Synthesis of laddersiloxanes by novel stereocontrolled approach

Article abstract of DOI:10.1016/j.jorganchem.2006.08.068

Three new pentacyclic laddersiloxanes were prepared by a new method utilizing single stereoisomer of disiloxanediol as a growing unit. Thus, one diastereomer of disiloxanediol, (RS)-[i-PrPhSi(OH)]₂O was isolated and treated with tetrachlorocycl

Full text of DOI:10.1016/j.jorganchem.2006.08.068

Journal of Organometallic Chemistry 692 (2007) 307–312
www.elsevier.com/locate/jorganchem
Synthesis of laddersiloxanes by novel stereocontrolled approach
*
Masafumi Unno , Tomoe Matsumoto, Hideyuki Matsumoto
Department of Applied Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
Received 28 February 2006; accepted 25 April 2006
Available online 30 August 2006
Abstract
Three new pentacyclic laddersiloxanes were prepared by a new method utilizing single stereoisomer of disiloxanediol as a growing
unit. Thus, one diastereomer of disiloxanediol, (RS)–[i-PrPhSi(OH)]2O was isolated and treated with tetrachlorocyclotetrasiloxane,
resulting in the formation of tricyclic laddersiloxanes with cis-Ph groups at terminals. All of the obtained isomers could be transformed
into pentacyclic laddersiloxanes by dephenylchlorination followed by the reaction with (RS)–[i-PrPhSi(OH)]2O. The structures of new
tricyclic and pentacyclic laddersiloxanes were determined by X-ray crystallography. The thermogravity properties of laddersiloxanes
depending on the molecular weight and structures were summarized.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Cyclic silanol; Silsesquioxane; Crystal structure; Laddersiloxane; Diastereomer
1. Introduction
seen in Scheme 2, when a diastereomeric mixture of dic-
hlorodisiloxane 2 is used, obtained laddersiloxanes possess
cis or trans conformation at the terminals. For the sake of
homologation, laddersiloxanes must possess cis-phenyl
groups at both terminals [6]; however, the ratio of this iso-
mer is statistically only 25% (50% at each terminal). This is
the reason why the yields of the laddersiloxanes decreased
by going to higher analogues. Our strategy to overcome
this problem is as follows. Out of three diastereomers of
dichlorosiloxanes, only RS-form satisfies cis-requisite.
When this isomer is separated and utilized in the reaction,
only laddersiloxanes capable of affording ring homologa-
tion can be obtained.
Recently, much effort has been directed toward the syn-
theses of ladder-type polysilsesquioxanes because of their
expected high thermal stability [1]. Although ladder skele-
ton have been claimed to generate under certain conditions
[2], their structures are not universally accepted [3]. Thus,
the construction of polysilsesquioxanes having true ladder
structures represents an important synthetic goal. As a part
of our program toward the synthesis of laddersiloxanes
(ladder silsesquioxanes with defined structure) [4], we have
recently reported the synthesis of pentacyclic laddersilox-
ane starting from cyclic tetraol 1 [5] and dichlorosiloxane
(2) (Scheme 1) [4a]. As shown in this Scheme, it is theoret-
ically possible to build up rings by repeating last two steps.
Because of the generation of stereoisomers, the yield of
each step must be optimized in order to construct longer
laddersiloxanes. In the synthesis of pentacyclic laddersilox-
anes, tricyclic laddersiloxanes were obtained in a good yield
(85%), however, pentacyclic 3 was not obtained in a satis-
factory yield (47%, mixture of five stereoisomers). This can
be explained by the generation of disadaptive isomers. As
2. Results and discussion
In the former synthesis, we used dichlorosiloxane 2 as a
growing unit (Scheme 3). However, separation of diastereo-
mers of 2 is seemingly difficult because dichloride 2 is liquid
and relatively unstable towards moisture. Then we exam-
ined the dihydrosiloxane 4, which can be transformed to
2. Unfortunately, the attempt for the separation of RS-
and RR/SS-isomers of 4 met with failure despite a consider-
able attempts. We then focused on diol 5. Fortunately in this
case, we could separate diastereomers with recrystallization
*
Corresponding author. Tel.: +81 277 30 1230; fax: +81 277 30 1236.
E-mail address: unno@chem.gunma-u.ac.jp (M. Unno).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.08.068

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M. Unno et al. / Journal of Organometallic Chemistry 692 (2007) 307–312
Scheme 1. Synthesis of pentacyclic laddersiloxane.
Scheme 2. Cis-requisite of laddersiloxane.
was limited. We combined these two separations, and
obtained pure 5a for the next step. The structure of 5a was
established on the basis of spectroscopic data, and further
corroborated by X-ray crystallography. Crystallographic
parameters are summarized in Table 1.
Scheme 3.
With desired pure diastereomer in hand, we first investi-
gated the preparation of tricyclic laddersiloxanes. As
shown in Scheme 5, tetrachlorocyclotetrasiloxane, gener-
ated from all-cis-[i-PrPhSiO]4, was treated with 5a, and
three stereoisomers of tricyclic laddersiloxanes 6a–6c were
or HPLC separation [7]. As illustrated in Scheme 4, repeat-
ing recrystallization from hexane afforded RS-isomer 5a in
gram quantities. Furthermore, separation with HPLC was
also applied for complete separation although the amount
Scheme 4. Separation of diastereomers.

M. Unno et al. / Journal of Organometallic Chemistry 692 (2007) 307–312
309
Table 1
Crystallographic data for 5a and 6a
5a
6a
Formula
C18H26Si2O3
346.57
Colorless platelet
0.30 · 0.30 · 0.10
Monoclinic
7.787(5)
C48H76Si8O10
1037.81
Colorless prism
0.50 · 0.50 · 0.50
Orthorhombic
30.926(1)
Molecular weight
Crystal description
Crystal size (mm)
Crystal system
˚
a (A)
˚
b (A)
21.80(1)
15.6122(7)
˚
c (A)
12.602(8)
111.59(1)
1989(2)
P21/a (#14)
4
11.9411(5)
b (ꢁ)
3
˚
V (A )
5765.4(4)
Pna21 (#33)
4
1.196
2.36
Space group
Z
Dcalc (g cmꢀ1
l (Mo Ka) (cmꢀ1
)
1.157
1.89
Fig. 1. The molecular structure and numbering of 6a with 30% probability
)
˚
thermal ellipsoids; selected bond lengths (A) and angles (ꢁ): Si(1)–O(8)
Number of reflections
measured
Number of independent
20162
27182
1.619(4), Si(1)–O(9) 1.618(4), Si(1)–O(1) 1.615(4), Si(6)–O(9) 1.633(4),
Si(6)–O(6) 1.634(4), Si(6)–O(5) 1.630(4), Si(7)–O(6) 1.616(4), Si(7)–O(7)
1.629(4), Si(2)–O(10) 1.626(4), Si(2)–O(1) 1.628(4), Si(2)–O(2) 1.621(4),
Si(8)–O(8) 1.630(4), Si(8)–O(7) 1.623(4), Si(3)–O(3) 1.625(4), Si(3)–O(2)
1.630(4), Si(4)–O(4) 1.627(4), Si(4)–O(3) 1.620(4), Si(5)–O(10) 1.620(5),
Si(5)–O(4) 1.610(5), Si(5)–O(5) 1.609(5); O(8)–Si(1) O(9) 109.8(2), O(8)–
Si(1)–O(1) 107.8(2), O(9)–Si(1)–O(1) 110.0(2), O(9)–Si(6)–O(6) 108.2(2),
O(6)–Si(6)–O(5) 107.9(2), O(6)–Si(7)–O(7) 110.8(2), O(10)–Si(2) –O(1)
110.6(2), O(10)–Si(2)–O(2) 109.3(2), O(1)–Si(2)–O(2) 106.8(2), O(8)–Si(8)–
O(7) 110.9(2), O(3)–Si(3)–O(2) 110.6(2), O(4)–Si(4)–O(3) 110.2(2), O(10)–
Si(5)–O(4) 110.1(2), O(10)–Si(5)–O(5) 109.0(2), O(4)–Si(5)–O(5) 108.5(3),
Si(1)–O(8)–Si(8) 150.6(3), Si(2)–O(10)–Si(5) 151.7(3), Si(1)–O(9)–Si(6)
151.3(3), Si(4)–O(4)–Si(5) 150.9(3), Si(1)–O(1)–Si(2) 145.5(3).
5966(0.156)
4999
11079(0.061)
11079
reflections (Rint
)
Number of observed
reflections (I > 2.00r(I))
Function minimized
Number of parameters
Reflections/parameters ratio
R1 (I > 2.00r(I))
P
P
2
2
wðF 2o ꢀ F 2c Þ
wðF 2o ꢀ F 2c Þ
209
596
23.92
0.094
0.115
0.85
18.59
0.068
0.240
2.02
wR2 (all reflections)
S
(D/r)max
0.00
2.69
ꢀ1.41
0.06
0.85
ꢀ0.72
ꢀ3
˚
˚
(Dq)max (e Aꢀ3
)
(Dq)min (e A
)
tained throughout the reactions and only laddersiloxanes
with syn-configuration in the center rings were obtained.
This is a good contrast to the case of oligosilanes, since
dearylchlorination of oligosilanes were reported to give
mixture of stereoisomers regardless whether cis or trans
isomer was employed [9] (see Table 2)
We then prepared pentacyclic laddersiloxanes. Because
of the limited amount of the tricyclic tetrachlorides, we
used isomeric mixture of 6a–6c. Dephenylchlorination fol-
lowed by the reaction with 5a again proceeded smoothly,
affording three pentacyclic laddersiloxanes 7a–7c in 4%,
8%, and 14% yields, respectively [10]. The HPL chromato-
gram at the end of the reaction clearly indicated the gener-
ation of only three isomers. It is noteworthy that all the
obtained. The yields were 3% (6a), 10% (6b), and 1% (6c).
Isomers 6b and 6c have previously been separated and
structurally determined by X-ray analyses [4a]; we identi-
fied them by comparing their spectra. Compound 6a is an
unknown isomer, thus crystallographic analysis was per-
formed, and structure was established. In Fig. 1, the molec-
ular structure is shown, and crystallographic parameters
are summarized in Table 1. Although the yields were not
optimized yet [8], generated laddersiloxanes possesses
cis-Ph groups in the terminal, and number of generated iso-
mers was diminished (six isomers when mixture of diaste-
reomers were used) [4a]. It should also be noted that
stereostructure (all-cis of starting compound) was main-
Scheme 5. Preparation of tricyclic laddersiloxanes.

310
M. Unno et al. / Journal of Organometallic Chemistry 692 (2007) 307–312
Table 2
Thermal properties of laddersiloxanes
Compounds
Td5 (ꢁC)
end of sublimation (ꢁC)
(i-Pr2SiO)4
Tricyclic Laddersiloxane
(in Ref. [4a])
202
253
345
390
Pentacyclic Laddersiloxane
(in Ref. [4a])
7 (isomeric mixture)
287
340
423
428
compounds possess anti–syn–anti framework, and those
could be generated from only 6a. Presumably, syn–syn
framework causes steric congestion, thus no other frame-
works were obtained. The stereochemistry of 7a–7c was
determined by X-ray crystallography (see Scheme 6).
Higher thermal stability was expected for laddersilox-
anes especially with longer framework. Table 1 summarizes
the result of thermogravity analysis; the result of thermog-
ravity analysis is shown in Fig. 2. Pentacyclic laddersilox-
ane 7 started subliming at 340 ꢁC, and lost its total
weight at 428 ꢁC. It showed higher Td5 (5% weight
decrease) and sublimation temperature than any ladders-
iloxanes previously reported. In addition, the thermal sta-
bility increased by adding rings to laddersiloxanes,
showing their advantage over existing chain polysiloxanes.
Interestingly, 7 showed more stability than previously
reported anti–syn–syn pentacyclic laddersiloxane [4a],
which is stereoisomer of 7. It should be noted that the ther-
mal properties depends on not only the molecular weights,
but structure of the framework.
Fig. 2. Thermogravity analysis of 7.
3. Experimental
3.1. General remarks
GLC analysis were performed on a SHIMADZU GC-
8A model using a 3.5 mm · 1000 mm glass column packed
with 10% silicon KF-96 on 60–80 mesh Celite 545sk. HPLC
analysis was carried out a JASCO 880-PU with a JASCO
CHEMICOSORB 4.6 mm · 250 mm 5-ODS-H column
detected by JASCO 875-UV. Preparative recycle-type
HPLC was carried out using JAI LC-908 with Chemco
20 mm · 250 mm 7-ODS column. FT-NMR spectra were
obtained on JEOL Model a-500 and k-500 (1H at
500.00 MHz, 13C at 125.65 MHz, and 29Si at 99.25 MHz).
Chemical shifts were reported as d unit (ppm) relative to
SiMe4, and residual solvents peaks were used for stan-
dards. For 29Si NMR, SiMe4 was used as an external stan-
In summary, heptacyclic laddersiloxane was prepared
for the first time by new synthetic method utilizing stereo-
controlled procedure. Further investigation towards the
synthesis of laddersiloxanes with various frameworks is
now due course.
Scheme 6. Preparation of pentacyclic laddersiloxanes.

M. Unno et al. / Journal of Organometallic Chemistry 692 (2007) 307–312
311
dard. EI mass spectrometry was performed with a JEOL
JMS-DX302 and JMS-700. Infrared spectra were measured
with a SHIMADZU FTIR-8700.
rated by preparative TLC (eluent: hexane/Et2O = 9/1) fol-
lowed by the separation with recycle-type HPLC (eluent:
MeOH/THF = 8/2) to give mainly three isomers of tricy-
clic laddersiloxanes (6a–6c). The yields were 3%, 10%,
1%, respectively. Compound 6a was structurally deter-
mined the by X-ray crystallography. The other compounds
(6b and 6c) were identified by the comparison with authen-
tic sample. exo-all-cis–syn (6a): colorless solid, mp 181–
3.2. Synthesis of 1,3-dihydroxy-1,3-diisopropyl-1,3-
diphenyldisiloxane (5)
Saturated aqueous NaHCO3 solution was added drop-
wise to vigorously stirred 1,3-dichloro-1,3-diisopropyl-1,3-
diphenyldisiloxane (15.1 g, 40.0 mmol) in ether (60 mL)
for 20 min at 0 ꢁC. Then, the reaction mixture was stirred
for 20 min at 0 ꢁC and was poured into water and the
organic layer was extracted with ether. The combined
organic phase was washed with water. The organic phase
was dried over anhydrous magnesium sulfate. Removing
the solvent gave 1,3-dihydroxy-1,3-diisopropyl-1,3-diph-
enyldisiloxane ([i-PrPhSi(OH)]2O) (5) (12.49 g, 90%). Most
proportion of RS-isomer contained was filtered off and
recrystallized from hexane. RS-1,3-dihydroxy-1,3-diisopro-
pyl-1,3-diphenyldisiloxane (5a): colorless solid, mp 101–
1
182 ꢁC; H NMR (CDCl3) d 0.620–1.273 (m, 56), 7.198–
7.533 (m, 20) ppm; 13C NMR (CDCl3) d 12.30, 14.68,
16.66, 16.76, 16.96, 127.37, 129.62, 134.23, 134.36 ppm;
29Si NMR (CDCl3) d-65.94, ꢀ33.17 ppm; MS (EI, 70 eV)
m/z (%) 993 (M+ꢀi-Pr, 28), 915 (3), 829 (6), 725 (8), 691
(7), 225 (100), 183 (23); IR (NaCl) m 3071, 3051, 2947,
2895, 2868, 1464, 1429, 1385, 1258, 1119, 1094, 1047,
993, 320, 887, 721, 700 cmꢀ1
.
3.2.3. Chlorination of tricyclic laddersiloxanes (6)
Chlorination of tricyclic laddersiloxanes was performed
according to our reported method [4a].
1
102 ꢁC; H NMR (DMSO-d6) d 0.87–0.88 (m, 14H), 6.69
(s, 2H), 7.32–7.62 (m, 10H) ppm; 13C NMR (DMSO-d6)
d 14.26, 17.06, 127.56, 129.52, 134.22, 136.48 ppm; 29Si
NMR (DMSO-d6) d-28.92 ppm; MS (70 eV) m/z (%) 303
(M+ꢀi-Pr, 100), 275 (4), 225 (10), 183 (13); IR (NaCl) m
3304, 3069, 3053, 3024, 3013, 2924, 2891, 2864, 1960,
1888, 1827, 1589, 1460, 1429, 1383, 1366, 1327, 1306,
1246, 1123, 1059, 1028, 988, 920, 889, 851, 739, 702,
3.2.4. Synthesis of 1,3,5,7,9,11,13,15,17,19,21,23-
dodecaisopropyl-9,11,21,23-tetraphenylpentacyclo-
[17.5.1.13,17.15,15.17,13]dodecasiloxanes (7)
A solution of [i-PrPhSi(OH)]2O (5b) [RS (>99% HPLC
purity) 0.20 g, 0.59 mmol] in pyridine (2.5 mL) was added
dropwise to a solution of tetrachlorotricyclic laddersilox-
ane (0.26 g, 0.29 mmol) in hexane (1 mL) for 25 min at
room temperature. The mixture was stirred for 16 h. The
reaction mixture was added to saturated aqueous NH4Cl
and hexane, and separated. The separated aqueous phase
was extracted with hexane. The organic phase was washed
with saturated aqueous NH4Cl, then dried over anhydrous
magnesium sulfate, and concentrated. The crude product
was separated by dry column chromatography (eluent: hex-
ane) followed by the separation with recycle-type HPLC
(eluent: MeOH/THF = 7/3) to give mainly three isomers
of pentacyclic laddersiloxanes (7a–7c), and the yields were
4%, 8%, and, 14% respectively. 7a: colorless solid, mp 195–
197 ꢁC; 1H NMR (CDCl3) d 0.71–1.10 (m, 84H), 7.18–7.50
(m, 20H) ppm; 13C NMR (CDCl3) d 11.85, 12.37, 14.48,
16.31, 16.62, 16.68, 16.84, 127.38, 129.62, 134.09,
134.58 ppm; 29Si NMR (CDCl3) d ꢀ66.01, ꢀ33.67 ppm;
MS (EI, 70 eV) m/z (%) 1375 (M+ꢀi-Pr, 100); IR (NaCl)
m 3071, 3051, 2947, 2895, 2868, 1466, 1429, 1385, 1364,
1364, 1259, 1165, 1121, 1069, 1042, 995, 920, 887, 770,
642 cmꢀ1
.
3.2.1. Preparation of 1,3,5,7-tetrachloro-1,3,5,7-
tetraisopropyltetracyclosiloxane
To a solution of all-cis-1,3,5,7-tetraisopropyl-1,3,5,7-tet-
raphenylcyclotetrasiloxane (0.37 g, 0.56 mmol) and anhy-
drous aluminum chloride (0.15 g, 1.13 mmol) in 15 mL of
benzene was passed hydrogen chloride for 30 min at room
temperature. Analysis by gas chromatography showed the
formation of 1,3,5,7-tetrachloro-1,3,5,7-tetraisopropyl-
cyclotetrasiloxane, then argon gas was bubbled. After fil-
tration of aluminum chloride, and removal of benzene,
the product was purified by Kugelrohr distillation to give
analytically pure 3 (0.226 g, 83%). Identification was made
by mass spectrum; MS (70 eV) m/z (%) 447 (M++2ꢀi-Pr,
100), 445 (M+ꢀi-Pr, 68), 363 (18).
3.2.2. Synthesis of syn-1,3,5,7,9,11,13,15-octaisopropyl-
5,7,13,15-tetraphenyltricyclo[9.5.1.13,9]octasiloxanes (6)
A solution of [i-PrPhSi(OH)]2O (5b) [RS (70% HPLC
purity) 0.28 g, 0.80 mmol] in pyridine (0.6 mL) was added
dropwise to a solution of tetrachlorocyclotetrasiloxane
(0.19 g, 0.39 mmol) in hexane (0.5 mL) for 3 min at 0 ꢁC.
The mixture was stirred for 14 h. The reaction mixture
was added to saturated aqueous NH4Cl and hexane, and
separated. The separated aqueous phase was extracted with
hexane. The organic phase was washed with saturated
aqueous NH4Cl, then dried over anhydrous magnesium
sulfate, and concentrated. The crude product was sepa-
1
700 cmꢀ1. 7b: colorless solid, mp 179–180 ꢁC; H NMR
(CDCl3) d 0.30–1.14 (m, 84H), 7.17–7.50 (m, 20H) ppm;
13C NMR (CDCl3) d 11.69, 11.91, 12.12, 12.31, 14.49,
14.70, 16.27, 16.39, 16.43, 16.52, 16.68, 16.81, 16.84,
16.87, 127.28, 127.39, 129.50, 129.61, 134.11, 134.17 ppm;
29Si NMR (CDCl3) d ꢀ66.23, ꢀ66.18, ꢀ65.86, ꢀ64.87,
ꢀ34.44, ꢀ33.60 ppm; MS (EI, 70 eV) m/z (%) 1375
(M+ꢀi-Pr, 100); IR (NaCl) m 3071, 3051, 2947, 2895,
2868, 1466, 1429, 1385, 1366, 1259, 1121, 1042, 995, 920,
887, 770, 700 cmꢀ1. 7c: colorless solid, mp 222–223 ꢁC;
1H NMR (CDCl3) d 0.24–1.25 (m, 84H), 7.17 (t, 8H,

312
M. Unno et al. / Journal of Organometallic Chemistry 692 (2007) 307–312
J = 7.3 Hz), 7.27 (tt, 4H, J = 7.3 Hz,1.5 Hz), 7.47 (dd, 8H,
J = 7.3 Hz, 1.5 Hz) ppm.; 13C NMR (CDCl3) d 11.62,
12.15, 14.71, 16.39, 16.47, 16.79, 16.93, 127.26, 129.41,
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2006.08.068.
129.47, 134.14 ppm; 29Si NMR (CDCl3)
d
ꢀ66.46,
References
ꢀ65.88, ꢀ34.44 ppm; MS (EI, 70 eV) m/z (%) 1375
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2866, 1464, 1429, 1385, 1364, 1259, 1121, 1096, 1068,
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[6] The result of MM2 calculation showed that trans-fused bicyclic
laddersiloxane was substantially less stable than cis-fused ones (due to
increasing bond stretch and bent energy). Additionally, no trans-fused
laddersiloxanes have been obtained in our previous experiments.
[7] (a) Separation of the stereoisomers of disiloxanediol ((PhMe(OH)-
Si)2O) appeared in the literature: A.W.P. Jarvie, A. Holt, J. Hickton,
J. Chem. Soc. (B) (1969) 978–979;
(b) M. Oishi, Y. Kawakami, Org. Lett. 1 (1999) 549–551.
[8] The HPLC analysis at the end of the reaction indicated the generation
of incompletely condensed siloxanes. Presumably, the steric shielding
of cyclic tetrachloride causes a lower yield. Although the R-value of
the crystallographic analysis is not satisfactory low, the stereostruc-
ture was unequivocally established.
Acknowledgement
Financial support for M.U. by grants from the Ministry
of Education, Culture, Sports, Science and Technology of
Japan (Grants-in-Aid Nos. 12640510 and 15350020) is
gratefully acknowledged.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for
the structures of compounds 5a and 6a reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no.
CCDC–605456 for 5a, and 605457 for 6a. Supplementary
[9] K. Tamao, M. Kumada, M. Ishikawa, J. Organomet. Chem. 31
(1971) 17–34.
[10] Because of the poor crystallinity, the X-ray analysis of 7a–7c have not
been fully accomplished. However, the structure of frameworks and
positions of substituents were unequivocally determined.