Preparation and oxidative coupling of bis[o-(hydrosilyl)phenyl]cuprates and bis[o-(fluorosilyl)phenyl]cuprates

Article abstract of DOI:10.1016/j.tetlet.2009.01.009

Bis[o-(hydrosilyl)phenyl]cuprates and bis[o-(fluorosilyl)phenyl]cuprates were prepared by reacting [o-(hydrosilyl)phenyl]lithiums and [o-(fluorosilyl)phenyl]lithiums, respectively, with copper salts, such as CuCN and Cu(OPiv)₂. The phenylcuprat

Full text of DOI:10.1016/j.tetlet.2009.01.009

Tetrahedron Letters 50 (2009) 1226–1228
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Tetrahedron Letters
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Preparation and oxidative coupling of bis[o-(hydrosilyl)phenyl]cuprates
and bis[o-(fluorosilyl)phenyl]cuprates
*
Atsushi Kawachi , Takuya Teranishi, Yohsuke Yamamoto
Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis[o-(hydrosilyl)phenyl]cuprates and bis[o-(fluorosilyl)phenyl]cuprates were prepared by reacting
[o-(hydrosilyl)phenyl]lithiums and [o-(fluorosilyl)phenyl]lithiums, respectively, with copper salts, such
as CuCN and Cu(OPiv)2. The phenylcuprates underwent oxidative coupling to afford 2,20-bis(hydrosi-
lyl)biphenyls and 2,20-bis(fluorosilyl)biphenyls.
Received 23 November 2008
Revised 22 December 2008
Accepted 8 January 2009
Available online 10 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
(o-Silylphenyl)lithium
Bis(o-silylphenyl)cuprate
Oxidative coupling
Biphenyl
1. Introduction
zene5 as an oxidant at ꢀ78 °C afforded the comparable yield of
7a (Table 1, entry 3), the yield of 7a was increased (74% yield)
Recently, we reported the preparation of [o-(hydrosilyl)phenyl]-
lithiums 1 (R = Me (a), Ph (b)) and [o-(fluorosilyl)phenyl]lithiums 2
(R = Me (a), Ph (b)) by Br–Li exchange of corresponding aryl bro-
mides 3 and 4, respectively, with tert-butyllithium (Scheme 1).1
Phenyllithiums 1 and 2 reacted with halosilanes and haloboranes to
form silicon-functionalized o-disilylbenzenes1b and o-(boryl)silyl-
benzenes.1c Then, we turned our attention to exchange of the
counter cation (Li+) in 1 and 2 with other metals in order to explore
the synthetic utility of the o-silylphenyl anions. Here, we report the
conversion of 1 and 2 into bis(o-silylphenyl)cuprates 5 and 6 and
the oxidative coupling of 5 and 6, forming silicon-functionalized
2,20-disilylbiphenyls 7 and 8.2 The aim of this work is to confirm
whether the hydrosilyl and fluorosilyl groups are compatible with
intramolecular phenylcuprates and survive oxidative coupling at
the ortho position.
First, we investigated the preparation of bis(o-silylphenyl)cup-
rates 5 and 6 and the oxidative coupling of them according to Lip-
shutz’s procedure (Table 1 and Scheme 1).3 [o-(Dimethylsilyl)
phenyl]lithium (1a) was prepared from phenyl bromide 3a with
tert-BuLi in Et2O at ꢀ78 °C (Scheme 1).1 Treatment of 1a with cop-
per cyanide (0.5 equiv) at the same temperature afforded bis[o-
(dimethylsilyl)phenyl](cyano)cuprate 5a.4 Introduction of gaseous
oxygen into the solution of 5a at ꢀ78 °C or ꢀ110 °C yielded 2,20-
bis(dimethylsilyl)biphenyl (7a) in moderate yields (Table 1, entries
1 and 2). Whereas treatment of 5a with 3 equiv of p-dinitroben-
when 5 equiv of p-dinitrobenzene was used (Table 1, entry 4).
The same treatment performed at ꢀ110 °C, however, resulted in
a lower yield (43% yield) (Table 1, entry 5). In contrast, bis[o-(flu-
orodimethylsilyl)phenyl]cuprate 6a afforded only a trace amount
of 8a, and dimerized product 9a was obtained as a major product
(8a/9a = 6/94 in 1H NMR spectra) under the same reaction condi-
tions (Scheme 1). The highly electrophilic fluorodimethylsilyl
group in 6a was easily attacked by the highly nucleophilic phenyl-
SiR X
SiR X
(a) or (b)
CuCN (x 0.5)
2
2
Et O or THF
−78 °C
2
Br
Li
3: X = H
4: X = F
R = Me (a), Ph (b)
1: X = H
2: X = F
SiR X
2
SiR X
O2N
NO2
(x 5)
2
−78 °C, 1 h
Cu(CN)Li
2
2
r.t.
XR Si
2
5: X = H
6: X = F
7a: R = Me, X = H 74%
52%
trace
39%
7b: R = Ph, X = H
8a: R = Me, X = F
8b: R = Ph, X = F
(a) for 3a and 4a: tert-BuLi (x 1)/ Et O/−78 °C, 2 h
2
(b) for 3b and 4b: tert-BuLi (x 2)/ THF/ −78 °C, 1 h
* Corresponding author. Tel.: +81 82 424 7431; fax: +81 82 424 0723.
E-mail address: kawachi@sci.hiroshima-u.ac.jp (A. Kawachi).
Scheme 1. Preparation and reaction of 5 and 6.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.009

A. Kawachi et al. / Tetrahedron Letters 50 (2009) 1226–1228
1227
Table 1
Oxidative coupling of bis[o-(dimethylsilyl)phenyl](cyano)cuprate 5a
SiMe F
2
SiMe F
2
MX
2
2a
Entry
Oxidant
Temp (°C)
Time (h)
Yield of 7a (%)
Et O
r.t.
2
M
1
2
3
4
5
O2
O2
78
ꢀ110
ꢀ78
0.5
0.5
1
1
1
48
54
45
74
43
temp., time
2
FMe Si
2
8a
10a-M
p-Dinitrobenzene (ꢁ3)
p-Dinitrobenzene (ꢁ5)
p-Dinitrobenzene (ꢁ5)
ꢀ78
Scheme 3. Preparation and reaction of 10a.
ꢀ110
Table 2
cuprate moiety in another molecule of 6a even at the low temper-
ature (Scheme 2).
Oxidative coupling of bis[o-(fluorodimethylsilyl)phenyl]metals 10a–M (M = Cu, Co)
Entry
MX2
Temp (°C)
Time (h)
8a:9a
Lipshutz’s procedure was also applicable to phenyl derivatives
1b and 2b. The reaction of phenyl bromides 3b and 4b with tert-
butyllithium (2 equiv) in THF at ꢀ78 °C provided phenyllithiums
1b and 2b,6 which were treated with copper cyanide (0.5 equiv)
to give phenylcuprates 5b and 6b, respectively (Scheme 1). Subse-
quent oxidation with p-dinitrobenzene (5 equiv) at ꢀ78 °C for 1 h
produced 2,20-bis(diphenylsilyl)biphenyls 7b in 52% yield and 8b
in 39% yield. Corresponding dimerized product 9b was not ob-
tained perhaps because sterically demanding phenyl groups on
the silicon retarded the nucleophilic substitution reaction in 6b
(Scheme 2).
1
2
3
4
CuCl2 (ꢁ2)
ꢀ78
ꢀ78
ꢀ78
ꢀ78
1
1
1
1
5:95
22:78
92:8
3:97
Cu(OTf)2 (ꢁ1)
Cu(OPiv)2 (ꢁ3)
CoCl2 (ꢁ0.5)
SiMe F
2
SiMe F
Cu(OPiv) (x 3)
2
2
2a
–60 °C, 24 h
Et O
2
Cu
2
–78 °C
FMe Si
2
In order to achieve oxidative coupling of the o-(fluorodimethyl-
silyl)phenyl anion, we examined several combinations of 2a with
copper(II) salts (MX2),7,8 in which bis[o-(fluorosilyl)phenyl]cuprate
10a–M (M = Cu) was postulated as an intermediate, as shown in
Scheme 3. It was expected that organocuprate 10a–M (M = Cu)
had lower reactivity toward the fluorosilyl group than cyanocup-
rate 6a, retarding the formation of 9a. In each reaction, phenyllithi-
um 2a was treated with MX2 in Et2O at ꢀ78 °C for 1 h,8 and then
the reaction mixture was allowed to warm to ambient tempera-
ture. The product ratio was determined by 1H NMR spectroscopy,
and the results are summarized in Table 2. Copper(II) dichloride8a
exclusively provided 9a (8a/9a = 5/95) (Table 2, entry 1), and cop-
per(II) bis(triflate) (Cu(OTf)2)8b also preferred the formation of 9a
to that of 8a (ca. 10% yield) (8a/9a = 22/78) (Table 2, entry 2). In
contrast, copper(II) bis(pivalate) (Cu(OPiv)2)8c,9 afforded 8a (ca.
40% yield) in higher yield than 9a (8a/9a = 92/8) (Table 2, entry
3). The other metal(II) salt, cobalt(II) dichloride,8d facilitated the
dimerization rather than the formation of 8a via 10a–M (M = Co),
giving 9a (8a/9a = 3/97) (Table 2, entry 4). We also treated 2a with
tris(acetylacetonate)iron(III) (Fe(acac)3)8e,10 (1.1 equiv) in Et2O
at ꢀ78 °C for 1 h, which preferred 8a (ca. 30% yield) to 9a
(8a/9a = 90/10), but a small amount of disiloxane 11 was formed
together. The reason for the successful result in the case of Cu
(OPiv)2 remains unclear, but it may be partly due to the high solu-
bility of copper(II) pivalate in Et2O, which leads to the complete
conversion of 2a into 10a–M (M = Cu) at the low temperature.
Finally, as the optimized conditions, we demonstrated that
treatment of 2a with Cu(OPiv)2 (3 equiv) in Et2O at ꢀ60 °C for
24 h provided biphenyl 8a in 66% yield together with a small
amount of 9a (8a/9a = 93/7) (Scheme 4).
8a 66%
10a-Cu
(8a/9a = 93/7)
Scheme 4. Oxidative coupling of 2a using Cu(OPiv)2.
ing silicon-functionalized 2,20-disilylbiphenyls 7 and 8 by oxidative
coupling. The present reactions demonstrate for the first time that
the hydrosilyl and fluorosilyl functionalities are compatible with
organocuprates at low temperatures and survive oxidative cou-
pling at the ortho position.
2. Experimental
Preparation of 5a and its oxidative coupling: A solution of tert-
BuLi in pentane (1.53 mol/L, 14.0 mL, 21.4 mmol) was added to a
solution of 3a (4.70 g, 21.8 mmol) in Et2O (40 mL) at ꢀ78 °C over
8 min. The reaction mixture was stirred at that temperature for
2 h. The resulting solution of 1a was transferred via a Teflon tube
to a suspension of CuCN (902 mg, 10.1 mmol) in Et2O (20 mL) at
ꢀ78 °C over 7 min. The reaction mixture was allowed to warm to
ambient temperature with vigorous stirring until the solution be-
came homogeneous (for ca. 16 min). Then the resulting solution
of 5a was cooled again to ꢀ78 °C. A suspension of p-dinitrobenzene
(17.0 g, 101 mmol) in Et2O (100 mL) was added to the solution of
5a at ꢀ78 °C over 15 min. The reaction mixture was stirred at the
same temperature for 1 h, and was then allowed to warm to room
temperature. The reaction mixture was filtered and the filtrate was
concentrated in vacuo. The resulting residue was diluted with hex-
ane (100 mL) and filtered. The filtrate was concentrated in vacuo to
give a brown oil (2.57 g). The oil was subjected to column chroma-
tography on silica gel (100 mL) using hexane as eluent (Rf = 0.6) to
give 7a (2.19 g, 74% yield) as a colorless oil. Compound 7a: 1H NMR
(CDCl3, d): 0.01 (d, J = 4 Hz, 6H), 0.03 (d, J = 4 Hz, 6H), 4.09 (sept,
J = 4 Hz, 2H), 7.21–7.23 (m, 2H), 7.33-7.40 (m, 4H), 7.58–7.60 (m,
2H). 13C NMR (CDCl3, d): ꢀ3.05, ꢀ3.02, 126.66, 128.45, 129.64,
134.65, 136.47, 149.64. 29Si{1H} NMR (CDCl3, d): ꢀ18.7 (d of sept,
In summary, we prepared bis[o-(hydrosilyl)phenyl]cuprates 5
and bis[o-(fluorosilyl)phenyl]cuprates 6, which lead to correspond-
R
R
SiR -F Cu
Si
2
1JSi–H = 191 Hz, JSi-H = 6 Hz). Anal. Calcd for C16H22Si2: C, 71.04;
2
Cu F-R Si
Si
2
H, 8.20. Found: C, 70.79; H, 8.24.
R
R
Preparation of 5b and its oxidative coupling: A solution of tert-
BuLi in pentane (1.58 mol/L, 1.3 mL, 2.0 mmol) was added to a
solution of 3b (339 mg, 1.0 mmol) in THF (2.0 mL) at ꢀ78 °C over
2 min. The reaction mixture was stirred at that temperature for
6a R = Me
6b R = Ph
9a R = Me
9b R = Ph
Scheme 2. Dimerization of 6.

1228
A. Kawachi et al. / Tetrahedron Letters 50 (2009) 1226–1228
1 h to give a solution of 1b. A suspension of CuCN (45 mg,
0.50 mmol) in THF (1.0 mL) was transferred via a Teflon tube to
the solution of 1b at ꢀ78 °C over 3 min. The reaction mixture
was allowed to warm to ambient temperature with vigorous stir-
ring until the solution became homogeneous (for ca. 3 min). The
resulting solution of 5b was cooled again to ꢀ78 °C. A suspension
of p-dinitrobenzene (840 mg, 5.0 mmol) in THF (3.0 mL) was added
to the solution of 5b at ꢀ78 °C over 2 min. The reaction mixture
was stirred at the same temperature for 1 h, and was then allowed
to warm to room temperature. The volume of the reaction mixture
was reduced to ca. 3 mL by evaporation. The concentrated reaction
mixture was diluted with hexane (10 mL) and filtered. The filtrate
was concentrated in vacuo to give a brown solid (353 mg). The so-
lid was subjected to column chromatography on silica gel (15 mL)
using hexane/AcOEt (20:1) as eluent (Rf = 0.23) to give a white so-
lid (247 mg), which was recrystallized from CH2Cl2/hexane to af-
ford 7b (134 mg, 52% yield) as colorless crystals. Compound 7b:
mp: 138–139 °C. 1H NMR (CDCl3, d): 4.94 (s, 2H), 6.94 (d,
J = 7 Hz, 2H), 7.16 (t, J = 7 Hz, 4H), 7.23–7.46 (m, 22H). 13C NMR
(CDCl3, d): 126.51, 127.77, 127.80, 128.83, 129.38, 129.40,
130.21, 132.66, 133.84, 134.26, 135.67, 135.85, 136.24, 149.87.
A suspension of Cu(OPiv)2 (421 mg, 1.58 mmol) in Et2O (1.0 mL)
was added to the resulting solution of 1a at ꢀ78 °C over 5 min.
The reaction mixture was stirred at ꢀ60 °C for 24 h, and was then
allowed to warm to room temperature. The reaction mixture was
diluted with hexane (10 mL) and filtered. The filtrate was concen-
trated in vacuo. The resulting residue was diluted with hexane
(10 mL) and filtered. The filtrate was concentrated in vacuo to give
a greenish oil (71 mg). The oil was subjected to bulb-to-bulb distil-
lation (115–125 °C (bath temperature)/0.1 mmHg) to yield 8a with
small amount of 9a (total 58 mg, 8a:9a = 93:7, 66% yield of 8a) as a
3
colorless oil. Compound 8a: 1H NMR (CDCl3, d): 0.00 (d, JF–H
=
3
8 Hz, 6H), 0.15 (d, JF–H = 8 Hz, 6H), 7.28–7.31 (m, 2H), 7.45–7.48
2
(m, 4H), 7.75–7.78 (m, 2H). 13C NMR (CDCl3, d): ꢀ0.73 (d, JC–F
=
2
16 Hz), 0.90 (d, JC–F = 16 Hz), 127.05, 129.44, 129.82, 134.11
3
2
3
(d, JC–F = 3 Hz), 135.67 (d, JC–F = 15 Hz), 148.54 (d, JC–F = 3 Hz).
19F NMR (CDCl3, d): ꢀ158.19 (sept, JF–H = 8 Hz). 29Si{1H} NMR
3
1
(CDCl3, d): 20.5 (d, JSi–F = 280 Hz). Anal. Calcd for C16H20F2Si2: C,
62.70; H, 6.58. Found: C, 62.41; H, 6.85.
Acknowledgments
29Si{1H} NMR (CDCl3, d): ꢀ22.9 (d, JSi–H = 206 Hz, JSi–H = 5 Hz).
Anal. Calcd for C36H30Si2: C, 83.34; H, 5.83. Found: C, 83.21; H, 5.75.
Preparation of 6b and its oxidative coupling: A solution of tert-
BuLi in pentane (1.58 mol/L, 1.3 mL, 2.0 mmol) was added to a
solution of 4b (360 mg, 1.0 mmol) in THF (2.0 mL) at ꢀ78 °C over
3 min. The reaction mixture was stirred at that temperature for
1 h to give a solution of 2b. A suspension of CuCN (45 mg,
0.50 mmol) in THF (1.0 mL) was transferred via a Teflon tube to
the solution of 2b at ꢀ78 °C over 3 min. The reaction mixture
was stirred at ꢀ78 °C for 12 min to give a homogeneous solution
of 6b. A suspension of p-dinitrobenzene (850 mg, 5.0 mmol) in
THF (3.0 mL) was added to the solution of 6b at ꢀ78 °C over
2 min. The reaction mixture was stirred at the same temperature
for 1 h, and was then allowed to warm to room temperature. The
volume of the reaction mixture was reduced to ca. 3 mL by evapo-
ration. The concentrated reaction mixture was diluted with hexane
(10 mL) and filtered. The filtrate was concentrated in vacuo to give
a brown solid (365 mg). This solid was washed with hexane to
yield pure 8b (108 mg, 39% yield) as colorless crystals. Compound
8b: mp: 139–140 °C. 1H NMR (CDCl3, d): 6.91 (d, J = 8 Hz, 2H), 7.14
(ddd, J = 8 Hz, 8 Hz, and 1 Hz, 2H), 7.23–7.34 (m, 18H), 7.38–7.43
(m, 4H), 7.48 (dd, J = 8 Hz and 1 Hz, 2H). 13C NMR (CDCl3, d):
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Area ‘Advanced Molecular Transformations of
Carbon Resources’ (No. 19020048) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
1
4
References and notes
1. (a) Kawachi, A.; Zaima, M.; Tani, A.; Yamamoto, Y. Chem. Lett. 2007, 36, 362; (b)
Kawachi, A.; Tani, A.; Machida, K.; Yamamoto, Y. Organometallics 2007, 26,
4697; (c) Kawachi, A.; Tani, A.; Shimada, J.; Yamamoto, Y. J. Am. Chem. Soc.
2008, 130, 4222.
2. Examples of Preparation of 2,20-disilylbiphenyls: (a) Kira, M.; Sakamoto, K.;
Sakurai, H. J. Am. Chem. Soc. 1983, 105, 7469; (b) Gross, U.; Kaufmann, D. Chem.
Ber. 1987, 120, 991; Binaphthyl analogs were also reported: (c) Fabris, F.;
Lucchi, O. D. J. Organomet. Chem. 1996, 509, 15; (d) Hoshi, T.; Shionoiri, H.;
Katano, M.; Suzuki, T.; Hagiwara, H. Tetrahedron: Asymmetry 2002, 13, 2167.
and references cited therein.
3. Lipshutz, B. H.; Siegmann, K.; Garcia, E.; Kayer, F. J. Am. Chem. Soc. 1993, 115,
9276.
4. These species are recognized as ‘cyano-Gilman cuprates’.
5. Tamao, K.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc. 1994, 116, 11715.
6. Because of the low solubility of phenyl derivatives 1b and 2b in Et2O, they were
prepared in THF: two equivalents of tert-BuLi were required to consume the
phenyl bromides completely.
7. In oxidative coupling of organometallic compounds using copper(II) salts, it is
postulated that the copper(II) salts serve as oxidants, see: Jukes, A. E. Adv.
Organomet. Chem 1974, 12, 215. and also Ref. 8a.
8. The stoichiometries were based on those given in the referenced papers, and
we believe that these stoichiometries are not far from the optimized ones:
CuCl2: (a) Wingerter, S.; Gornitzka, H.; Bertrand, G.; Stalke, D. Eur. J. Inorg.
Chem. 1999, 173; Cu(OTf)2: (b) Kobayashi, Y.; Taguchi, T.; Tokuno, E.
Tetrahedron Lett. 1977, 42, 3741; Cu(OPiv)2: (c) Muci, A. R.; Campos, K. R.;
Evans, D. A. J. Am. Chem. Soc. 1995, 117, 9075; CoCl2: (d) Koenig, K. E.; Lein, G.
M.; Stuckler, P.; Kaneda, T.; Cram, D. J. J. Am. Chem. Soc. 1979, 101, 3553;
Fe(acac)3: (e) Tsuji, H.; Komatsu, S.; Kanda, T.; Umehara, T.; Saeki, T.; Tamao, K.
Chem. Lett. 2006, 35, 758.
3
126.40, 127.71, 129.69, 130.08, 130.36 (d, JC–F = 14 Hz), 132.15
2
2
2
(d, JC–F = 16 Hz), 133.33 (d, JC–F = 17 Hz), 133.71 (d, JC–F
=
17 Hz), 134.46, 134.48, 135.10, 135.97, 136.00, 149.30. 19F NMR
(CDCl3, d): ꢀ165.67 (s). 29Si{1H} NMR (CDCl3, d): ꢀ4.7 (d, JSi–
1
F = 283 Hz). Anal. Calcd for C36H28F2Si2: C, 77.94; H, 5.09. Found:
C, 77.54; H, 5.30.
Preparation of 10a and its oxidative coupling: A solution of tert-
BuLi in pentane (1.53 mol/L, 0.35 mL, 0.54 mmol) was added to a
solution of 4a (124 mg, 0.53 mmol) in Et2O (10 mL) at ꢀ78 °C over
3 min. The reaction mixture was stirred at that temperature for 2 h.
9. Muto, Y.; Hirashima, N.; Tokii, T.; Kato, M.; Suzuki, I. Bull. Chem. Soc. Jpn. 1986,
59, 3762.
10. Charles, R. G.; Pawlikowski, M. A. J. Phys. Chem. 1958, 62, 440.