An efficient approach to monophenyl-functionalized octasilsesquioxanes

Article abstract of DOI:10.1002/ejic.200900012

By controlling the ratio of N-bromosuccinimide (NBS) to tri-chloro(p-tolyl)silane, [4-(bromomethyl)phenyl]trichlorosilane and trichloro[4-(tribromomethyl)phenyl]sLlane were selectively prepared for the first time. Then, by treating the above two trichloro

Full text of DOI:10.1002/ejic.200900012

SHORT COMMUNICATION
DOI: 10.1002/ejic.200900012
An Efficient Approach to Monophenyl-Functionalized Octasilsesquioxanes
Hongzhi Liu,*[a] Shin-ichi Kondo,[b] Nobuhiro Takeda,[b] and Masafumi Unno*[b]
Keywords: Functionalization / Bromination / Cage silsesquioxanes / Synthesis design
By controlling the ratio of N-bromosuccinimide (NBS) to tri-
chloro(p-tolyl)silane, [4-(bromomethyl)phenyl]trichlorosilane
and trichloro[4-(tribromomethyl)phenyl]silane were selec-
tively prepared for the first time. Then, by treating the
above two trichlorosilanes with incompletely condensed
silsesquioxane, R7Si7O9(OH)3, a (monobromomethyl)phenyl-
tethered octasilsesquioxane and (tribromomethyl)phenyl-
tethered octasilsesquioxane were easily obtained. Finally,
the resulting two compounds were further hydrolyzed to
form
a (monohydroxymethyl)phenyl- and monocarboxy-
phenyl-functionalized octasilsesquioxane, respectively.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
though some monofunctionalized T8 compounds are now
available, to the best of our knowledge, monocarboxy-
phenyl-functionalized T8 has not been reported. The carb-
oxylate substituents render it a unique and novel nanosized
building block (NBB) for hybrid materials, because they
provide new linkage options for the synthesis of such mate-
rials. As a model compound, it will play a different, but
also very important role in basic and applied research, com-
parable to that of an octacarboxyl cage.[25] We report here
the first preparation of monocarboxyphenyl-functionalized
Introduction
Recently, polyhedral oligomeric silsesquioxanes (POSS)
of the general formula (RSiO3/2)n, as well as spherosilicates
(especially silyl-substituted ones), (R3SiO)8Si8O12 (Q8
M
)
(Scheme 1), have received great interest because they can be
widely used as excellent platforms for new hybrid materi-
als,[1–6] homogeneous models for silica supports,[7,8] sup-
ports for organometallic catalysts,[9–11] biocompatible drug
carriers,[12] and also as anion receptors.[13,14] The incorpora-
tion of cages into polymers produces nanocomposites with
improved properties such as glass transition temperature,
mechanical strength, thermal and chemical stability, and
ease of processing.[15–18] Early in the development of silses-
quioxane chemistry, Feher and co-workers discovered that
incompletely condensed cages with the formula
R7Si7O9(OH)3 (R = iBu, Ph, Me, cHex, cyclopentyl, etc.)
could be isolated and then capped with an organosilane
(RЈSiX3) reagent to produce a wide variety of octasilses-
quioxanes (T8) with one uniquely modified apex.[19–21] The
iBu substituent was chosen in this paper. In order to take
advantage of this class of compounds further, effective ap-
proaches to specific trichlorosilanes appear very important
in the course of preparation of monofunctionalized T8. Es-
pecially when hydrosilylation, the typical method to intro-
duce functional groups to the T8 cage, is not effective, ex-
ploring new chlorosilanes appears necessary.[22–24] Al-
Scheme 1. The structures of POSS (typically, T8) and spherosilicate.
[a] Advanced Technology Research Center (ATEC), Gunma Uni-
versity
Tenjin-cho 1-5-1, Kiryu, Gunma 376-8515, Japan
E-mail: liu-hongzhi@hotmail.com
[b] Department of Chemistry and Chemical Biology and Inter-
national Education and Research Center for Silicon Science
(Silicon Center), Graduate School of Engineering, Gunma Uni-
versity
Tenjin-cho 1-5-1, Kiryu, Gunma 376-8515, Japan
E-mail: unno@chem-bio.gunma-u.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Scheme 2. Synthesis of 2 and 3.
Eur. J. Inorg. Chem. 2009, 1317–1319
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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H. Liu, S. Kondo, N. Takeda, M. Unno
SHORT COMMUNICATION
T8 and some other T8 compounds (Scheme 2 and Scheme 3) ported.[27,28] However, this method is problematic in indus-
based on a novel synthetic concept for the selective bromi- trial use, as Cl2 is toxic and its handling is potentially haz-
nation of trichloro(p-tolyl)silane.
ardous. Here, we report a milder and more convenient way
to halogenate trichloro(p-tolyl)silane with N-bromosuc-
cinimide (NBS). NBS is a useful reagent for the bromina-
tion of carbons at allylic and benzylic positions. The bromi-
nation of tolyl derivatives produced (bromo-, dibromo-, and
tribromomethyl)phenyl derivatives, depending on the
amount of NBS used.[29,30] Presently, however, no research
deals with the preparation of brominated chloro(tolyl)-
silanes by reaction with NBS. We found trichloro(p-tolyl)-
silane could be brominated by NBS to form [4-(bromo-
methyl)phenyl]trichlorosilane and trichloro[4-(tribromo-
methyl)phenyl]silane (Scheme 2). The control of the ratio
of NBS to trichloro(p-tolyl)silane is the key factor for the
selective synthesis of these brominated compounds. It
should be pointed out that even the use of one equivalent
or less NBS produced trichloro[4-(dibromomethyl)phenyl]-
silane and 3, along with the main product 2. The addition
of excess NBS was required to prepare 3. However, tri-
chloro[4-(dibromomethyl)phenyl]silane always remains in
the final product, even after a prolonged reaction. By opti-
mizing the reaction condition and separation, we obtained
2 and 3 in satisfactory yields. The isolation of trichloro[4-
(dibromomethyl)phenyl]silane by the reaction of 1 with two
equivalents of NBS did not work, because it is difficult to
separate it from the reaction mixture.
This method is general, milder, and could be widely ap-
plied to the bromination of other chlorosilanes [for exam-
ple, chlorodimethyl(tolyl)silane]. These brominated chlo-
rosilanes will play an important role in preparing new mole-
cules and materials.
Scheme 3. Synthetic routes to functionalized T8.
We used the above two chlorosilanes (2 and 3) to react
with 4[20,31] and succeeded in obtaining the desired com-
pounds (6 and 8) in high yields (Scheme 3). The resulting
Results and Discussion
The starting point of our investigation was monotolyl- two compounds could be further hydrolyzed to form
tethered T8 5, which was expected to produce monocarb- (monohydroxymethyl)phenyl- and monocarboxyphenyl-
oxy-functionalized T8 and other derivatives.[26] Although an functionalized T8 (7 and 9). At first, we attempted to obtain
1
octacarboxyl spherosilicate was successfully prepared 7 by hydrolysis of 6 with AgNO3; however, the H NMR
through complete bromination of an octatolyl-tethered spectrum showed that only a mixture of alcohol and nitrate
spherosilicate following hydrolysis,[25] all attempts to prepare ester was obtained. It was efficient to use AgClO4 for the
monocarboxy-functionalized T8 via 5 by following a similar hydrolysis of 6 to yield 7 (Figure 1). The effectiveness of this
method failed. This failure was due to the production of a reaction should be ascribed to the greater nucleophilicity of
–
– [32]
mixture of monobromo, dibromo, and tribromo derivatives NO3 than that of ClO4 .
Target compound 9 could be
and the inability to purify the crude product, which should easily prepared by hydrolysis of 8 in the presence of AgNO3
be ascribed to the low reactivity of the tolyl group in 5 due and formic acid (Figure 2). These new T8 enrich the pool of
to steric hindrance of the large cage core. That is to say, the silsesquioxanes. The versatility and reactivity of these three
tolyl group in 5 is directly linked to the T8 cage, and it is derivatives (6, 7, and 9) make them very useful precursors
difficult to obtain the complete tribromo derivative. In con- for the preparation of new compounds. They are also very
trast, tolyl groups in spherosilicate are not linked to the cage useful in polymer preparation. For example, 4-(bro-
directly, appearing more reactive and easy to be brominated momethyl)phenyl-functionalized T8 6 will act as an initiator
completely. Thus, it also represents a challenge to synthesize in atom transfer radical polymerization (ATRP),[33,34] and
such a compound. Therefore, we need to seek a new syn- (monohydroxymethyl)phenyl-functionalized T8 7 will initi-
thetic strategy. Just as described above, specific trichloro- ate ring opening metathesis polymerization (ROMP).[35] As
silanes would help resolve this problem.
for monocarboxyphenyl-functionalized T8 9, it can form
Previously, the preparation of trichloro[4-(chloromethyl)- hybrid polyesters or polyamides by condensation polymeri-
phenyl]silane and trichloro[4-(trichloromethyl)phenyl]silane zation; in addition, this compound will produce other deri-
by halogenation of trichloro(p-tolyl)silane with Cl2 was re- vatives, which further react to form polymers.
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Eur. J. Inorg. Chem. 2009, 1317–1319

Monophenyl-Functionalized Octasilsesquioxanes
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Acknowledgments
This work was partially supported by Gunma Association for Sili-
con Science and Technology. We acknowledge the generous gift of
silicon compounds by Shin-Etsu Chemical and Momentive Per-
formance Materials.
[1] R. Tamaki, Y. Tanaka, M. Z. Asuncion, J. Choi, R. M. Laine,
Received: January 6, 2009
Published Online: March 2, 2009
J. Am. Chem. Soc. 2001, 123, 12416–12417.
Eur. J. Inorg. Chem. 2009, 1317–1319
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