Synthesis and electronic spectra of 2,5-bis-(2-arylethenyl)siloles

Article abstract of DOI:10.1023/A:1017951201553

A new series of 2,5-bis(arylethenyl)siloles la-d was prepared by the reaction of the corresponding zirconacyclopentadienes with H₂SiCl₂ in CH₂Cl₂. Absorption maxima of siloles la-d were observed in the significantly longer wavelength region compared to those of distyryl-substituted benzenes and heteroles. The emission spectrum of distyrylsilole la showed a large Stokes shift compared with distyrylsilole 2.

Full text of DOI:10.1023/A:1017951201553

Chemistry of Heterocyclic Compounds, Vol. 37, No. 11, 2001
SYNTHESIS AND ELECTRONIC
SPECTRA OF 2,5-BIS-
(2-ARYLETHENYL)SILOLES*
K. Kanno and M. Kira
A new series of 2,5-bis(arylethenyl)siloles 1a-d was prepared by the reaction of the corresponding
zirconacyclopentadienes with H2SiCl2 in CH2Cl2. Absorption maxima of siloles 1a-d were observed in
the significantly longer wavelength region compared to those of distyryl-substituted benzenes and
heteroles. The emission spectrum of distyrylsilole 1a showed a large Stokes shift compared with
distyrylsilole 2.
Keywords: 2,5-divinylsilole, silacyclopentadiene, silole, zirconacyclopentadiene, absorption spectrum,
emission spectrum.
Much attention has been focused on siloles (silacyclopentadienes) in the field of material science, due to
their unique electronic properties [1-7]. Since siloles among various heteroles have exceptionally low-lying
LUMO levels due to effective *– * interaction [8], the incorporation of silole units into -conjugated
σ
π
π
molecules is expected to cause significant lowering of the LUMO levels, hence narrowing their band gaps
[9-13].
Herein we report a novel synthetic method for a series of 2,5-bis(arylethenyl)siloles 1a-d and
their electronic spectra. Although vinyl-substituted siloles should be useful models for silole-containing
conjugated polyenes, there have been very few studies of the synthesis of this type of siloles reported so far,
with the exception of a study of the synthesis of 3,4-diphenyl-2,5-distyrylsilole (2) by Tamao and co-workers
[14].
Ph
Ph
Si
Si
Ar
Ph
Ar
Ph
Me
1a–d
Me
Me
Me
2
1 a Ar = Ph, b Ar = 4-MeOC6H4, c Ar = 4-ClC6H4, d Ar = 2-Thienyl
_______
* Dedicated to Professor E. Lukevics on the occasion of his 65th birthday.
__________________________________________________________________________________________
Department of Chemistry, Graduate School of Science, Tohiku University, Aoba-ku, Sendai 980-8578,
Japan; e-mail: mkira@si.chem.tohoku.ac.jp. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11,
pp. 1519-1523, November, 2001. Original article submitted October 23, 2001.
1388
0009-3122/01/3711-1388$25.00©2001 Plenum Publishing Corporation

Results and Discussion
Synthesis of 2,5-bis(2-arylethenyl)siloles. We have recently found that the direct reactions of
zirconacyclopentadienes with sterically unhindered halosilanes afford a convenient and useful method for the
synthesis of siloles [15]. This method was found to be applicable for the synthesis of a series of 2,5-bis(2-
arylethenyl)siloles 1a-d (Scheme 1). Bis-enynes 3a-d as precursors were prepared by the Sonogashira coupling
reactions [16, 17] of 1,7-octadiyne with the corresponding (E)-1-bromo-2-arylethylenes in good yields. The
reactions of the enynes with 3 equivalents of Cp2ZrBu2 gave the corresponding zirconacyclopentadienes 4a-d
[18-22]. Without isolation, the zirconacycles were treated with H2SiCl2 in CH2Cl2 to afford the corresponding
siloles in moderate yields (43-50%).
Scheme 1
Ar
n-BuMgCl (6 equiv)
+
Cp2ZrCl2
THF, –80 oC to rt, 0.5 d
(3 equiv)
Ar
3a–d
THF
H2SiCl2
1a–d
0 oC, 1 h
CH2Cl2
Ar
Ar
Zr
4a–d
1 a Ar = Ph (42%); b Ar = 4-MeOC6H4 (43%); c Ar = 4-ClC6H4 (50%); d Ar = 2-Thienyl (46%)
(yields based on the starting bis-enyne)
Since Cp2ZrBu2 is known to decompose at room temperature [18], an excess amount of Cp2ZrBu2 is
required to afford siloles 1a-d in reasonable yields. When one equivalent of Cp2ZrBu2 was employed in the
reactions with bis-enynes 3a-d, the isolated yields of 1a-d were significantly decreased to ca. 10% with ca. 40%
recovery of the bis-enynes.
We have recently observed the Pd-catalyzed chlorination of the Si–H functions of siloles [23].
Application of this method (PdCl2, allyl chloride, room temperature, 12 h) to the functionalization of 1a failed,
however, to give a complex product mixture.
Absorption and Emission Spectra
The absorption and emission spectral data of siloles 1a-d in CH2Cl2 at room temperature are shown in
Table 1. The absorption maxima of 1a-d were observed at around 440-470 nm and their extinction coefficients
were more than 30,000. The emission maxima appeared at around 580-620 nm with quantum yields of 0.01-0.17
[24, 25]. While the aromatic substituent effects on the electronic spectra of 1a-c were not remarkable,
thienyl-substituted silole 1d showed absorption and emission maxima at much longer wavelengths than those
of 1a-c.
1389

TABLE 1. Absorption and emission spectral data of bis(2-arylethenyl)-
siloles 1a-d in CH2Cl2 at room temperature
Absorption
λmax, nm
, cm-1·M-1
Emission
λmax, nm
Silole
Ar
ε
Φf
Ph
444
456
449
465
35500
31900
33600
36300
578
589
582
624
0.17
0.09
0.15
0.01
1a
1b
1c
1d
4-MeOC6H4
4-ClC6H4
2-Thienyl
The absorption maxima of siloles 1a-d were observed at much longer wavelengths compared to those of
p-distyryl-substituted benzene [26] and heteroles [27, 28]. The results indicate that the incorporation of silole
units into vinylene-linked -conjugated molecules decreases their HOMO-LUMO gaps effectively due to the
π
*– * interaction in siloles.
σ
π
The absorption and emission maxima of silole 1a showed the red-shift of 9 and 51 nm, respectively,
from those of 2. The larger Stokes shift of 1a is suggestive of the more significant conformational difference
between the ground and the excited states of 1a, while further work should be required to elucidate the origin of
the difference between the geometric and electronic structure of 1a and 2.
EXPERIMENTAL
General. All reactions were carried out under an argon atmosphere in dry solvents and in anhydrous
1
13
29
conditions unless otherwise noted. H (300 MHz), C (75.4 MHz), and Si (59.6 MHz) NMR spectra were
recorded on a Bruker AC300P spectrometer. Chemical shifts are based on the residual solvent resonances. Mass
spectra and high-resolution mass spectra were obtained in JEOL JMS-600W and JEOL JMS D-300 mass
spectrometers. UV spectra were measured by a Hewlett Packard G1103A (8453E) spectrometer or a Milton Roy
MR-3000 spectrometer. Emission spectra were recorded on a Hitachi Fluorescence Spectrophotometer 850.
GC analyses were done using a SHIMADZU GC-8A packed with SE-30 (5 and 15% in 5-mm glass columns).
Silica gel column chromatography was carried out using Merck silica gel 60 (particle size 63-200 m) or
µ
Wakosil C-300 (particle size 40-64 m). Thin-layer chromatography was carried out using Merck silica gel plate
µ
60 G-254.
Materials. Reagents and spectral grade solvents were purchased and used without further purification
unless otherwise noted. Anhydrous tetrahydrofuran (THF) was obtained by distillation from sodium
benzophenone, or as a commercial source (KANTO Chemical). Dichloromethane (CH2Cl2) was dried by
distillation from phosphorous oxide followed by distillation from calcium hydride. Hexane and toluene were
distilled from sodium under argon. (E)-1-Bromo-2-(4-methoxyphenyl)ethylene [29] and 1-bromo-2-(2-
thienyl)ethylene [30, 31] were prepared by the literature methods.
E
To a 100 ml three-necked flask with a
Synthesis of ( )-1-Bromo-2-(4-chlorophenyl)ethylene.
condenser, 4-chloro- , -dibromostyrene [15] (11.07 g, 37.3 mmol), diethyl phosphite (10.38 g, 75.2 mmol),
β β
°
and triethylamine (7.92 g, 78.2 mmol) were added and stirred at 50 C for 12 h [31]. After cooled to room
temperature, the mixture was diluted with ether. White precipitate formed and was removed by filtration. The
1
filtrate was concentrated and purified by flash chromatography to afford colorless crystals (7.63 g). H NMR
analysis of the crystals revealed that the E/Z ratio was 91/9. The crystals were further purified by crystallization
from EtOH to afford pure E-isomer of the title compound (6.07 g, 75%). The spectral data were consistent with
the literature values [33].
1390

General Procedure for the Synthesis of 1,12-Diaryldoceca-1,11-dien-3,9-diynes 3a-d. Synthesis of
3a (Ar = Ph). Typically, in a 50 ml two-necked flask, a mixture of Pd(PPh3)4 (1.15 g, 0.995 mmol),
-bromostyrene (mixture of isomers, E/Z = 9/1, 8.15 g, 40.1 mmol of E-isomer), and 1,7-octadiyne in 40 ml of
β
pyrrolidine was stirred at room temperature overnight. At this time, a fluffy precipitate formed. The suspension
was treated with ice-cooled saturated aq. NH4Cl and extracted with ether (in the case of 3b and 3c, CHCl3 was
used instead of ether for extraction). The organic layer was washed with water, saturated aq. NaHCO3, and
brine, and then dried over anhydrous Na2SO4. After removal of the solvent, the residue was subjected to flash
chromatography (hexane:toluene = 5:1 to 4:1) to afford a yellow solid. Pure 3a was obtained by recrystallization
from ethanol as colorless crystals (3.96 g, 64% yield).
1
:
3a. Yield 64%; mp 76-77°C. H NMR (CDCl3) 7.39-7.21 (10H, m); 6.87 (2H, d, J = 16 Hz); 6.14 (2H,
13
dt, J = 16, 2 Hz); 2.49-2.35 (4H, m); 1.79-1.64 ppm (4H, m). C NMR (CDCl3): 140.2, 136.5, 128.6, 128.2,
126.0, 108.8, 92.3, 80.1, 27.9, 19.2 ppm. MS (70 eV), m/z (%): 310 (M+, 80), 267 (40), 219 (67), 191 (51),
156 (33), 141 (46), 128 (31), 115 (100), 91 (40). Found, %: C 92.79; H 7.15. C24H22. Calculated, %: C 92.86;
H 7.14.
1
):
3b. Yield 82%; mp 117-119°C. H NMR (CDCl3 7.28 (4H, dt, J = 9, 3 Hz); 6.83 (4H, dt, J = 9, 3 Hz);
6.81 (2H, d, J = 16 Hz); 6.00 (2H, dt, J = 16, 2 Hz); 3.79 (6H, s); 2.45-2.40 (4H, m); 1.74-1.65 ppm (4H, m).
13C NMR (CDCl3): 159.8, 139.7, 129.4, 127.3, 114.1, 106.4, 91.5, 80.3, 55.3, 28.0, 19.2 ppm. MS (70 eV),
m/z (%): 370 (M+, 84), 249 (22), 141 (39), 129 (23), 128 (100), 127 (27), 121 (67), 115 (37). Found, %:
C 84.03; H 7.16. C26H26O2. Calculated, %: C 84.29; H 7.07.
1
):
3c. Yield 77%; mp 136°C. H NMR (CDCl3 7.26 (8H, s); 6.79 (2H, d, J = 16 Hz); 6.10 (2H, dt, J = 16,
2 Hz); 2.44-2.36 (4H, m); 1.74-1.66 ppm (4H, m). 13C NMR (CDCl3): 138.8, 135.0, 133.9, 128.8, 127.2, 109.4,
93.0, 79.9, 27.8, 19.2 ppm. MS (70 eV), m/z (%): 380 (M++2, 31), 378 (M+, 38), 265 (24), 253 (23), 218 (20),
204 (21), 203 (30), 202 (21), 175 (28), 165 (39), 153 (42), 152 (38), 151 (32), 149 (52), 141 (29), 149 (54),
139 (100), 127 (28), 125 (34), 115 (37). Found, %: C 75.88; H 5.51. C24H24Cl2. Calculated, %: C 75.99; H 5.31.
1
3d. Yield 48%; mp 68°C. H NMR (CDCl3): 7.18-7.09 (2H, m); 7.00-6.89 (4H, m); 6.96 (2H, d,
13
J = 16 Hz); 5.96 (2H, dt, J = 16, 2 Hz); 2.49-2.30 (4H, m); 1.76-1.60 ppm (4H, m). C NMR (CDCl3): 141.6,
133.0, 127.6, 126.4, 124.8, 108.0, 92.7, 79.8, 27.8, 19.2 ppm. MS (70 eV), m/z (%): 324 (M++2, 13),
322 (M+, 93), 293 (23), 279 (27), 225 (32), 210 (22), 197 (42), 184 (24), 147 (100), 134 (35), 128 (24), 121 (63),
115 (60), 103 (44), 102 (24), 97 (54), 77 (35), 63 (21). Found, %: C 74.36; H 5.76. C20H18S2. Calculated, %:
C 74.49; H 5.63.
E
General Procedure for the Synthesis of 2,5-Bis[( )-2-arylethenyl]-3,4-tetramethylenesiloles 1a-d.
Synthesis of 1a (Ar = Ph). Typically, in a Schlenk flask, to a solution of Cp2ZrCl2 (888 mg, 3.04 mmol) and
1,12-diphenyldodeca-1,11-dien-3,9-diyne (3a, 310 mg, 1.00 mmol) in THF (10 ml) was added n-BuMgCl
(1.17 M THF solution, 5.2 ml, 6.1 mmol) via a syringe over 3 min at -80°C and the mixture was slowly warmed
to room temperature over 0.5 days with stirring. The solvent was removed in vacuo and 20 ml of CH2Cl2 was
added to the residue. H2SiCl2 (ca. 1 ml at below -10°C) was added to the mixture via vacuum transfer and the
resulting mixture was stirred at 0°C for 1 h under Ar atmosphere. After removal of the volatiles, the residue was
treated with 20 ml of CH2Cl2 and filtered by a glass filter with Celite. The filtrate was passed through a pad of
florisil. Removal of the solvent gave an orange solid contaminated with a significant amount of Cp2ZrCl2. The
residual Cp2ZrCl2 was removed by passing through a short pad of silica gel (deactivated with 10 wt. % of water;
CH2Cl2 was used as an eluent). Removal of the solvent afforded an orange solid, which was recrystallized from
toluene at -20°C to yield the pure title compound (160 mg).
CAUTION: Dichlorosilane is a flammable gas and exposure in air should be avoided.
1a. Yield 47%, red-orange crystals; mp 191-193°C. 1H NMR (CDCl3): 7.48-7.41 (4 H, m); 7.3-7.27 (4H,
m); 7.28 (2H, d, J = 16 Hz); 7.23-7.16 (2H, m); 6.62 (2H, d, J = 16 Hz); 4.67 (2H, s); 2.77-2.63 (4H, m);
1.77-1.63 ppm (4H, m). 13C NMR (CDCl3): 156.2, 138.0, 132.1, 129.7, 128.6 127.2, 126.3, 126.2, 27.7,
29
23.0 ppm. Si NMR (CDCl3): -44.9 ppm. MS (70 eV), m/z (%): 340 (M+, 100), 236 (14), 235 (10), 165 (12),
143 (12), 131 (17), 105 (19), 91 (17).
1391

1b. Yield 43%, red-orange crystals; mp 201-203°C. 1H NMR (CDCl3): 7.37 (4H, d, J = 9 Hz); 7.13 (2H,
d, J = 16 Hz); 6.84 (4H, d, J = 9 Hz); 6.55 (2H, d, J = 16 Hz); 4.63 (2H, s); 3.80 (6H, s); 2.75-2.58 (4H, m);
13
1.76-1.60 ppm (4H, m). C NMR (CDCl3): 159.0, 155.2, 131.4, 131.0, 129.3, 127.5, 124.4, 114.1, 55.3, 27.7,
29
23.1 ppm. Si NMR (CDCl3): -45.2 ppm. MS (70 eV), m/z (%): 400 (M+, 62), 326 (56), 251 (25), 161 (28),
139 (31), 137 (37), 121 (100), 91 (41), 73 (86), 44 (53), 40 (76). Found, %: C 78.08; H 7.12. C26H28O2Si.
Calculated, %: C 77.96; H 7.05.
1c. Yield 50%, red-orange crystals; mp 220-222°C. 1H NMR (CDCl3): 7.35 (4H, d, J = 9 Hz); 7.26 (4H,
d, J = 9 Hz); 7.22 (2H, d, J = 16 Hz); 6.53 (2H, d, J = 16 Hz); 4.62 (2H, s); 2.76-2.62 (4H, m); 1.75-1.64 ppm
13
29
(4H, m). C NMR (CDCl3): 156.7, 136.5, 132.7, 130.8, 129.7, 128.8, 127.4, 126.7, 27.7, 22.9 ppm. Si NMR
(CDCl3): -44.9 ppm. MS (70 eV), m/z (%): 412 (M++4, 18), 410 (M++2, 92), 408 (M+, 100), 282 (20), 280 (18),
270 (19), 215 (21), 205 (20), 179 (25), 165 (27), 139 (19), 115 (23). Found, %: C 70.65; H 5.58. C24H22Cl2Si.
Calculated, %: C 70.41; H 5.42.
1
1d. Yield 46%, red crystals; mp 150-152°C. H NMR (CDCl3): 7.16-7.10 (2H, m); 7.05 (2H, d,
J = 15 Hz); 7.00-6.91 (4H, m); 6.72 (2H, d, J = 15 Hz); 4.60 (2H, s); 2.72-2.57 (4H, m); 1.74-1.59 ppm (4H, m).
13
29
):
C NMR (CDCl3): 156.1, 143.7, 129.3, 127.7, 126.2, 125.6, 124.9, 124.1, 27.7, 22.9 ppm. Si NMR (CDCl3
-45.2 ppm. MS (70 eV), m/z (%): 354 (M++2, 17), 352 (M+, 100), 242 (11), 165 (12), 136 (11), 115 (15),
111 (14), 110(13), 97 (42), 91 (11), 84 (14). Found, %: C 68.53; H 5.89. C20H20S2Si. Calculated, %: C 68.13;
H 5.72.
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