3-Bis(4-methylphenyl)-1,1,3,3-tetra(prop-2-enyl)-
disiloxane (2d): colorless liquid. 1H NMR (400 MHz,
CDCl3): δ = 1.86–1.88 (m, 8 H), 2.35 (s, 6 H), 4.86–4.92 (m,
8 H), 5.73–5.80 (m, 4 H), 7.17 (d, J = 7.2 Hz, 4 H), 7.43 (d,
J = 7.2 Hz, 4 H). 13C NMR (100 MHz, CDCl3): δ = 21.5,
23.4, 114.6, 128.5, 132.7, 133.4, 133.7, 139.5. HRMS (EI+):
m/z [M]+ calcd for C26H34OSi2: 418.2148; found: 418.2152.
(22) The role of MeCN is currently unclear. For the reports on the
formation of N-methyl- and N-ethylnitrilium ions with
MeCN in the presence of MR, see: (a) Meerwein, H.;
Laasch, P.; Mersch, R.; Spille, J. Chem. Ber. 1956, 89, 209.
(b) Gordon, J.; Turrell, G. J. Org. Chem. 1959, 24, 269.
(23) Bourget, L.; Mutin, P. H.; Vioux, A.; Frances, J. M.
J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2415.
(24) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114,
6697.
(20) Maegawa, Y.; Nagano, T.; Yabuno, T.; Nakagawa, H.;
Shimada, T. Tetrahedron 2007, 63, 11467.
(21) Typical Experimental Procedure for the Synthesis of
Symmetrical Disiloxane (Table 1, entries 2, 8 and 9):
A dry and nitrogen-flushed 10-mL screw-capped vial was
charged with alkoxysilane (1.0 mmol), MeCN (5.0 mL)
followed by the addition of K2CO3 (138.2 mg, 1.0 mmol)
and Me3OBF4 (148 mg, 1.0 mmol) or Et3OBF4 (190 mg, 1.0
mmol) or Et3OPF6 (248 mg, 1.0 mmol). The reaction
mixture was stirred at 100 °C for 90 min and quenched by
dropwise addition of H2O (1.0–2.0 mL). It was then diluted
with CH2Cl2 and the organic layer was washed with brine,
dried over anhyd MgSO4, and evaporated under reduced
pressure. The crude mixture was purified by chromatog-
(25) (a) Olah, G. A.; Li, X.-Y.; Wang, Q.; Rasul, G.; Prakash,
G. K. S. J. Am. Chem. Soc. 1995, 117, 8962. (b) Olah, G. A.;
Doggweiler, H.; Felberg, J. D.; Frohlich, S. J. Org. Chem.
1985, 50, 4847.
(26) Charpentier, P. A.; Li, X.; Sui, R. Langmuir 2009, 25, 3748.
(27) For detailed procedures, see the Supporting Information.
Synlett 2012, 23, 1633–1638
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