chosen as a model compound, and cross-coupling reactions of
4-(trans-4-n-propylcyclohexyl)phenyltrimethoxysilane 1a with
1-bromo-3,4-difluorobenzene 3a were attempted.6 Arylsilane
1a was stirred with KF in DMF and then the reaction mixture
was reacted with 3a for 3 h at 120 °C in the presence of a
catalytic amount of palladium(ii) acetate [Pd(OAc)2]. However,
the yield of the desired product 4a was only 2% (Table 1, entry
1). When the fluoride ion source and the solvent were changed
to tetrabutylammonium fluoride (TBAF)† and THF–DMF
mixed solvents, respectively, the yield of 4a increased to 49%
(entry 2). In addition, changing of the catalyst to tetrakis-
(triphenylphosphine)palladium(0) [(Ph3P)4Pd] gave 4a in 71%
yield (entry 3). Under theses conditions, reactions were carried
out in several solvents (entries 4–6), and toluene was found to
be the best solvent (entry 6).
The reaction of 4-bromoanisole 3b with 1a gave not only the
desired 4b (61%) but also the demethoxylated product 4c (29%;
entry 2). Although the mechanism of demethoxylation has not
been elucidated, it is most probable that demethoxylation occurs
before the formation of the new sp2–sp2 bond because the
reaction of the reverse combination of entry 2 showed no
formation of 4c (entry 10).
In these reactions, homo-coupling products were not detected
and other byproducts were also very rare, which made
purification of the products straightforward. Another advantage
of this reaction system is that 1 is very resistant to hydrolysis,
and thus can be stored for a long time without special
precautions to exclude moisture.
Footnotes
Although the yield of 4a was 46% when Pd(OAc)2 was used
as a catalyst, the addition of Ph3P had a remarkable effect
(entries 8–11). The best molar ratio of Pd(OAc)2 and Ph3P was
1:3, which gave 4a in 90% yield (entry 10). From the above
results, it can be seen that (Ph3P)4Pd and Pd(OAc)2–Ph3P are
good catalysts for the reaction. Taking the price, stability and
simplicity of handling into account, Pd(OAc)2–Ph3P is the more
practical.
The reactions of several arylsilanes 1 with aryl bromides
were investigated next under the conditions used in Table 1,
entry 10 (Scheme 2). They proceeded with high chem-
oselectivity and the results are summarized in Table 2.
All reactions gave the desired biaryl derivatives in good
yields except entry 2. In particular, when 3a was used, 4a, 4f,
and 4i were obtained in more than 82% yield within 10 h
(entries 1, 5 and 7). In general, as liquid crystalline compounds
having fluorine atoms on their aromatic rings are known to
exhibit good properties, the present reaction is very useful for
the preparation of fluorinated liquid crystalline compounds.6,7
* E-mail: qyk13654@niftyserve.or.jp
† TBAF containing 5 mass% water was purchased from Aldrich and used
after drying with molecular sieves 4 Å.
References
1 D. Demus, H. Demus and H. Zaschke, Flu¨ssige Kristalle in Tabellen,
VEB Deutsche Verlag fu¨r Grundstoff Industrie, Leipzig, 1976; D. Demus
and H. Zaschke, Flu¨ssige Kristalle in Tabellen II, VEB Deutsche Verlag
fu¨r Grundstoff Industrie, Leipzig, 1984; G. W. Gray and G. R. Luckhurst,
The Molecular Physics of Liquid Crystal, Academic Press, New York,
London, Toronto, Sydney and San Francisco, 1979.
2 For reviews of the preparation of biaryls, see (a) M. T. Sainsbury,
Tetrahedron, 1980, 36, 3327; (b) G. Bringmann, R. Walter and R.
Weirich, Angew. Chem., Int. Ed. Engl., 1990, 29, 977;. (c) T. Hiyama and
Y. Hatanaka, Pure Appl. Chem., 1994, 66, 1471; (d) D. W. Knight,
Comprehensive Organic Synthesis, Pergamon Press, Oxford, New York,
Seoul and Tokyo, vol. 3, pp. 499-516, 1991; (e) N. Miyaura and A.
Suzuki, Synth. Org. Chem. Jpn., 1988, 46, 848; (f) A. Suzuki and N.
Miyaura, J. Synth. Org. Chem. Jpn., 1993, 51, 1043.
3 Y. Hatanaka and T. Hiyama, J. Synth. Org. Chem. Jpn., 1990, 48, 834; Y.
Hatanaka, K. Goda, Y. Okahara and T. Hiyama, Tetrahedron, 1994, 50,
8301.
4 For the reactions of organometallic reagents with tetrachlorosilane, see:
L. W. Breed and W. J. Haggerty, J. Org. Chem., 1960, 25, 126; W. Laarz,
M. Schulten, R. Boese and D. Blaeser, Z. Naturforsch. B, Chem. Sci.,
1992, 47, 1233; N. Auner, R. Probst, F. Hahn and E. Herdtweck,
J. Organomet. Chem., 1993, 459, 25.
5 For the reactions of organometallic reagents with tetramethoxysilane,
see: T. G. Selin and R. West, J. Am. Chem. Soc., 1962, 84, 1856; R.
Tacke, A. Mentlage-Falten, H. Linoh and S. Magada, Z. Naturforsch., B:
Anorg Chem., Org. Chem., 1986, 41B, 649;A. Hosomi, S. Kohra, K.
Ogata, T. Yanagi and Y. Tominaga, J. Org. Chem., 1990, 55, 2415.
6 Y. Goto, T. Ogawa, S. Sawada and S. Sugimori, Mol. Cryst. Liq. Cryst.,
1991, 209, 1.
7 H. J. Plach, B. Rieger, E. Poetsch and V. Reiffenrath, Proc. 10th Int. Disp.
Res. Conf., 1990, 136; G. Weber, U. Finkenzeller, T. Geelhaar,
H. J. Plach, B. Rieger and L. Pohl, Liq. Cryst., 1989, 5, 1381; H. J. Plach,
G. Weber and B. Rieger, Proc. SID Int. Symp., 1990, 91.
Table 2 Preparation of a variety of biaryl compoundsa 4
Entry
1
3
t/h
Product (% yield)
1b
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1b
1c
1d
1e
1e
1f
3a
3b
3c
3d
3a
3e
3a
3f
3
10
25
30
10
30
30
30
30
30
30
4a (90)
4b (61), 4c (29)
4d (74)
4e (87)
4f (92)
4g (84), 4h (4)
4i (82)
4j (72)
4k (84)
4b (75)
3g
3h
3i
10
11
1g
4f (78)
a
Pd(OAc)2 (5 mol%)-Ph3P (15 mol%) was used as catalyst and the
reactions were carried out in toluene. b The same data as shown in Table 1,
entry 10.
Received in Cambridge, UK, 13th March 1997; Com.
7/01770G
1310
Chem. Commun., 1997