Cleavage of C-Si bond by intramolecular nucleophilic attack: Lithiation-promoted formation of siloles from 1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives

Article abstract of DOI:10.1016/j.tetlet.2004.11.098

Lithiation of 1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives with t-BuLi afforded substituted siloles in high yields. Cleavage of one of the three C-Si bonds took place via intramolecular nucleophilic substitution. Selective cleavage was observed when the silyl group possessed different substituents. Results showed that vinyl and phenyl substituents on Si were substituted much more easily than methyl groups, whilst a methyl group was exclusively deleted from an i-Pr-SiMe₂ moiety.

Full text of DOI:10.1016/j.tetlet.2004.11.098

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 499–501
Cleavage of C–Si bond by intramolecular nucleophilic attack:
lithiation-promoted formation of siloles from
1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives
Zhihui Wang,a Hongyun Fanga and Zhenfeng Xia,b,
*
aKey Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
College of Chemistry, Peking University, Beijing 100871, China
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
Received 13 September 2004; revised 2 November 2004; accepted 5 November 2004
Available online 7 December 2004
Abstract—Lithiation of 1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives with t-BuLi afforded substituted siloles in high
yields. Cleavage of one of the three C–Si bonds took place via intramolecular nucleophilic substitution. Selective cleavage was
observed when the silyl group possessed different substituents. Results showed that vinyl and phenyl substituents on Si were substi-
tuted much more easily than methyl groups, whilst a methyl group was exclusively deleted from an i-Pr–SiMe2 moiety.
Ó 2004 Elsevier Ltd. All rights reserved.
R
R
Nucleophilic activation and substitution of organosil-
icon groups are fundamental in organosilicon chemistry
and in developing synthetic methods.1 Pentavalent orga-
nosilicates have been proposed as key intermediates in
organolithium mediated nucleophilic activation and
substitution reactions of organosilicon compounds
(Eq. 1).1–6
2)
R
R
R
R
NaHCO3
SiR1R2R3
Li
SiR1R2R3
H
R
R
R
3: 0%
1)
2 t-BuLi
- 50 oC, 1h
2
R
R
SiR1R2R3
Br
desired monolithio
reagent
R
1
2)
R
R
R
NaHCO3
Nu-
SiMe2
1a: R = Et,
Nu SiR3
R'
Nu SiR3
+ R'
R' SiR3
ð1Þ
R1 = R2 = R3 = Me
R
4
Our group has been recently interested in reaction chem-
istry of 1-lithio-1,3-butadiene derivatives.7–11 These
monolithio reagents used in this laboratory are usually
generated in situ from their corresponding monohalo
compounds and t-BuLi. However, when 1-bromo-4-
trimethylsilyl-1,3-butadiene 1a (R = Et, R1 = R2 =
R3 = Me) was treated with 2 equiv of t-BuLi as usually
done, no formation of the desired monolithio reagent
2 was observed (Scheme 1). Instead, interestingly, silole
derivative 4a (R = Et) was obtained in 88% yield
(Scheme 1). In this paper, we would like to report an
alternative procedure for the preparation of silole deriv-
4a: R = Et, 88% yield
Scheme 1. Lithiation-promoted formation of siloles from 1-bromo-4-
trisubstituted silyl-1,3-butadiene derivatives.
atives and preliminary results on organolithium-pro-
moted substituent-dependent selective cleavage of C–Si
bonds.
Multi-substituted silacyclopentadienes (siloles) are use-
ful compounds for the study of Si-containing materi-
als.12 Thus, preparative methods for these compounds
have been of interest. Several papers have appeared
reporting the synthesis of siloles from typical reactions
of dihalosilanes with 1,4-dilithio-1,3-dienes.12 The reac-
tion reported here (Scheme 1) represents a new method
Keywords: Nucleophilic substitution; Organosilicon compounds; Sil-
oles; Pentaorganosilicates.
*
Corresponding author. Tel.: +86 10 6275 9728; fax: +86 10 6275
1708; e-mail: zfxi@pku.edu.cn
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.098

500
Z. Wang et al. / Tetrahedron Letters 46 (2005) 499–501
Table 1. Formation of siloles by lithiation of 1-bromo-4-trisubstituted
silyl-1,3-butadiene derivativesa
warm-up
MeLi
RLi
[Li(THF)4]+
SiMe3
Br
SiMe2
Yield of 4/%b
SiMe3
- 100 o
THF
C
Bromo compound 1
Product 4
Et
Et
Et
5
Et
88 (60)
SiMe3
Br
pentaorganosilicates
SiMe2
4a
4b
1a
Et
Et
Et
Et
Scheme 2.
Et
Et
Et
Et
Et
Et
SiEt3
Br
1b
(82)
SiEt2
trap the organolithium species to understand the reac-
tion mechanism for our reaction. The experimental pro-
cedure is given in Scheme 3. Table 2 demonstrates
results obtained in cases of SiMe3 and SiEt3, in which
MeLi and EtLi are proposed to be generated in situ,
respectively.
Et
Et
Et
Et
Et
Et
Et
Et
SiMe2
Br
1c
99 (80)
SiMe 4c
Et
Et
Et
In cases of 1c,d, and 1e which possess different substitu-
ents on the Si atom, trap of organolithium compounds
generated in situ following the procedure shown in
Scheme 3 with aldehydes was also carried out (Fig. 1).
In case of 1c, only 6a was isolated in 75% yield, which
was consistent with the observation of formation of 4c
(Table 1) as the only silole product. In cases of 1d and
1e, a mixture of two alcohols were isolated, respectively.
Alcohols 6d and 6e with vinyl group or phenyl group
were obtained as the major products, while 6a was ob-
tained in both cases as the minor products (Fig. 1).
Et
SiMe2
Et
Et
Et
Et
SiMe2
Br
1d
1e
1f
69
4a
4a
4d
Et
Et
Et
Et
Ph
SiMe2
Br
Et
Et
Et
Et
78
SiMe2
Et
Pr
Et
Pr
Pr
Pr
Pr
Pr
SiMe3
Br
65 (48)
SiMe2
Pr
Pr
a Reaction conditions are given in Scheme 1.
b GC yields. Isolated yields are given in parentheses.
R
R
SiR'2
R
R
R
R
1) 2 t-BuLi
SiR'3
Br
+
R'Li
R'
- 50 oC, 1h
R
R
for the preparation of multi-substituted siloles via lithi-
ation-promoted nucleophilic substitution of organosil-
icon compounds. Representative results are given in
Table 1. A variety of substituents on the Si atom, such
as methyl, ethyl, isopropyl, vinyl, and phenyl could be
used to afford silole derivatives in good to excellent
yields.
4
1
R
R
2) 1.2 equiv.
4-Ph-PhCHO
SiR'2
+
Ph
C OH
- 50 oC to r. t., 1h
3) H+
R
H
R
4
6
Scheme 3. Trapping of organolithium compounds generated in situ.
In the case of 1c, silole 4c with a methyl group and an
isopropyl group was obtained in excellent yield.13 Obvi-
ously, one of the two methyl groups in 1c was substi-
tuted, leaving the bigger isopropyl group untouched.
However, in cases of 1d and 1e, although other siloles
were detected by GC–MS, 4a was formed as the major
product, indicating that the vinyl group in 1d and the
phenyl group in 1e were much more readily substituted
than methyl groups. Kumada and co-workers have stud-
ied substitution reaction of siloles.3 They found that
methyl groups on Si atoms of siloles could be replaced
by butyl groups in the presence of an excess of BuLi.3
Pentaorganosilicates were proposed as the intermediates
for such substitution reaction. Klumpp and co-workers
reported first observation of pentaorganosilicates 5 by
low-temperature NMR spectroscopy in a special case
(Scheme 2).6
Table 2. Trapping of MeLi and EtLi with aldehydesa
Bromo compound 1
Yield of 4/%b
4a: (60)
Yield of 6/%b
Et
Et
6a: R0 = Me (64)
SiMe3
Br
1a
Et
Et
Et
Et
SiEt3
Br
4b: (82)
4d: (48)
6b: R0 = Et (83)
1b
Et
Et
Pr
Pr
SiMe3
Br
6a: R0 = Me (71)
1f
Pr
Pr
a Reaction conditions are given in Scheme 3.
b Isolated yields.
Since a new organolithium species has been proposed to
be generated in such substitution reaction, we tried to

Z. Wang et al. / Tetrahedron Letters 46 (2005) 499–501
501
Et
References and notes
Et
Et
Me
OH
H
SiMe2
Br
Ph
Ph
Ph
Ph
OH
OH
1. (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C.
Chem. Rev. 1993, 93, 1371–1448, and references cited
therein; (b) Hermanns, J.; Schmidt, B. J. Chem. Soc.,
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3. Ishikawa, M.; Tabohashi, T.; Sugisawa, H.; Nishimura,
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See also:Okinoshima, H.; Yamamoto, K.; Kumada, M. J.
Am. Chem. Soc. 1972, 94, 9263–9264.
H
H
6c
6a
Et
1c
Et
75% isolated yield
0%
Me
OH
H
Et
Et
SiMe2
Br
Ph
6d
6a
Et
53%
20%
1d
Et
4. Damrauer, R.; Danahey, S. E. Organometallics 1986, 5,
1490–1494.
Ph
SiMe2
Br
Et
Et
Me
OH
H
Ph
OH
Ph
5. (a) Gevorgyan, V.; Borisova, L.; Lukevics, E. J. Organo-
met. Chem. 1992, 441, 381–387; (b) Rot, N.; Nijbacker, T.;
Kroon, R.; de Kanter, F. J. J.; Bickelhaupt, F.; Lutz, M.;
Spek, A. L. Organometallics 2000, 19, 1319–1324.
6. (a) de Keijzer, A. H. J. F.; de Kanter, F. J. J.; Schakel, M.;
Schmitz, R. F.; Klumpp, G. W. Angew. Chem., Int. Ed.
1996, 35, 1127–1128; (b) de Keijzer, A. H. J. F.; de Kanter,
F. J. J.; Schakel, M.; Osinga, V. P.; Klumpp, G. W. J.
Organomet. Chem. 1997, 548, 29–32; (c) Couzijn, E. P. A.;
Schakel, M.; de Kanter, F. J. J.; Ehlers, A. W.; Lutz, M.;
Spek, A. L.; Lammertsma, K. Angew. Chem., Int. Ed.
2004, 43, 3440–3442.
7. Preparative methods for 1-bromo-1,3-diene derivatives,
see: (a) Takahashi, T.; Kondakov, D. Y.; Xi, Z.; Suzuki,
N. J. Am. Chem. Soc. 1995, 117, 5871–5872; (b) Taka-
hashi, T.; Sun, W.; Xi, C.; Ubayama, H.; Xi, Z. Tetra-
hedron 1998, 54, 715–726; (c) Ubayama, H.; Sun, W.; Xi,
Z.; Takahashi, T. Chem. Commun. 1998, 1931–1932.
8. Chen, J.; Song, Q.; Wang, C.; Xi, Z. J. Am. Chem. Soc.
2002, 124, 6238–6239.
H
Et
6e
6a
83%
11%
1e
Figure 1.
1) 2 t-BuLi
Me
SiMe2R
Br
Si
R'
- 50 oC, 1h
2) NaHCO3
7
8
8a: R' = Me, GC yield 92%
8b: R' = i-Pr, GC yield 99%
isolated yield 86%,
7a: R = Me
7b: R = i-Pr
7c: R = Ph
8a: R' = Me, isolated yield 86%
Scheme 4.
9. Song, Q.; Li, Z.; Chen, J.; Wang, C.; Xi, Z. Org. Lett.
2002, 4, 4627–4629.
10. Wang, C.; Chen, J.; Song, Q.; Li, Z.; Xi, Z. Arkivoc
2003(2), 155–164.
11. Fang, H.; Song, Q.; Wang, Z.; Xi, Z. Tetrahedron Lett.
2004, 45, 5159–5162.
To compare with the system reported by Klumpp and
co-workers,6 we prepared bromides 7 with methyl, iso-
propyl, and phenyl groups, respectively on the Si atom,
and investigated their selectivity of cleavage of Si–C
bonds following our procedure (Scheme 4). Results ob-
tained for this system were consistent with those shown
in Table 1.
12. For preparation of siloles from reactions of R2SiCl2 and
1,4-dilithio-1,3-dienes, see: (a) Dubac, J.; Laporterie, A.;
Manuel, G. Chem. Rev. 1990, 90, 215–263; (b) Ashe, A. J.,
III; Kampf, J. W.; Al-Taweel, S. M. J. Am. Chem. Soc.
1992, 114, 372–374; (c) Ashe, A. J., III; Al-Ahmad, S.;
Pilotek, S.; Puranik, D. B.; Elschenbroich, C.; Behrendt,
A. Organometallics 1995, 14, 2689–2698; (d) Bankwitz, U.;
Sohn, H.; Powell, D. R.; West, R. J. Organomet. Chem.
1995, 499, C7–C9; (e) Freeman, W. P.; Tilley, T. D.;
Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc.
1996, 118, 10457–10468; (f) Yamaguchi, S.; Jin, R.;
Tamao, K. J. Organomet. Chem. 1998, 559, 73–80; (g)
Xi, C.; Huo, S.; Afifi, T. H.; Hara, R.; Takahashi, T.
Tetrahedron Lett. 1997, 38, 4099–4102.
13. A typical experimental procedure for the formation of 4c.
To a solution of 1c (1.0 mmol) in Et2O (5 mL) was added
t-BuLi (1.5 M pentane solution, 2.0 mmol) at ꢀ50 °C and
the mixture was stirred for 1 h at the same temperature.
The reaction mixture was quenched with aqueous
NaHCO3 and extracted with diethyl ether. The extract
was washed with brine and dried over MgSO4. The solvent
was evaporated in vacuo and the residue was purified by
chromatography on silica gel to afford 4c as colorless
liquid, isolated yield 80% (200 mg), GC yield 99%. NMR
data for 4c: 1H NMR (CDCl3, SiMe4): d 0.19 (s, 3H),
0.94–1.05 (m, 18H), 2.12–2.41 (m, 9H). 13C NMR (CDCl3,
SiMe4): d ꢀ6.01, 12.90, 14.85, 15.30, 17.67, 20.64, 22.56,
136.42, 154.54. HRMS calcd for C16H30Si 250.2117.
Found 250.2121.
In summary, we have developed a new method for prep-
aration of multi-substituted siloles. A pentaorganosili-
cate is proposed to be the intermediate for this
reaction. Preliminary results show that lithiation-pro-
moted nucleophilic substitution reactions of organosil-
icon compounds are substituent dependent. This
observation is expected to have useful applications for
organic synthesis. Further investigation into the reaction
mechanism and applications are in progress.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (20172003, 20232010,
20328201), the Major State Basic Research Develop-
ment Program (G2000077502-D), and Dow Corning
Corporation. Cheung Kong Scholars Programm, Qiu
Shi Science and Technologies Foundation, and BASF
are gratefully acknowledged.