Lithio siloles: Facile synthesis and applications

Article abstract of DOI:10.1021/ja070404s

Lithio siloles of diversified structures and substitution patterns were highly efficiently formed from readily available silyl 1,4-dilithio-1,3-butadienes via novel intramolecular skeletal rearrangements. Copyright

Full text of DOI:10.1021/ja070404s

Published on Web 02/23/2007
Lithio Siloles: Facile Synthesis and Applications
Chao Wang,† Qian Luo,† Hui Sun,† Xiangyu Guo,† and Zhenfeng Xi*,†,‡
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,
and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, Shanghai 200032, China
Received January 19, 2007; E-mail: zfxi@pku.edu.cn
Scheme 1
Functionalized siloles have become ever more attractive in recent
years, since these types of compounds have practical applications
including new π-system organic materials of electronic and opto-
electronic devices.1-3 Among synthetic methods for siloles, the
Tamao-Yamaguchi method for in situ generation of 2,5-dilithio
siloles and lithio benzosiloles has been demonstrated to be
particularly important, because a wide variety of functionalized
siloles can be readily synthesized by further applications of 2,5-
dilithio siloles and lithio benzosiloles.1,2 In this Communication,
we would like to report a facile synthesis of a new type of lithio
siloles 2 from readily available silyl 1,4-dilithio 1,3-butadiene
derivative 1,4,5 via novel reaction mechanisms (eq 1). These lithio
siloles 2 are more general in terms of substitution patterns and
synthetic methods, affording diversified structures, as demonstrated
by their further applications.
of functionalized siloles in one-pot. Diversified utilization of the
unique structures of thus obtained siloles can be further expected.
The reaction mechanism is very intriguing. Given in Scheme 2
1,4-Bis(trimethylsilyl)-2,3-diphenyl-1,4-dilithio-1,3-diene 1a (1
are two proposed mechanisms for this useful reaction. First, the
mmol) in 5 mL of diethyl ether was quantitatively generated in
electrocyclic ring-closing of 1 giving 6 is assumed to be a key step
situ from its corresponding 1,4-diiodo compound 3a (1 mmol) and
for path a.6 It has been well-documented that (1Z,3Z)-1,4-dilithio-
t-BuLi (4 mmol) at -78 °C (Scheme 1).4,5 The solution was then
heated to reflux in the presence of HMPA and maintained for 1 h.
The reaction was very clean, affording the R-silylated silole
derivative 4a in 89% isolated yield upon hydrolysis with water.
When the reaction mixture was quenched with D2O instead, the
deuterated compound 4aD was obtained in 88% isolated yield with
D incorporation more than 98%. Similarly, the dibutyl substituted
1b and the dihexyl 1c also afforded siloles 4b and 4c in 85% and
1,3-butadienes like 1 generally take the s-cis conformation and a
double-bridged dilithium structure,7,8 which is proposed to be the
crucial factor leading to the unprecedented facile electrocyclic
formation of 6.6 For path b, the reversion of the stereochemistry at
the terminal carbon bonding with a Li atom and a SiMe3 group of
1 forming 8 is assumed to be the essential step. Configurational
E/Z isomerization of 1-(trimethylsilyl)-1-alkenyllithium compounds
has been reported in the literature.9,10 Then, an intramolecular
86% isolated yields, respectively. Termination of the reaction with
anionic attack on the adjacent SiMe3 group followed by release of
I2 afforded 2-iodo-5-trimethylsilyl silole 5a in 73% isolated yield.
The structure of 5a has been determined by single-crystal X-ray
structural analysis (CCDC 631085).
MeLi might result in the formation of 2-lithiosiloles 2.11 When the
reaction was carried out without HMPA, no formation of 2 was
Other types of silylated butadienes were found to undergo similar
reactions affording their corresponding siloles (Figure 1). Interest-
ingly, the monosilylated butadiene 1e and 1f could also proceed
this reaction highly selectively to form silole 4e and 4f in 82% and
55% isolated yields, respectively.
All the above results indicated that 1,4-dilithio-1,3-dienes 1
underwent novel intramolecular skeletal rearrangements affording
the very useful lithio silole derivatives 2.
As demonstrated in Figure 2, these readily and efficiently
generated lithiosiloles 2 can be applied for the synthesis of a variety
† Peking University.
‡ State Key Laboratory of Organometallic Chemistry.
Figure 1.
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10.1021/ja070404s CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
and structural diversity; (3) formed via novel reaction patterns.
Further investigation into the reaction mechanism, scope, and
applications is in progress.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China and the Major State Basic
Research Development Program (Grant 2006CB806105). The
Cheung Kong Scholars Programme, Qiu Shi Science & Technolo-
gies Foundation, Dow Corning Corporation, and BASF are grate-
fully acknowledged.
Supporting Information Available: Experimental details, copies
1
of H and 13C NMR spectra for all isolated compounds and crystal-
lographic data for 5a. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 2. One-pot synthesis of 5 via 2a and (a) SiMe3Cl; (b) SiPh3Cl; (c)
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Scheme 2
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In conclusion, we report in this paper a new type of lithio siloles,
which can be complementary to those Tamao-Yamaguchi re-
agents.1,2 These new lithio siloles have the following features: (1)
readily available; (2) more general in terms of substitution patterns
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