New catalytic route to borasiloxanes

Article abstract of DOI:10.1039/b801013g

A new, highly selective method for effective synthesis of boryl silyl ethers (borasiloxanes) via O-borylation of silanols with vinylboronates catalyzed by the Ru-H complexes [RuHCl(CO)(PCy₃)₂] and [RuHCl(CO)(PPh₃)₃

Full text of DOI:10.1039/b801013g

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New catalytic route to borasiloxanesw
Bogdan Marciniec* and Je˛drzej Walkowiak
Received (in Cambridge, UK) 18th January 2008, Accepted 15th April 2008
First published as an Advance Article on the web 9th May 2008
DOI: 10.1039/b801013g
A new, highly selective method for effective synthesis of boryl
silyl ethers (borasiloxanes) via O-borylation of silanols with
vinylboronates catalyzed by the Ru–H complexes [RuHCl(CO)-
(PCy3)2] and [RuHCl(CO)(PPh3)3] is described.
O-borylation of silanols, which can be useful in the synthesis
of boryl silyl ethers (borasiloxanes) as well as in borylation
strategy of inorganic and organometallic materials. Molecular
borasiloxanes have been the most useful precursors of boro-
silicate materials used in many areas.10 Conventional ap-
proaches to compounds containing B–O–Si linkages involve
condensation of boronic acids with chlorosilanes and chloro-
siloxanes11 (catalyzed by tertiary amines), alkoxysilanes12 and
silanols.13 In these processes, borasiloxanes are prepared by
azeotropic removal of water from the reaction mixture. Also
condensation of chloroborates14 and alkoxyborates15 with
silanols can lead to desired products. Some of these processes
are restricted by the necessity of removal of corrosive HCl and
H2O or by instability of the substrates towards moisture.
Here, we present a new catalytic reaction that involves
activation of the O–H bond in silanols by vinylboronates
proceeding in the presence of ruthenium hydrogen catalysts
yielding silyl boryl ethers with elimination of ethylene
(eqn (3)).
In the last two decades we have developed a new type of TM
catalyzed reaction of vinyl-substituted organosilicon com-
pounds with a variety of olefins called the silylative coupling
or trans-silylation, occurring in the presence of complexes
containing initially or generating an M–H and M–Si bond
(for a recent review see ref. 1). This reaction has become a very
valuable synthetic tool in the preparation of vinyl-functiona-
lized molecular and macromolecular organosilicon com-
pounds. However, this mode of reactivity seems to be
general and is also exhibited by vinylboronates (trans-
borylation)2a and vinylgermanes (trans-germylation).2b The
general scheme is as follows (eqn (1)).
ð1Þ
ð3Þ
The mechanistic aspects of b-boryl and b-silyl elimination
occuring in these processes based on DFT calculations have
been recently reported.3a,b
The borylation reaction was first examined in the presence of
[RuHCl(CO)(PCy3)2] (I) (1–2 mol%), which is well-known as
the most effective catalyst in metallative coupling of olefins
and acetylenes9 in an open and closed (sealed ampoules)
system in toluene, under argon, 60–130 1C (See Tables 1
and 2).
The reaction, well-recognized as a new catalytic route for
activation of QC–H bond of olefins and QC–E bond of
vinylmetalloid derivatives (where E = Si, B, Ge and others)
with ethylene evolution has been recently extended to the
activation of RC–H4,5 (for
E = Si, Ge), QCaryl–H
(E = Si)6 as well as –OH bonds of alcohols7 and silanols,8
particularly by vinylsilanes (for review see ref. 9). Those recent
experiments emphasize that vinylsilicon compounds function
as silylative agents and hydrogen acceptors.9 O-Silylation of
silanols with vinylsilanes, proceeding in the presence of ruthe-
nium complex leads to formation of unsymmetric siloxanes via
elimination of ethylene8 (eqn (2)).
Since a two- or threefold excess of vinylboronate was
necessary to achieve quantitative or very high conversion of
silanols, under such conditions, a competitive homo-coupling
of vinylboronates occurs (see eqn (4)) to give bis(boryl)ethenes
as by-products. However, this by-product can be separated by
column chromatography.
ð2Þ
ð4Þ
The aim of this work was to check the possibility of extending
the role of vinylborane by its use as a hydrogen acceptor in
All of the reactions catalysed by [RuHCl(CO)(PCy3)2] (I),
except those with triphenylsilanol as a reagent, proceeded in
very good yields and selectivity. In order to eliminate the side
reaction, another ruthenium complex [RuHCl(CO)(PPh3)3]
(II) was used, which is well-known as an inactive catalyst in
the homo-coupling of vinylboron compounds and allowed us
Department of Organometallic Chemistry, Faculty of Chemistry,
´
Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
E-mail: marcinb@amu.edu.pl; Fax: +48(61)8291508;
Tel: +48(61)8291366
w Electronic supplementary information (ESI) available: Experimental
and spectroscopic data. See DOI: 10.1039/b801013g
ꢀc
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Table 1 Borylation of silanols (HOSiR3) by vinylboronates catalyzed by [RuHCl(CO)(PCy3)2] (I)
Borane
Silanol (HOSiR3)
[Ru] : [Si] : [B]
T/1C
t/h
Silanol conversionb (%)
Selectivity A : Bb (%)
HOSi(i-Pr3)
HOSi(OSiMe3)3
HOSiMe2Ph
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 2.5
10ꢁ2 : 1 : 2.5
110
130
60
100
130
130
60
18
18
3
100
95
100
100
0
99 : 1
100 : 0
95 : 5
97 : 3
—
6
HO-SiPh3
HOSi(O-t-Bu)3
HOSiMe2(t-Bu)
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 2
18
18
6
100
100
100 : 0
99 : 1
HOSi(i-Pr3)a
HOSi(OSiMe3)3
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 3
b
130
130
100
130
100
18
18
18
18
18
88
91
96
82
91 : 9
93 : 7
91 : 9
96 : 4
99 : 1
a
HOSiMe2Pha
HOSi(O-t-Bu)3
a
HOSiMe2(t-Bu)a
100
a
Reaction carried out in a closed system. Determined by GC; A : B = borasiloxane : bis(boryl)ethene.
Table 2 Quantitative borylation of silanols by vinylboronates catalyzed by [RuHCl(CO)(PPh3)3] (II)
Borane
Silanol HOSiR3
[Ru] : [Si] : [B]
T/1C
t/h
Silanol conversionb (isolated yield) (%)
HOSi(i-Pr3)
HOSi(OSiMe3)3
HOSiMe2Ph
2 ꢂ 10ꢁ2 : 1 : 2
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 2
2 ꢂ 10ꢁ2 : 1 : 2
100
100
100
60
130
60
6
18
6
6
18
6
100 (78)
100 (69)
100 (67)
88
71
100 (72)
100 (80)
HOSi(O-t-Bu)3
HOSiMe2(t-Bu)
HOSiEt3
60
6
HOSi(i-Pr3)a
HOSi(OSiMe3)3
HOSiMe2Ph
HOSi(O-t-Bu)3
HOSiMe2(t-Bu)
HOSiEt3
2 ꢂ 10ꢁ2 : 1 : 2
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 3
2 ꢂ 10ꢁ2 : 1 : 2.5
2 ꢂ 10ꢁ2 : 1 : 2
2 ꢂ 10ꢁ2 : 1 : 2
100
130
60
130
60
6
18
6
18
6
100 (75)
68
96
73
100 (82)
100 (67)
a
a
60
6
a
b
Reaction carried out in a closed system. Determined by GC.
to obtain quantitative conversion of silanol into desired
borasiloxane (Table 2).
1H NMR examination confirmed the formation of the Ru–H
complex (triplet signal at ꢁ24.3 ppm) providing a convenient
test for the oxidative addition of the silanol into the Ru–B
bond and subsequent reductive elimination of borasiloxane.
The postulated mechanism (omitting the competitive vinyl-
boronate homo-coupling side reaction) is given in Scheme 1.
In conclusion, we have developed a new catalytic route for
effective borylation of silanols with vinylboronates in the
presence of ruthenium hydride catalysts, in which vinylboro-
nate was used as a borylating agent and hydrogen acceptor to
form a borasiloxane with evolution of ethylene.
Many of the borasiloxanes were isolated (see Table 2) and
characterized spectroscopically (1H, 13C, 11B, 29Si NMR, GC-
MS) and unisolated compounds were identified by GC-MS
spectroscopy.
On the basis of the results obtained and our previous reports
on silylation of silanols8 and trans-borylation of alkenes with
vinylboronates,2a the insertion–elimination mechanism of cat-
alysis is proposed for this process. While the insertion of
vinylboronate into a ruthenium–hydride bond was previously
reported,2a in order to provide an evidence for the second-half
of the catalytic cycle (Scheme 1) the stoichiometric reaction of
silanol with Ru–B complexes was examined.
For this purpose [Ru(BO2C6H4)Cl(CO)(PCy3)2] (III) was
prepared2a and tested in equimolar reaction in the presence of
1.5-fold excess of tert-butyldimethylsilanol. The reaction
yielded the respective borasiloxane and hydride complex
(identified by NMR spectroscopy) according to eqn (5):
Scheme 1 Proposed mechanism for O-borylation of silanols with
vinylboronates.
ð5Þ
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This work was supported by the project of The Ministry of
Science and Higher Education (Poland) PBZ-KBN 118/T09/17.
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Soraru, N. Dallabona, Ch. Gervais and F. Babonneau, Chem.
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