Synthesis of 1,2-bis(trimethylsilyl)benzene derivatives from 1,2-dichlorobenzenes using a hybrid metal Mg/CuCl in the presence of LiCl in 1,3-dimethyl-2-imidazolidinone

Article abstract of DOI:10.1021/jo4000866

A practical and safe synthesis of 1,2-bis(trimethylsilyl)benzene from 1,2-dichlorobenzene and Me₃SiCl was achieved by use of a hybrid metal of Mg and CuCl in the presence of LiCl in 1,3-dimethyl-2-imidazolidinone (DMI). This method does not req

Full text of DOI:10.1021/jo4000866

Note
pubs.acs.org/joc
Synthesis of 1,2-Bis(trimethylsilyl)benzene Derivatives from 1,2-
Dichlorobenzenes Using a Hybrid Metal Mg/CuCl in the Presence of
LiCl in 1,3-Dimethyl-2-imidazolidinone
Tsugio Kitamura,*,† Keisuke Gondo,† and Toshimasa Katagiri‡
†Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga
840-8502, Japan
‡Department of Applied Chemistry, Faculty of Engineering, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530,
Japan
S
* Supporting Information
ABSTRACT: A practical and safe synthesis of 1,2-bis(trimethylsilyl)-
benzene from 1,2-dichlorobenzene and Me3SiCl was achieved by use of a
hybrid metal of Mg and CuCl in the presence of LiCl in 1,3-dimethyl-2-
imidazolidinone (DMI). This method does not require a toxic HMPA,
provides a high yield of the product under mild conditions, and is also
applied to synthesis of substituted 1,2-bis(trimethylsilyl)benzenes and poly(trimethylsilyl)benzenes.
Scheme 1. Representative Procedure for Synthesis of 1
rylsilanes are useful intermediates in organic synthesis and
utilized for several functional group transformations,1 and
A
palladium-catalyzed Hiyama coupling reactions.2 Recently,
much attention has been paid to organosilicone compounds
as functional materials applicable for optoelectronic devices due
to the presence of σ*−π* conjugation.3 In addition, it has been
suggested that bifunctional disilylbenzene derivatives are
valuable compounds for key materials as a building block for
functional materials.4
1,2-Bis(trimethylsilyl)benzene (1) is a useful starting material
for preparing an efficient hypervalent iodine−benzyne
precursor, (phenyl)[2-(trimethylsilyl)phenyl]iodonium triflate
(2).5 The 1,2-bis(trimethylsilyl)benzenes are also used for
synthesis of 9,10-dihydro-9,10-diboraanthracene as the building
block of luminescent boron-containing polymers6 and func-
tional materials7 and 9,10-dimethyl-9,10-diboraanthracene,
which is a powerful Lewis acid catalyst.8 Furthermore, synthesis
of functionalized 1,2-bis(trimethylsilyl)benzenes has been
furnished by C−H activation and functionalization of the
parent 1,2-bis(trimethylsilyl)benzene.9 Although there are
interesting applications, the chemistry of 1,2-disilylbenzene
derivatives has not been greatly studied. This is because 1,2-
bis(trimethylsilyl)benzene has some serious problems in the
synthesis. Therefore, development of synthetic methods of
silylbenzenes is a challenging research subject which will be
indispensable to development of promising organosilicon
chemistry, organic synthesis, and functional materials.
For these reasons, some alternative methods using 1,2-
dibromobenzene, instead of 1,2-dichlorobenzene, have been
recently reported for synthesis of 1,2-bis(trimethylsilyl)-
benzene, as outlined in Scheme 2. Bettinger and Filthaus
reported synthesis of 1,2-bis(trimethylsilyl)benzene using a
specific and controlled procedure: lithiation of 1,2-dibromo-
benzene with t-BuLi cooled at −70 °C in THF/Et2O at −120
°C followed by silylation with Me3SiOTf cooled at −70 °C.12
This procedure needs very low temperature conditions,
expensive chemicals, and inconvenient operation.
Wegner et al. reported more convenient methods for
synthesis of 1,2-bis(trimethylsilyl)benzene.13 One method
uses Mg activated with DIBAL-H. Another one includes Mg
activated with FeCl3 and DIBAL-H in the presence of TMEDA.
However, the yields of bis(trimethylsilyl)benzene are 37 and
41%, respectively. Wagner et al. reported improved methods
using Rieke Mg and Mg activated by 1,2-dibromoethane,14
giving 1,2-bis(trimethylsilyl)benzene in 65 and 62% yields,
respectively.
Although the synthesis of 1,2-bis(trimethylsilyl)benzene has
been performed by several methods shown in Scheme 2, most
methods do not give the product in a high yield, and an
expensive 1,2-dibromobenzene is essential as starting material
in any case. The method using lithiation with t-BuLi needs
special and inconvenient operation. Therefore, the desirable
Previously, we reported the most reliable and convenient
method for synthesis of 1,2-bis(trimethylsilyl)benzene, which is
furnished by the reaction of 1,2-dichlorobenzene with
chlorotrimethylsilane in the presence of Mg and a catalytic
amount of iodine in HMPA (Scheme 1).5,10 However, this
method suffers the use of toxic, carcinogenic HMPA11 as
solvent and the severe conditions of high temperature and long
reaction time.
Received: January 21, 2013
© XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Note
a
Scheme 2. Recently Reported Methods for Synthesis of 1
Table 1. Optimization of the Reaction Conditions
b
yield (%)
entry
LiCl (mmol)
temp (°C)
1
5
1
2
3
4
5
6
7
0
0
rt
90
7
60
11
3
50
44
52
60
72
53
84
89
45
2
0
120
90
1
0
4
90
0
8
90
0
12
8
90
0
c
8
90
0
d
9
8
90
0
d
d
g
,
e
f
10
11
12
8
90
12
67
0
,
8
90
100
0
a
Conditions: Mg turnings (8 mmol), CuCl (1 mmol), LiCl, 3 (1
mmol), Me3SiCl (16 mmol), and DMI (10 mL) for 17 h at 90 °C.
b
c
Yields were determined by 1H NMR. Mg powder (99.0%) (8 mmol)
d
was used instead of Mg turnings. Mg power (99.9%) (8 mmol) was
used instead of Mg turnings. DMI (2 mL) was used. DMI (5 mL)
and THF (5 mL) were used. Conditions: 3 (1 mmol), Me3SiCl (4
e
f
g
mmol), Mg (4 mmol), a catalytic amount of I2, and DMI (10 mL) at
100 °C for 48 h.
synthetic method should involve a convenient procedure, a
high yield of the product, and a cheap 1,2-dichlorobenzene as a
starting material without toxic HMPA. However, to the best of
our knowledge, there are no reports on such a safe and cheap
method of preparing 1,2-bis(trimethylsilyl)benzenes starting
from 1,2-dichlorobenzenes. Recently, Katagiri et al. reported
that a reagent composed of Mg and CuCl caused reductive
silylation of trifluoroacetates in 1,3-dimethyl-2-imidazolizinone
(DMI).15 Inspired by this result, we envisaged a hybrid
Grignard reagent prepared from Mg and CuCl for synthesis of
1,2-bis(trimethylsilyl)benzene from 1,2-dichlorobenzene. Here,
we report for the first time an efficient synthesis of 1,2-
bis(trimethylsilyl)benzene from 1,2-dichlorobenzene without
HMPA as a solvent.
First, we examined the reaction temperature roughly under
the conditions of Mg turnings and CuCl in DMI (Table 1,
entries 1−3). The reaction of 1,2-dichlorobenzene with
chlorotrimethylsilane at room temperature for 17 h gave 1,2-
bis(trimethylsilyl)benzene (1) and 1-chloro-2-trimethylsilyl-
benzene (5) in 7 and 60% yields, respectively. The reaction
at 90 °C for 17 h afforded the product 1 in 50% yield, but
further increasing the temperature to 120 °C resulted in a slight
decrease in the yield of 1 (44%).
Since Krasovskiy and Knochel clarified that addition of LiCl
to i-PrMgCl or s-BuMgCl promotes selective halogen−metal
exchange,16 we expected that the addition of LiCl would assist
the silylation reaction more effectively. Under the above
conditions at 90 °C, the addition of LiCl was examined (Table
1, entries 4−7). Surprisingly, we observed that 1,2-bis-
(trimethylsilyl)benzene was only formed in 52% yield when
an equimolar amount of LiCl was added. Since the formation of
1-chloro-2-(trimethylsilyl)benzene was controlled completely,
the addition effect of LiCl is critical. When the quantity of LiCl
was increased to 4 or 8 mmol, the yield of the product also
increased, but further increments of LiCl decreased the yield.
The best result was obtained when LiCl of 8 mmol was used.
Next, we examined three kinds of magnesium available
commercially, i.e., Mg turnings, Mg powder (99.0%), and Mg
powder (99.9%) (Table 1, entries 8 and 9). Although a good
yield of the product was obtained even by using Mg turnings,
use of Mg powder gave the better result. When Mg powder
with high purity was used, the product was formed in 89%
yield.
When the amount of DMI was decreased to 2 mL, the yield
of the product was decreased to 47% and the monosilylated
product was also formed (entry 10). Use of a 1:1 mixture of
DMI and THF drastically decreased the product and afforded
the monosilylated product in 67% yield (entry 11), suggesting
that addition of THF was not desirable as solvent. Furthermore,
we examined the reaction under the original conditions5 using
DMI instead of HMPA (entry 12). However, none of the
product was formed, and 1,2-dichlorobenzene was recovered.
Therefore, the addition of CuCl and LiCl is essential to
promote the bis-silylation.
Since the optimum temperature in the presence of LiCl was
not examined in detail, the temperature effect was examined
again. The results are given in Table 2. In order to compare
with the conditions at 90 °C (entry 4), the reaction in a
temperature lower than 90 °C was considered. The reaction at
0 °C afforded 1 and 5 in 12 and 67% yields, respectively (entry
1). At 25 °C, 1 and 5 were formed in 43 and 56% yields,
respectively (entry 2). When the reaction was conducted at still
higher 55 °C, 1 was only formed in 93% yield (entry 3). These
results indicate that the bis-silylation reaction takes place
stepwise through the monosilylated product. To recognize the
utility of this method, the reaction was applied a 50 mmol scale
(entry 4). This result shows that this reaction can be scaled up
easily.
With the optimized conditions for the disilylation reaction in
hand, we explored the scope of this reaction, as shown in Table
3. As a substrate of the bis-silylation reaction, we chose
dichlorotoluenes (6a and 6b), dichloroanisoles (6c and 6d),
1,2-dichloro-4-fluorodichlorobenzene (6e), 1,2,4-trichloroben-
B
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a
applicable for polysilylation. Although these reactions are
preliminary experiments, these results suggest that the present
procedure is useful for the silylation reaction of various
dichlorobenzene derivatives.
Table 2. Effect of Temperature in the Presence of LiCl
b
b
The bis-silylation reaction is explained as follows. A hybrid
metal prepared from Mg powder and CuCl showed a drastic
behavior in the bis-silylation reaction of 1,2-dichlorobenzene
derivatives using DMI solvent in the presence of LiCl. The role
of CuCl is considered to enhance the reduction ability by
depositing Cu(0) on the magnesium surface.15 Addition of LiCl
in DMI increases the polarity of the solvent system to promote
electron transfer. In addition, as suggested by Krasovskiy and
Knochel,16 the addition of LiCl may break the polymeric
aggregate of Grignard reagents generated in situ to increase the
reactivity. As a result, electron transfer efficiently takes place to
produce 2-chlorophenylmagnesium chloride, which reacts with
chlorotrimethylsilane to give 1-chloro-2-(trimethylsilyl)benzene
(5). The similar reaction of 5 successively occurs to form 1,2-
bis(trimethylsilyl)benzene (1). The successive process is
supported by the selective formation of monosilylated product
5 in the reaction at low temperature. In addition, we examined
the reaction of 1,2-dibromobenzene under the present
conditions to compare the reactivity. 1,2-Dibromobenzene
showed a high reactivity and reacted even at 0 °C with a good
yield.
entry
temp (°C)
yield of 1 (%)
yield of 5 (%)
1
2
3
0
25
55
55
90
12
43
93
87
89
67
56
0
c
4
d
0
5
0
a
Conditions: Mg powder (8 mmol), CuCl (1 mmol), LiCl (8 mmol),
3 (1 mmol), Me3SiCl (16 mmol), and DMI (10 mL) for 17 h. Yields
were determined by H NMR. Mg powder (400 mmol), CuCl (50
mmol), LiCl (400 mmol), 3 (50 mmol), Me3SiCl (800 mmol), and
b
c
1
d
DMI (300 mL) were used. Same as Table 1, entry 9.
a
Table 3. Silylation of 1,1-Dichlorobenzene Derivatives
In summary, we have demonstrated that a hybrid Grignard
reagent composed Mg and Cu is highly effective for bis-
silylation of 1,2-dichlorobenzene with chlorotrimethylsilane in
DMI. This approach provides an effective synthesis of 1,2-
bis(trimethylsilyl)benzene with a high yield. A number of
previously inaccessible but potentially useful substituted 1,2-
disilylbenzenes can now be synthesized in a straightforward
synthesis. Therefore, it is the most outstanding method in the
synthesis of 1,2-bis(trimethylsilyl)benzene reported to date.
This method is also applicable for synthesis of substituted 1,2-
bis(trimethylsilyl)benzene derivatives and poly(trimethylsilyl)-
benzenes.
EXPERIMENTAL SECTION
■
Reaction of 1,2-Dichlorobenzene (3) with Chlorotrimethyl-
silane in the Presence of Mg, CuCl, and LiCl in DMI. To a
mixture of Mg turnings (8 mmol), CuCl (1 mmol), LiCl (the amount
given in Table 1), and chlorotrimethylsilane (16 mmol) in DMI (10
mL) was added 3 (1 mmol). The mixture was gradually heated to the
temperature described in Table 1 and stirred for 17 h. After being
cooled to room temperature, the reaction mixture was poured into a
mixture of saturated NaHCO3, hexane, and ice. The resulting
precipitates were filtered off, and the filtrate was extracted with
hexane. The combined organic extract was washed with brine, dried
over anhydrous Na2SO4, and concentrated by a rotary evaporator. The
yield of the products was determined by 1H NMR. The products were
separated by column chromatography on silica gel with hexane as
eluent.
a
Conditions: Mg powder (8 mmol), CuCl (1 mmol), LiCl (8 mmol),
substrate (1 mmol), Me3SiCl (16 mmol), and DMI (10 mL) for 17 h.
b
c
Isolated yields by column chromatography. Reaction time, 36 h.
zene (8), and 1,2,4,5-tetrachlorobenzene (9). When the
reaction of 6a was conducted under the optimized conditions,
3,4-bis(trimethylsilyl)toluene (7a) was obtained in 92% yield.
However, a similar reaction of 6b decreased the yield (50%) of
the corresponding bis-silylated toluene 7b presumably due to
the steric hindrance. Similar results were obtained in the case of
dichloroanisoles, which gave the corresponding bis-silylated
anisoles (7c and 7d) in 87 and 56% yields, respectively. The
reaction of a fluorinated dichlorobenzene, 1,2-dichloro-4-
fluorobenzene (6e), also proceeded efficiently to afford 1-
fluoro-3,4-bis(trimethylsilyl)benzene (7e) in 88% yield. Even in
the case of 1,2,4-trichlorobenzene (8) and 1,2,4,5-tetrachlor-
obenzene (9), use of the same conditions as above gave 1,2,4-
tris(trimethylsilyl)benzene (10) and 1,2,4,5-tetrakis-
(trimethylsilyl)benzene (11) in 88 and 78% yields, respectively.
This result suggests that the present silylation reaction is also
1,2-Bis(trimethylsilyl)benzene (1).5b The product was obtained
in 93% yield (Table 2, entry 3) as a colorless oil: 1H NMR (300 MHz,
CDCl3) δ 0.37 (s, 18H), 7.31−7.34 (m, 2H), 7.65−7.68 (m, 2H); 13
NMR (75 MHz, CDCl3) δ 2.5, 128.2, 135.6, 146.1.
C
1-Chloro-2-(trimethylsilyl)benzene (5).17 The product was
1
obtained in 60% yield (Table 1, entry 1) as colorless oil: H NMR
(300 MHz, CDCl3) δ 0.38 (s, 9H), 7.20−7.35 (m, 3H), 7.43−7.46 (m,
1H); 13C NMR (75 MHz, CDCl3) δ 0.8, 125.9, 129.2, 130.5, 135.6,
138.7, 141.0.
General Procedure for Preparation of 1,2-Bis(trimethylsilyl)-
arenes. To a mixture of Mg powder (8 mmol), CuCl (1 mmol), LiCl
(8 mmol), DMI (10 mL), and chlorotrimethylsilane (16 mmol) was
added 1,2-dichloroarene (1 mmol). The mixture was gradually heated
C
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The Journal of Organic Chemistry
Note
to 55 °C and stirred at that temperature for 17 h. After being cooled to
room temperature, the reaction mixture was poured into a mixture of
saturated NaHCO3, hexane, and ice. The resulting precipitates were
filtered off, and the filtrate was extracted with hexane. The combined
organic extract was washed with brine, dried over anhydrous Na2SO4,
and concentrated by a rotary evaporator. The crude product was
purified by column chromatography on silica gel with hexane or
hexane/CH2Cl2 as eluent or distilled under reduced pressure.
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1
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1
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7.35 (dd, J = 3.0, 10.8 Hz, 1H), 7.63 (dd, J = 6.6, 8.4 Hz, 1H); 13C
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1
as colorless crystals: mp 41−42 °C; yield 0.259 g (88%); H NMR
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J = 7.5 Hz, 1H), 7.65 (d, J = 7.5 Hz, 1H), 7.83 (s, 1H); 13C NMR (100
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1,2,4,5-Tetrakis(trimethylsilyl)benzene (11).19 The product was
obtained as colorless crystals: mp 169−170 °C; yield 0.286 g (78%);
1H NMR (300 MHz, CDCl3) δ 0.37 (s, 36H), 7.96 (s, 2H); 13C NMR
(75 MHz, CDCl3) δ 1.8, 141.5, 144.8.
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ASSOCIATED CONTENT
■
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S
* Supporting Information
1H and 13C NMR spectra of new products 7c and 7d. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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1995, 499, 63−71.
AUTHOR INFORMATION
■
Corresponding Author
*Tel/Fax: 0952-28-8550. E-mail: kitamura@cc.saga-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was partly supported by A-STEP from Japan
Science and Technology Agency. We also acknowledge Mitsui
Chemicals, Inc., (DMI) and Shin-Etsu Chemical Co.
(chlorotrimethylsilane) for their generous gifts.
D
dx.doi.org/10.1021/jo4000866 | J. Org. Chem. XXXX, XXX, XXX−XXX