CL-150692
Received: July 22, 2015 | Accepted: August 10, 2015 | Web Released: August 21, 2015
Facile Synthesis of Cyclic Fluorosiloxanes
Naoki Oguri, Nobuhiro Takeda, and Masafumi Unno*
Department of Chemistry and Chemical Biology, Graduate School of Science and Technology,
Gunma University, Kiryu, Gunma 376-8515
(E-mail: unno@gunma-u.ac.jp)
Novel 1,3,5,7-tetrafluorocyclotetrasiloxanes were synthe-
cyclic halosiloxanes by column chromatography or by distil-
lation because of high boiling points. Hence, facile selective
preparations of cyclic halosiloxanes are strongly desired for
selective Si-O-Si bond formation.
Feher et al. have reported facile fluorination reaction of
incompletely-condensed siloxane bearing silanol groups with
high yields by use of HBF4 and BF3¢OEt2.11 Fluorosilanes could
also be precursors to Si-O-Si bond formation as reported by
Klingebiel and Graalmann.12
sized from cyclotetrasiloxanetetraol by a facile synthetic
method. By adjusting the amount of the fluorinating reagent,
synthesis of a single isomer of 1,3,5,7-tetrafluorocyclotetrasi-
loxanes as well as the preparation of all four isomers were
accomplished. The products are expected to serve as potential
precursors to not only well-defined silsesquioxanes but also
asymmetric cyclic siloxanes.
In this paper, we report facile syntheses of cyclic fluoro-
siloxanes by a ubiquitous fluorinating reagent, which would
be a powerful method to prepare precursors to well-defined
silsesquioxanes.
Fluorination reactions of silanols have been so far reported
with only HF13 or BF3¢OEt2.14 Cleaving Si-O-Si by HF is
possible by generating naked fluoride ion. Although Feher’s
results indicated that BF3¢OEt2 was deemed an adequate
fluorinating reagent for fluorination of silanols with siloxane
frameworks, additional application for silanols was not known
yet.
Recently, ladder and cubic silsesquioxanes have attracted
considerable interest mainly due to their high thermal stability.
During the last decade, we have reported the syntheses of ladder
oligosilsesquioxanes with high thermal stability starting from
cyclic silanol, all-cis-cyclotetrasiloxanetetraol.1 Gunji’s group
has also synthesized industry favorable methyl-substituted
ladder oligosilsesquioxanes and polysilsesquioxanes.2 Kaneko
et al. have developed a preparation method for ladder poly-
silsesquioxanes with ammonium groups, which are soluble in
water, by sol-gel method.3
In these syntheses of well-defined materials, the selection of
suitable starting material is important. Among possible starting
compounds, cyclic silanols have been shown to be most
effective. Stereoselective synthesis of all-cis-1,3,5,7-cyclotetra-
siloxanetetraol with various substituents has been reported by
many groups.4 Gunji, Abe, and co-workers have synthesized
cis-trans-cis-1,3,5,7-tetraisocyanato-1,3,5,7-tetramethylcyclotetra-
siloxane.5 Quite recently, Kaneko et al. reported the selective
preparation of cis-trans-cis cyclic tetrasiloxanes with propylam-
monium trifluoromethanesulfonate.6
Cage silsesquioxanes are also accessible from cyclic
silanols, and octasilsesquioxanes4b,7 and hexasilsesquioxanes1c
have been synthesized from all-cis-cyclotetrasiloxanetetraols.
However, syntheses of cage silsesquioxanes with different
substitutes, which are expected to show controlled macroscopic
properties,8 are still challenging. Cyclic silanols are a half part
of octasilsesquioxanes, and serve as a good starting material for
mixed-substituted cages. Suitable reaction counterparts of cyclic
silanols are cyclic halosiloxanes to afford cage silsesquioxanes
with different substituents. A decade ago, our group synthesized
and separated all four isomers of (i-PrSiClO)4, which were
reacted with water to give the cyclic silanol, however this
synthesis demanded stepwise procedures in addition to the
stereoisomer separation with HPLC.9 Cyclic bromosiloxane, cis-
trans-cis-tetrabromotetramethylcyclotetrasiloxane, has also been
prepared.10 This bromosiloxane spontaneously hydrolyzed in air
to afford ladder polysilsesquioxane. As described above, chloro-
and bromosiloxanes are unstable to moisture, and syntheti-
cally difficult to obtain. Therefore, utilization of these cyclic
halosiloxanes is not suitable for facile syntheses of well-defined
silsesquioxanes. As expected, it is also difficult to separate these
To ascertain that BF3¢OEt2 is an adequate reagent, we have
conducted a series of experiments on the fluorinations of simple
silanols with BF3¢OEt2 to give corresponding fluorosilane
(Table 1). We reacted the silanols with an equivalent amount
of BF3¢OEt2 with stirring at room temperature for 1 day. The
1
results of MS and H, 19F, and 29Si NMR spectra demonstrated
the formation of corresponding fluorosilane in each reaction,
and conversion ratios of silanols to fluorosilanes could be
1
determined by H NMR spectrum. These spectra were consisted
with previously reported data. The conversion ratio of t-
BuPh2SiOH, a silanol with bulky substituent, suggests the
difficulty of fluorination of sterically hindered silanols (Table 1,
Entry 1).
The less hindered silanols, with triphenyl or methyldiphenyl
group, were converted to corresponding fluorosilane with
quantitative conversion ratio (Table 1, Entry 2 and 3). The
series of experiments indicated that BF3¢OEt2 is assumed to
be a potent fluorinating reagent unless a silanol has bulky
substituents.
Next, we tried to fluorinate all-cis cyclic silanol. All-cis-
(PhSi(OH)O)4 and all-cis-(i-BuSi(OH)O)4 were synthesized
Table 1. Fluorination of silanols by BF3¢OEt2
BF3 • OEt2
R3Si–OH
R3Si–F
r.t., 1 d
Entry
R3
Solvent
Conversion
1
2
3
t-BuPh2
MePh2
Ph3
Et2O
THF
THF
25%
quant.
quant.
© 2015 The Chemical Society of Japan