Synthesis of functionalized siloles from Si-tethered diynes

Article abstract of DOI:10.1016/j.tetlet.2009.01.152

A new synthetic itinerary to silole from Si-tethered diynes is reported. In this protocol, the Si-tethered diyne manifests definitely the reactivity of monoyne to form the lithio silole via zirconacyclopetadiene, 1,4-diiodo-1,3-butadiene, and the correspo

Full text of DOI:10.1016/j.tetlet.2009.01.152

Tetrahedron Letters 50 (2009) 3213–3215
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Synthesis of functionalized siloles from Si-tethered diynes
Qian Luo a, Li Gu a, Chao Wang a, Junhui Liu a, Wenxiong Zhang a, Zhenfeng Xi a,b,
*
a Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
College of Chemistry, Peking University, Beijing 100871, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new synthetic itinerary to silole from Si-tethered diynes is reported. In this protocol, the Si-tethered
diyne manifests definitely the reactivity of monoyne to form the lithio silole via zirconacyclopetadiene,
1,4-diiodo-1,3-butadiene, and the corresponding dilithiodiene successively. Lithio siloles thus obtained
above could be easily functionalized to give various types of silole derivatives. Complex structure like
bridged bis-silole compounds could also be constructed by this process.
Received 13 January 2009
Accepted 30 January 2009
Available online 5 February 2009
Keywords:
Silole
Ó 2009 Elsevier Ltd. All rights reserved.
Phenyl-bridged bis-silole
1,4-Dilithio-1,3-butadiene
1,4-Diiodo-1,3-butadiene
Si-tethered diynes
Zirconacyclopetadiene
Cleavage of Si–C bond
Silacyclopentadiene, also named silole, has attracted much
attention as a useful building block of -conjugated organic mate-
We have reported that Si-tethered diynes 1 can beheave as
mono triple bond to react with Negishi reagent (Cp2ZrnBu2), yield-
ing zirconacyclopetadiene 6 which can change into the corre-
sponding 1,4-diiodo-1,3-butadiene 3 by treating with CuCl and
p
rials for electronic and opto-electronic devices due to its unique
electronic structure.1–5 A breakthrough of the synthetic method
for silole was made by Tamao and Yamaguchi, in which they re-
duced Si-tethered diynes 1 to prepare in situ 2,5-dilithio siloles 2
or lithio benzosilole 20 (Scheme 1).2 Those lithio siloles are very
important, because a wide variety of functionalized siloles can be
readily synthesized by further transformation. Yet owing to the
nature of the Tamao–Yamaguchi method, the substituent on diyne
or the silole ring can only be the aromatic group. Recently, during
our continuous research on the reactivity of 1,4-dilithio-1,3-buta-
diene (di-Li reagent for short),6 we found a facile protocol for syn-
thesis of a new type of lithio siloles from readily available silyl di-Li
reagents.6f These lithio siloles are more general in terms of substi-
tution patterns and synthetic methods, affording diversified silole
derivatives. As a consecutive interest in this field, we would like
to report herein a novel reaction pathway of diyne 1 to synthesize
lithio silole 5 through a short route, including zirconium-mediated
reductive homo-coupling of diyne 17–9 and skeletal rearrangement
of silyl di-Li reagent 4, as clarified briefly in Scheme 1. Different
from the Tamao–Yamaguchi method, in this protocol diyne 1 not
only exhibited the reactive property of monoyne with a compara-
tively broad substitution tolerance, but also can be used to con-
struct complex framework like bridged bis-silole compounds.
t
I2.8e,10 Lithiation of the di-iodide 3 with BuLi (4 equiv) at ꢀ78 °C
in THF can give the di-Li reagent 4 quantitatively. The solution
was then allowed to warm to room temperature and kept for 1 h
before quenched with water. a-Silylated silole 7 was then obtained
in excellent yield.11 When D2O was used instead to terminate the
reaction of 3a, the deuterated compound 7aD was isolated in
90% yield with D incorporation more than 99%. On one hand, the
reaction is very selective, providing 7 as the only product without
observation of 70, which indicates that release of PhC„CLi is much
easier than that of MeLi;3d,12 On the other hand, unlike Tamao–
Yamaguchi method, n-propyl substituted silole 7b can also be ob-
tained in high yield from diyne 1b smoothly, implying the electro-
donating group like alkyl can also be utilized in this protocol. In-
deed, the in situ generated lithio silole and alkynyl lithium were
trapped at one time by adding 4-biphenylcarboxaldehyde to form
the alcohol 8a and 9, respectively, when 3a was used (Scheme 2).
Diyne 1a–b could also react with another different alkyne to
form unsymmetric zirconacyclopentadiene 6c–e13 and mono sily-
lated diiodobutadiene 3c–e in sequence,12 and the last ones can
undergo similar reaction after lithiation, to give specifically the ex-
pected siloles 7c–e as the only products in excellent yield, in spite
of different types of substituent on the alkyne 1 (Scheme 3).
Scheme 4 demonstrated the satisfying reactivity of these readily
and efficiently prepared lithio siloles 5. For example, 5a can be
* Corresponding author. Tel./fax: +86 10 6275 9728.
E-mail address: zfxi@pku.edu.cn (Z. Xi).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.152

3214
Q. Luo et al. / Tetrahedron Letters 50 (2009) 3213–3215
Tamao-Yamaguchi
method
new route
monoyne model
diyne model
R
R
R
SiR'2
SiR'2
Li
SiR'2
Li
Li
SiR'2
''ZrCp2"
R
Z
R
R
Z
R
tBuLi
Naph-Li
I2 / CuCl
Z
Z
Li
Li
I
I
SiR'2
or
SiMe2
Li
Z
R
Ar
1
Z
RC CLi
R
Z
Z
5
4
3
2
2'
Scheme 1.
1 eq. nBuLi
THF, -78 oC, 1 h
1 eq.
R2
0.5 eq.
Cp2ZrCl2
R2
2 eq. EtMgBr
Cp2ZrCl2
R
R
-20 oC, 3 h
Me2Si
THF
3 eq. I2
2 eq. CuCl
R
R
-78 oC, 1 h
0.5 eq. Cp2ZrnBu2
50 oC, 1 h
R1
1eq.
R2
SiMe2
ZrCp2
R1
3 eq. I2
R1
ZrCp2
Me2Si
R1
Me2Si
Me2Si
6a~b
R1
R
1a: R = Ph
R2
I
I
R
2 eq. CuCl
1b: R = nPr
ZrCp2
R2
SiMe2
50 oC, 1 h
R2
R2
R
R
R2
R1
1a: R1 = Ph
1b: R1 nPr
6c~e
R1
R
Me2Si
Me2Si
3c~e
Me2Si
=
R
R
4 eq. tBuLi
R
R
I
I
Li
Li
SiMe2
Me2Si
R1
Li
SiMe2
R2
THF
THF
r.t., 1 h
R1
R2
R
R
4 eq. tBuLi
THF
H2O
-78 oC, 0.5 h
Li
Li
Li
5a~b
Me2Si
3a~b
Me2Si
r.t., 1 h
R2
4a~b
-78 oC, 0.5 h
R
R
R2
5c~e
4c~e
R
R
H
Me2Si
R
Me2Si
R1
7c: R1 = Ph, R2 = Ph, 86%
R
R
X2O
7d: R1 = Ph, R2 nPr, 88%
7e: R1 nPr, R2 = Ph, 81%
=
SiMe2
R2
SiMe2
Si
R
R2
=
R
Me
X
H
7a'~b'
not observed
7a, R = Ph, X = H, 91%
7aD, R = Ph, X = D, 90%,
D > 99%
Scheme 3.
7b, R = nPr, X = H, 86%
Ph
Ph
Ph
Me2Si
Me2Si
Ph
Me2Si
Ph
Ph
Ph
E+
OH
Ar
2 eq. ArCHO H2O
SiMe2
SiMe2
SiMe2
+
Ph
Ph
Ph
E
Li
9, 82%
5a
8
Ar
Ar :
Ph
HO
8a, 86%
Ph
Ph
Ph
Me2Si
Me2Si
Me2Si
Scheme 2.
Ph
Ph
Ph
Ph
Ph
Ph
SiMe2
SiMe2
SiMe2
applied under different conditions to make a variety of functional-
ized siloles 8a–f in one-pot with high efficiency.
OH
HO
O
8b, 94%
E+ = 4'-bipheny CHO E+ = 4'-tolyl COCl E+ = CO2
O
Ph
Me
8a, 86%
8c, 71%
It is known that molecules with enhanced p-conjugation always
lead to a set of desirable properties, such as more intense lumines-
cence or higher carrier mobility.1,14 One of the promising methods
to enlarge the conjugation is to join up the low-conjugated sections
with various types of linkers.2g,15 Based on this concept, 1,4-dialky-
nylbenzene 10 was employed as the precursor of the linker to react
with diyne 1a, offering the tetra-iodide 3f in 61% yield (Scheme 5).
Treating this compound with similar condition as illustrated above,
the acquisitive bridged bis-silole derivative 7f was successfully
gained in good yield. Stopping the reaction with D2O also gave
the relevant deutero product 7fD, which revealed the existence
of intermediate 5f. Compounds of this kind which contain large
conjugation and complex structure are usually hard to make by or-
dinary method. Therefore, application of such lithio siloles as a
practical synthon can be further expected.
Ph
Ph
Ph
Me2Si
Me2Si
Me2Si
Ph
Ph
Ph
Ph
Ph
Ph
SiMe2
SiMe2
SiMe2
I
8f, 82%
E+ = I2
Me
8d, 94%
E+ = MeI
SiMe3
8e, 93%
E+ = Me3SiCl
Scheme 4.
intermediates formed in situ are useful synthons for diversified
preparation of siloles. Furthermore, structurally interesting phe-
nyl-bridged bis-silole compound has also been synthesized effi-
ciently through this process.
In summary, a new itinerary to synthesize functionalized siloles
from Si-tethered diynes has been discussed in this Letter. The lithio

Q. Luo et al. / Tetrahedron Letters 50 (2009) 3213–3215
3215
nPr
Ph
nPr
Ph
nPr
nPr
Ph
nPr
nPr
Ph
2 eq.
Ph
Ph 6 eq. I2
2 eq. CuCl
10
Cp2ZrnBu2
Me2Si
SiMe2
Zr
Zr
+
toluene
Me2Si
SiMe2
SiMe2
Me2Si
I
I
I
I
Cp2
Cp2
50 oC, 1 h
6f
3f, 61%
1a
1a
Ph
Ph
Ph
Ph
nPr
Ph
Ph
Ph
nPr
nPr
nPr
nPr
nPr
Ph
8 eq. tBuLi
Ph
Li
Ph
Li
Ph
Ph
X
X2O
Me2Si
SiMe2
Li Li
Li Li
r.t., 1 h
THF
Si
Si
Me2
Si
Me2
Si
Me2
X
Me2
-78 oC, 0.5 h
4f
5f
Ph
Ph
7f, X = H, 72%
7fD, X = D, 74%, D > 95%
Scheme 5.
5. (a) Ohmura, T.; Masuda, K.; Suginome, M. J. Am. Chem. Soc. 2008, 130, 1526–
1527; (b) Ilies, L.; Tsuji, H.; Sato, Y.; Nakamura, E. J. Am. Chem. Soc. 2008, 130,
4240–4241.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (20632010, 20821062, 20872005) and the Major
State Basic Research Development Program (2006CB806105). The
Cheung Kong Scholars Programme, Qiu Shi Science and Technolo-
gies Foundation, Dow Corning Corporation, and BASF are gratefully
acknowledged.
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Supplementary data
Experimental details and characterization data for all new com-
pounds and copies of 1H NMR and 13C NMR spectra for all new
compounds are available. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.01.152.
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was added t-BuLi (4.0 mmol, 1.5 M in pentane). After this reaction mixture was
stirred at ꢀ78 °C for 30 min, it was warmed to room temperature and
maintained for 1 h. The above reaction mixture was quenched with water and
extracted with Et2O. The extract was washed with brine and dried over MgSO4.
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products 7. Compound 7a: colorless wax, 91% yield (382 mg); 1H NMR
(300 MHz, CDCl3, Me4Si, 25 °C): d 0.03 (s, 6H, CH3), 0.47 (s, 6H, CH3), 6.29 (s,
1H, CH), 6.94–7.46 (m, 15H, CH); 13C NMR (75 MHz, CDCl3, Me4Si, 25 °C): d
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(1 quat. C), 140.91 (1 quat. C), 142.32 (1 quat. C), 161.78 (1 quat. C), 168.14 (1
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