Silylation of silanols with vinylsilanes catalyzed by a ruthenium complex

Article abstract of DOI:10.1016/j.tetlet.2007.12.091

A new ruthenium complex-catalyzed O-silylation of silanols with vinylsilanes leading to siloxane bond formation with the evolution of ethylene is described. A maximum conversion of silanol is reached using an excess of vinylsilane to also yield the product of the homo-coupling of the latter. Under the optimum conditions, when vinylsilane with at least one ethoxy substituent is used, the reaction gives exclusively unsymmetrical siloxanes.

Full text of DOI:10.1016/j.tetlet.2007.12.091

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1310–1313
Silylation of silanols with vinylsilanes catalyzed by a ruthenium complex
*
´
Bogdan Marciniec , Piotr Pawluc, Grzegorz Hreczycho, Anna Macina, Martyna Madalska
Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Received 26 October 2007; revised 11 December 2007; accepted 19 December 2007
Available online 11 January 2008
Abstract
A new ruthenium complex-catalyzed O-silylation of silanols with vinylsilanes leading to siloxane bond formation with the evolution of
ethylene is described. A maximum conversion of silanol is reached using an excess of vinylsilane to also yield the product of the homo-
coupling of the latter. Under the optimum conditions, when vinylsilane with at least one ethoxy substituent is used, the reaction gives
exclusively unsymmetrical siloxanes.
Ó 2007 Published by Elsevier Ltd.
The silylative coupling of olefins with vinyl-substituted
organosilicon compounds, which we have developed in
the last two decades as a new effective catalytic activation
of the @C–H bond of olefins and @C–Si bond of organo-
silicon compounds (generally occurring in the presence of
complexes containing M–H and M–Si bonds) appears to
be a valuable synthetic tool in the preparation of vinyl-
substituted organosilicon reagents and polymers.1 This
mode of catalytic reactivity has also been extended to vinyl-
silicon compounds of other sp,2 that is, @Caryl–H bonds2
and sp-hybridized carbon–hydrogen bonds3 as well as O–
H bonds of alcohols.4 The mechanism of this new general
reaction in which vinylsilicon compounds function as silyl-
ating agents and hydrogen acceptors has been proved to
involve the insertion of the vinyl–Si bond into the TM–H
bond (where TM = Ru, Rh, Ir or Co) and b-Si transfer
to the metal with the elimination of ethylene and genera-
tion of a TM–Si bond. Next migratory insertion of an
alkene (or alkyne) into the TM–Si bond and b-H transfer
to the metal eliminate the substituted silylethene (or silyl-
ethyne), respectively.5
addition of alkene6 or benzyl alcohol4 to the metal centre
followed by the insertion of vinylsilicon compounds into
a new M–H bond, forming M–Si bonds with the elimina-
tion of ethylene. The final step is the elimination of the
silylation product and regeneration of the precursor. Very
recently, this mode of vinylsilane reactivity was applied in
[RhCl(coe)2]2-catalyzed silylation of hydrogen chloride
affording a chlorosilane.7
The aim of this work is to use the new role of vinylsil-
anes as hydrogen acceptors in O-silylation of silanols,
which has a great potential in the hydrophobization strat-
egy of inorganic (and organometallic) materials as well as
in the synthesis of siloxane derivatives.
Conventional approaches to unsymmetrical siloxanes
involve condensation of silanols with chloro-, amino-, acyl-
oxy- or alkoxysilanes, co-hydrolysis of two chloro- or alk-
oxysilanes and the reaction of silanolates with halosilanes.8
Not only stoichiometric reactions, but also catalytic
methods such as rhodium(I)-catalyzed dehydrocoupling
of silanols with hydrosilanes,9 or dealkylative coupling of
hydrosilanes with alkoxysilanes10 were reported for disilox-
ane bond formation. Unsymmetrical siloxanes were also
obtained from silanols and hydrosilanes using a phase
transfer catalytic system.11 However, some of these
processes are restricted by the necessity of the removal of
corrosive HCl, H2O or explosive (H2) stoichiometric
by-products and by the instability of the silicon substrates
When rhodium and ruthenium complexes (e.g.,
RhCl(PPh3)3, Ru3(CO)12, [Rh(cod)(OSiMe3)]2) are used
as catalysts, the initiating process involves an oxidative
*
Corresponding author. Tel.: +48 618291366; fax: +48 618291508.
E-mail address: bogdan.marciniec@amu.edu.pl (B. Marciniec).
0040-4039/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2007.12.091

B. Marciniec et al. / Tetrahedron Letters 49 (2008) 1310–1313
1311
towards moisture. Besides, non-symmetric disiloxanes can-
not be produced effectively using some of these methods.
Herein, we present a new catalytic reaction that involves
activation of the O–H bond in silanols by vinylsilanes
occurring in the presence of a ruthenium–hydride cata-
lyst yielding siloxanes via the elimination of ethylene
(Scheme 1).
Since the vinylsilane has to be used in excess, the unsym-
metrical siloxanes can be accompanied by the products of
vinylsilane homo-coupling (Scheme 2).
The silylation of silanols by vinylsilanes with the forma-
tion of ethylene as a by-product could be very attractive
since the starting materials for this process are often com-
mercially available and inexpensive and ethylene can be
easily removed from the reaction mixture.
with maximum 90% conversion of the respective silanol to
yield a mixture of siloxane and 1,2-bis(dimethylphenyl-
silyl)-ethene as a result of homo-coupling of vinyldimethyl-
phenylsilane (Scheme 2), a two- or fourfold excess of
vinylsilane was necessary to achieve complete conversion
of the silanol. Although the excess of vinylsilane in the
presence of a ruthenium–hydride complex facilitates com-
petitive formation of the vinylsilane homo-coupling prod-
uct, its use also enables quantitative conversion of the
silanol to the silylation product. Moreover, the accompa-
nying bis(silyl)ethenes can be easily separated by column
chromatography or simple evaporation from the reaction
mixture (when vinyltrimethylsilane is used as a silylating
agent and volatile 1,2-bis(trimethylsilyl)ethene is formed).
Most of the reactions examined proceeded in good yield
to give unsymmetrical siloxanes accompanied by 1,2-
bis(silyl)ethene.12 However, the reaction required a longer
time (24–48 h) than the previously reported silylation of
alkenes5 or alkynes.3 While vinyltrimethylsilane and vinyl-
dimethylphenylsilane reacted efficiently in this process, the
silylation of silanols by vinyl(siloxy)silanes was less effec-
tive (Table 1, entries 12–13). It is worth noting that the pro-
cess described led to exclusive formation of unsymmetrical
siloxanes (without vinylsilane homo-coupling products),
when a vinylsilane with at least one ethoxy substituent
was used (Table 1, entries 8–10).
The silylation reactions were examined in the presence
of
a ruthenium–hydride catalyst: RuHCl(CO)(PCy3)2
(2 mol %)—known to be active in silylative coupling of
vinylsilanes with olefins and alkynes,5 in an open or closed
(sealed ampoules) system in toluene (110–120 °C), under
argon.
Since the equimolar reaction of vinyldimethyl-phenylsi-
lane with selected silanols (Table 1, entries 1–2) proceeded
[Ru-H]
R'3Si-O-SiR3
R'3SiO H
+
SiR3
-
Regardless of the substrate ratio, the silylation reaction
between
bulky
vinyltris(trimethylsiloxy)silane
and
R,R' = alkyl, aryl, siloxy, alkoxy
tris(trimethylsiloxy)silanol (Table 1 entry 14) did not occur.
Most of the siloxane products were isolated and character-
ized spectroscopically.13
Scheme 1.
On the basis of the results obtained and our previous
reports on the silylation of alkenes and alkynes by vinylsil-
anes,5 an insertion–elimination mechanism is postulated
for this process. The reversible insertion of vinylsilane into
a ruthenium–hydride bond was previously reported by
Wakatsuki et al.14 to provide evidence for one half of the
[Ru-H]
SiR3
2
SiR3
R3Si
-
R = alkyl, aryl, siloxy
Scheme 2.
Table 1
Silylation of silanols (HOSiR0 ) by vinylsilanes catalyzed by RuHCl(CO)(PCy3)2
3
Entry
Vinylsilane SiR3
Silanol HOSiR03
Molar ratio
[CH2@CHSiR3]/[HOSiR03]
Silanol
conversion (%)b
Time (h)
Yieldb
(isolated) (%)
1a
2a
3a
4a
5
6
7
8
9
10
11
12
13
14
SiMe2Ph
SiMe2Ph
SiMe2Ph
SiMe2Ph
SiMe3
SiMe3
SiMe3
Si(OEt)Me2
Si(OEt)2Me
Si(OEt)2Me
SiMe2(OSiMe3)
SiMe2(OSiMe3)
SiMe(OSiMe3)2
Si(OSiMe3)3
HOSi(OSiMe3)3
HOSi(i-Pr)3
HOSi(OSiMe3)3
HOSi(i-Pr)3
HOSi(OSiMe3)3
HOSi(i-Pr)3
HOSiMe2Ph
HOSi(OSiMe3)3
HOSi(OSiMe3)3
HOSiMe2Ph
HOSiMe2Ph
HOSi(i-Pr)3
1:1
1:1
2:1
2:1
4:1
4:1
4:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
89
90
99
24
24
24
24
24
24
24
48
48
48
48
48
48
48
89 (58)
90 (60)
99 (69)
99 (67)
99 (78)
98 (86)
99 (83)
99 (79)
99 (67)
98
99
100
100
100
100
100
100
100
49
99
49
78
0
HOSiMe2Ph
HOSi(OSiMe3)3
78
0
Reaction conditions: toluene, 110–120 °C, [Silanol]/[Catalyst] = 1:2 ꢀ 10ꢁ2 mol.
a
Reaction carried out in an open system.
Calculated by GC.
b

1312
B. Marciniec et al. / Tetrahedron Letters 49 (2008) 1310–1313
OSiMe3
O Si OSiMe3
OSiMe3
Me
OSiMe3
+
H
Ph Si O Si OSiMe3
[Ru] H
+
[Ru] SiMe2Ph
Me
OSiMe3
1
2
[Ru] = Ru(Cl)(CO)(PPh3)2
Scheme 3.
[Ru]
H
O
References and notes
R'3Si
SiR3
SiR3
1. (a) Marciniec, B.; Pietraszuk, C. In Handbook of Metathesis; Grubbs,
R., Ed.; Verlag Chemie, 2003, Chapter 2.13; (b) Marciniec, B.;
Pietraszuk, C. Curr. Org. Chem. 2003, 7, 691–735.
2. Kakiuchi, F.; Matsumoto, M.; Sonoda, M.; Fukuyama, T.; Chatani,
N.; Murai, S.; Furukawa, N.; Seki, Y. Chem. Lett. 2000, 750–
751.
H
H
H
H
[Ru]
SiR3
[Ru] OSiR'3
SiR3
H
3. Marciniec, B.; Dudziec, B.; Kownacki, I. Angew. Chem., Int. Ed. 2006,
45, 8180–8184.
4. Park, J.-W.; Chang, H.-J.; Jun, C.-H. Synlett 2006, 771–775.
5. (a) Marciniec, B. Coord. Chem. Rev. 2005, 249, 2374–2390; (b)
Marciniec, B. Acc. Chem. Res. 2007, 40, 943–952.
R'3SiO
H
[Ru] SiR3
Scheme 4. Mechanism of the O-silylation of silanols with vinylsilanes.
_
6. Marciniec, B.; Kownacki, I.; Kubicki, M.; Krzyzanowski, P.; Wal-
_
czuk, E.; Błazejewska-Chadyniak, P. In Perspectives in Organometal-
catalytic cycle. To supply evidence for the second half, the
stoichiometric reaction of silanol with a Ru–Si complex
was examined (Scheme 3).
lic Chemistry; Screttas, C. G., Steele, B. R., Eds.; RSC: Cambridge,
2003; pp 253–264.
7. Park, J.-W.; Jun, C.-H. Org. Lett. 2007, 9, 4073–4076.
8. Brook, M. A. Silicon in Organic, Organometallic and Polymer
Chemistry; Wiley, 2000; pp 257–308, 466–472.
Therefore, the reaction of equimolar amounts of the
ruthenium–silyl complex Ru(SiMe2Ph)(Cl)(CO)(PPh3)2
1
9. Michalska, Z. M. Transition Met. Chem. 1980, 5, 125–130.
10. Chojnowski, J.; Rubinsztajn, S.; Cella, J. A.; Fortuniak, W.; Cypryk,
M.; Kuriata, J.; Kazmierski, K. Organometallics 2005, 24, 6077–6084.
11. Abele, R.; Abele, E.; Fleisher, M.; Grinberga, S.; Lukevics, E. J.
Organomet. Chem. 2003, 686, 52–57.
12. Representative experimental procedure: The glass reactor was charged
under argon with dry and deoxygenated silanol (5 ꢀ 10ꢁ4 mol),
vinylsilane (5 ꢀ 10ꢁ4–2 ꢀ 10ꢁ3 mol), toluene (1 mL) and RuHCl-
(CO)(PCy3)2 (1 ꢀ 10ꢁ5 mol) under the conditions given in Table 1.
The reactor was sealed and reaction mixture was heated at 120 °C for
24–48 h. When vinyldimethyl-phenylsilane was used as the silylating
agent, the reaction occurs at 110 °C in an open system under a flow of
argon. After the silanol disappearance was confirmed by GC, the
solvent was removed, and the crude product was purified by column
chromatography (silica gel/hexane) to give the corresponding
siloxane.
13. Spectroscopic data of selected products:1,1,1,5,5-Pentamethyl-5-
phenyl-3,3-bis(trimethylsiloxy)-trisiloxane (Table 1, entry 1). 1H
NMR (300 MHz, CDCl3) d (ppm): 0.09 (s, 27H, SiCH3), 0.30 (s,
6H, SiCH3), 7.21–7.24 (m, 3H, Ph), 7.55–7.60 (m, 2H, Ph). 13C NMR
(75 MHz, CDCl3) d (ppm): 0.4 (SiCH3), 2.2 (SiCH3), 128.1 128.3,
129.7, 133.4, (Ph). MS (EI) m/z: 431 (M+ꢁ15, 50%), 369 (15), 343
(50), 327 (50), 281 (100), 266 (10), 250 (10), 250 (10), 208 (10), 135
(25), 73 (35). Anal. Calcd for C17H38O4Si5: C, 45.69; H, 8.57. Found:
C, 45.46; H, 8.75.
and tris(trimethylsiloxy)silanol was carried out at room
temperature and the reaction progress was monitored by
1H NMR and GCMS.15
Analysis of the reaction mixture by GCMS revealed the
formation of unsymmetrical siloxane 2 after 0.5 h (Scheme
3) and free triphenylphosphine as a result of decoordina-
tion of the starting ruthenium–silyl complex 1. As in the
previously reported reaction of a ruthenium–silyl complex
with silylacetylene,3 1H NMR examination confirmed the
formation of a complex containing the Ru–H bond (dt sig-
nals at ꢁ5.43 ppm). Although only trace amounts of the
1
ruthenium–hydride complex were observed by H NMR
spectroscopy in this stoichiometric test, the above experi-
ment provides convincing evidence for the oxidative addi-
tion of the silanol into the Ru–Si bond. The proposed
mechanism (omitting the competitive vinylsilane homo-
coupling process) is given in Scheme 4.
In conclusion, we have developed a new catalytic route
for efficient O-silylation of silanols with vinylsilanes, in
which vinylsilane acts as a silylating agent and hydrogen
acceptor to form a disiloxane bond with the evolution of
ethylene. Further application of this protocol to the synthe-
sis of polysiloxanes as well as hydrophobization of silica
surfaces is currently under study.
1,1,1-Triisopropyl-3,3,3-trimethyldisiloxane (Table 1, entry 6). 1H
NMR (300 MHz, CDCl3) d (ppm): 0.09 (s, 9H, SiCH3), 0.92 (m, 21H,
SiCH(CH3)2). 13C NMR (75 MHz, CDCl3,) d (ppm): 0.4 (SiCH3),
12.9 (SiCH(CH3)2), 18.2 (SiCH(CH3)2). MS (EI) m/z: 246 (M+, 2%),
231 (80), 203 (85), 189 (25), 175 (20), 161 (100), 147 (35), 133 (80), 119
(50), 103 (15), 73 (45). Anal. Calcd for C12H30OSi2: C, 58.46; H, 12.27.
Found: C, 58.71; H, 12.39.
Acknowledgement
1,1-Diethoxy-1,5,5,5-tetramethyl-3,3-bis(trimethylsiloxy)-trisiloxane
(Table 1, entry 9). 1H NMR (300 MHz, CDCl3) d (ppm): 0.11 (s,
30H, SiCH3), 1.20 (t, 6H, J = 7 Hz, OCH2CH3), 3.80 (q, 4H,
J = 7 Hz, OCH2CH3). 13C NMR (75 MHz, CDCl3) d (ppm): ꢁ5.4
(SiCH3), 1.6 (OSiCH3), 18.3 (OCH2CH3), 58.0 (OCH2CH3) MS (EI)
This work was made possible by a Grant PBZ-KBN
118/T09/17 from the Ministry of Science and Higher Edu-
cation (Poland).

B. Marciniec et al. / Tetrahedron Letters 49 (2008) 1310–1313
1313
m/z: 429 (M+ꢁ15, 25%), 399 (30), 342 (100), 325 (45), 267 (20), 73
(75). Anal. Calcd for C14H40O6Si5: C, 37.80; H, 9.06. Found: C, 37.92;
H, 9.19.
15. In an NMR tube, 0.01 g (1.2 ꢀ 10ꢁ5 mol) of Ru(SiMe2Ph)(Cl)-
(CO)(PPh3)2, dodecane (internal standard) and 0.5 mL of C6D6 were
placed under argon. Next, 3.7 mg (1.2 ꢀ 10ꢁ5 mol) of tris(trimethyl-
siloxy)silanol was added and the reaction was monitored by 1H NMR
and GCMS at room temperature.
14. Wakatsuki, Y.; Yamazaki, H.; Nakano, M.; Yamamoto, Y. J. Chem.
Soc., Chem. Commun. 1991, 703–704.