Synthesis and spectroscopic study of silacyclyne-substituted phenyleneethynylenes

Article abstract of DOI:10.1016/j.tetlet.2009.03.163

A series of 1,1-dimethyl-4,5:8,9-dibenzo-1-silacycloundeca-4,8-diene-2,6,10-triyne (DST)-substituted phenyleneethynylenes were successfully synthesized by reaction of Me₂SiCl₂ with dimagnesium dianions which had been prepared from 2,

Full text of DOI:10.1016/j.tetlet.2009.03.163

Tetrahedron Letters 50 (2009) 2860–2864
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and spectroscopic study of silacyclyne-substituted
phenyleneethynylenes
a
a
b
Guoliang Mao a, Akihiro Orita a, , Daisuke Matsuo , Takayoshi Hirate , Tetsuo Iwanaga ,
*
Shinji Toyota b, Junzo Otera a,
*
a Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan
b Department of Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1,1-dimethyl-4,5:8,9-dibenzo-1-silacycloundeca-4,8-diene-2,6,10-triyne (DST)-substituted
phenyleneethynylenes were successfully synthesized by reaction of Me2SiCl2 with dimagnesium dianions
which had been prepared from 2,20-diethynyl(diphenylethyne) derivatives by treatment with 2 equiv of
MeMgBr. All the products were white-to-pale yellow powder stable enough to handle in the air, and their
photoluminescence spectra were recorded both in solution and in solid state.
Received 2 February 2009
Revised 4 March 2009
Accepted 5 March 2009
Available online 28 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Alkynes
Silacyclynes
Sulfones
Aldehydes
Eliminations
Great attention has been focused on dehydrobenzoannulene
derivatives1 because of their unique electronic2 and optical proper-
ties3 as well as key building blocks for two-dimensional carbon
networks.4 Although aromatic compounds having highly conju-
tion protocol in combination with Sonogashira coupling.10 We
also reveal their optical properties in solution and in the solid
state,11 and bathochromic shifts in UV–vis absorption spectra in
comparison with the corresponding unsubstituted linear
phenyleneethynylenes.
gated
p-system are promising as emission materials of OLED (or-
ganic light-emitting diode)5 because of their small HOMO-LUMO
band gaps, dehydrobenzo[12]annulenes emitted only weak fluo-
rescence in solution when UV light was irradiated.6 In our contin-
uing study on syntheses of acetylene derivatives and their optical
Firstly, we synthesized 1,1-dimethyl-4,5:8,9-dibenzo-1-silacyc-
loundeca-4,8-diene-2,6,10-triyne (DST, 1a) by using 2,20-dib-
romodiphenylethyne as starting compound which can be easily
obtained from the one-shot double elimination reaction between
2-bromobenzyl sulfone 2 and 2-bromobenzaldehyde 3 (Scheme
1). Addition of an excess amount of lithium hexamethyldisilazide
(LiHMDS) to a THF solution of 2, 3 and diethyl chlorophosphate
afforded 4 in 75% yield. Dibromide 4 was transformed into 5 by
Sonogashira coupling with trimethylsilylethyne. The TMS-pro-
tected acetylene 5 underwent desilylation by treatment with
K2CO3/MeOH to give 6. Treatment of 6 with methyl magnesium
bromide and dichlorodimethylsilane at 50–60 °C provided 1a in
58% yield. Column chromatography on silica gel enabled facile sep-
aration of DST (1a) from the remaining starting compound 6 and
other byproducts to achieve satisfactory results of elemental anal-
ysis. DST (1a) is air-stable white powder with mp 210–212 °C, and
its 13C NMR in CDCl3 indicated three acetylenic carbons at 92.3,
98.5 and 107.2 ppm.
properties,7 we disclosed that the
p-conjugated system in ethy-
*
nylsilane is expanded through
r (C–Si)-p (C„C) interaction to en-
hance fluorescence emission.8 We envisaged that introduction of
dimethylsilylbis(ethynylene) group(s) to phenyleneethynylene ar-
ray could achieve expansion of two-dimensional
system in terms of
p
-conjugated
*
r (C–Si)-p (C„C) interaction and/or enhanced
planarity of phenyleneethynylene array induced by restricted
rotation of benzene rings9 to effect facile control of fluorescence
wavelength keeping a high quantum yield. We have already
established a double elimination protocol of b-substituted sulf-
ones for synthesis of functionalized diarylethynes.7 Herein are
described syntheses of a series of 1,1-dimethyl-4,5:8,9-dibenzo-
1-silacycloundeca-4,8-diene-2,6,10-triyne
(DST)-substituted
phenyleneethynylenes by taking advantage of the double elimina-
Secondly, fused DST derivative 1b was prepared from commer-
cially available 2,6-dibromoaniline (7) (Scheme 2). The aniline 7
was transformed to 1,3-dibromo-2-iodobenzene (8) by Sandmeyer
iodination. Sonogashira coupling of 8 with 2-methyl-3-butyn-2-ol
* Corresponding authors. Tel.: +81 86 256 9533; fax: +81 86 256 4292 (A.O.); tel.:
+81 86 256 9525; fax: +81 86 256 4292 (J.O.).
E-mail addresses: orita@high.ous.ac.jp (A. Orita), otera@high.ous.ac.jp (J. Otera).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.163

G. Mao et al. / Tetrahedron Letters 50 (2009) 2860–2864
2861
LiHMDS
Br
Br
OHC
ClP(O)(OEt)2
LiHMDS
THF
+
Br
Br
-HOP(O)(OEt)2
SO2Ph
Br
Br
PhSO2 OP(O)(OEt)2
3
2
PhSO2
TMS TMS
TMS (2.4 eq),
Pd(PPh3)4, CuI
Br
Br
LiHMDS
-PhSO2H
i-Pr2NH, toluene,
75 °C, 12 h
4 75%
5 90%
Me Me
Si
MeMgBr
(2.0 eq)
Me2SiCl2
K2CO3
THF, MeOH,
rt, 2 h
THF,
THF,
60 °C, 12 h
50 °C, 1 h
6
90%
1a
58% (DST)
Scheme 1.
OH
Br
Br
Br
Pd(PPh3)4, CuI
Et3N,
NaNO2
HCl, H2O
KI
KOH
benzene
NH2
I
OH
reflux, 13.5 h
Br
8 86%
Br
Br
7
9
82%
TMS TMS
Br
TMS,
Pd(PPh3)4, CuI
Br
Br
Br
8,
K2CO3
Pd(PPh3)4, CuI
THF,
toluene,
Et3N,
MeOH,
rt, 2 h
Br
69%
i-Pr2NH
reflux, 17 h
Br
10
11 73%
TMS TMS
12
63%
Me Me
Si
MeMgBr
(2.0 eq)
Me2SiCl2
THF,
THF,
60 °C, 12 h
50 °C, 1 h
Si
Me Me
13
90%
1b 57%
Scheme 2.
afforded 9, and base-catalyzed deprotection of
9
furnished
mixture of bis-, tri- and tetra-adducts was obtained. The fused
DST derivative 1b was obtained by desilylation of tetra-adduct
12 followed by treatment of MeMgBr/Me2SiCl2.
DST-substituted phenyleneethynylenes 1c–f were prepared
successfully by virtue of the double elimination reaction and Sono-
gashira coupling as shown in Schemes 3–5.12
terminal acetylene 10 (Scheme 2). Sonogashira coupling of 10 with
8 afforded bis(1,3-dibromophenyl)ethyne (11) in 73% yield. Sono-
gashira coupling of 11 with trimethylsilylethyne was rather slug-
gish to require subjection of the crude products to the same
coupling reaction again for completion. Otherwise, an inseparable
Ph,
Br
Br
Br
Br
Br
Pd(PPh3)4, CuI
HCl
KOAc
CHO
I
I
CHO
reflux,
AcOH,
i-Pr2NH, toluene,
16
1 h
85%
reflux, 14 h
rt, 12 h
14
15 71%
TMS TMS
TMS,
Pd(PPh3)4, CuI
LiHMDS
ClP(O)(OEt)2
Br
Br
2
+
16
THF, 0 °C-rt, 2 h
toluene,
18
17
80%
67%
i-Pr2NH
Me Me
Si
MeMgBr
(2.0 eq)
Me2SiCl2
K2CO3
THF, MeOH,
rt, 2 h
THF,
THF,
60 °C, 12 h
1c
57%
50 °C, 1 h
90%
Scheme 3.

2862
G. Mao et al. / Tetrahedron Letters 50 (2009) 2860–2864
Br
TMS
TMS
TMS
20
TMS,
Br
Br
Br
K2CO3
Pd(PPh3)4, CuI
Pd(PPh3)4, CuI
I
I
THF, MeOH,
rt, 2 h
toluene, i-Pr2NH
i-Pr2NH, toluene,
Br
19
rt, 12 h
Br
Br
21
79%
TMS
22
77%
Si
MeMgBr
(2.0 eq)
Me2SiCl2
THF,
reflux, 12 h
60 °C, 1 h
1d
51%
23
74%
Si
Scheme 4.
In order to evaluate electronic effect of silacyclyne on phenyle-
neethynylenes, UV–vis absorption spectra were recorded for 1a
and 1b in CH2Cl2 (1.0 ꢀ 10ꢁ5 mol/L) (Fig. 1). Silacyclynes are stable
enough, and no decomposition of 1a nor 1b was observed in mea-
surement of UV–vis absorption spectra. While 1a showed a sharp
vibronic spectrum, 1b recorded 20 nm red-shifted absorption
bands at 250–300 nm and 310–360 nm indicative of a larger ex-
respectively. Such bathochromic shift supports again that intro-
duction of DST component enables efficient
of phenyleneethynylenes.
p-system expansion
32
33
panded
p system.
Figure 2 illustrates three profiles of UV–vis absorption of 1,4-
di(phenylethynyl)benzene (32), 1c and 1d in CH2Cl2. While 32
exhibited a broad vibronic absorption in the region of 270–
350 nm, incorporation of the DST moiety/moieties induced batho-
chromic shift of absorption maxima of the longest wavelength by
21 nm (341–362 nm) and 39 nm (341–380 nm) in 1c and 1d,
While bis(4-(phenylethynyl)phenyl)ethyne (33) showed an in-
tense vibronic absorption band in the region of 270–380 nm, sila-
cyclynes 1e and 1f exhibited strong absorption bands from 285
to 395 nm and from 300 to 400 nm, respectively (Fig. 3). It should
Br
Br
Pd(PPh3)4, CuI
+
I
SO2Ph
i-Pr2NH, toluene,
SO2Ph
24 89%
rt, 12 h
TMS TMS
LiHMDS
Br
Br
TMS,
Pd(PPh3)4, CuI
K2CO3
ClP(O)(OEt)2
24 + 3
THF, MeOH,
rt, 2 h
THF,
toluene, i-Pr2NH
25
76%
26 81%
0 °C-rt, 2 h
Si
MeMgBr
(2.0 eq)
Me2SiCl2
THF,
reflux, 12 h
1e
50%
27 90%
60 °C, 1 h
Br
Br
Pd(PPh3)4, CuI
+
I
SO2Ph
i-Pr2NH, toluene,
SO2Ph
28
87%
rt, 12 h
TMS TMS
TMS,
LiHMDS
Br
Br
Br
OHC
ClP(O)(OEt)2
Pd(PPh3)4, CuI
+
28
THF,
toluene, i-Pr2NH
30 86%
29 81%
16
0 °C-rt, 2 h
Si
MeMgBr
(2.0 eq)
K2CO3
Me2SiCl2
THF,
THF, MeOH,
rt, 2 h
reflux, 12 h
1f
56%
60 °C, 1 h
31 96%
Scheme 5.

G. Mao et al. / Tetrahedron Letters 50 (2009) 2860–2864
2863
Table 1
Fluorescence properties of 1a–f, diphenylethyne, 32 and 33
Emission in
Excitation
wavelength CH2Cl2
UF in Emission in UF in
a
CH2Cl2 (nm)
the solid
the solid
statea
(9.4 ꢀ 10ꢁ7 M) (nm)
state (nm)
Diphenylethyne Weak emission 288
—
359
375
397
394
451
451
444
471
473
0.41
0.14
0.24
0.59
0.37
0.41
1.00
0.18
0.17
1a
1b
32
1c
1d
33
1e
1f
342
362
353
371
390
375
388
396
265
284
319
272
299
340
377
384
0.52
0.35
0.86
0.67
0.85
0.99
0.86
0.88
a
Absolute quantum yields were recorded by an integration sphere (Hamamatsu
photonics C9920-02).
Figure 1. UV–vis absorption spectra of 1a and 1b.
silacyclyne 1b in solution (UPL in CH2Cl2, 1a 0.52, 1b 0.35), 1b
showed longer wavelength of emission maximum than 1a. In 1a
and 1b, introduction of dimethylsilylbis(ethynylene) group to in-
nately non-emissive diphenylethyne brought about efficient
expansion of
p-conjugated system inducing effective emission of
1a and 1b. Such expansion of
p-conjugated system was observed
in other cyclic derivatives 1c–f. Silacyclynes 1c and 1d showed
emission maxima at 371 and 390 nm, respectively, and these emis-
sion maxima were red shifted by 18 and 37 nm in comparison with
emission maximum of 32. Fluorescence quantum yield of 1d
(UF = 0.85) is larger than that of 1c (UF = 0.67). Silacyclynes (1e,
1f) exhibited strong emission as well, and the internal silacyclyne
1f showed larger red shift of emission maximum (396 nm) than the
terminal silacyclyne 1e (388 nm). This larger red shift of 1f would
be induced by larger bathochromic shift in UV–vis absorption spec-
tra. Notably, in sharp contrast to dehydrobenzo[12]annulenes,
silacyclynes 1a–f enables tuning of wavelengths in photolumines-
cence with high fluorescence quantum yields.
Figure 2. UV–vis absorption spectra of 1c, 1b and 32.
When UV-light was irradiated, the silacyclynes 1a–f emitted
strong fluorescence in the solid state. Although fluorescence quan-
tum yields of 1a–f were smaller than those of the corresponding
parent linear phenyleneethynylenes, the wavelengths of emission
maxima of 1a–f underwent bathochromic shift.13
We succeeded in preparation of a variety of silacyclynes by
reaction of dichlorodimethylsilane with 2,20-bis(magnesioethy-
nyl)diphenylethynes which had been derived by treatment of
2,20-di(ethynyl)diphenylethynes with MeMgBr. These silacyclynes
exhibited intense absorption bands in UV–vis spectroscopy, and
the absorption maxima of the longest wavelength showed batho-
be noted that introduction of silacyclyne to the central position (1f)
gave rise to a larger bathochromic shift than introduction to the
terminal position (1e).
Emission spectra of 1a–f, diphenylethyne, 32 and 33 were re-
corded in CH2Cl2 solution (9.4 ꢀ 10ꢁ7 M) and in the solid state at
room temperature. All the fluorescence properties of these com-
pounds are summarized in Table 1. The excitation profiles of
1a–f are compatible with those of UV–vis absorption. Although
the monocyclyne 1a showed higher quantum yield than fused
chromic shift in proportion to the lengths of p-conjugated system
in phenyleneethynylene arrays. When irradiated by UV light, a set
of silacyclynes emitted strong fluorescence in moderate to high
fluorescence quantum yields both in CH2Cl2 and in the solid state.
Application of the silacyclynes to emission material of electrolumi-
nescence device is under investigation.
Acknowledgements
This work was supported by the Grant-in-Aid for Scientific
Research from MEXT, Japan, Okayama Prefecture Industrial Promo-
tion Foundation and Electric Technology Research Foundation of
Chugoku.
Supplementary data
Full synthetic details for the preparation of 1a–f and PL profiles
of 1a–f and their precursors 5, 12, 18, 26 and 30 are provided.
Figure 3. UV–vis absorption spectra of 1e, 1f and 33.

2864
G. Mao et al. / Tetrahedron Letters 50 (2009) 2860–2864
6. It has been reported that
a fluorescence quantum yield of dehy-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.03.163.
drobenzo[12]annulene is 0.15: Janecka-Styrcz, K.; Lipinski, J.; Ruziewicz, Z. J.
Lumin. 1978, 17, 83. For substituted dehydrobenzo[12]annulenes, fluorescence
quantum yields are 0.08 and 0.06: see Ref. 2e.
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p-conjugated material induces a low fluorescence quantum
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