NHC-catalyzed dehydrogenative self-coupling of diphenylsilane: A facile synthesis of octaphenylcyclotetra(siloxane)

Article abstract of DOI:10.1016/j.tetlet.2011.09.028

A unique application of the CuIPr N-heterocyclic carbene (NHC) to the dehydrogenative self-coupling of diphenylsilane has been discovered. This transformation is carried out open to air at room temperature, yielding octaphenylcyclotetra(siloxane) quantitatively in 1 h. This preparation constitutes a significant improvement over existing methods for the preparation of this compound and demonstrates a novel mode of reactivity for CuIPr. The diphenylsilanone tetramer is the precursor to a number of industrially significant polymers.

Full text of DOI:10.1016/j.tetlet.2011.09.028

Tetrahedron Letters 52 (2011) 6130–6132
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
NHC-catalyzed dehydrogenative self-coupling of diphenylsilane: a facile
synthesis of octaphenylcyclotetra(siloxane)
⇑
Abigail Albright, Robert E. Gawley
University of Arkansas, Department of Chemistry and Biochemistry, Fayetteville, AR 72701, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A unique application of the CuIPr N-heterocyclic carbene (NHC) to the dehydrogenative self-coupling of
diphenylsilane has been discovered. This transformation is carried out open to air at room temperature,
yielding octaphenylcyclotetra(siloxane) quantitatively in 1 h. This preparation constitutes a significant
improvement over existing methods for the preparation of this compound and demonstrates a novel
mode of reactivity for CuIPr. The diphenylsilanone tetramer is the precursor to a number of industrially
significant polymers.
Received 19 August 2011
Revised 31 August 2011
Accepted 6 September 2011
Available online 12 September 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Octaphenylcyclotetra(siloxane)
Siloxane polymer
Molecular oxygen
NHC
HCuIPr
Introduction
In addition to their application in polymer chemistry, silanols
such as the one depicted in Scheme 1 are useful in their own right
Siloxane polymers can be synthesized as fluids, gels, elastomers,
and resins and have a wide variety of applications in both industry
and academia (Fig. 1).1 Straight chain poly(dimethyl-siloxane)
(PDMS) is a representative silicone polymer with a number of useful
properties including high gas permeability, a small dielectric con-
stant, a very flexible backbone, and general chemical and physiolog-
ical inertness.2 Replacement of the methyl groups of PDMS with
alternate organic substituents is of interest because it allows for
fine-tuning of the properties of the polymer.3 Octaphenylcyclotet-
ra(siloxane) or (Ph2SiO)4, is a common reagent for the introduction
of the diphenylsiloxane moiety into a variety of copolymers or it
can be polymerized independently to yield polydiphenylsiloxane
(PDPS). Recent work has shown that the incorporation of diph-
enylsiloxane groups into polyimidesiloxane (PIS) polymers provides
for significantly improved compatibility between the segments of
the copolymer and allowed for manipulation of the thermal stabili-
ties and mechanical properties.4
as reagents in cross coupling reactions.7 Strict pH control prevents
self-condensation to disiloxanes and higher order oligomers.8
Although still widely prepared by the hydrolysis of the chlorosil-
anes, recent efforts have focused on the metal-catalyzed conver-
sion of organosilanes to silanols. A number of promising systems
employing rhenium, ruthenium, iridium, and silver have been
developed,9 most of which employ water or hydrogen peroxide
as the oxygen source. Comparatively few methods employing
molecular oxygen have been reported.10
Results
The copper N,N0-bis(2,6-diisopropylphenyl)imidazol-2-ylidine
(CuIPr) NHC is an air and moisture stable catalyst that has been ap-
plied in a wide variety of reactions,11 but its application to silane
oxidation has not, to our knowledge, been reported previously.
During an investigation into the NHC–CuH mediated hydrosilyla-
tion of ketones,14 we observed the formation of a highly crystalline
Most of the previously published syntheses of octaphenylcyclo-
tetra(siloxane), 1, require lengthy reaction times and high tempera-
tures.5 These syntheses, as well as the typical industrial preparation
of 1, involve either the hydrolysis of diphenyldichlorosilanes or
diphenyldialkoxysilanes. The general reaction is depicted in Scheme
1, with the best-reported reaction conditions included.6
O
O
O
O
Si
Si
Si
Si
n
n
n
n
Me
Me
PDMS
Ph
Ph
PDPS
Ph
Me Me
Ph
Copolymer
⇑
Corresponding author. Tel.: +1 479 575 6933; fax: +1 479 575 5178.
E-mail address: bgawley@uark.edu (R.E. Gawley).
Figure 1. Representative siloxane polymers.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.028

A. Albright, R. E. Gawley / Tetrahedron Letters 52 (2011) 6130–6132
6131
Ph
Ph
Et3N
Toluene
Ph
O
Si
Ph
Ph
Ph
Si
O
O
Si
HO
OH
Si
Ph
Ph
Si
O
H2O
Toluene
Ph
1, 75% yield
Ph
Cl
Ph
Cl
Ph
Si
Scheme 1. Prior synthesis of compound 1.
byproduct when rigorous exclusion of air was neglected. Indeed,
our first observation of the byproduct occurred upon its precipita-
tion from the crude hydrosilylation reaction mixture as a diamond
shaped crystal over 2 cm in length! Upon isolation and character-
ization, we discovered it to be octaphenylcyclotetra(siloxane), 1.
Formation of the siloxane tetramer can be avoided by carrying
out hydrosilylation under inert atmosphere. Alternatively, siloxane
1 can be obtained exclusively by simply stirring diphenylsilane and
NaOt-Bu in THF containing 3 mol % CuIPr-Cl, open to air, under
which conditions diphenyl silane is completely consumed in 1h
at room temperature and siloxane 1 is produced quantitatively
(Scheme 2). The evolution of gas, presumably hydrogen, is
observed.
Figure 2. XRD structure of compound 1.
Conclusion
In summary, an extremely facile synthesis of octaphenylcyclo-
tetra(siloxane) was discovered and, perhaps more notably, an
interesting mode of reactivity for CuIPr NHC. The new synthesis
of siloxane 1 provides a significant improvement over those cur-
rently in the literature and may be adaptable to a larger scale.
The identity of siloxane 1 was established by NMR, XRD, and
combustion and melting point analysis. Very high quality colorless
cubic crystals were obtained by slow diffusion of hexanes into
dichloromethane at low temperature. The triclinic unit cell
Experimental
General
(a = 10.726 Å, b = 10.7624 Å, c = 19.125 Å,
a = 83.694°, b = 83.026°,
c
= 76.213°) was determined to have the P1 space group, and after
refinement, a final R value of 0.0402 was obtained. Details are
found in the Supplementary data. Our crystal structure is illus-
trated in Figure 2, which is in complete agreement with that previ-
ously reported.5,6
It is likely that when the CuIPr is first treated with a tert-butox-
ide base it forms the copper alkoxide, which then reacts with the
silane to give the corresponding copper hydride. The formation of
IPr copper hydride in this manner is supported by its previously re-
ported activity.12 Notably, copper (I) hydride complexes have been
reported to catalyze silicon–oxygen bond formation between orga-
nosilanes and alcohols.13
All reagents and solvents were used as received, unless indi-
cated below. Tetrahydrofuran (THF) was distilled from Na/benzo-
phenone ketyl under N2 immediately prior to use. 1H NMR
spectra were obtained at 300 or 400 MHz in CDCl3 unless other-
wise noted. Coupling constants (J), are reported in Hz. X-ray dif-
fraction was carried out on a Bruker SMART X2S. The CuIPr was
prepared according to literature methods.17
Synthesis of octaphenylcyclotetrasiloxane, 1
However, as this transformation proceeds smoothly even under
anhydrous conditions, it apparently employs dioxygen as a reac-
tant. The mechanism of the CuIPr-catalyzed formation of 1 is not
obvious; there is precedence for the coordination of both molecu-
lar oxygen15 and organosilanes16 to NHCs. It is likely that the cat-
alytic cycle yields diphenylsilanediol, which then undergoes
dehydrogenative self-coupling to yield 1.
To a scintillation vial equipped with a stir bar was added CuIPr
(14.6 mg, 0.030 mmol), sodium tert-butoxide (11.5 mg, 0.12
mmol), and THF (5 mL). The reaction vessel was left open to air.
The heterogeneous solution was stirred for 10 min before the addi-
tion of diphenyl silane (0.186 mL, 1.0 mmol). An immediate color
change to bright yellow was observed and stirring was continued
for 1 h, over which time the reaction solution darkened signifi-
cantly. The reaction solution was then treated with charcoal, run
through a plug of silica and celite, and recrystallized from dichloro-
methane and hexanes. The resultant highly crystalline white solid
was isolated by filtration (0.25 mmol, 0.197 g, 100%). 1HNMR
(CDCl3): 7.23 (16H, t, J = 7.33), 7.39 (8H, tt, J = 7.49), 7.53 (16H,
dd, J = 7.33). 13C NMR (CDCl3): 127.63 (CH), 130.04 (C), 134.40
(CH). Anal Calcd for C48H40O4Si4: C, 72.68; H, 5.08. Found: C,
72.62; H, 5.00.
Ph
N
Ph
Cu
Cl
Ph
O
Si
N
Ph
Si
O
O
+ H2
H
H
, 3 mol%
Si
Ph
Ph
Acknowledgments
Si
NaOtBu, 12 mol %
O2, THF, RT, 1 h
Si
O
Ph
Ph
We thank the Arkansas Biosciences Institute and the National
Science Foundation (CHE 1011788) for direct support of this work.
Core facilities were funded by the Arkansas Biosciences Institute
and the National Institutes of Health (P30RR 031154).
Ph
Ph
1
100% yield
Scheme 2. NHC-catalyzed oxidation of diphenylsilane.

6132
A. Albright, R. E. Gawley / Tetrahedron Letters 52 (2011) 6130–6132
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Spectroscopic data for compound 1 as well all XRD data can be
found in supporting information. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.09.028.
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