Synthesis of organosilicon polymers by using the Lewis-acid-catalyzed trans-allylsilylation of alkynes

Article abstract of DOI:10.1016/j.tetlet.2004.11.048

The HfCl₄ catalyzed polymerization of allyl(4-ethynylphenyl) dimethylsilane (1a) proceeded smoothly to give poly[dimethyl-(1,4-pentadienyl) phenylsilane] (2a), and the HfCl₄ catalyzed reaction of 1-allyl-2-(4-ethynyphenyl)-1,1,2,2-tetramethyldisilane (1b) afforded poly[1,1,2,2-tetramethyl-1-(1,4-pentadienyl)-2-phenyldisilane] (2b). These polymerization reactions proceeded via the stereoselective trans-allylsilylation of the 4-ethynyl group attached to the aromatic ring.

Full text of DOI:10.1016/j.tetlet.2004.11.048

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 27–30
Synthesis of organosilicon polymers by using the
Lewis-acid-catalyzed trans-allylsilylation of alkynes
*
Naoki Asao, Hisamitsu Tomeba and Yoshinori Yamamoto
*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received 24 September 2004; revised 1 November 2004; accepted 8 November 2004
Abstract—The HfCl4 catalyzed polymerization of allyl(4-ethynylphenyl)dimethylsilane (1a) proceeded smoothly to give poly-
[
4
dimethyl-(1,4-pentadienyl)phenylsilane] (2a), and the HfCl catalyzed reaction of 1-allyl-2-(4-ethynyphenyl)-1,1,2,2-tetramethyl-
disilane (1b) afforded poly[1,1,2,2-tetramethyl-1-(1,4-pentadienyl)-2-phenyldisilane] (2b). These polymerization reactions proceeded
via the stereoselective trans-allylsilylation of the 4-ethynyl group attached to the aromatic ring.
ꢀ
2004 Elsevier Ltd. All rights reserved.
There has been considerable interest in the chemistry of
organosilicon polymers since they are potentially useful
as functional materials, such as photoresists, semicon-
ductors, hole-transporting materials, and precursors of
silicon carbide. In particular, polymers consisting of
oligosilane units and p-electron systems, which are ar-
ranged alternately in the main chain, exhibit orbital
interactions between the silicon–silicon unit and p-elec-
tron system, which is known as r–p conjugation, lead-
ing to extended conjugation along the polymer
For the past few years, we have studied stereoselective
carbometallation of alkynes, and the Lewis-acid-cata-
lyzed trans-selective carbosilylation of unactivated al-
kynes using organosilicon reagents, such as allylsilanes,
vinylsilanes, propargylsilanes, allenylsilanes, and arylsil-
anes has been achieved. It is well known that the carbo-
metallation of alkynes using R–M (M = Cu, Li, Mg,
transition metals, etc.) proceeds in the cis-fashion to give
8
the corresponding cis-alkenyl metals stereoselectively.
However, the Lewis-acid-catalyzed addition of R–Si to
alkynes proceeds in a trans-manner and the correspond-
ing trans-alkenylsilanes are obtained as the sole product
as shown in Eq. 1. We have extended this Lewis-acid-
catalyzed methodology to the synthesis of organosilicon
1
,2
7
3
–5
chain.
organosilicon polymers shown in Figure 1 have been re-
Although a number of synthetic methods for
6
ported, little attention has been paid to the Z-selective
synthesis of the alkenylsilicon polymers.
Si
Si
Si
Si Si
Si Si
Si
S
O
n
n
n
n
(
Alkenyl Si-Si, E/Z mixture)
(Alkynyl Si-Si)
(Heteroaromatic Si-Si)
Si
Si
Si Si
Si Si
n
n
n
(
Aryl Si-Si)
(Biphenyl Si-Si)
(Naphthyl Si-Si)
Figure 1.
Keywords: Organosilicon polymer; r–p Conjugation; Allylsilylation; Lewis-acid catalyst; Carbometallation; Hafnium chloride.
*
Corresponding authors. Tel.: +81 22 217 6581; fax: +81 22 217 6784; e-mail addresses: asao@mail.tains.tohoku.ac.jp;
yoshi@yamamoto1.chem.tohoku.ac.jp
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.048

28
N. Asao et al. / Tetrahedron Letters 46 (2005) 27–30
polymers including r–p conjugated systems; The HfCl4
catalyzed reaction of allyl(4-ethynylphenyl)-dimethyl-
silane (1a) gave poly[dimethyl(1,4-pentadienyl)-phenyl-
silane] (2a), and the HfCl4 catalyzed reaction of 1-
allyl-2-(4-ethynylphenyl)-1,1,2,2-tetramethyldisilane (1b)
afforded poly[1,1,2,2-tetramethyl-1-(1,4-pentadienyl)-
(4-trimethylsilanylethynyl-phenyl)-disilane 6 in 77%
yield. Desilylation of 6 produced 1b in 65% yield
(Scheme 2).
First, we examined the intermolecular reaction of the
allyldisilane 7 with phenylacetylene in order to deter-
mine whether the Lewis-acid-catalyzed addition of the
allyldisilane proceeds similarly to that of allyltrimeth-
ylsilane (Eq. 4). The reaction proceeded smoothly in
the trans-fashion and the corresponding allylsilylation
product 8 was obtained as the sole product in 62%
2
-phenyldisilane] (2b) (Eqs. 2 and 3).
R
cat. Lewis acid
+
R SiR'
3
ð1Þ
ð2Þ
SiR'
3
R: allyl, vinyl, ...
9
yield. The stereochemistry of 8 was unambiguously
determined by NOE experiments; irradiation of the
methyl protons on the dimethylsilyl group enhanced
the signal of the ortho-protons of the phenyl group
Si
Si
cat. HfCl4
(9.5% NOE), whereas no enhancement of any signals
2 2
CH Cl
of the methylene protons on the allyl group was
observed.
n
1
a
2a
ð4Þ
Since the model study demonstrated that the allyldi-
silane, as well as the allylsilane, added to the alkyne regio-
and stereoselectively, we next examined the polymeriza-
tion of 1a and 1b using HfCl as catalyst in CH Cl and
ð3Þ
4
2
2
the results are summarized in Table 1. The reaction of 1a
proceeded smoothly at room temperature to give a
crude polymer, which was purified by precipitation into
i-PrOH to afford 2a in 57% yield (entry 1). Polymeriza-
tion of 1b was also performed under the same conditions
as above and the corresponding polymer 2b was ob-
tained in 47% yield (entry 2). The structures of 2a and
The requisite starting monomers 1a and 1b were pre-
pared as shown in Schemes 1 and 2. Sonogashira
coupling of 1-bromo-4-iodobenzene 3 with trimeth-
ylsilylacetylene gave (4-bromo-phenylethynyl)trimeth-
ylsilane 4 in 98% yield. Treatment of 4 with n-BuLi,
followed by addition of allylchlorodimethylsilane affor-
ded 1-(allyldimethyl-silanyl)-4-trimethylsilanylethynyl-
benzene 5 in 93% yield. Desilylation of 5 with aqueous
KOH produced 1a in 90% yield (Scheme 1). On the
other hand, treatment of 4 with n-BuLi, followed by
the addition of 1,2-dichlorotetramethyldisilane and allyl
magnesium chloride gave 1-allyl-1,1,2,2-tetramethyl-2-
1
13
2b were determined by H and C NMR spectroscopy,
in which the signal of the olefinic protons neighboring
silicon of 2b showed good correspondence with those
10
of (Z)-2-phenyl-1-(trimethylsilyl)-1,4-pentadiene Z-9
and the model compound 8; Z-9, 5.59ppm; E-9,
12
11
5
.94ppm; 8, 5.62ppm; 2a, 5.75ppm; 2b, 5.53ppm.
The olefinic protons of the Z-derivatives (Z-9, 8, and
Br
a
Br
4
Table 1. The HfCl -catalyzed polymerization of 1a and 1b
Me
3
Si
1. n-BuLi
b
c
n
c
w
Entry
1
2
Yield
M
M
w n
M /M
cat. Pd
2.
SiMe
2
Cl
I
9
8%
Si
93%
4
4
4
4
3
Me Si
3
4
1
2
1a
1b
2a
2b
57
47
1.0 · 10
1.2 · 10
3.2 · 10
4.1 · 10
3.2
3.4
a
KOH
0%
Si
The reaction was performed using 1 in the presence of HfCl
4
9
2 2
(20mol%) in CH Cl at room temperature for 3.5h.
Isolated yield.
b
c
Me
3
Si
5
1a
Determined by GPC using the standard polystyrene calibration
curve.
Scheme 1.
Si
1
. n-BuLi
Si
Si
KOH
5%
4
Si
2
. (Me
.
2
ClSi)
MgCl Me
2
6
3
3
Si
6
1b
77%
Scheme 2.

N. Asao et al. / Tetrahedron Letters 46 (2005) 27–30
29
2
b) showed similar chemical shifts, but the proton of the
5. Bassindale, A. R.; Taylor, P. G. In The Chemistry of
Organic Silicon Compounds; Patai, S., Rappoport, Z.,
Eds.; Wiley: New York, 1991; Chapter 14.
E-derivative (E-9) appeared in lower field than that of
the Z-derivative; 5.94 versus 5.59ppm. The stereochem-
istry of 2a was not so unambiguous because 5.75ppm
was just in between the model values of 5.59 and
6
. For reviews, see: (a) Yamaguchi, S.; Tamao, K. Silicon-
Containing Polymers 2000, 461–498; (b) Ohshita, J.;
Kunai, A. Acta Polym. 1998, 49, 379–403.
5
.94ppm. However, only a single peak appeared in the
7
. (a) Asao, N.; Yoshikawa, E.; Yamamoto, Y. J. Org.
Chem. 1996, 61, 4874–4875; (b) Yoshikawa, E.; Gevorg-
yan, V.; Asao, N.; Yamamoto, Y. J. Am. Chem. Soc. 1997,
119, 6781–6786; (c) Asao, N.; Shimada, T.; Yamamoto, Y.
J. Am. Chem. Soc. 1999, 121, 3797–3798; (d) Asao, N.;
Yamamoto, Y. Bull. Chem. Soc. Jpn. 2000, 73, 1071–1087;
1
olefinic region of H NMR spectra of 2a and we con-
cluded that 2a took the Z-configuration based on the
general rule for the Lewis-acid catalyzed addition (see
Eq. 1) and on the analogy of 2b. Both polymers are sol-
uble in common organic solvents, such as ethers and
halocarbons, but are insoluble in methanol.
(
e) Yoshikawa, E.; Kasahara, M.; Asao, N.; Yamamoto,
Y. Tetrahedron Lett. 2000, 41, 4499–4502; (f) Asao, N.;
Nabatame, K.; Yamamoto, Y. Chem. Lett. 2001, 22, 982–
The molecular weights (M ) of 2a–b were found to be
n
1
9
83; (g) Asao, N.; Shimada, T.; Shimada, T.; Yamamoto,
0,000–12,000 and their molecular weight distributions
Y. J. Am. Chem. Soc. 2001, 123, 10899–10902, For a
review, see: (h) Asao, N.; Yamamoto, Y. Bull. Chem. Soc.
Jpn. 2000, 73, 1071–1087.
ranged from 3.2 to 3.4 based on GPC analysis using the
standard polystyrene calibration curve. Recently, Mori
et al. reported that the stereodivergent synthesis of E-
and Z-organosilicon polymers, which was accomplished
by Rh-catalyzed hydrosilylation of alkynes. However,
to the best of our knowledge, this is the first example of
the stereocontrolled synthesis of r–p conjugated organo-
silicon polymers containing a Z-alkenylsilane moiety.
8
. For reviews, see: (a) Knochel, P. In Comprehensive
Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, 1991; Vol. 4, pp 865–911; (b) Yamam-
oto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207–2293; (c)
Marek, I.; Normant, J. F. In Carbometallation Reactions
in Metal Catalyzed Cross-Coupling Reactions; Diederich,
F., Stang, P., Eds.; Wiley VCH: New York, 1998; pp 271–
1
3
3
5
37; (d) Fallis, A. G.; Forgione, P. Tetrahedron 2001, 57,
899–5913.
The preparation of 2b is representative. To a suspension
of HfCl (64mg, 20mol%) in CH Cl (2mL) was added
4
2
2
1
9
. Compound 8: H NMR (600MHz, CDCl ) d 7.45–7.25
3
1
b (259mg, 1mmol) with vigorous stirring at room tem-
(
5
m, 5H), 5.85 (ddt, J = 6.6, 9.0, 18.0Hz, 1H), 5.62 (s, 1H),
.10–5.02 (m, 2H), 3.20 (dd, J = 1.2, 6.6Hz, 2H), 0.02 (s,
perature. The mixture was stirred for 3.5h and then was
poured into i-PrOH (50mL). After additional stirring
for 20min, the mixture was filtered through a pad of
Celite. A precipitate remained on the Celite, which was
washed with i-PrOH, and was dissolved in CH Cl .
1
3
9H), ꢀ0.15 (s, 6H); C NMR (100MHz, CDCl ) d 156.7,
3
143.9, 136.0, 128.0, 127.7, 126.9, 126.5, 116.2, 47.0, ꢀ1.9,
ꢀ2.6.
1
1
0. See Ref. 7b. (Z)-2-Phenyl-1-(trimethylsilyl)-1,4-pentadiene
2
2
1
Z-9: H NMR (CDCl , 270MHz) d 7.30–7.25 (m, 3H),
7
5
The solvents were evaporated to give a crude material
containing significant amounts of i-PrOH. To remove
i-PrOH completely, the crude product was again dis-
solved in CH Cl , the solvents were evaporated, and this
3
.17–7.13 (m, 2H), 5.83 (ddt, J = 7.0, 9.2, 17.9Hz, 1H),
.59 (t, J = 1.5Hz, 1H), 5.02–4.98 (m, 2H), 3.21 (ddd,
13
J = 1.5, 2.7, 7.0Hz, 2H), ꢀ0.19 (s, 9H); C NMR
2
2
(
67.9MHz, CDCl ) d 157.3, 143.9, 135.8, 127.88, 127.86,
3
procedure was repeated five times. The material was
dried in vacuo and 2b was obtained as a brown gum
1
27.7, 126.9, 116.4, 46.8, 0.1.
1. The spectral data of (E)-2-phenyl-1-(trimethylsilyl)-1,4-
pentadiene E-9 was reported, see: Chatani, N.; Amishiro,
N.; Murai, S. J. Am. Chem. Soc. 1991, 113, 7778–7780. E-
(
122mg) in 47% yield.
1
We are now in a position to synthesize organosilicon
polymers, bearing (Z)-alkenylsilane moieties, in a regio-
and stereoselective manner using the Lewis-acid-cata-
lyzed trans-allylsilylation of alkynes. While the silicon–
silicon bond of disilanes is easily cleaved by a transition
9: H NMR (CDCl
3
, 270MHz) d 7.45–7.42 (m, 2H), 7.33–
7
1
1
.21 (m, 3H), 5.94 (s, 1H), 5.80 (ddt, J = 1.7, 10.1, 17.1Hz,
H), 5.06 (dt, J = 1.7, 17.1Hz, 1H), 5.00 (dt, J = 1.7,
0.1Hz, 1H), 3.39 (dt, J = 1.7, 5.9Hz, 2H), 0.20 (s, 9H);
1
3
C NMR (67.9MHz, CDCl
29.44, 128.08, 127.27, 126.23, 116.06, 38.72, 0.20.
3
) d 154.01, 143.36, 136.53,
1
1
4
metal catalyst, such an undesired reaction does not oc-
cur in the present Lewis-acid-catalyzed reaction. Further
studies on the properties of these polymers and exten-
sion of the present method to the preparation of other
functional materials are underway.
1
1
2. Compound 2a: H NMR (300MHz, CDCl ) d 7.70–6.70
3
(
(
br, 4H), 6.10–5.50 (br, 2H), 5.20–4.90 (br, 2H), 3.35–3.00
br, 2H), 0.20–0.14 (br, 6H); C NMR (75.5MHz,
1
3
CDCl ) d 158.8, 143.8, 139.3, 135.7, 133.0, 127.1, 125.7,
3
ꢀ
1
116.5, 46.6, ꢀ1.2; IR (KBr) 2955, 1591, 1247, 835cm
.
1
Compound 2b: H NMR (300MHz, CDCl
3
) d 7.55–7.16
br, 2H), 7.16–6.78 (br, 2H), 5.93–5.65 (br, 1H), 5.53 (br,
(
1
H), 5.14–4.87 (br, 2H), 3.28–2.99 (br, 2H), 0.25 (br, 6H),
1
3
References and notes
ꢀ0.23 (br, 6H); C NMR (75.5MHz, CDCl ) d 157.3,
3
1
43.8, 138.2, 136.0, 133.3, 127.3, 125.8, 116.3, 46.7, ꢀ2.6,
ꢀ
1
1
2
. Zeldin, M.; Wynne, K. J.; Allcock, H. R. Inorganic
and Organometallic Polymers; ACS Symposium Series,
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ꢀ3.7; IR (KBr) 2951, 1589, 1244, 829cm ; UV–vis
(CHCl ) k /nm (e) 252nm (12,000).
3
max
13. (a) Mori, A.; Takahisa, E.; Yamamura, Y.; Kato, T.;
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. Sakurai, H. J. Organomet. Chem. 1980, 200, 261.

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