Testing the functional tolerance of the Piers-Rubinsztajn reaction: A new strategy for functional silicones

Article abstract of DOI:10.1039/c0cc00369g

The Piers-Rubinsztajn reaction, involving B(C₆F ₅)₃-catalyzed siloxane formation from hydrosilanes + alkoxysilanes, is tolerant of a wide variety of functional groups, and provides a generic strategy for the preparation of structurally complex functional silicones.

Full text of DOI:10.1039/c0cc00369g

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Testing the functional tolerance of the Piers–Rubinsztajn reaction:
a new strategy for functional siliconesw
John B. Grande, David B. Thompson,z Ferdinand Gonzaga and Michael A. Brook*
Received 13th March 2010, Accepted 24th April 2010
First published as an Advance Article on the web 28th May 2010
DOI: 10.1039/c0cc00369g
The Piers–Rubinsztajn reaction, involving B(C6F5)3-catalyzed
siloxane formation from hydrosilanes + alkoxysilanes, is
tolerant of a wide variety of functional groups, and provides a
generic strategy for the preparation of structurally complex
Fig. 2 Strategy for preparing organofunctional silicones.
functional silicones.
Based on such a mechanism, it is expected—and can be
The sensitivity of siloxane bonds to acid and base-catalyzed
inferred from Piers’ pioneering studies of the use of the
redistribution and other degradation reactions makes the
catalyst in synthetic organic transformations, such as the
synthesis of complex, well-defined silicone structures difficult.1
reduction of ketones by hydrosilylation13—that strong Lewis
Recently, we2 and others3–6 have examined the dehydrocarbo-
bases will complex effectively with B(C6F5)3, suppressing
native condensation of alkoxysilanes with hydrosilanes
reactions with weaker binding groups. We have undertaken
(R3SiH + R0OSiR00 - R3SiOSiR0 0 + R0H). The process,
3
3
a study of the functional tolerance of the Piers–Rubinsztajn
reaction to establish the synthetic limitations imposed by
organic functional groups on the process, whether present
on the hydrosilane or alkoxysilane component (Fig. 2).y
Lewis bases. The presence of amino groups in the reaction
mixture completely suppressed the Piers–Rubinsztajn reaction:
there was no change in starting materials when 3-aminopropyl-
trimethoxysilane was combined with pentamethyldisiloxane 3
(Fig. 3A). As has been shown in examples from organic
synthesis,14 sufficiently strong Lewis bases complex the boron,
essentially irreversibly, preventing the formation of 1. However,
useful reactions readily occurred with weaker Lewis bases.
Thiols cleanly react with hydrosilanes to produce both
hydrogen and S–Si bonds,15 as was confirmed by the reaction
of dodecanethiol with 3. The reaction, however, is slightly less
explicit when both thiols and alkoxysilanes are present in the
same reaction mixture, as in the case of the stoichiometric
reaction of 3-mercaptotrimethoxysilane with pentamethyl-
disiloxane (Fig. 3B). When performed in dichloromethane,
little reaction of any type appeared to occur. However, in
hexane, complete disappearance of the thiol moiety was
accompanied by partial loss of the alkoxysilane groups
(about 10%). Lowering the temperature did not enhance the
kinetic selectivity, as no reaction occurred. However, it is
possible to convert all alkoxysilanes to siloxanes when excess
hydrosilane (4.5 equivalents) is used. It should be possible to
catalyzed by B(C6F5)3—the Piers–Rubinsztajn reaction7–10
—
permits the rapid assembly of complex 3D silicone structures
with high fidelity: degradation of the silicone framework does
not normally occur.
Silicones are often conjugated with more polar species to
yield a wide range of materials of significant industrial
importance, such as surfactants, liquid crystals, foaming or
anti-foaming agents, among others.1,11 Thus, in order to be of
practical importance, the ability of the Piers–Rubinsztajn
reaction to perform in the presence of common organic
functional groups needs to be addressed.
The mechanistic sequence (Fig. 1) involving the formation
of siloxanes, with the concomitant production of alkanes, is
thought to involve the complexation of hydrosilane with the
Lewis acidic boron 1 (Fig. 1a–c). Displacement of hydride by
alkoxysilane leads to the key intermediate 2 from which both
metathetic processes (c)12 and the desired reduction occur (b).
Fig. 1 Proposed mechanism for B(C6F5)3-catalyzed siloxane formation.
Department of Chemistry and Chemical Biology, McMaster
University, 1280 Main St. W., Hamilton ON, Canada L8S 4M1.
E-mail: mabrook@mcmaster.ca; Fax: + 1 905 522 2509;
Tel: + 1 905 525 9140 ext. 23483
w Electronic supplementary information (ESI) available: Preparative
descriptions and spectroscopic data for the compounds in Table 1, and
compounds 6–8. See DOI: 10.1039/c0cc00369g
z Current address: Dr David Thompson, NSERC Postdoctoral
Fellow, LANXESS Inc. Global Butyl R&D, 999 Collip Circle,
London ON, Canada N6G 0J3; E-mail: david.thompson@lanxess.com.
Fig. 3 Lewis basic functional groups in siloxane formation.
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selectively liberate the thiol from compound 4 (n = 0) by the
facile hydrolysis of the S-silyl group.16
Epoxides are widely used as monomers, adhesion promoters
and linkers. Ganachaud et al. demonstrated that, in the
presence of water, B(C6F5)3 will catalyze epoxide ring opening
by carboxylic acids.17 Under anhydrous conditions at room
temperature, treatment of 3-(glycidoxy)propyltrimethoxy-
silane with pentamethyldisiloxane in the presence of catalytic
amounts of B(C6F5)3 led to selective reductive epoxide ring
opening (Fig. 3C). Even in the presence of excess silane,
however, there was no evidence of any reaction at the
methoxysilane groups to give siloxanes.
Fig. 5 Concise preparation of organofunctional silicones.
Haloalkanes. Chloro- and iodoalkane-containing silicones
were examined for their ability to participate in the
Piers–Rubinsztajn reaction (Fig. 4A, Table 1, entries 1–2).
For example, the reaction of phenyldimethylsilane with
chloropropyltriethoxysilane in the presence of 0.15 mol% of
B(C6F5)3 occurred rapidly after an induction time of less than
two minutes to give 5 in 93% isolated yield (Table 1, entry 1,
Fig. 5). In all the examples described, the reaction proceeds
after short induction times, and with no evidence of unwanted
side reactions. The ability to further exploit such compounds
was demonstrated using azide/alkyne click chemistry: the
chloroalkylsilicone was treated with sodium azide at 90 1C
to give 6, which in turn underwent a facile 1,3-dipolar
cycloaddition with diethyl acetylenedicarboxylate at room
temperature to give 7.18
Hydrosilylation. Transition metal-catalyzed hydrosilylation
is one of the key methods to make silicon–carbon bonds.19
B(C6F5)3 has also been reported to catalyze hydrosilylation
reactions (e.g., R3SiH + H2CQCHR0 - R3SiCH2CH2R0),
but only at catalyst concentrations that are relatively high
(B5 mol%).20 Siloxane formation from the Piers–Rubinsztajn
reaction is not diverted by hydrosilylation processes irrespective
of whether the SiH and alkenyl groups are on different
(Fig. 4B, Table 1, entries 3–6) or the same (Table 1, entries 7, 8)
silicon atoms. This provides an important synthetic opportunity
because it is possible using dehydrocarbonative condensation to
create complex alkenyl-modified silicones at exceptionally low
boron catalyst loadings that can be subsequently modified in a
platinum-catalyzed hydrosilylation reaction. With appropriate
substituents the two reactions—hydrosilylation and the Piers–
Rubinsztajn—are orthogonal, as shown in the preparation of
compound 8 (Table 1, entry 3, Fig. 6).
Fig. 4 Compatible functional groups.
Table 1 Reaction of hydrosilanes with vinyl- or allyl-functionalized
trialkoxysilanes
E R, R
R0
Ph
R0 0
X
Timea,d Yield
1
2
Me
Et Cl
Me I
120
60
93
73
Me, Phc CHQCH2
With alkenyl- and haloalkylsilane starting materials, the
Piers–Rubinsztajn process leads to complex, functional
silicones in high yield, with no apparent by-products, and
excellent fidelity. No evidence of metathesis during synthesis,
or redistribution after or during silicone formation was
observed. Complete displacement of all of the alkoxy groups
on tri- (T) and tetrafunctional (Q) silanes occurred (Table 1).
With the exception of highly hindered compounds, which have
E R, R
R0
R0 0
Y
Timea,d Yield
3
4
5
6
Me
OTMSb
Et CHQCH2
Me
Et CH2CHQCH2 180
300
180
89
90
87
94
OTMSb Me
Me
OTMS Me
OTMS
Me
300
E R, R
Y
R0 0
Timea Yield
7
8
Me OSi(CH3)2CHQCH2 Et
Me. Phc CHQCH2
Et
120
150
75
89
a
b
Typical catalyst loadings of 0.05 mol%. TMS = trimethylsilyl.
d
Three different ligands were used: R = Me, Ph. Induction time in
c
seconds.
Fig. 6 Iterative silicone assembly.
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not yet been tested, this protocol is suitable for the reaction of
mono- (M) and difunctional (D) functional alkoxysilanes.
Therefore, the assembly of complex silicone structures of
various structural complexity (MDTQz21 resins) is readily
achieved. Moreover, silicone formation using this process
can be complemented by subsequent organic reactions leading
to yet more complex silicones by iterative use of the coupling
reaction followed by hydrosilylation, nucleophilic substitution,
or click chemistry. An examination of the degree to which
these processes can be used to explicitly assemble large
molecules will be the focus of future accounts.
3 J. Chojnowski, S. Rubinsztajn, W. Fortuniak and J. Kurjata,
J. Inorg. Organomet. Polym. Mater., 2007, 17, 173–187.
4 J. Cella and S. Rubinsztajn, Macromolecules, 2008, 41, 6965–6971.
5 J. Chojnowski, S. Rubinsztajn, W. Fortuniak and J. Kurjata,
Macromolecules, 2008, 41, 7352–7358.
6 S. Rubinsztajn and J. A. Cella, Macromolecules, 2005, 38,
1061–1063.
7 D. J. Parks, J. M. Blackwell and W. E. Piers, J. Org. Chem., 2000,
65, 3090–3098.
8 S. Rubinsztajn and J. Cella, Polym. Prep., 2004, 45, 635–636.
9 S. Rubinsztajn and J. A. Cella, European Patent Application,
General Electric, WO2005118682, 2005.
10 S. Rubinsztajn and J. A. Cella, US Patent 7064173, General
Electric, USA, 2006.
11 G. E. LeGrow and L. J. Petroff, in Silicone Surfactants,
ed. R. M. Hill, Marcel Dekker, New York, 1999, pp. 49–64.
12 J. Kurjata, W. Fortuniak, S. Rubinsztajn and J. Chojnowski, Eur.
Polym. J., 2009, 45, 3372–3379.
13 W. E. Piers, Advances in Organometallic Chemistry, Elsevier
Academic Press, San Diego, 2005, pp. 1–76.
In summary, the condensation of alkoxy- and hydrosilanes
catalyzed by B(C6F5)3 efficiently occurs to give functional
siloxanes. While epoxides only underwent ring opening reduction,
and thiols compete with condensation with alkoxysilanes, reac-
tions with haloalkyl and alkenylsiloxanes led to the controlled
assembly of complex functional silicones in a few steps.
14 J. M. Blackwell, E. R. Sonmor, T. Scoccitti and W. E. Piers, Org.
Lett., 2000, 2, 3921–3923.
15 D. J. Harrison, D. R. Edwards, R. McDonald and L. Rosenberg,
Dalton Trans., 2008, 3401–3411.
Notes and references
16 A. Degl’innocenti and A. Capperucci, J. Sulfur Chem., 1998, 20,
279–395.
17 E. Pouget, E. H. Garcia and F. Ganachaud, Macromol. Rapid
Commun., 2008, 29, 425–430.
18 F. Gonzaga, G. Yu and M. A. Brook, Chem. Commun., 2009,
1730–1732.
19 J. B. G. Marceniec, W. Urbaniak and Z. W. Kornetka, Compre-
hensive Handbook on Hydrosilylation Chemistry, Pergamon,
Oxford, 1992.
y An examination of the scope of this reaction will be the subject of a
separate manuscript.
z General electric silicone nomenclature: M = Me3Si, D = Me2SiO2/2
,
T = MeSiO3/2 and Q = SiO4/2. The subscript nomenclature is used to
denote, for example with SiO4/2, that there are four single bonds to
oxygen from silicon, and that each oxygen bonds to another silicon
through a single bond, i.e., Si(OSi)4 rather than SiO2, which might
imply SiQO double bonds.
1 M. A. Brook, Silicon in Organic, Organometallic and Polymer
Chemistry, Wiley, New York, 2000, pp. 256–308.
2 D. B. Thompson and M. A. Brook, J. Am. Chem. Soc., 2008, 130,
32–33.
20 M. Rubin, T. Schwier and V. Gevorgyan, J. Org. Chem., 2002, 67,
1936–1940.
21 M. A. Brook, Silicon in Organic, Organometallic and Polymer
Chemistry, Wiley, New York, 2000, pp. 1–26.
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4990 | Chem. Commun., 2010, 46, 4988–4990