Synthesis of biaryls via unusual deoxygenative dimerization of 1,4-epoxy-1,4-dihydroarenes catalyzed by palladium complexes

Article abstract of DOI:10.1021/ol0069204

matrix presented Treatment of various 1,4-epoxy-1,4-dihydroarenes with trichlorosilane in toluene in the presence of a palladium complex affords the corresponding biaryls in good to excellent yields. The process appears to occur via a novel palladium-catalyzed hydrosilylative dimerization of 1,4-epoxy-1,4-dihydroarenes and subsequent elimination of HOSiCl₃ and H₂O.

Full text of DOI:10.1021/ol0069204

ORGANIC
LETTERS
2
001
Vol. 3, No. 6
11-814
Synthesis of Biaryls via Unusual
Deoxygenative Dimerization of
8
1,4-Epoxy-1,4-dihydroarenes Catalyzed
by Palladium Complexes
Huai-Ting Shih, Hung-Hsin Shih, and Chien-Hong Cheng*
Department of Chemistry, Tsing Hua UniVersity, Hsinchu, Taiwan 300
chcheng@mx.nthu.edu.tw
Received November 23, 2000
ABSTRACT
Treatment of various 1,4-epoxy-1,4-dihydroarenes with trichlorosilane in toluene in the presence of a palladium complex affords the corresponding
biaryls in good to excellent yields. The process appears to occur via a novel palladium-catalyzed hydrosilylative dimerization of 1,4-epoxy-
3 2
1,4-dihydroarenes and subsequent elimination of HOSiCl and H O.
Metal-mediated addition of a H-Si bond to an unsaturated
carbon-carbon bond is a powerful process for the synthesis
catalysts for these reactions. Activated bicyclic olefins,
particularly 7-oxa- and 7-azanorbornadienes, show rich
addition chemistry in the presence of metal complexes.
1
6-8
of various alkyl and vinyl silanes. The hydrosilylation also
2
,3
4,5a
5
applies to dienes, diynes, and enynes leading to new
Recently, we observed that these olefins undergo homo [2
2
9
10
11
carbocycles. Palladium complexes are known to be active
+ 2], cross [2 + 2], and [2 + 2 + 2] cyclization in the
presence of nickel complexes. Additions of an electrophile
or nucleophile to activated bicyclic olefins catalyzed by
palladium or nickel complexes are also known.6 Our
(1) (a) Speier, J. L. AdV. Organmetal. Chem. 1979, 17, 407. (b) Chalk,
-8
A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. (c) Colvin, E. W.
Silicon in Organic Synthesis; Butterworth: London, 1981; p 44. (d) Crabtree,
R. H. The Organometallic Chemistry of The Transition Metals, 2nd ed.;
Wiely: New York, 1994; p 229. (e) Tanke, R. S.; Crabtree, R. H. J. Am.
Chem. Soc. 1990, 112, 7984. (f) Weber, W. P. Silicon Reagents for Organic
Synthesis; Springer-Verlag: Berlin, 1983; p 79. (g) Takeuchi, R.; Nitta, S.;
Watanabe, D. J. Org. Chem. 1995, 60, 3045. (h) Fleming, I. Chem. Soc.
ReV. 1981, 110, 83. (i) Molander, G. A.; Dowdy, E. D.; Noll, B. C.
Organometallics 1998, 17, 3754.
(6) (a) Duan, J. P.; Cheng, H. C. Orgaometallics 1995, 14, 1608. (b)
Duan, J. P.; Cheng, H. C. Tetrahedron Lett. 1993, 25, 4019. (c) Moinet,
C.; Fiaud, J. C. Tetrahedron Lett. 1995, 36, 2051. (d) Fugami, K.; Hagiwara,
S.; Kosugi, M. Synlett 1998, 477.
(7) (a) Feng, C. C.; Nandi, M.; Sambaiah, T.; Cheng, C. H. J. Org. Chem.
1999, 64, 3538.
(
2) (a) Molander, G. A.; Nichols, P. J. J. Am. Chem. Soc. 1995, 117,
415. (b) Widenhoefer, R. A.; Perch, N. P. Org. Lett. 1999, 1, 1103. (c)
Stengone, C. N.; Widenhoefer, R. A. Tetrahedron Lett. 1999, 40, 1451.
3) (a) Widenhoefer, R. A.; Decarli, M. A. J. Am. Chem. Soc. 1998,
1
1
6
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(8) (a) Lautens, M.; Fagnou, K.; Taylor, M. Org. Lett. 2000, 2, 1677.
(b) Lautens, M.; Fagnou, K.; Rovis, T. J. Am. Chem. Soc. 2000, 122, 5650.
(c) Lautens, M.; Fagnou, K.; Rovis, T. J. Am. Chem. Soc. 1997, 119, 11090.
(d) Lautens, M.; Renaud, J. L.; Hiebert, S. J. Am. Chem. Soc. 2000, 122,
1804. (e) Lautens, M.; Hiebert, S.; Renaud, J. L. Org. Lett. 2000, 2, 1971.
(f) Lautens, M.; Fillion, E. J. Org. Chem. 1998, 63, 647. (g) Lautens, M.;
Rovis, T. Tetrahedron 1998, 54, 1107.
(9) Huang, D. J.; Cheng, C. H. J. Organomet. Chem. 1995, 490, C1.
(10) Huang, D. J.; Rayabarapu, D. K.; Li, L. P.; Sambaiah, T.; Cheng.
C. H. Chem. Eur. J. 2000, 6, 3706.
4
(
20, 3805. (b) Molander, G. A.; Dowdy, E. D.; Schumann, H. J. Org. Chem.
998, 63, 3386. (c) Molander, G. A.; Julius, M, J. J. Org. Chem. 1992, 57,
347. (d) Perch, N. P.; Widenhoefer, R. A. J. Am. Chem. Soc. 1999, 121,
960.
(4) Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1989, 111,
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6
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5) (a) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc. 1992,
(11) (a) Sambaiah, T.; Huang, D. J.; Cheng, C. H. J. Chem. Soc., Perkin
Trans. 1 2000, 195. (b) Huang, D. J.; Sambaiah, T.; Cheng, C. H. New J.
Chem. 1998, 22, 1147.
1
1
14, 6580. (b) Molander, G. A.; Retsch, W. H. J. Am. Chem. Soc. 1997,
19, 8817.
1
0.1021/ol0069204 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/01/2001

interest in the development of new metal-mediated addition
reactions of bicyclic olefins has led us to explore the addition
of the H-Si bond to bicyclic olefins. Herein, we report a
novel method for the synthesis of various substituted biaryls
from hydrosilylative dimerization of 7-oxanorbornadienes
catalyzed by palladium complexes. This regioselective
catalytic reaction proceeds extremely rapidly via an unusual
reductive dimerization of 7-oxanorbornadienes (Scheme 1).
is completely regioselective; no other regioisomers of 3a and
2a were observed. Control experiments show that no 3a and
2a were formed in the absence of either Pd(dba) or
2
trichlorosilane (entries 2 and 3). Products 3a and 2a were
characterized by NMR and mass data.
To further understand the nature of the present palladium-
catalyzed reaction, the effect of silane, palladium catalyst,
ligand, and solvent used on the product yield was examined.
The results are listed in Table 1. Similar to tichlorosilane,
dichloromethylsilane is active for the conversion of 1a to
2
a and 3a (entry 4). However, chlorodimethylsilane and
triethylsilane do not give the expected products (entries 5-6).
In addition to Pd(dba) , other palladium complexes, Pd-
CH CN) Cl and Pd(OAc) , are also very active catalysts,
but Pd(PPh Cl and Pd(dppe) Cl are completely inactive
Scheme 1
2
(
3
2
2
2
3
)
2
2
2
2
for the reaction (entries 16-19). Instead, 1a underwent ring
opening to give 1-naphthol, when these phosphine complexes
were used. Similarly, addition of PPh
containing Pd(dba) greatly inhibits the formation of 3a and
a (entries 12-13). The choice of solvent is crucial to the
3
to the catalytic solution
2
2
success of the catalytic reaction. Toluene appears to be the
best among the solvents employed. THF and dichloro-
methane gave 3a and 2a in 76 and 85% yields, respectively
The reaction of 7-oxabenzonorbornadiene (1a) with tri-
chlorosilane was carried out in toluene in the presence of
.5 mol % of Pd(dba) at ambient temperature for ca. 1 min.
2
A black precipitate was observed at the end of the reaction.
2
(entries 1, 14-15). Solvents with a high coordinating ability
such as acetonitrile, DMF, and DMSO afford none of the
desired products.
The ratio of 3a and 2a is also affected by the reaction
time and the reagent used. Treating 1a and trichlorosilane
Analysis of the reacted solution showed that 1a was
12
completely converted to 2a and 2,2′-binaphthyl (3a) in 99/1
ratio in 86% combined yield (Table 1. entry 1). The reaction
2
in the presence of Pd(dba) at 0 °C (entry 7) for 3 min gave
a 2a/3a ratio of 95/5. Further stirring of the solution led to
a slow conversion of 2a to 3a (entry 8). Addition of zinc
powder or silica gel to the reaction solution enhances the
transformation (entries 9-11). Treatment of 2a with only
trichlorosilane and silica gel led to the formation of 3a,
smoothly indicating that the dehydration is not a palladium-
catalyzed reaction.
The mechanism for the present binaphthyl formation is
still ambiguous, but presumably binaphthyl 3a is from the
dehydration of the corresponding oxabenzonorbornene 2a.
On the basis of the observed products and known palladium
chemistry, we propose two plausible pathways for the
formation of 2a. The first one as shown in Scheme 2 involves
the coordination of two molecules of 1a to a palladium(0)
species to give a palladium(II) cycle. Reaction of this five-
Table 1. Effect of Reaction Conditions on the Reaction of
7
-Oxabenzonorbornadiene (1a) with Trichlorosilane
time
yield, %b
entrya
catalyst
Pd(dba)2
silanes
solvents (min)
(2a /3a )
1
2
3
4
5
6
7
8
9
HSiCl3
HSiCl3
toluene
toluene
toluene
toluene
1
30
30
1
5
60
3
60
60
60
86 (99/1)
0
0
85 (89/11)
0
0
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2c
Pd(dba)2d
Pd(dba)2c
Pd(dba)2/PPh3 HSiCl3
Pd(dba)2/2PPh3 HSiCl3
Pd(dba)2
Pd(dba)2
HSiCl2Me
HSiClMe2 toluene
HSiEt3
HSiCl3
HSiCl3
HSiCl3
HSiCl3
HSiCl3
toluene
toluene
toluene
toluene
toluene
95 (95/5)
94 (88/12)
96 (11/89)
98 (8/92)
98 (0/100)
28 (88/12)e
33 (100/0)f
76 (99/1)
85 (100/0)
82 (98/2)
78 (95/5)
0g
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
toluene 360
toluene
toluene
THF
CH2Cl2
toluene
toluene
toluene
toluene
5
30
1
1
1
1
30
30
HSiCl3
HSiCl3
Scheme 2
Pd(MeCN)2Cl2 HSiCl3
Pd(OAc)2
Pd(PPh3)2Cl2
Pd(dppe)Cl2
HSiCl3
HSiCl3
HSiCl3
0g
a
Reaction conditions: 1a (1.00 mmol), silane (2.5 mmol), Pd(dba)2 (0.025
mmol), solvent (2 mL). Entries 7-11 were carried out at 0 °C for 1 min
and then left at ambient temperature; the others were at ambient temperature
b
only. Total yields of 2a and 3a. The ratio (2a/3a) is given in parentheses
1
c
and was determined by H NMR. 2 equiv of Zn powder was added in the
d
e
reaction. 120 mg of silica gel was added to the reaction. 49% of 1a was
observed from H NMR. f 1-Naphthol in 67% yield was obtained. g 1-
Naphthol in quantitatively yield was obtained.
1
812
Org. Lett., Vol. 3, No. 6, 2001

membered palladocycle with HSiCl via σ-bond metathesis
3
and subsequent rearrangement gives product 2a. The forma-
Table 2. Results of the Reaction of
tion of metallacycles from two unsaturated carbon-carbon
1,4-Epoxy-1,4-Dihydroarenes with Trichlorosilane
13
bonds is known and is proposed as a key step in many
metal-catalyzed reactions.9
-11,14
In addition, σ-bond metath-
esis between a palladium(II)-carbon bond and a Si-H bond
has been proposed previously.3 The other possible pathways
a,b
3
involve the oxidative addition of HSiCl to a Pd(0) species
as a key step. Subsequent coordination of 7-oxa-
norbornadiene molecules, insertion into Pd-H and Pd-
carbon bonds, â-oxyelimination, and elimination of HOSiCl
3
lead to the formation 2a (Scheme 3). Both pathways appear
Scheme 3
to account for the formation of 2a from 1a and HSiCl
3
satisfactorily. The actual mechanism for the present catalytic
reaction requires further studies, but the first mechanism
(Scheme 2) seems to explain better the coupling of two
molecules of 1 to give 2 or 3.
The binaphthyl formation is successfully extended to
substituted 7-oxabenzonorbornadienes which were synthe-
sized from the corresponding substituted benzynes and
furan.1 The results are summarized in Table 2. Thus 1b
bearing a methyl group at the bridgehead reacts with
trichlorosilane under reaction conditions similar to those for
the formation of compound 3a to give isomers 3b and 3b′
in a 74/26 ratio. In principle, there are three possible isomers,
5
3b, 3b′ and 3b′′, for this reaction. Scheme 3 shows the
plausible intermediates that react with trichlorosilane to give
these three isomers. Because of the rigid nature of 7-oxa-
benzonorbornadiene, these palladocycle intermediates are
expected to be rigid too. The steric repulsion between the
bridgehead groups of the 7-oxabenzonorbornadiene moieties
in each intermediate is likely a dominating factor for the
(
12) (a) Hooker, S. C.; Fieser, L. F. J. Am. Chem. Soc. 1936, 58, 1216.
b) Andersen, N. G.; Maddaford, S. P.; Keay, B. A. J. Org. Chem. 1996,
1, 9556.
(
6
(
13) Canty, A. J. In ComprehensiVe Organometallic Chemistry II; Abel,
E. E., Stone, F. G. A., Wilkinson, G., Eds; Pergamon Press: Oxford, 1995;
Vol 9, Chapter 5.
a
(
14) (a) Sambaiah, T.; Li, L. P.; Huang, D. J.; Lin, C. H.; Rayabarapu,
Reaction conditions for entries 1, 4, 5, and 8: 1 (1.00 mmol), HSiCl3
D. K.; Cheng. C. H. J. Org. Chem. 1999, 64, 3663. (b) Ikeda, S.; Mori, N.;
Sato, Y. J. Am. Chem. Soc. 1997, 119, 4779.
(2.5 mmol), Zn powder (2.00 mmol), Pd(dba) (0.025 mmol) and toluene
(2 mL); ambient temperature with stirring for 6-8 h. For entries 2, 3, and
6: 1 (1.00 mmol), HSiCl (2.5 mmol), Pd(dba) (0.025 mmol), toluene (2
mL); ambient temperature with stirring for ca. 1 min. The ratios were
determined by1 H NMR. c Isolated yields; yields in parentheses were
determined by H NMR using a standard internal method.
2
(15) 1a is commercially available. 1b and 1c: Gribble, G. W.; Kelly,
3
2
b
W. J. Tetrahedron Lett. 1981, 22, 2475. 1d and 1e: Giles, R. G. F.; Hughes,
A. B.; Sargent, M. V. J. Chem. Soc., Perkin Trans. 1 1991, 1581. 1f: Hart,
H.; Lai, C. Y.; Nwokogu, G. C.; Shamouilian, S. Tetrahedron 1987, 43,
1
5
203. 1g and 1h: Banciu, M. D.; Brown, R. F. C.; Coulston, K. J.;
Eastwood, F. W.; Jurss, C. Aust. J. Chem. 1996, 965.
Org. Lett., Vol. 3, No. 6, 2001
813

observed selectivity.16 For example, in intermediate III
Dimethyl-substituted benzonorbornadiene 1c also undergoes
reductive dimerization to afford the expected tetramethyl-
binaphthyl albeit in lower yield. Substitution on the aromatic
ring of 7-oxabenzonorbornadiene does not affect the reduc-
tive dimerization but does influence the ratio of 2/3.
Substrates 1d and 1e both bearing electron-donating alkoxy
groups react smoothly with trichlorosilane to afford the
corresponding biaryls in 87 and 92% yields. In contrast,
dibromo-substituted 7-oxabenzonorbornadiene gave only the
reductive coupling product 2f. Attempts to dehydrate this
product by using zinc metal powder or silica gel failed. Two
other 1,4-epoxy-1,4-dihydroarenes, 1g and 1h, were tested
for the present biaryl formation reaction. The reaction of 1g
with trichlorosilane proceeded smoothly to form three
(Scheme 4), the two bridgehead methyl groups trans to the
Scheme 4
1
7
isomers, 3g, 3g′, and 3g′′, in an ca. 1/2/1 ratio in 88%
combined yield. The reaction of 1h with trichlorosilane gave
1
8
a single product, 3h, in excellent yield.
We have observed an unusual palladium-catalyzed biaryl
formation from 1,4-epoxy-1,4-dihydroarenes and trichloro-
silane via a reductive dimerization intermediate. This method
is very effective for the synthesis of various biaryls that
generally show high photoluminescence. Studies on the
application of these products as materials in an organic light
emitting diode (OLED) are underway.
Acknowledgment. We thank the National Science Coun-
cil of the Republic of China (NSC 89-2119-M-007-010) for
support of this research.
palladium center are expected to exhibit much stronger
repulsion than that in the other intermediates. Thus, the
formation of III is least likely. The formation of 3b as the
major product may be explained on the basis of the fact that
intermediate I which has two hydrogen atoms at the
bridgeheads trans to the palladium center is most favorable.
Supporting Information Available: Experimental pro-
cedures and spectral data for all compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
(
16) The stereochemistry of intermediates I-III is also expected to affect
OL0069204
the steric energy. In all these possible intermediates, only the stereochemistry
with both 1a moieties cis to each other and the palladium center exo to
these two moieties is considered. This stereochemistry facilitates further
reaction of the intermediates with trichlorosilane due to less steric hindrance.
(17) These compounds are known, see: Badger, G. M.; Donnelly, J. K.;
Spotswood, T. M. Aust. J. Chem. 1964, 17, 8.
(18) Buu-Hoi, N. P.; Lavit-Lamy, M. D. Bull. Soc. Chim. Fr. 1966, 2500.
814
Org. Lett., Vol. 3, No. 6, 2001