A new selective approach to unsymmetrical siloxanes and germasiloxanes via O-metalation of silanols with 2-methylallylsilanes and 2-methylallylgermanes

Article abstract of DOI:10.1039/c1nj20624a

A scandium(iii) trifluoromethanesulfonate-catalyzed O-metalation of silanols with 2-methylallylsilanes and 2-methylallylgermanes leading to siloxane or germasiloxane bond formation under mild conditions with evolution of isobutylene is described.

Full text of DOI:10.1039/c1nj20624a

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LETTER
A new selective approach to unsymmetrical siloxanes and germasiloxanes
via O-metalation of silanols with 2-methylallylsilanes and
2-methylallylgermanes
´
Grzegorz Hreczycho,* Piotr Pawluc and Bogdan Marciniec
Received (in Victoria, Australia) 17th July 2011, Accepted 29th September 2011
DOI: 10.1039/c1nj20624a
A scandium(III) trifluoromethanesulfonate-catalyzed O-metalation
of silanols with 2-methylallylsilanes and 2-methylallylgermanes
leading to siloxane or germasiloxane bond formation under mild
conditions with evolution of isobutylene is described.
iodine10 or trifluoromethanesulfonic acid11 and scandium(III)
trifluoromethanesulfonate12 leading to silyl ethers as well as
the silylation of SiO–H groups of the silica surface with
2-methylallylsilanes catalyzed by scandium(III) trifluoromethane-
sulfonate.13 Nevertheless, to the best of our knowledge, no
examples of a method for the synthesis of functionalized,
unsymmetrical disiloxanes or germasiloxanes from the corres-
ponding silanols and 2-methylallylsilanes/2-methylallylgermanes
in the presence of a catalytic amount of Sc(OTf)3 have been
described to date.
The functionalized, unsymmetrical disiloxanes and germasil-
oxanes have attracted much attention as precursors for the
preparation of various functional materials, such as liquid
crystalline polymers or resins for fabrication of high refractive
index and low dielectric constant materials and components of
biocompatible oils.1
Herein, we report a new facile and selective protocol for
the synthesis of functionalized, unsymmetrical disiloxanes and
germasiloxanes that involves activation of the O–H bond
in silanols by 2-methylallylsilanes or 2-methylallylgermanes
occurring in the presence of scandium(III) trifluoromethane-
sulfonate with elimination of isobutylene (Scheme 1).
The metalation of silanols by 2-methylallylsilanes or 2-methyl-
allylgermanes with the formation of isobutylene as a neutral and
harmless by-product could be very attractive since a wide variety
of silanols is commercially available. The silyl- and germyl-
substituted olefins were easily prepared by the reaction of
(2-methylallyl)magnesium chloride with the corresponding
chlorosilane or chlorogermane.
Conventional approaches to unsymmetrical siloxanes and
germasiloxanes involve condensation of silanols with chloro-,
amino-, acyloxy- or alkoxysilanes/germanes and cohydrolysis
of two chloro- or alkoxysilanes/germanes.2 In recent years not
only stoichiometric reactions, but also catalytic methods such
as rhodium(I)-catalyzed dehydrocoupling of silanols with
hydrosilanes,3 or dealkylative coupling of hydrosilanes with
alkoxysilanes4 have been reported for disiloxane bond formation.
Unsymmetrical siloxanes were also obtained from silanols and
hydrosilanes using a phase transfer catalytic system.5 The
catalytic method for SiO–Ge bond formation based on the
B(C6F5)3-catalyzed dealkylative coupling of hydrosilanes with
alkoxygermanes or hydrogermanes with alkoxysilanes has also
been reported.6
The silylation reactions were examined in the presence of
scandium trifluoromethanesulfonate (Sc(OTf)3) known to be
active in the silylation of alcohols and SiO–H groups on the
silica surface with methylallylsilanes.9–13 Both reported reac-
tions were performed under similar conditions (catalyst loading
ꢀ0.5–5 mol%, MeCN or EtCN as a solvent, 0.5–12 h, room
temperature).
During the course of our recent studies on the catalytic
activation of O–H bonds of silanols and alcohols by vinyl-
metalloids we have developed new catalytic routes for effi-
cient O-silylation of silanols with vinylsilanes catalyzed by
[RuHCl(CO)(PCy3)2]7 and O-germylation of silanols with
vinylgermanes catalyzed by [Ru3(CO)12].8 Recently we have
focused our research on the use of 2-methylallylsilanes and
2-methylallylgermanes as metalating agents and hydrogen
acceptors for the O-metalation of silanols to form disiloxane
or germasiloxane bonds, respectively.
Our initial studies were carried out with the tris(trimethyl-
siloxy)silanol and 2-methylallyldimethyl-vinylsilane. After
several attempts we found that the O-silylation reaction of
There are several reports on the successful silylation of
alcohols with allylsilanes catalyzed by p-toluenesulfonic acid9
Department of Organometallic Chemistry, Faculty of Chemistry,
Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
E-mail: g.h@amu.edu.pl; Fax: +48 61 8291508
Scheme 1 The O-metalation of silanols by 2-methylallylsilanes-
(germanes) catalyzed by Sc(OTf)3.
c
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New J. Chem., 2011, 35, 2743–2746 2743

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tris(trimethylsiloxy)silanol (1 equiv.) with 2-methylallyldimethyl-
vinylsilane (1.2 equiv.) in the presence of 2 mol% Sc(OTf)3 in
an open system conducted in acetonitrile at room temperature
afforded exclusively 1,1,1,5,5-pentamethyl-5-vinyl-3,3-bis(trimethyl-
siloxy)-trisiloxane (1) after 1 hour (Table 1, entry 1).
However, in some cases when triethylsilanol and dimethylphenyl-
silanol were used, a silanol homo-condensation by-product
was observed (20–30%). However two- or three-fold excess of
olefin and decreasing the reaction temperature to 0 1C allowed
minimizing the formation of side products, which could be
easily separated by column chromatography. All germasil-
oxane and siloxane products were isolated and characterized
spectroscopically.
The presence of a Sc(OTf)3 catalyst is crucial for this
transformation. In the absence of a Lewis acidic catalyst, no
reaction took place. As may be expected, lowering of the
catalyst loading to 1 mol% resulted in decrease of the reaction
rate (74% conversion of tris(trimethylsiloxy)silanol after
4 hours), but not of product selectivity.
The attractiveness of this selective O-germylation and
O-silylation of silanols by allylsilanes and allylgermanes stems
from the fact that it permits synthesis of functional compounds
with hydrogen atoms or vinyl groups, which may be used as
reagents for further catalytic transformations.14
These optimal conditions were applied to other silanols
(triethylsilanol, dimethylphenylsilanol, and triisopropylsilanol),
2-methylallylsilanes (2-methyl-allyltriisopropylsilane, 2-methyl-
allyltris(trimethylsiloxy)-silane, 2-methylallyldiiso-propylsilane)
and 2-methyl-allyltriethylgermane providing the desired products
in high isolated yields (79–96%) (Scheme 1, Table 1).
As an extension to the present study we investigated the
hydrosilylation reaction of 1,1,1,5,5-pentamethyl-5-vinyl-3,3-
bis(trimethylsiloxy)trisiloxane (1) with heptamethyltrisiloxane
in the presence of a platinum Karstedt catalyst known for
its high catalytic activity for a number of hydrosilylation
reactions.15,16 The reaction was carried out under typical
Most of the reactions examined proceeded selectively in very
good yield to form the respective siloxanes or germasiloxanes.
Table 1 Reaction of silanols with 2-methylallylsilanes and 2-methylallyltriethylgermane catalyzed by Sc(OTf)3
Molar ratio
[CH2QC(CH3)CH2ER3] : [HOSiR03]
Isolated
yield (%)
Entry
1
–ER3
–SiR03
Product
–SiMe2CHQCH2
–Si(OSiMe3)3
1.2 : 1
95
2
3
4
–SiMe2CHQCH2
–SiMe2CHQCH2
–SiMe2CHQCH2
–Si(i-Pr)3
–SiEt3
1.2 : 1
3 : 1
97
83a
81a
–SiMe2Ph
3 : 1
5
6
7
–Si(OSiMe3)3
–Si(OSiMe3)3
–Si(OSiMe3)3
–Si(OSiMe3)3
–Si(i-Pr)3
–SiEt3
1.2 : 1
1.2 : 1
3 : 1
92
95
86a
8
–Si(i-Pr)3
–Si(i-Pr)3
–Si(i-Pr)3
–SiH(i-Pr)2
–GeEt3
–Si(OSiMe3)3
–Si(i-Pr)3
3 : 1
93
98
79a
90
94
96
9
1.2 : 1
1.2 : 1
1.2 : 1
1.2 : 1
1.2 : 1
10
11
12
13
–SiMe2Ph
–Si(OSiMe3)3
–Si(OSiMe3)3
–Si(i-Pr)3
–GeEt3
14
15
–GeEt3
–GeEt3
–SiEt3
2 : 1
2 : 1
a
88a
80a
–SiMe2Ph
Reaction conditions: acetonitrile (0.1 M), room temperature, catalyst loading—2 mol%. Reaction performed at 0 1C.
c
2744 New J. Chem., 2011, 35, 2743–2746
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conditions used for hydrosilylation of vinylsiloxanes (toluene,
60 1C, 4 h), and the product was isolated as a viscous oil with
excellent yield (94%).
product was purified by column chromatography on silica gel,
eluting with n-hexane to give corresponding compounds 1–14.
1,1,1,5,5-Pentamethyl-5-vinyl-3,3-bis(trimethylsiloxy)-trisiloxane
(1). 1H NMR (300 MHz, C6D6); d (ppm): 0.23 (s, 27H, OSiCH3),
0.29 (s, 6H, SiCH3), 5.81–5.90 (dd, J = 4.0, 20.2 Hz, 1H,
CHQCH2), 5.93–6.00 (dd, J = 4.0, 14.9 Hz, 1H, CHQCH2),
6.21–6.33 (dd, J = 14.9, 20.2 Hz, 1H, CHQCH2). 13C NMR
(75 MHz, C6D6): d (ppm) 0.3 (SiCH3), 1.8 (OSiCH3), 132.2
(CHQCH2), 139.2 (CHQCH2). Anal. calcd for C13H36O4Si5:
C, 39.34; H, 9.14%. Found: C, 39.49; H, 9.06%.
The trisiloxane (1) can be also functionalized via the silylative
coupling reaction. The silylation reaction was examined in the
presence of a ruthenium-hydride catalyst: RuHCl(CO)(PCy3)2—
known to be active in silylative coupling of vinylsilanes with
olefins.17 In a typical procedure, trisiloxane (1), trimethylvinyl-
silane (1 : 3 ratio) and RuHCl(CO)(PCy3)2 catalyst (2 mol%)
were dissolved in toluene (0.1 M concentration) and heated in a
Schlenk bomb flask fitted with a plug valve at 110 1C for 24 h to
yield functionalized vinylsiloxane with good yield (89%) and
high stereoselectivity (E/Z = 98/2) (Scheme 2).
1
1,1-Dimethyl-3,3,3-triisopropyl-1-vinyldisiloxane (2). H NMR
(300 MHz, C6D6): d (ppm) 0.24 (s, 6H, SiCH3), 0.94–1.03
(m, 3H, SiCH), 1.07–1.09 (m, 18H, SiCH(CH3)2), 5.73–5.79
(dd, J = 4.0, 20.2 Hz, 1H, CHQCH2), 5.89–5.94 (dd, J =
4.0, 14.9 Hz, 1H, CHQCH2), 6.18–6.27 (dd, J = 14.9, 20.2 Hz,
1H, CHQCH2). 13C NMR (75 MHz, C6D6): d (ppm) 0.7
(SiCH3), 13.1 (SiCH), 18.1 (SiCH(CH3)2), 131.7 (CHQCH2),
140.0 (CHQCH2). Anal. calcd for C13H30OSi2: C, 60.39;
H, 11.70%. Found: C, 60.50; H, 11.62%.
In conclusion, we have developed a new catalytic route
for efficient O-silylation and O-germylation of silanols with
2-methylallylsilanes and 2-methylallylgermanes, in which the
unsaturated compound acts as a metalating agent and hydrogen
acceptor to form the SiO–Si or SiO–Ge bond with evolution
of isobutylene. Mild conditions, good functional group com-
patibility and the simplicity of the experimental technique of
this new catalytic approach to functionalized siloxanes and
germasiloxanes are favorable features of the reaction. Further
application of this protocol for the synthesis of polysiloxanes is
currently under study.
1,1,1-Triethyl-3,3-dimethyl-3-vinyldisiloxane (3). 1H NMR
(300 MHz, C6D6): d (ppm) 0.24 (s, 6H, SiCH3), 0.61–0.66
(q, 6H, SiCH2), 0.97–1.03 (t, 9H, SiCH2CH3), 5.72–5.78
(dd, J = 3.9, 20.1 Hz, 1H, CHQCH2), 5.90–5.94 (dd, J =
3.9, 14.8 Hz, 1H, CHQCH2), 6.18–6.26 (dd, J = 14.8, 20.1 Hz,
1H CHQCH2). 13C NMR (75 MHz, C6D6): d (ppm) 0.7
(SiCH3), 7.5 (SiCH2), 8.8 (SiCH2CH3), 131.8 (CHQCH2),
139.8 (CHQCH2). Anal. calcd for C10H24OSi2: C, 55.48; H,
11.18%. Found: C, 55.69; H, 11.09%.
Experimental section
The reagents and Sc(OTf)3 as well as the Karstedt catalyst
used for experiments were purchased from Sigma-Aldrich
Co. and Gelest Inc. and used without further purification.
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were
recorded on a Varian XL 300 spectrometer using C6D6 as a
solvent. GC analyses were performed on a Varian 3400 with a
Megabore column (30 m) and TCD.
1,1,3,3-Tetramethyl-1-phenyl-3-vinyldisiloxane (4). 1H NMR
(300 MHz, C6D6): d (ppm) 0.29 (s, 6H, SiCH3), 0.44 (s, 6H,
SiCH3), 5.73–5.79 (dd, J = 4.0, 20.2 Hz, 1H, CHQCH2),
5.92–5.96 (dd, J = 4.0, 14.7 Hz, 1H, CHQCH2), 6.19–6.27
(dd, J = 14.7, 20.2 Hz, 1H, CHQCH2), 7.23–7.32 (m, 3H,
Ph), 7.56–7.61 (m, 2H, Ph). 13C NMR (75 MHz, C6D6):
d (ppm) 0.3, 1.8 (SiCH3), 128.1, 129.4, 130.5, 134.5 (Ph), 130.9
(CHQCH2), 139.2 (CHQCH2). Anal. calcd for C12H20OSi2: C,
60.95; H, 8.53%. Found: C, 60.89; H, 8.42%.
Synthesis of siloxanes and germasiloxanes
The structures of synthesized siloxanes and germasiloxanes 5,
11 and 14 were confirmed by GC-MS and NMR spectroscopy
matching data reported in the literature.8,18
General method. A mixture consisting of 1.5 mmol of silanol,
1.8–4.5 mmol of 2-methylallylsilanes or 2-methylallyltriethyl-
germane (according to conditions indicated in Table 1), 14.8 mg
(0.03 mmol) of Sc(OTf)3 and 15 ml of acetonitrile were placed in
a 50 ml one-necked round-bottomed flask and stirred at room
temperature (for triethylsilanol and dimethylphenylsilanol at
0 1C) for 1 h. After this time the solvent was evaporated and the
1,1,1-Trimethyl-5,5,5-triisopropyl-3,3-bis(trimethyl-siloxy)tri-
siloxane (6). 1H NMR (300 MHz, C6D6): d (ppm) 0.24 (s, 27H,
OSiCH3), 0.93–1.07 (m, 3H, SiCH), 1.15–1.19 (m, 18H,
SiCH(CH3)2). 13C NMR (75 MHz, C6D6): d (ppm) 1.8 (SiCH3),
13.1 (SiCH), 18.1 (SiCH(CH3)2). Anal. calcd for C18H48O4Si5: C,
46.10; H, 10.32%. Found: C, 46.12; H, 10.40%.
1,1,1-Triethyl-5,5,5-trimethyl-3,3-bis(trimethylsiloxy)-trisiloxane
(7). 1H NMR (300 MHz, C6D6): d (ppm) 0.25 (s, 27H, OSiCH3),
0.60–0.64 (q, 6H, SiCH2), 1.00–1.04 (m, 9H, SiCH2CH3). 13C
NMR (75 MHz, C6D6): d (ppm) 1.8 (SiCH3), 6.6 (SiCH2CH3),
7.7 (SiCH2CH3), Anal. calcd for C15H42O4Si5: C, 42.20; H,
9.92%. Found: C, 42.13; H, 9.81%.
Hexaisopropyldisiloxane (8). 1H NMR (300 MHz, C6D6):
d (ppm) 0.93–1.16 (m, 42H, SiCH, SiCH(CH3)2). 13C NMR
(75 MHz, C6D6): d (ppm) 13.8 (SiCH), 18.4 (SiCH(CH3)2).
Anal. calcd for C18H42OSi2: C, 65.37; H, 12.80%. Found:
C, 65.25; H, 12.41%.
Scheme 2 Synthetic application of 1,1,1,5,5-pentamethyl-5-vinyl-3,3-
bis(trimethylsiloxy)trisiloxane.
c
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1,1-Dimethyl-1-phenyl-3,3,3-triisopropyldisiloxane (9). 1H NMR
(300 MHz, C6D6): d (ppm) 0.39 (s, 6H, SiCH3), 0.93–1.02 (m, 3H,
SiCH), 1.06–1.10 (m, 18H, SiCH(CH3)2), 7.24–7.33 (m, 3H, Ph),
7.56–7.62 (m, 2H, Ph). 13C NMR (75 MHz, C6D6): d (ppm) 1.7
(SiCH3), 13.1 (SiCH), 18.2 (SiCH(CH3)2), 128.2, 129.4, 130.4,
134.2 (Ph). Anal. calcd for C17H32OSi2: C, 66.16; H, 10.45%.
Found: C, 66.19; H, 10.39%.
at 110 1C for 24 h. Next, after cooling the reaction mixture
to room temperature, the solvent was evaporated and the
product was purified by column chromatography on silica
gel, eluting with n-hexane to give the product in 89% yield as
1
a colorless liquid. H NMR (300 MHz, C6D6): d (ppm) 0.15
(s, 9H, SiCH3), 0.24 (s, 27H, OSiCH3), 0.34 (s, 6H, SiCH3),
6.82 (d, J = 22.6, 1H, CHQCH), 6.95 (d, J = 22.6, 1H,
CHQCH). 13C NMR (75 MHz, C6D6): d (ppm) ꢀ1.6, 1.8, 1.9
(SiCH3), 149.7, 152.0 (SiCHQ). Anal. calcd for C16H44O4Si6: C,
40.97; H, 9.46%. Found: C, 41.15; H, 9.41%.
1,1,1-Trimethyl-5-hydrido-5,5-triisopropyl-3,3-bis(tri-methyl-
1
siloxy)trisiloxane (10). H NMR (300 MHz, C6D6): d (ppm)
0.24 (s, 27H, OSiCH3), 0.96–1.06 (m, 2H, SiCH), 1.10–1.18
(m, 12H, SiCH(CH3)2), 4.56 (s, 1H, SiH). 13C NMR (75 MHz,
C6D6): d (ppm) 1.8 (OSiCH3), 13.3 (SiCH), 17.3 (SiCH(CH3)2).
Anal. calcd for C15H42O4Si5: C, 42.20; H, 9.92%. Found: C,
41.92; H, 9.80%.
Acknowledgements
This work was made possible by a grant IP2010 013070
(Iuventus Plus) from Ministry of Science and Higher Education
(Poland).
Triethyl(triisopropylsiloxy)germane (12). 1H NMR (300 MHz,
C6D6): d (ppm) 0.82–0.85 (q, 6H, GeCH2), 1.01–1.18 (m, 30H,
GeCH2CH3, SiCH and SiCH(CH3)2) 13C NMR (75 MHz,
C6D6): d (ppm) 8.2 (GeCH2CH3), 9.1 (GeCH2CH3), 13.9 (SiCH),
18.6 (SiCH(CH3)2). Anal. calcd for C15H36GeOSi: C, 54.07; H,
10.89%. Found: C, 53.95; H, 10.78%.
Notes and references
1 (a) Silicon-Containing Polymers, ed. R. G. Jones, W. Ando and
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M. J. Loboda, E. Mc Quiston and L. Rodriguez, WO,
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2 (a) M. A. Brook, Silicon in Organic, Organometallic and Polymer
Chemistry, Wiley, 2000, pp. 257–308 and 466–472; (b) P. Thornton,
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Mazzah, M. Noltemeyer and H. W. Roesky, Z. Anorg. Allg.
Chem., 1991, 604, 93; (f) H. Schmidbaur and H. Hussek,
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3 Z. M. Michalska, Transition Met. Chem. (London), 1980, 5, 125.
4 J. Chojnowski, S. Rubinsztajn, J. A. Cella, W. Fortuniak, M. Cypryk,
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6 L. Ignatovich, V. Muravenko, S. Grinberga and E. Lukevics,
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´
14 B. Marciniec, H. Maciejewski, C. Pietraszuk and P. Pawluc,
Triethyl(triethylsiloxy)germane (13). 1H NMR (300 MHz,
C6D6): d (ppm) 0.60–0.64 (q, 6H, SiCH2), 0.81–0.84 (q, 6H,
GeCH2), 1.00–1.11 (m, 18H, GeCH2CH3, and SiCH2CH3).
13C NMR (75 MHz, C6D6): d (ppm) 6.5 (SiCH2CH3), 7.6
(SiCH2CH3), 8.1 (GeCH2CH3), 8.9 (GeCH2CH3). Anal. calcd
for C12H30GeOSi: C, 49.51; H, 10.39%. Found: C, 49.45; H,
10.44%.
The hydrosilylation reaction of 1,1,1,5,5-pentamethyl-5-vinyl-3,3-
bis(trimethylsiloxy)-trisiloxane with heptamethyltrisiloxane
A mixture consisting of 0.2 g (0.5 mmol) of 1,1,1,5,5-
pentamethyl-5-vinyl-3,3-bis(trimethylsiloxy)trisiloxane, 0.12 g
(0.55 mmol) of heptamethyltrisiloxane, 5 ꢁ 10ꢀ6 mol of
platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex
and 5 ml of toluene was placed in a 20 ml Schlenk bomb flask
fitted with a plug valve and heated at 60 1C for 4 h. Next, after
cooling the reaction mixture to room temperature, the solvent
was evaporated and the product was purified by column
chromatography on silica gel, eluting with n-hexane to give
the product in 94% yield as a colorless liquid.
1H NMR (300 MHz, C6D6): d (ppm) 0.15–0.30 (m, 54H,
OSiCH3), 0.67–0.78 (m, 4H, SiCH2). 13C NMR (75 MHz,
C6D6): d (ppm) ꢀ0.5 (SiCH3), 1.8, 2.1 (SiCH3), 9.4, 9.9
(SiCH2). Anal. calcd for C20H58O6Si8: C, 38.78; H, 9.44%.
Found: C, 38.92; H, 9.39%.
´
, G. Hreczycho, A. Macina and
´
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3,3-bis(trimethylsiloxy)trisiloxane with trimethylvinylsilane
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35, 687.
17 B. Marciniec, Acc. Chem. Res., 2007, 40, 943.
18 K. Kuroda, T. Koine and C. Kato, J. Chem. Soc., Dalton Trans.,
1981, 1957.
A mixture consisting of 0.2 g (0.5 mmol) of 1,1,1,5,5-
pentamethyl-5-vinyl-3,3-bis(trimethylsiloxy)trisiloxane, 0.15 g
(1.5 mmol) of trimethylvinylsilane, 0.01 mmol of
[RuHCl(CO)(PCy3)2] and 5 ml of toluene was placed in a
20 ml Schlenk bomb flask fitted with a plug valve and heated
c
2746 New J. Chem., 2011, 35, 2743–2746
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011