Stereoisomers of 1,3,5,7-tetrahydroxy-1,3,5,7- tetraisopropylcyclotetrasiloxane: Synthesis and structures in the crystal

Article abstract of DOI:10.1021/ja043894m

The first synthesis of all four stereoisomers of 1,3,5,7-tetrahydroxy-1,3, 5,7-tetraisopropylcyclotetrasiloxane, [i-PrSiO(OH)]₄ (all-trans-, cis-cis-trans-, cis-trans-cis-, and all-cis-1), is presented. The starting compounds, all-trans-, cis-c

Full text of DOI:10.1021/ja043894m

Published on Web 01/29/2005
Stereoisomers of
1,3,5,7-Tetrahydroxy-1,3,5,7-tetraisopropylcyclotetrasiloxane:
Synthesis and Structures in the Crystal
Masafumi Unno,* Yasuaki Kawaguchi, Yukiko Kishimoto, and Hideyuki Matsumoto*
Contribution from the Department of Nano-Material Systems, Graduate School of Engineering,
Gunma UniVersity, Kiryu, Gunma 376-8515, Japan
Received October 7, 2004; E-mail: unno@chem.gunma-u.ac.jp
Abstract: The first synthesis of all four stereoisomers of 1,3,5,7-tetrahydroxy-1,3,5,7-tetraisopropylcy-
clotetrasiloxane, [i-PrSiO(OH)]4 (all-trans-, cis-cis-trans-, cis-trans-cis-, and all-cis-1), is presented. The
starting compounds, all-trans-, cis-cis-trans-, cis-trans-cis-, and all-cis-1,3,5,7-tetraaryl-1,3,5,7-tetraisopro-
pylcyclotetrasiloxanes, were prepared by the hydrolysis of the corresponding arylisopropyldichlorosilanes,
i-PrArSiCl2 (Ar ) Ph, p-tolyl), and subsequent separation of isomers. A combination of dephenylchlorination
of tetraarylcyclotetrasiloxanes and the following hydrolysis proved to be an efficient method for the
stereospecific transformation of aryl-substituted cyclotetrasiloxanes into (i-PrSiO(OH))4. For example,
treatment of cis-trans-cis-1,3,5,7-tetraphenyl-1,3,5,7-tetraisopropylcyclotetrasilane with HCl and AlCl3,
followed by hydrolysis in the presence of pyridine, resulted in the exclusive formation of cis-trans-cis-1 in
92% yield. The structures of cis-cis-trans-1, cis-trans-cis-1, and all-cis-1 were determined by X-ray
crystallography. All isomers were found to construct unique packing structures by intermolecular hydrogen
bonding; cis-trans-cis-1 composed an infinite antiladder structure, and cis-cis-trans-1 formed a sheetlike
structure.
Introduction
best knowledge. This result is explained by the following two
facts: (1) the absence of a synthetic pathway to other isomers;
In the past decade, there was a great deal of investigation on
the synthesis and applications of polysilanols.1 Among such
polysilanols, 1,3,5,7-tetrahydroxy-1,3,5,7-tetraorganocyclotet-
rasiloxanes have shown their utility as precursors of polysil-
sesquioxanes with cage and ladder structures.2-5 The first
synthesis of cyclotetrasiloxanetetraols was reported in 1965,
when Brown and Vogt reported that 1,3,5,7-tetrahydroxy-
1,3,5,7-tetraphenylcyclotetrasiloxane and 1,3,5,7-tetrahydroxy-
1,3,5,7-tetracyclohexylcyclotetrasiloxane were obtained by the
hydrolytic condensation of trichlorophenylsilane and trichloro-
cyclohexylsilane, respectively.6 They assigned the structure of
both compounds to be all-cis isomer from the IR spectra of
their derivatives, and later, the structure of (PhSiO(OH))4 was
confirmed by Feher in 1996 by X-ray crystallography.7 These
are the only examples of cyclotetrasiloxanetetraols, and no
isomers other than the all-cis form have been reported to our
and (2) the possibility that condensation of the silanepolyols
causes difficulties in the separation and purification step.
As a part of our work related to silsesquioxane chemistry,8
we have prepared (i-PrSiO(OH))4 with a stepwise transformation
from i-PrPhSiCl2.3 Again, the obtained isomer was only all-cis
form, and we attributed this rather unusual (sterically or sta-
tistically unfavorable) selectivity to the stabilization by hydrogen
bonding at the final stage of the reaction. Incidentally, this tetraol
is very versatile as the precursors of various siloxanes; we have
reported the preparation of tricyclic laddersiloxanes,3 pentacyclic
laddersiloxanes,4 hexasilsesquioxanes,3 octasilsesquioxanes,2 and
supramolecular nanosize aggregates.5 This remarkable diversity
prompted us to prepare other isomers of cyclotetrasiloxanetet-
raols, which are also considered to be potential precursors of
various siloxanes, for example, ladder silsesquioxanes. Regard-
ing the other stereoisomers, a Russian group reported the
isomerization of all-cis-(PhSiO(OH))4 with Me3SiCl to give an
equilibrium mixture of all isomers.9,10 Other than this report,
there have been no examples thus far.
(1) (a) Lickiss, P. D. AdV. Inorg. Chem. 1995, 42, 147. (b) Lickiss, P. D. In
The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig,
Z., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3, Chapter 12. (c) Feher, F.
J.; Budzichowski, T. A. Polyhedron 1995, 14, 3239. (d) Duchateau, R.
Chem. ReV. 2002, 102, 3525. (e) The Chemistry of Organic Silicon
Compounds; Jutzi, P., Schubert, U., Eds.; Wiley-VCH: Weinheim,
Germany, 2003; Chapter III.
(2) Unno, M.; Takada, K.; Matsumoto, H. Chem. Lett. 1998, 489.
(3) Unno, M.; Suto, A.; Takada, K.; Matsumoto, H. Bull. Chem. Soc. Jpn.
2000, 73, 215.
(4) Unno, M.; Suto, A.; Matsumoto, H. J. Am. Chem. Soc. 2002, 124, 1574.
(5) Unno, M.; Takada, K.; Matsumoto, H. Chem. Lett. 2000, 242.
(6) (a) Brown, J. F., Jr.; Vogt, L. H., Jr. J. Am. Chem. Soc. 1965, 87, 4313. (b)
Brown, J. F., Jr. J. Am. Chem. Soc. 1965, 87, 4317.
(7) Feher, F. J.; Schwab, J. J.; Soulivong, D.; Ziller, J. W. Main Group Chem.
1997, 2, 123.
(8) (a) Unno, M.; Shamsul, B. A.; Saito, H.; Matsumoto, H. Organometallics
1996, 15, 2413. (b) Unno, M.; Shamsul, B. A.; Arai, M.; Takada, K.;
Tanaka, R.; Matsumoto, H. Appl. Organomet. Chem. 1999, 13, 1. (c) Unno,
M.; Imai, Y.; Matsumoto, H. Silicon Chem. 2003, 2, 175. (d) Unno, M.;
Tanaka, T.; Matsumoto, H. J. Organomet. Chem. 2003, 686, 175. (e) Unno,
M.; Matsumoto, T.; Mochizuki, K.; Higuchi, K.; Goto, M.; Matsumoto,
H. J. Organomet. Chem. 2003, 685, 156.
(9) Klement’ev, I. Yu.; Shklover, V. E.; Kulish, M. A.; Tikhonov, V. S.;
Volkova, E. V. Dokl. Akad. Nauk SSSR 1981, 259, 1371.
(10) Unfortunately, our attempts to reproduce this result have not been successful;
trials to separate the products met with the decomposition.
9
2256
J. AM. CHEM. SOC. 2005, 127, 2256-2263
10.1021/ja043894m CCC: $30.25 © 2005 American Chemical Society

Synthesis of Cyclotetrasiloxanetetraol
A R T I C L E S
Scheme 1. Strategy for the Synthesis of All Isomers of Cyclotetrasiloxanetetraols
Cyclotetrasiloxanetetraol, especially the phenyl-substituted
one, lacks enough stability to be chromatographically separated
or purified. Feher also mentioned its instability in his successful
crystallographical identification of all-cis-(PhSiO(OH))4 with
careful handling.7 In contrast, all-cis-(i-PrSiO(OH))4 was stable
in air or in basic or acidic media. Thus, we chose isopropyl
groups as substituents in this project. One expected problem
was the separation of isomers. We have shown that reverse-
phase HPLC was very effective in separating stereoisomers.4
However, this method could not be applied to alkyl-substituted
silsesquioxanes because of the lack of an appropriate detector
for these compounds.
Confronted with the fact that separating isomers of cyclotet-
rasiloxanetetraols is difficult, we devised a strategy as illustrated
in Scheme 1. By overcoming the following two synthetic
hurdles, we were able to accomplish this task. (1) The first
hydrolytic condensation gives all four isomers of (i-PrArSiO)4,
and they are separated. (2) Two consecutive reactions (dearyl-
chlorination and hydrolysis) proceed while maintaining the
stereostructures.
The stereochemistry of organosilicon compounds has been
intensively studied,11 and several reports described the stereo-
chemistry of dearylchlorination of silicon compounds. Kumada’s
group reported that dephenylchlorination of cis- and trans-1,2-
dimethyl-1,2-diphenyl-1,2-disilacyclohexane always gave a
mixture of isomers regardless of whether pure cis or trans
isomer was employed, indicating the reaction proceeded in a
nonstereospecific manner.12 They also observed that isomeric
pure 1,2-difluoro-1,2-dimethyl-1,2-disilacyclohexane underwent
epimerization slowly by the action of atmospheric moisture or
rapidly with ethanol, and suggested that this epimerization was
responsible for the nonstereospecific dearylhalogenation. The
facile racemization of (+) or (-)-naphthylphenylchlorosilane
resulting in a mixture of diastereomers in the mentholysis
was also reported.11d Additionally, racemization of halosi-
lanes was reported to be enhanced by the addition of nucleo-
philes (e.g., HMPA, DMF, or pyridine).13 Although the hy-
drolysis of acyclic chlorosilanes is recognized to proceed
invariably in inversion of configuration,14 Corriu demon-
strated that in the presence of nucleophiles, the hydrolysis
of methyl(1-naphthyl)phenylchlorosilane took place with re-
tention of configuration,15 and this was rationalized by a
recent theoretical study.16 All of these previous results imply
that the stereospecific preparation of cyclic silanols is a diffi-
cult task. Nonetheless, these reports deal with methyl-substi-
tuted silanes, and the bulkiness of the substituents has consider-
able effect on the stability against racemization. With these
considerations in mind, we adopted isopropyl groups for
substituents.
In our recent synthesis of pentacyclic laddersiloxane,4 we
succeeded in the transformation of phenyl groups to chlorine
atoms with the siloxane rings remaining intact. Although the
stereostructure of a ladder framework was maintained, stereo-
chemistry at the silicon atoms was not elucidated. In the present
paper, we employed phenyl and p-tolyl groups on the silicon
atoms, and stereospecific transformation via chlorosiloxanes to
silanols was demonstrated. As expected, the obtained cyclic
silanols were shown to compose a variety of hydrogen-bonded
aggregates.
Results and Discussion
Preparation of Aryl-Substituted Cyclotetrasiloxanes. Al-
though preparations of cyclotetrasiloxanes bearing two different
(11) For reviews, see: (a) Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In The
Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Z., Eds.;
Wiley: Chichester, U.K., 1998; Vol. 2, Chapter 9. (b) Chuit, C.; Corriu,
R. J. P.; Reye, C.; Young, J. V. Chem. ReV. 1993, 93, 1371-1448. (c)
Holmes, R. R. Chem. ReV. 1990, 90, 17. (d) Corriu, R. J. P.; Guerin, C.;
Moreau, J. J. E. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; Chapter 4.
(12) Tamao, K.; Kumada, M.; Ishikawa, M. J. Organomet. Chem. 1971, 31,
17.
(13) Bassindale, A. R.; Lau, J. C.-Y.; Taylor, P. G. J. Organomet. Chem. 1995,
490, 75.
(14) See ref 11a, p 496.
(15) Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Organomet. Chem. 1978,
154, 33.
(16) Ignatyev, I.; Schaefer, H. F., III. Organometallics 2001, 20, 3113.
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2257

A R T I C L E S
Unno et al.
Scheme 2. Preparation and Separation of the Isomers of (i-PrPhSiO)4
2
substituents have been known for years,17 only stereoisomers
of (MePhSiO)4 have been separated to date. The first syn-
thesis of this compound was reported in 1948 by Lewis,18
followed by the report in which partial separation and IR mea-
surement were performed.19 Eighteen years later, isolation of a
cis-trans-cis isomer was effected by distillation and fractional
crystallization over a period of 2 months.20 Since then, structure
spectroscopic methods. Fortunately, two of the isomers (2a and
2d) gave suitable single crystals, and the structure was assigned
to cis-trans-cis and all-cis isomer, respectively. In consequence,
all four isomers were unequivocally determined. Crystal struc-
tures are shown in Figure 2, and the result of crystallographic
data is summarized in Table 1.
The ratio of four isomers (22:57:11:10) is similar to that of
the statistical abundance (25:50:12.5:12.5), but all-cis and all-
trans isomers were less formed than in the case of (MePhSiO)4
(ratio was 20:50:13:17).17c In this reaction, cis-cis-trans isomer
2b was generated predominantly (46%), whereas cis-trans-cis
isomer 2a, which is mostly desired for the construction of
laddersiloxanes, was obtained in 18% yield. Since we could
not observe any equilibrium between isomers for 2a-d, we then
tried p-tolyl as an aryl substituent, aiming for a better yield of
the cis-trans-cis isomer. The reaction is depicted in Scheme 3;
the HPLC chart is shown in Figure 3, and all four isomers were
separated. Again, compound 3b was assigned to be cis-cis-trans
isomer from the NMR spectra, and the structures of the
remaining three isomers were determined by X-ray crystal-
lography. The result of analysis is shown in Figure 4 and Table
1. Although it was expected that the isomer ratio might not be
affected (steric environment around the silicon atoms seems to
be barely different), yields of isomers (22, 32, 11, and 8% for
3a-d, respectively) were different; we obtained cis-trans-cis
isomer with improved yield.
1
elucidation by H NMR21 and X-ray crystallographic analysis
of a cis-trans-cis isomer have been accomplished.22 However,
the molecular structures of other isomers have not yet been
determined, and only a few reactions starting from a single
stereoisomer have emerged.23 It is thus worthwhile to determine
the structures of all isomers and to transfer them into the
versatile compounds. In view of the fact that facile separation
and further transformation may demand stability, isopropyl-
substituted compounds are more favorable.
The reaction of i-PrPhSiCl4 with aqueous KOH or NaOH
proceeded smoothly to give cyclotetrasiloxane, (i-PrPhSiO)4 (2),
as a mixture of isomers (Scheme 2). The reverse-phase HPLC
chromatogram at the end of the reaction is shown in Figure 1.
This chart clearly indicates that all four isomers were generated
in the reaction. For methyl-substituted cyclic siloxanes, KOH
cleaves Si-O bonds and equilibrium was observed.18 By
repeating the reactions under conditions of various scales,
concentrations, and reaction time, we observed no difference
in the ratio of isomers. Thus, the reaction promptly came to the
final mixture, and no further equilibrium occurred. This is
another advantage for isopropyl-substituted siloxanes, showing
the stability of their frameworks. The separation of four isomers
was performed by recycle-type HPLC, and 2a (18%), 2b (46%),
2c (9%), and 2d (8%) were obtained. It is noteworthy that the
total yield (81%) is quite high, compared to the result of
(MePhSiO)4. In the latter case, a trimer (D3) and pentamer (D5)
were also obtained as well as polymeric compounds, and thus
the yield was lower.19 One of the isomers, the cis-cis-trans
isomer, could be assigned to 2b from the NMR spectra (three
different silicon atoms and substituents). However, the structures
of the remaining three isomers could not be determined by
Structure of (i-PrArSiO)4. The structural parameters for
cyclotetrasiloxane rings for 2a, 2d, 3a, 3c, and 3d are sum-
marized in Table 2. Average Si-O bond lengths and O-Si-O
bond angles are identical within errors, whereas the Si-O-Si
angle varies from 147.6 to 159.6°, showing flexibility of the
bonding. Additionally, dihedral angles of four Si atoms in the
(17) (a) Sakiyama, M.; Nishizawa, Y.; Okawara, R. Bull. Chem. Soc. Jpn. 1965,
38, 2182. (b) Pelletier, E.; Harrod, J. F. Can. J. Chem. 1983, 61, 762. (c)
Harrod, J. F.; Pelletier, E. Organometallics 1984, 3, 1064. (d) Park, M. J.;
Yim, E. S.; Lee, S. J.; Park, M. K.; Han, B. H. Main Group Met. Chem.
1999, 22, 713.
(18) Lewis, R. N. J. Am. Chem. Soc. 1948, 70, 1115.
(19) Young, C. W.; Servais, P. C.; Currie, C. C.; Hunter, J. J. Am. Chem. Soc.
1948, 70, 3758.
(20) (a) Hickton, H. J.; Holt, A.; Homer, J.; Jarvie, A. W. J. Chem. Soc. C
1966, 149. (b) Vardosanidze, Ts. N.; Andrianov, K. A.; Nogaideli, A. I.;
Yakushkina, S. E.; Rukhadze, N. P. Vysokomol. Soedin., A 1969, 11, 514.
(21) (a) Homer, J.; Jarvie, A. W.; Holt, A.; Hickton, H. J. J. Chem. Soc. B
1967, 67. (b) Williams, D. E.; Ronk, G. M.; Spielvogel, D. E. J. Organomet.
Chem. 1974, 69, 69. (c) Pelletier, E.; Harrod, J. F. Organometallics 1984,
3, 1070.
(22) (a) Pierron, E. D.; Hobbs, C. F.; Parker, D. L.; Lester, D.; Bauer, D. J. J.
Heterocycl. Chem. 1966, 3, 533. (b) Cherenkova, O. I.; Alekseev, N. V.;
Gusev, A. I. Zh. Strukt. Khim. 1975, 16, 504. (c) So¨derholm, M. Acta Chem.
Scand., Ser. B 1978, B32, 171.
(23) (a) Sommer, L. H.; Frye, C. L. J. Am. Chem. Soc. 1960, 82, 3796. (b) ref
14.
Figure 1. HPL chromatogram at the end of the reaction (ODS, MeOH).
9
2258 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

Synthesis of Cyclotetrasiloxanetetraol
A R T I C L E S
Figure 2. ORTEP drawings of compounds 2a and 2d derived from X-ray crystallographic analysis.
Table 1. Crystallographic Data for 2a, 2d, 3a, 3c, and 3d
2a
2d
3a
3c
3d
formula
C36H48O4Si4
657.11
monoclinic
P21/n
13.393(1)
18.750(2)
14.822(1)
C36H48O4Si4
657.11
monoclinic
P21/a
19.782(2)
10.279(1)
19.450(2)
C40H56O4Si4
713.22
triclinic
P1h
10.0898(2)
10.8738(2)
11.1989(2)
75.44(1)
67.62(1)
67.20(1)
1039.5(1)
1
C40H56O4Si4
713.22
triclinic
P1h
10.8849(4)
12.3267(5)
15.1563(6)
78.373(5)
81.219(5)
77.132(5)
2056.7(2)
2
C40H56O4Si4
713.22
monoclinic
Cc
10.785(1)
18.662(2)
21.071(3)
formula weight
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
94.359(3)
105.265(3)
97.383(4)
γ, deg
V, Å3
3711.4(6)
4
3815.5(7)
4
4205.7(9)
4
Z
µ, mm-1
2θ max, deg
independent reflns
observed reflns
solution
R1
0.195
64.6
0.190
64.7
0.179
64.6
5379
4410
SIR92
0.065
0.181
0.181
64.6
10664
9550
SIR97
0.041
0.112
0.177
64.3
10774
8821
SIR97
0.047
0.145
1.12
11075
9751
SIR97
0.094
0.275
0.98
10145
6957
SIR92
0.051
0.159
1.52
wR2
S
1.31
1.08
(∆/σ)max
(∆F)max, e Å-3
(∆F)min, e Å-3
No. of params
0.0010
0.48
-0.50
399
0.0020
0.98
-0.88
389
0.0000
0.73
-0.58
246
0.0010
0.38
-0.39
435
0.0000
0.38
-0.42
435
Scheme 3. Preparation and Separation of the Isomers of (i-Pr(p-Tol)SiO)4
3
eight-membered rings showed a wide range of values. For
example, cis-trans-cis-(i-Pr(p-Tol)SiO)4 (3a) adopted a planar
structure; the dihedral angle was 0°. As a result, the Si-O-Si
angles became wider in order to release the strain enforced by
the eclipsed conformation of the adjacent substituents. On the
other hand, large dihedral angles substantially decrease the
repulsion of the vicinal substituents, resulting in smaller
Si-O-Si angles, as seen in the case of all-cis 3d. Only one
crystallographic report for (RArSiO)4 cyclotetrasiloxanes has
appeared prior to our results, that is, cis-trans-cis-(MePhSiO)4.22
This compound adopts a nearly planar structure, and average
Si-O bond lengths and O-Si-O and Si-O-Si bond angles
were 1.622 Å, 109.6°, and 150.2°, respectively. Interesting to
note is that these values are very close to those of isopro-
pyl-substituted cyclotetrasiloxanes, indicating the steric conges-
tion is not severe for this system. Similar to the isomers of
laddersiloxanes,4 the melting points vary, 80-199 °C for
(i-PrPhSiO)4 and 70-127 °C for (i-Pr(p-Tol)SiO)4.
Dearylchlorination Reaction. Dephenylchlorination of oli-
gosilanes has been known for years11 and is widely adopted.
However, this reaction has not been applied to siloxanes because
HCl or HBr was known to cleave Si-O bonds.17a Quite recently,
we have shown that this reactivity is observed only for
methylsiloxanes, and we successfully transformed phenyl groups
on isopropyl-substituted cyclic siloxanes to chlorine atoms in
high yields by the action of HCl and AlCl3.4 Thus, we applied
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2259

A R T I C L E S
Unno et al.
Scheme 4. Dearylchlorination of (i-PrPhSiO)4 2 (Mixture of
Isomers)
Scheme 5. One-Step Synthesis of Cyclotetrasiloxanetetraols 1
from Four Isomers of 2 and 3
Figure 3. HPL chromatogram at the end of the reaction (ODS, MeOH).
Direct Synthesis of Cyclotetrasiloxanetetraol from
(i-PrArSiO)4. The separated isomers of 2a-d and 3a-d were
subjected to dearylchlorination followed by hydrolysis. The
reaction condition for the first step was the same as that
described above; then the ethereal solution of tetrachlorides was
treated with water. After usual workup, recrystallization of the
crude product gave pure tetraols (Scheme 5). In every reaction,
we were pleased to observe the formation of only single isomers.
All of the spectroscopic analysis clearly indicated generation
of the target tetraols. Regarding stereostructures, the assignment
was accomplished as follows. First, cis-cis-trans isomer 1b was
determined by NMR spectra as in the case of 2b and 3b. Second,
we have already reported the synthesis and structure of an all-
cis isomer,2,5 thus 1d could be assigned by the comparison of
its spectra and melting point. The structures of the remaining
two isomers, all-trans and cis-trans-cis, are only unequivocally
established by crystallographic analysis. Unfortunately, the
single crystals of the all-trans isomer could not be obtained after
repeated trials; however, the structure determination of cis-trans-
cis isomer 1a enabled us to assign all four isomers. The
stereostructure around the silicon atom was maintained perfectly
in these reactions, and thus we succeeded in obtaining all of
the isomers in pure forms.
Figure 4. ORTEP representation of compounds 3a, 3c, and 3d.
this reaction to 2 (mixture of isomers). In 30 min at 25 °C, the
starting material completely disappeared, and workup followed
by Kugelrohr distillation gave the target tetrachloride in 95%
yield (Scheme 4). The spectroscopic analysis (NMR, MS, and
IR) indicated the generation of the desired tetrachlorides. The
obtained compound is stable in this procedure; however, it
slowly decomposed with moisture in air. Since the yield is
excellent and further purification seemed unnecessary, we
decided not to separate the tetrachloride but to obtain the tetraol
in a single procedure from tetraarylcyclotetrasiloxanes.
Structure of Cyclotetrasiloxanetetraols. The molecular
structures are shown in Figure 5, and crystallographic data are
summarized in Table 3. The structural parameters (Table 2) are
all within the normal range for siloxanes. The most interesting
point of cyclotetrasiloxanetetraols is their flexible framework
and four hydroxyl groups that enable multiple connections by
Table 2. Comparison of Structural Parameters of (Si-O)4 Rings
compounds
Si
−O (Å)
Si
−O
−Si (deg)
O
−Si−O (deg)
dihedral angles (deg)
cis-trans-cis-(i-PrPhSiO)4, 2a
all-cis-(i-PrPhSiO)4, 2d
1.629
1.623
1.624
1.632
1.631
1.622
1.612
1.614
1.611
1.593
1.622
153.2
154.2
159.6
148.0
147.6
150.2
147.5
156.9
150.4
152.7
150.6
110.9
110.2
110.2
110.3
110.6
109.6
109.6
109.7
109.8
110.4
109.9
41.7, 41.5
27.4, 26.2
0
34.6, 34.6
48.5, 49.7
NA
NA
cis-trans-cis-(i-Pr(p-Tol)SiO)4, 3a
all-trans-(i-Pr(p-Tol)SiO)4, 3c
all-cis-(i-Pr(p-Tol)SiO)4, 3d
a
cis-trans-cis-(MePhSiO)4
all-cis-(Ph(OH)SiO)4
cis-trans-cis-(i-PrSi(OH)O)4, 1a
cis-cis-trans-(i-PrSi(OH)O)4, 1b
all-cis-(i-Pr(OH)SiO)4, 1db
all-cis-(i-Pr(OH)SiO)4, 1dc
0
47.4. 46.2, 45.6, 43.7
42.2, 44.3
36.3-48.7 (avg. 41.7)
a From ref 22. b Cocrystallized with water. From ref 2. c Cocrystallized with i-Pr2Si(OH)2. From ref 5.
9
2260 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

Synthesis of Cyclotetrasiloxanetetraol
A R T I C L E S
Figure 5. ORTEP structures of tetraols 1a and 1b.
Table 3. Crystallographic Data for 1a and 1b
1a
1b
formula
C12H32O8Si4
416.72
triclinic
P1h
6.5367(2)
8.6085(1)
10.8444(5)
72.974(8)
82.221(9)
72.695(8)
553.38(5)
1
C12H32O8Si4
416.72
monoclinic
P21/a
14.719(3)
20.237(2)
14.891(1)
formula weight
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
94.52(1)
77.132(5)
4421(2)
8
γ, deg
V, Å3
Z
µ, mm-1
2θ max, deg
independent reflns
observed reflns
solution
R1
wR2
S
(∆/σ)max
(∆F)max, e Å-3
(∆F)min, e Å-3
No. of params
0.299
64.1
2764
2260
SIR92
0.060
0.172
1.533
0.0000
0.72
0.300
55.0
10481
3242
SHELXS86
0.077
0.084
0.831
0.0020
0.51
Figure 7. Top and side views of the hydrogen-bonded array found in the
crystal structure of 1b (Si: green, O: red, C: gray, H: white).
-0.66
126
-0.46
498
Figure 8. Detailed scheme of the hydrogen bonding of 1a and 1b (Si:
green, O: red). Hydrogens on carbon atoms are omitted for clarity.
tubelike structure with i-Pr2Si(OH)2.5 Similarly, 1a and 1b are
proven to construct unique crystal structures. Notably, cis-trans-
cis isomer 1a possesses a planar (Si-O)4 framework, and an
infinite ladder structure was crafted in the crystal with two
hydroxyl groups side-by-side. The packing diagram shown in
Figure 6 represents its beautiful composition. Moreover, this
structure indicates the possibility of access to the all-anti ladder
polysiloxane. We are currently investigating its transformation,
including solid-state reaction. For cis-cis-trans-(i-PrSi(OH)O)4
isomer 1b, four hydroxyl groups reside in nearly the same plane
reflecting the large dihedral angles, and the molecular structures
compose sheetlike aggregates. The packing scheme is shown
in Figure 7. It is also fascinating if this could lead to the sheetlike
polysiloxane. The details of the hydrogen bonding are depicted
in Figure 8.
Figure 6. Top and side views of the hydrogen-bonded array found in the
crystal structure of 1a (Si: green, O: red, C: gray, H: white).
hydrogen bonding. As shown in Table 2, dihedral angles of 1a,
1b, and 1d are different.
We have reported the structure of all-cis isomer 1d, which
composes sphere-type aggregates with water2 and a nanosize
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2261

A R T I C L E S
Unno et al.
3024, 3015, 3003, 1591, 1464, 1429, 1385, 1364, 1246, 1161, 1123,
1084, 1061, 1028, 993, 918, 883, 854, 741, 700, 644, 623, 615, 569.
2c: Colorless plate, mp 198-199 °C; H NMR (CDCl3) δ 0.85 (d,
Conclusion
1
We have developed a simple procedure for the synthesis of
cyclic silanols and prepared the formerly inaccessible three
isomers of cyclotetrasiloxanetetraol (1). Four isomers of
(i-PrArSiO)4 (2 and 3) were obtained, and they were successfully
transferred to (i-PrSiO(OH))4 (1) while maintaining the stereo-
structure in excellent yields. The structures of cis-trans-cis- and
cis-cis-trans-(i-PrSiO(OH))4 were established unequivocally by
X-ray crystallography, and their stimulating packing structures
by hydrogen bonding were revealed.
24H, J ) 7.0 Hz), 0.94 (sept, 4H, J ) 7.0 Hz), 7.31 (t, 8H, J ) 7.0
Hz), 7.38 (t, 4H, J ) 7.0 Hz), 7.66 (d, 8H, J ) 7.0 Hz); 13C NMR
(CDCl3) δ 14.79, 16.67, 127.55, 129.70, 134.03, 134.99; 29Si NMR
(CDCl3) δ -32.56; MS (70 eV) m/z (%) 613 (M+ - i-Pr, 100); IR
(NaCl) ν 3069, 3047, 2961, 2913, 2893, 1956, 1886, 1825, 1773, 1591,
1463, 1427, 1246, 1124, 1086, 1061, 1028, 993, 887, 743, 719, 700,
1
650. 2d: Colorless plate, mp 150-151 °C; H NMR (CDCl3) δ 1.55
(d, 24H, J ) 6.8 Hz), 1.62 (sept, 4H, J ) 6.8 Hz), 7.40 (t, 8H, J ) 7.0
Hz), 7.58 (t, 4H, J ) 7.0 Hz), 7.67 (d, 8H, J ) 7.0 Hz); 13C NMR
(CDCl3) δ 14.91, 16.90, 127.15, 127.22, 129.30, 129.35, 134.02, 134.09,
134.20 (overlap); 29Si NMR (CDCl3) δ -32.59; MS (70 eV) m/z (%)
613 (M+ - i-Pr, 100); IR (NaCl) ν 3071, 3049, 3009, 2947, 2893,
2864, 1464, 1427, 1385, 1362, 1248, 1121, 1088, 1065, 1028, 997,
918, 885, 739, 721, 698, 644, 596, 563.
Experimental Section
All reagents were obtained from commercial sources and used
without additional purification. Ether and tetrahydrofuran were distilled
from sodium benzophenone ketyl under argon. Analytical HPLC was
performed in a JASCO 875UV/880PU with a Chemco 4.6 mm × 250
mm 5-ODS-H column. Preparative recycle-type HPLC was carried out
using JAI LC-908 with a Chemco 20 mm × 250 mm 7-ODS-H column.
Analytical GC was measured on a Shimadzu GC-8A with a column
(KF-96 10% on Celite 545sk). Fourier transform nuclear magnetic
resonance spectra were obtained by a JEOL Model AL-500 (1H at
500.00 MHz, 13C at 125.65 MHz, and 29Si at 99.25 MHz). Chemical
shifts were reported as δ units (parts per million) relative to SiMe4,
and residual solvents peaks were used for standard. Electron impact
mass spectrometry was performed with a JEOL JMS-DX302. Infrared
spectra were measured with a Shimadzu FTIR-8700.
Synthesis of 1,3,5,7-Tetraisopropyl-1,3,5,7-tetra(p-tolyl)cyclotet-
rasiloxane (3). A solution of i-Pr(p-Tol)SiCl2 (1.2 g, 5.1 mmol) in
THF (10 mL) was treated with aqueous NaOH solution (10 mL, 4.5
mol/L) in a similar manner (reflux, 36 h) as that described above. The
isomeric mixture of 3 was separated by recycle-type HPLC (ODS,
MeOH) to afford 3a (cis-trans-cis, 201 mg, 22%), 3b (cis-cis-trans,
290 mg, 32%), 3c (all-trans, 99 mg, 11%), and 3d (all-cis, 68 mg,
8%). 3a: Colorless solid, mp 69-70 °C; 1H NMR (CDCl3) δ 0.88 (d,
24H, J ) 1.2 Hz), 0.89 (br s, 4H), 2.37 (s, 4H), 7.16 (d, 8H, J ) 7.7
Hz), 7.56 (d, 8H, J ) 7.7 Hz); 13C NMR (CDCl3) δ 14.98, 16.71, 21.58,
128.29, 131.46, 134.28, 139.37; 29Si NMR (CDCl3) δ -32.17; MS (70
eV) m/z (%) 669 (M+ - i-Pr, 100): IR (NaCl) ν 3069, 3034, 3013,
2943, 2924, 2893, 1607, 1464, 1383, 1246, 1161, 1117, 1084, 1063,
1022, 993, 883, 800, 721, 679, 658, 583, 540. 3b: Colorless semisolid;
1H NMR (CDCl3) δ 0.80 (s, 7H), 1.10 (d, 12H, J ) 7.2 Hz), 1.12 (d,
6H, J ) 7.2 Hz), 1.18 (m, 3H), 2.36 (s, 1H), 2.38 (s, 2H), 2.45 (s, 1H),
7.08 (d, 6H, J ) 7.7 Hz), 7.31 (d, 2H, J ) 7.7 Hz), 7.48 (d, 6H, J )
7.7 Hz), 7.74 (d, 2H, J ) 7.7 Hz); 13C NMR (CDCl3) δ 14.86, 15.09,
15.10, 16.55, 16.81, 16.91, 21.49, 128.15 (overlap), 128.36, 131.17
Preparation of Dichloroisopropylphenylsilane. A THF solution
of i-PrMgCl (1.8 mol/L, 350 mL) was added dropwise to a solution of
PhSiCl3 (133.3 g, 0.63 mol) in THF (450 mL) at 0 °C for 4 h. This
solution was heated to reflux for 2 h. After filtration and removal of
THF, distillation (113 °C/20 mmHg) gave target material (115.1 g,
83%). Spectral data: 1H NMR (CDCl3) δ 1.14 (d, 6H), 1.50 (sept,
1H), 7.48 (m, 3H), 7.72 (d, 1H), 7.74 (d, 1H); 13C NMR (CDCl3) δ
16.09, 19.13, 128.24, 131.47, 133.82; 29Si NMR (99 MHz, CDCl3) δ
-21.49; IR NaCl) ν 3074, 3055, 2955, 2934, 2870, 1960, 1888, 1819,
1771, 1591, 1464, 1429, 1117, 1070, 997, 881, 741, 712, 696, 664,
621, 571, 534, 519.
(overlap), 131.68, 134.11, 134.22, 134.32, 139.13, 139.19, 139.47; 29
-
Si NMR (CDCl3) δ -32.25, -32.14, -32.09; MS (70 eV) m/z (%)
669 (M+ - i-Pr, 100); IR (NaCl) ν 3069, 3013, 2945, 2924, 2866,
1607, 1464, 1385, 1246, 1117, 1084, 1063, 1022, 993, 883, 800, 719,
679, 658, 583. 3c: Colorless solid, mp 125-127 °C; 1H NMR (CDCl3)
δ 0.86 (d, 24H, J ) 7.2 Hz), 0.93 (sept, 4H, J ) 7.2 Hz), 2.36 (s, 4H),
7.13 (d, 8H, J ) 7.6 Hz), 7.55 (d, 8H, J ) 7.6 Hz); 13C NMR (CDCl3)
δ 14.95, 16.77, 21.61, 128.29, 131.59, 134.09, 139.36; 29Si NMR
(CDCl3) δ -32.37; MS (70 eV) m/z (%) 669 (M+ - i-Pr, 100); IR
(NaCl) ν 3036, 3015, 2943, 2891, 2864, 2847, 1651, 1502, 1464, 1392,
1364, 1346, 1309, 1248, 1215, 1188, 1119, 1057, 1022, 988, 918, 883,
847, 802, 758, 717, 679, 659, 608, 584, 542. 3d: Colorless solid, mp
Synthesis of 1,3,5,7-Tetraisopropyl-1,3,5,7-tetraphenylcyclotet-
rasiloxane (2). Aqueous KOH solution (6 mL, 4.2 mol/L) was added
to a solution of i-PrPhSiCl2 (2.02 g, 9.0 mmol) in THF (6 mL). The
reaction mixture was heated to reflux until GC indicated complete
consumption of the starting dichloride (24 h). After being cooled to
ambient temperature, the reaction mixture was diluted with water and
extracted three times with benzene. The combined organic extracts were
dried over anhydrous magnesium sulfate and concentrated. The isomeric
mixture of 2 was separated by recycle-type HPLC (ODS, MeOH) to
afford 2a (cis-trans-cis, 269 mg, 18%), 2b (cis-cis-trans, 683 mg, 46%),
2c (all-trans, 141 mg, 9%), and 2d (all-cis, 125 mg, 8%). 2a: Colorless
1
118-119 °C; H NMR (CDCl3) δ 1.10 (d, 24H, J ) 6.5 Hz), 1.13
(sept, 4H, J ) 6.5 Hz), 2.24 (s, 12H), 6.83 (d, 8H, J ) 7.6 Hz), 7.15
(d, 8H, J ) 7.6 Hz); 13C NMR (CDCl3) δ 15.07, 16.93, 21.49, 127.91,
130.82, 134.14, 138.85; 29Si NMR (CDCl3) δ -32.31; MS (70 eV)
m/z (%) 669 (M+ - i-Pr, 100); IR (NaCl) ν 3069, 3034, 3013, 2945,
2923, 2893, 2864, 1607, 1464, 1392, 1383, 1259, 1244, 1161, 1119,
1082, 1061, 1022, 989, 885, 800, 719, 692, 675, 656, 606, 586, 548,
505.
1
plate, mp 122-124 °C; H NMR (CDCl3) δ 1.02 (s, 28H), 7.44 (t,
12H, J ) 6.7 Hz), 7.49 (d, 8H, J ) 7.3 Hz), 7.73 (dd, 8H, J ) 6.7, 7.3
Hz); 13C NMR (CDCl3) δ 14.95, 16.65, 127.55, 129.73, 134.20, 134.86;
29Si NMR (CDCl3) δ -32.21; MS (70 eV) m/z (%) 613 (M+ - i-Pr,
100); IR (NaCl) ν 3071, 3051, 3003, 2943, 2893, 1591, 1464, 1429,
1382, 1364, 1246, 1161, 1123, 1082, 1061, 1028, 993, 918, 883, 854,
741, 721, 700, 644, 625, 615, 569. 2b: Colorless plate, mp 79-80 °C;
1H NMR (CDCl3) δ 0.76 (d, 6H, J ) 5.2 Hz), 0.79 (sept, 1H, J ) 5.2
Hz), 1.08 (d, 6H, J ) 7.7 Hz), 1.09 (d, 6H, J ) 7.7 Hz), 1.11 (d, 6H,
J ) 7.7 Hz), 1.20 (sept, 3H, J ) 7.7 Hz), 7.16 (t, 2H, J ) 7.1 Hz),
7.19 (t, 4H, J ) 6.8 Hz), 7.26 (t, 1H, J ) 6.7 Hz), 7.29 (t, 2H, J ) 7.1
Hz), 7.45 (m, 3H), 7.57 (d, 2H, J ) 6.7 Hz), 7.59 (d, 4H, J ) 6.8 Hz),
7.86 (m, 2H); 13C NMR (CDCl3) δ 14.81, 15.03 (overlap), 16.48, 16.78,
16.89, 127.38, 127.46, 127.63, 129.61 (overlap), 129.82, 134.04, 134.13,
134.25, 134.54, 134.57, 135.12; 29Si NMR (CDCl3) δ -32.25, -32.11;
MS (70 eV) m/z (%) 613 (M+ - i-Pr, 100); IR (NaCl) ν 3071, 3051,
Synthesis of 1,3,5,7-Tetrachloro-1,3,5,7-tetraisopropylcyclotet-
rasiloxane (mixture of isomers). To a solution of (i-PrPhSiO)4 (5.1
g, 7.8 mmol, mixture of isomers) and anhydrous AlCl3 (2.1 g, 15.9
mmol) in benzene (90 mL) was passed HCl gas for 30 min. Acetone
was added to the solution to quench aluminum chloride, and argon gas
was bubbled. After filtration and removal of THF, Kugelrohr distillation
gave 3.6 g (95%) of target tetrachloride. (i-PrClSiO)4: Colorless liquid,
bp 130 °C/2 mmHg; MS (70 eV) m/z (%) 443 (M+ - i-Pr, 100); IR
(NaCl) ν 2955, 2874, 1466, 1389, 1369, 1256, 1103, 1067, 1001, 924,
885, 681, 648, 509.
9
2262 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

Synthesis of Cyclotetrasiloxanetetraol
A R T I C L E S
1
Synthesis of 1,3,5,7-Tetrahydroxy-1,3,5,7-tetraisopropylcyclotet-
rasiloxane from Tetrachloride (mixture of isomers). Water (0.5 g,
29.5 mmol) was added dropwise to (i-PrClSiO)4 (mixture of isomers,
3.6 g, 7.4 mmol) and aniline (2.7 g, 29.5 mmol) in 50 mL of ethyl
ether at 0 °C for 1 h. The solution was stirred at 0 °C for an additional
1 h. Water and ethyl ether were added to the solution, and the aqueous
phase was extracted three times with ethyl ether. The combined organic
phase was dried over anhydrous magnesium sulfate and evaporated.
Recrystallized from ethyl ether gave (i-PrSi(OH)O)4 (2.6 g, 84%).
General Procedure for the Synthesis of Tetraol 1a-d. To a
solution of (i-PrPhSiO)4 (2) or (i-Pr(p-Tol)SiO)4 (3) and AlCl3 (2.0
equiv) in benzene was passed HCl gas for 30 min. Acetone was added
to the solution to quench AlCl3, and argon gas was bubbled. After
filtration, water (4.1 equiv) was added dropwise to the solution for 5
min at 0 °C, then the mixture was stirred for an additional 30 min.
Water and ethyl ether were added to the solution, and the aqueous phase
was extracted three times with ethyl ether. The combined organic phase
was dried over anhydrous magnesium sulfate and concentrated.
Recrystallization from ethyl ether gave pure tetraol 1.
160-161 °C; H NMR (DMSO-d6) δ 0.70 (sept, 4H, J ) 7.3 Hz),
0.94 (d, 24H, J ) 7.3 Hz), 6.24 (s, 4H); 13C NMR (DMSO-d6) δ 12.24,
17.17; 29Si NMR (DMSO-d6) δ -58.46; MS (70 eV) m/z (%) 373 (M+
- i-Pr, 10), 355 (100); IR (NaCl) ν 3288, 2949, 2870, 1468, 1389,
1367, 1259, 1067, 1001, 876, 727, 652, 604. Anal. Calcd for
C12H32O8Si4: C, 34.59; H, 7.74. Found: C, 34.64; H, 7.80.
Synthesis of All-cis-1,3,5,7-tetrahydroxy-1,3,5,7-tetraisopropyl-
cyclotetrasiloxane (1d). Compound 1d was prepared by following the
general procedure employing all-cis-(i-PrPhSiO)4 (2d, 99 mg, 0.15
mmol) and AlCl3 (40 mg, 0.30 mmol) in 5 mL of benzene. Recrystal-
lized from ethyl ether gave all-cis tetraol, 1d (58 mg, 93%). The reaction
from all-cis-(i-Pr(p-Tol)SiO)4 (3d, 68 mg, 0.10 mmol) with AlCl3 (26
mg, 0.20 mmol) in 5 mL of benzene proceeded similarly to give 1d
(36 mg, 90%). This compound was identified by the comparison with
the authentic sample.2,5
X-ray Structure Analysis (General Procedure). Single crystals
were coated and mounted on a glass fiber using epoxy adhesive and
were cooled by a Rigaku nitrogen-flow low-temperature apparatus. The
measurement was performed on a Rigaku RAXIS-IV imaging plate
diffractometer (1b was measured with Rigaku AFC-7S) using graphite
monochromated Mo KR radiation. Indexing was performed from four
stills that were exposed for 60 s. A sweep of data was done using φ
oscillations in 0.5° steps. The exposure rate was 180.0 s/deg. The
detector swing angle was -0.15°. The crystal-to-detector distance was
99.89 mm. Readout was performed in the 0.100 mm pixel mode. The
structure was solved by direct methods24 and expanded using Fourier
techniques. The non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were refined isotropically. All calculations were
performed using the CrystalStructure crystallographic software package,
except for refinement, which was performed using SHELXL-97.25
Synthesis of cis-trans-cis-1,3,5,7-Tetrahydroxy-1,3,5,7-tetraiso-
propylcyclotetrasiloxane (1a). Compound 1a was prepared by fol-
lowing the general procedure employing cis-trans-cis-(i-PrPhSiO)4 (2a,
101 mg, 0.15 mmol) and AlCl3 (290 mg, 2.2 mmol) in 6 mL of benzene.
Recrystallization from hexane gave 1a (59.0 mg, 92%) as a colorless
1
solid: mp 186-187 °C; H NMR (DMSO-d6) δ 0.72 (sep, 4H, J )
6.0 Hz), 0.95 (d, 24H, J ) 6.0 Hz), 6.23 (s, 4H); 13C NMR (DMSO-
d6) δ 12.50, 17.38; 29Si NMR (DMSO-d6) δ -58.97; MS (70 eV) m/z
(%) 373 (M+ - i-Pr, 6), 355 (100); IR (NaCl) ν 3275, 2949, 2870,
1628, 1466, 1387, 1367, 1258, 1109, 1067, 999, 872, 893, 731, 651,
619. Anal. Calcd for C12H32O8Si4: C, 34.59; H, 7.74. Found: C, 34.59;
H, 7.76.
Acknowledgment. Financial support for M.U. by a grant from
the Ministry of Education, Culture, Sports, Science and Tech-
nology of Japan (Grants-in-Aid Nos. 12640510 and 15036213:
Reaction Control of Dynamic Complexes) is gratefully ac-
knowledged.
Synthesis of cis-cis-trans-1,3,5,7-Tetrahydroxy-1,3,5,7-tetraiso-
propylcyclotetrasiloxane (1b). Compound 1b was prepared by fol-
lowing the general procedure employing cis-cis-trans-(i-Pr(p-Tol)SiO)4
(3b, 434 mg, 0.61 mmol) and AlCl3 (465 mg, 3.5 mmol) in 8 mL of
benzene. Recrystallized from hexane gave 1b (214 mg, 84%) as a
1
colorless solid: mp 157-158 °C; H NMR (DMSO-d6) δ 0.72 (m,
Supporting Information Available: Crystallographic infor-
mation for 1a, 1b, 2a, 2d, 3a, 3c, and 3d. This material is
available free of charge via the Internet at http://pubs.acs.org.
4H), 0.95 (d, 12H, J ) 7.0 Hz), 0.96 (d, 6H, J ) 7.0 Hz), 0.97 (d, 6H,
J ) 7.0 Hz), 6.22 (s, 1H), 6.23 (s, 1H), 6.26 (s, 2H); 13C NMR (DMSO-
d6) δ 12.51 (overlap), 17.39, 17.41, 17.47; 29Si NMR (DMSO-d6) δ
-59.69, -59.12, -59.00; MS (70 eV) m/z (%) 373 (M+ - i-Pr, 2),
355 (43), 72 (100); IR (NaCl) ν 3261, 2947, 2870, 1466, 1387, 1259,
1165, 1103, 1067, 1001, 874, 802, 729, 652, 603. Anal. Calcd for
C12H32O8Si4: C, 34.59; H, 7.74. Found: C, 34.69; H, 7.50.
Synthesis of All-trans-1,3,5,7-tetrahydroxy-1,3,5,7-tetraisopro-
pylcyclotetrasiloxane (1c). Compound 1c was prepared by following
the general procedure employing all-trans-(i-PrPhSiO)4 (2c, 3.73 g, 5.6
mmol) and AlCl3 (1.5 g, 11.4 mmol) in 50 mL of benzene. Recrystal-
lized from ethyl ether gave 1c (2.41 g, 97%) as a colorless solid: mp
JA043894M
(24) (a) Sheldrick, G. M. SHELXS-86. In Crystallographic Computing 3;
Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University Press:
Oxford, 1985; p 175. (b) Altomare, A.; Burla, M. C.; Camalli, M.;
Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. SIR92. J. Appl.
Crystallogr. 1994, 27, 435. (c) Altomare, A.; Burla, M.; Camalli, M.;
Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A.; Polidori, G.;
Spagna, R. SIR97. J. Appl. Crystallogr. 1999, 32, 115.
(25) Sheldrick, G. M. SHELXL-97; Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2263