Proline catalyzes direct C-H arylations of unactivated arenes

Article abstract of DOI:10.1021/jo3008594

Several amino acids were tested to catalyze the transition-metal-free direct C-H arylation of unactivated benzene derivatives. Among them, proline was found to be an excellent catalyst for the cross-coupling between aryl halides and unactivated arenes. The reaction presumably involves an aryl radical anion as the intermediate based on several experiments. The reaction using this catalyst system offers an option toward establishing an environmentally benign and cost-effective route to biaryls.

Full text of DOI:10.1021/jo3008594

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Article
Proline-Catalyzes Direct C-H Arylations of Unactivated Arenes
Kouichi Tanimoto, Minoru Ueno, Kazutaka Takeda, Mitsunori Kirihata, and Shinji Tanimori
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 24 Aug 2012
Downloaded from http://pubs.acs.org on August 25, 2012
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Proline-Catalyzes Direct C-H Arylations of Unactivated Arenes
Kouichi Tanimoro, Minoru Ueno, Kazutaka Takeda, Mitsunori Kirihata, Shinji
Tanimori*
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Department of Bioscience and Informatics, Graduate School of Life and Environmental
Sciences, Osaka Prefecture University, 1-1 Gakuencho, Nakaku, Sakai, Osaka 599-8531,
Japan
tanimori@bioinfo.osakafu-u.ac.jp
ABSTRACT: Several amino acids were tested to catalyze the transition-metal-free
direct C-H arylation of unactivated benzene derivatives. Among them, proline was
found to be an excellent catalyst for the cross-coupling between aryl halides and
unactivated arenes. The reaction presumably involves an aryl radical anion as the
intermediate based on several experiments. The reaction using this catalyst system
offers an option toward establishing an environmentally benign and cost-effective route
to biaryls.
INTRODUCTION
Biaryl compounds are frequently found in nature as biologically active molecules,
1
such like antibiotics and enzyme inhibitors, for the treatment of hypertension as well as
bipolar disorders. Biaryl units are also an important structural motif for the
2
development of pharmaceuticals and functionalized materials. Consequently, the
3
4
formation of aryl-aryl bonds has attracted much interest over the past decades.
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The traditional synthesis of biaryl compounds involves the transition-metal-catalyzed
5
arylation reactions of organometallic reagents with haloarenes.
However,
organometallic reagents are usually unstable and need to be initially synthesized from
benzene. Recently, the transition-metal catalyzed arylation between unactivated arenes
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and aryl halides through substitution at the C-H bonds has been developed. These
methods still require a transition metal catalyst which sometimes consists of forming
impurities in the product. A remarkable breakthrough has been conducted by Itami
and co-workers who found that potassium t-butoxide alone can promote the biaryl
coupling of electron-deficient nitrogen heterocycles and haloarenes during microwave
7
8
irradiation.
The combinations of an organic catalyst including DMEDA,
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,10-phenanthroline and derivatives , and quinoline-1-amino-2-carboxylic acid and
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1
KOt-Bu and/or NaOt-Bu were subsequently identified as effective catalyst systems for
promoting the transition-metal-free cross coupling reactions of aryl halides with
unactivated arenes. Organocatalysis emerged as a very rapidly growing area for recent
12
chemical syntheses because of its environmental friendliness.
We now describe an
amino acid-promoted method for accessing biaryl compounds via the direct C-H
arylation of unactivated arenes.
RESULTS AND DISCUSSION
4-Iodoanisole 1a was selected as a model substrate for the reaction of benzene 2 in
the presence of a series of amino acids. Utilizing proline as a catalyst (20 mol%), the
reaction afforded the desired 4-methoxybiphenyl 3a in only 4% yield along with the
recovered 4-iodoanisole 1a (86%) at 80 ℃ for 24 h in the presence of KOt-Bu in a
pressure-resistant tube (Table 1, entry 1). Gratifyingly, the yields increased to 47%
and 87 % for the same reactions when the temperature was increased 120 and 150 ℃,
respectively (entries 2 and 3). Other amino acids and derivatives were also tested as
listed in entries 4 to 12. Most of the examined amino acids displayed low to moderate
efficiencies except for sarcosine, which had an effect comparable to that of proline and
found to be the catalyst of choice for the arylation reaction (entry 9). Binol and
1
,2-diaminocyclohexanediylbis(2-pyridinecarboxamide) L1 also acted as promoters
albeit in moderate yields (entries 13 and 14). It was found that the choice and the
presence of a base are crucial for this transformation. Notably, the reactions with
K CO , K PO , and NaO-t-Bu and no base did not entirely take place (entries 15-17 and
2
3
3
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1). The amount of base (3 equiv.) would also be important for the complete
consumption of the stating materials (entry 3 vs. entry 19). A control experiment
without the use of catalyst resulted in a decreased yield (entry 20).
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Table 1. Optimization of reaction conditions for the reaction with 4-iodoanisole 1 and benzene 2
b
entry
catalyst
proline
base
t (℃)
40
yield (%)
1
2
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
4
proline
120
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
47
87
62
38
63
6
3
proline
4
2-phenylglycine
phenylalanine
leucine
5
6
7
valine
8
pipecolic acid
sarcosine
N,N’-dimethylglycine
trans-4-OH-proline
prolinamide
BINOL
61
80
48
53
49
46
43
0
9
10
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1
2
13
1
1
1
1
4
5
6
7
L1
proline
K
2
CO
3
proline
K
3
PO
4
0
proline
NaOt-Bu
KOt-Bu
0
1
9
proline
150
55
(2 equiv)
2
2
0
1
none
KOt-Bu
150
150
35
0
proline
None
a
Unless otherwise stated, reactions were carried out with 1 (0.50 mmol), 2 (4 mL), catalyst (0.10 mmol),
b
and base (1.50 mmol) under argon atmosphere in a 35 mL pressure-resistant tube.
Isolated yield.
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With the optimized conditions in hand (entries 3 and/or 9, Table 1), we next examined
the substrate scope for this transformation and the results are shown in Table 2. The
p-substituted iodoarenes in spite of the electron-donating and electron-withdrawing
nature of the substituents were found to be excellent substrates for the arylation to
provide the corresponding biaryls in 71 to 87% yields (entries 1, 4, 7 and 10). For
entry 4, p-terphenyl was also formed in 14% yield. On the other hand, iodoarenes
bearing o- and m-substituents produced moderate yields (38 to 63%) of biaryls under
the same reaction conditions (entries 2, 3, 5, 6, 8, 9 and 11). Regarding entry 6,
1,2-diphenylbenzene was also isolated in 13% yield. Better conversions were
observed when the reactions were carried out with sarcosine instead of proline (in the
case of entry 2), at elevated temperature (180 ℃, entry 3), and for a prolonged reaction
time (48 h, entry 12). The low reactivities of the o-substituted iodoarenes would be
attributed to the steric effects. The instability of the aryl radicals at the m-position of
the substituents by the inductive effects would be responsible for the low reactivities of
the m-substituted iodoarenes (vide infra). Aryl bromide was less reactive for this
transformation (entry 12).
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Heterocyclic holoarenes were also tested (Table 2). 2-Iodopyridine was smoothly
converted into 2-phenylpyridine by the reaction of benzene in an excellent yield (83%,
entry 13). However, the 3-iodo- and 4-iodopyridines were found to be less reactive
(entries 14 and 15). The intermediate aryl radical formed from 2-iodopyridine would
be stabilized by a neighboring nitrogen atom in the former case to achieve the good
conversion. Such an effect is impossible in the latter cases. No transformation was
observed when 2-iodothiophene was applied to the reaction of benzene (not shown) in
spite of the 3-iodothiophene being transformed into 3-phenylthiophene in 69% yield
(entry 16).
a
Table 2. Proline-catalyzed direct arylation of benzene with Ar-I
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entry
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3
yield (%)
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d
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d
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f
80
3
g
g
48 (57)
3h
g
43 (53)
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l
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3n
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o
a
Reaction conditions: 4-iodoanisole (0.50 mmol), L-proline (0.10 mmol), and KOt-Bu (1.50 mmol) in
benzene (4 mL) at 150 ℃ under argon atmosphere in a 35 mL pressure-resistant tube.
b
Isolated yield.
p-Terphenyl (14%) was detected.
c
f
d
e
Sarcosine was employed instead of proline.
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Reaction at 180 ℃.
Yields based upon consumed haroarene 1.
g
h
,2-Diphenylbenzene (13%) was detected.
Reaction for
4
8 h.
Next, a series of arene variations were also examined in this reaction (Table 3).
Mono-substituted arenes, such as benzonitrile and toluene, afforded a mixture of
regioisomers favouring the ortho isomers (entries 1 and 2). When both coupling
partners are electron rich (entry 2), presumably due to the instability of the intermediate
biaryl radical (vide infra), the reaction was ineffective.
Reactions with
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,4-difluorobenzene and pyradine afforded the desired biaryls in 48 and 70% yields,
respectively (entries 3 and 4).
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a
Table 3. Proline-catalyzed direct arylation of various arenes with Ar-I
b
c
entry
1
2
3
yield [%]
ratio of o/m/p
1
74
56/17/23
3
p
q
2
3
4
44
48
70
55/27/18
3
-
3
r
-
3s
a
Reaction conditions: 4-iodoanisole (0.50 mmol), L-proline (0.10 mmol), and KOt-Bu (1.50 mmol) in
b
benzene (4 mL) at 150 ℃ under argon atmosphere in a 35 mL pressure-resistant tube.
Isolated yield.
c
The ratio was determined by NMR analysis.
The coupling reaction presumably involves an aryl radical anion as the
7-11
intermediate.
To verify this proposal, a radical-trapping experiment was carried out
using TEMPO (1 equiv., Table 4, entry 1), a typical radical scavenger, for the reaction of
-iodoanisole and benzene. The reaction rate was significantly suppressed to afford
4
only a 10% yield of the biaryl 3a. On the other hand, the presence of AIBN (0.4
equiv.), known as a thermal-free radical initiator, promoted the conversion in the
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absence of proline (entry 2). When the reaction was performed without both AIBN
and proline as a control experiment, the biaryl 3a was formed in 35% yield (Table 1,
entry 20). To examine the effect of trace metal impurities, the transformation with a
copper salt was examined which resulted in a lower yield than in the case without
copper (entry 3 vs. Table 2, entry 1). These observations strongly suggest the
operation of a radical pathway for these conversions.
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Table 4. Arylation reaction with a radical-trapping agent, a radical initiator, and a transition
metal
a
b
entry
catalyst
proline
none
additive (equiv.)
TEMPO (1.0)
AIBN (0.4)
yield (%)
1
2
3
10
60
75
proline
CuI (0.1)
a
Reaction conditions: 4-Iodoanisole (0.50 mmol), L-proline (0.10 mmol) or none, and KOt-Bu (1.50
b
mmol) in benzene (4 mL) at 150 ℃ under argon atmosphere in a 35 mL pressure-resistant tube.
c
Isolated yield.
Without proline.
We next tested the competition experiment using an equimolar amount of benzene
along with benzene-d to estimate the rate-determining step in this arylation reaction
6
(Table 5). The observed K /K (1.27) ratio was low which indicated that the
H D
rate-determining step is not the deprotonation step but is the single electron-transfer
process.
a
6
Table 5. A competition reaction between benzene and benzene-d under the consdition
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entry
benzene (equiv.)
benzene-d
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(equiv.)
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yield of 3-d
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(%)
H D
K /K
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2
3
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0
45
90
0
84
-
65
71
0
1.27
-
-
90
87
a
Reaction conditions: 4-iodoanisole (0.50 mmol), L-proline (0.10 mmol), and KOt-Bu (1.50 mmol) in
b
benzene (2 mL) and benzene-d
Isolated yield.
6
at 150 ℃ under argon atmosphere in a 35 mL pressure-resistant tube.
To gain further insight into the mechanism of the transformation, a competition
reaction in the presence of an equimolar amount of the electron-sufficient and
electron-deficient haroarenes 1a and 1b with benzene was investigated. A mixture of
4
-iodoanisole 1a and 4-fluoroiodobenzene 1b was reacted with benzene for 1 h to form
biaryls 3a and 3d favoring 3d being formed from the electron-deficient substrate 1b
entry 1). Individual reactions for the haroarenes 1a and 1b under the same conditions
(
afforded the yields of 31 and 53 %, respectively (entries 2 and 3). These phenomena
would suggest that the reaction involves a single-electron transfer step because the
electron-deficient haroarene would be a better acceptor for the electron.
a
Table 6. A competition reaction between electron-rich and electron–poor haroarene with benzene
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entry
1a (equiv.)
1b (equiv.)
yield of 3a (%)
yield of 3d (%)
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0
53
49
-
59
-
1.0
53
a
Reaction conditions: 4-iodoanisole (0.50 mmol), L-proline (0.10 mmol), and KOt-Bu (1.50 mmol) in
b
benzene (4 mL) at 150 ℃ under argon atmosphere in a 35 mL pressure-resistant tube.
Isolated yield.
Based upon this information, we propose the following possible mechanism (Scheme
1
). An electron transfer from the chelate 4 derived from KOt-Bu and proline would
afford the radical anion 6, which would be transferred to the aryl radical 7 by releasing
the halonium ion. The addition of benzene to 7 followed by deprotonation would
convert the aryl radical 7 to the biaryl radical anion 8, which would form the final
product 3 by electron transfer to the haroarene to regenerate the radical anion 6.
Scheme 1. Proposed catalyst cycle
Finally, a gram-scale experiment targeting the synthesis of 1-pentyl-4-phenylbenzene
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0, a key building block for the preparation of liquid crystals, has been achieved
(Scheme 2). The reaction of 4-iodopenylbenzene (1.37 g, 5.0 mmol) with benzene (30
mL) in the presence of L-proline (20 mol%) and t-BuOK (3 equiv.) afforded the desired
biaryl 10 (0.85 g, 3.8 mmol) in 76% isolated yield.
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Scheme 2. A gram scale synthesis of 1-pentyl-4-phenylbenzene
CONCLUSION
In conclusion, we have discovered a protocol for the transition-metal-free synthesis of
biaryls based upon the utilization of amino acids (L-proline and sarcosine) as
organocatalysts with unactivated benzenes as the substrate throughout the direct C-H
bond functionalization. This method avoids the use of toxic transition-metal catalyst
and organometallic reagents. Therefore, the present results would provide a new
insight into the realization of environmentally benign and cost-effective access to biaryl
compounds.
EXPERIMENTAL SECTION
General Remarks. NMR spectra were recorded on a spectrometer, operating at
1
13
00 MHz for H NMR and 100 MHz for C NMR. Chemical shifts in CDCl
4
3
are
1 13
(7.26 ppm for H NMR and 77.0 ppm for C
reported on the
δ
scale relative to CHCl
3
NMR) as an internal reference. The following abbreviations are used to multiplicities:
‘s’ (singlet), ‘d’ (doublet), ‘t’ (triplet), ‘m’ (multiplet), ‘br’ (brosd). Column
chromatography was carried out with silica gel (230-400 mesh). Reactions were
carried out in dry solvents under a nitrogen atmosphere.
General Procedure: Cross-coupling of Aryl Iodides with Arenes. Aryl iodides
(0.5 mmol ) and L-proline (0.1 mmol, 20 mol%) were added to a 35 mL oven-dried
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pressure-resistant tube with a tap. KOt-Bu (1.5 mmol, 3.0 equiv.) was then added.
Benzene (4 mL) was added to the tubes using a syringe. The mixture was then stirred
in a sealed tube under argon atmosphere. Then the mixture was stirred at 150
℃ until
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complete consumption of starting material was monitored by TLC. After completion
of the reaction, the mixture was diluted with ethyl acetate, passed through a fritted glass
filter to remove the inorganic salts and solvent was removed with the aid of rotary
evaporator. The residue was purified by column chromatography on silica gel using
hexane/ethyl acetate as eluent to provide the desired product.
11a
4-Methoxy-biphenyl (3a: Table 1, Entry 3).
Eluent hexane/ethyl acetate (22:1).
1
1a
A white solid. Yield, 80.6 mg, 87%. Mp 86-88
℃ (lit. 85.6-86.8 ℃); R 0.13
f
1
（
hexane）; H NMR （400 MHz, CDCl ）
δ
7.62-7.49 (m, 4H ), 7.42 (t, 2H, J=7.7 Hz),
3
13
7
.31 (d, 1H, J=7.2 Hz), 6.98 (d, 2H, J=8.5 Hz), 3.86 (s, 3H, OCH ). C-NMR (100
3
MHz , CDCl
3
) δ 159.1, 140.8, 133.7, 128.7 (2C), 128.1 (2C), 126.7 (2C), 126.6, 114.2
(2C), 55.3.
11a
-Methoxy-biphenyl (3b: Table 2, Entry 2).
3
Eluent hexane/ethyl acetate (22:1).
1
A colorless oil. Yield, 45.6 mg, 64 %. R 0.13（hexane）; H NMR （400 MHz,
f
CDCl
3
）δ 7.59 (d, 2H, J=7.3 Hz), 7.43 (t, 2H, J=7.4 Hz), 7.39-7.31 (m, 2H), 7.18 (d, 1H,
J=7.8 Hz), 7.15-7.10 (m, 1H), 6.90 (dd, 1H, J=2.1, 8.1 Hz), 3.86 (s, 3H, OCH3).
1
3
C-NMR (100 MHz , CDCl
3
)
δ
159.9, 142.7, 141.1, 129.7, 128.7 (2C), 127.4, 127.2
(2C), 119.7, 112.9, 112.6, 55.3.
11a
2-Methoxy-biphenyl (3c: Table 2, Entry 3).
Eluent hexane/ethyl acetate (22:1).
1
A colorless oil. Yield, 65.2 mg, 70 %. R
f
0.13（hexane）; H NMR （400 MHz,
CDCl
3
）
δ
7.55-7.53 (m, 1H), 7.53-7.49 (m, 1H), 7.41 (t, 2H, J=7.4 Hz),7.35-7.29 (m,
13
). C-NMR (100 MHz , CDCl
3
H), 7.06-6.95 (m, 2H), 3.81 (s, 3H, OCH
3
3
)
δ
156.4,
138.5, 130.8, 130.7, 129.5 (2C), 128.6, 128.0 (2C), 126.9, 120.8, 111.2, 55.5.
11a
4-Fluoro-biphenyl (3d: Table 2, Entry 4).
Eluent hexane/ethyl acetate (22:1).
1
1a
A white solid. Yield, 61.0 mg, 71 %. Mp 74-76
℃ (lit. 71.1-72.5 ℃); R 0.39
f
1
（
hexane ）; H NMR（400 MHz, CDCl
3
）δ
7.59-7.50 (m, 4H), 7.49-7.40 (m, 2H), 7.34
1
3
(t, 1H, J=7.6 Hz), 7.18-7.08 (m, 2H). C-NMR (100 MHz , CDCl
3
)
δ
163.7, 161.2,
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40.2, 137.3, 137.3, 128.8, 128.7, 128.6, 127.2, 127.0, 115.7, 115.5.
14
3
-Fluoro-biphenyl (3e: Table 2, Entry 5) .
A white solid. Yield, 32.9 mg, 38 %. R
CDCl
Eluent hexane/ethyl acetate (22:1).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
f
0.39（hexane）; H NMR （400 MHz,
3
）
δ
7.59-7.55 (m, 2H), 7.48-7.42 (m, 2H), 7.41-7.34 (m, 3H), 7.31-7.26 (m, 1H).
1
3
1
163.2 (d, JCF= 245.0 Hz), 143.4, 139.9 (d, JCF= 2.5
4
C-NMR (100 MHz , CDCl
3
)
δ
3
Hz), 130.2 (d, J = 8.3 Hz), 128.9, 127.8, 127.1, 122.7 (d, J = 2.5 Hz), 114.1(d, J =
4
2
CF
CF
CF
20.7 Hz), 113.9.
15
2-Fluoro-biphenyl (3f: Table 2, Entry 6).
Eluent hexane/ethyl acetate (22:1).
15
A white solid. Yield, 46.7 mg, 54 %. Mp 71-72
℃ (lit. 70-72 ℃); R 0.39
f
1
（
hexane ）; H NMR （400 MHz, CDCl ）
δ
7.57-7.50 (m, 2H), 7.48-7.40 (m, 3H),
3
1
.40-7.26 (m, 2H), 7.25-7.10 (m, 2H). C-NMR (100 MHz , CDCl
3
2
159.8 (d, JCF
7
3
)
δ
=
3
47.5 Hz), 135.8, 130.8 (d, J = 3.3 Hz), 129.1 (d, J = 8.3 Hz), 129.0, 129.0, 128.9,
2
2
CF
CF
3
128.4, 127.6, 124.3 (d, J = 4.1 Hz), 116.1 (d, J = 22.3Hz).
2
CF
CF
11a
4-Methyl-biphenyl (3g: Table 2, Entry 7) .
Eluent hexane/ethyl acetate (22:1).
11a
A white solid. Yield, 67.1 mg, 80 %. Mp 46-47
℃ (lit. 42.7-44.2 ℃); R 0.35
f
1
（
hexane ）; H NMR （400 MHz, CDCl
3
）
δ
7.58 (d, 2H, J=7.6 Hz), 7.49 (d, 2H,
J=8.0 Hz), 7.43 (t, 2H, J=7.6 Hz), 7.32 (t, 1H, J=7.3 Hz), 7.25 (d, 2H, J=7.8 Hz), 2.40
1
3
(s, 3H, CH
3
). C-NMR (100 MHz , CDCl
3
)
δ
141.1, 138.3, 137.0, 129.5 (2C), 128.7
(2C), 127.0 (2C), 127.0 (3C), 21.1.
11a
3
-Methyl-biphenyl (3h: Table 2, Entry 8).
A colorless oil. Yield, 40.5 mg, 48 %. R
CDCl
Hz), 2.42 (s, 3H, CH
128.0 (3C), 127.2 (2C), 127.1, 124.3, 21.6.
Eluent hexane/ethyl acetate (22:1).
1
f
0.49（hexane）; H NMR （400 MHz,
3
）δ 7.62-7.56 (m, 2H), 7.47-7.38 (m, 4H), 7.38-7.30 (m, 2H), 7.16 (d, 1H, J=7.1
1
3
3
). C-NMR (100 MHz , CDCl
3
)
δ
141.3, 141.2, 138.3, 128.7, 128.6,
11a
2
-Methyl-biphenyl (3i: Table 2, Entry 9).
A colorless oil. Yield, 40.7 mg, 43 %. R
CDCl
Eluent hexane/ethyl acetate (22:1).
1
f
0.25（hexane）; H NMR （400 MHz,
3
）
δ
7.47-7.38 (m, 2H ), 7.38-7.30 (m, 3H), 7.29-7.20 (m, 4H), 2.28 (s, 3H, CH
3
).
1
3
C-NMR (100 MHz , CDCl δ 141.9, 135.3, 130.3, 129.8, 129.2 (2C), 128.0 (2C),
3
)
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5
5
5
5
6
1
27.2, 127.1, 126.7, 125.8, 20.5.
1
6
4-Phenyl-1-tert-butylbenzene (3j: Table 2, Entry 10).
Eluent hexane/ethyl
1
℃ (lit. 51 ℃); Rf
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
acetate (22:1). A white solid. Yield, 76.0 mg, 72 %. Mp 48-51
1
0.2 6（ hexane ）; H NMR（400 MHz, CDCl
3
）
δ
7.62-7.56 (m, 2H), 7.56-7.50 (m, 2H),
13
.50-7.40 (m, 4H), 7.36-7.30 (m, 1H), 1.37 (s, 9H, C(CH ) ). C-NMR (100 MHz ,
7
3 3
CDCl
3
)
δ
150.2, 141.0, 138.3, 128.7 (2C), 127.0 (2C), 127.0, 126.8 (2C), 125.7 (2C),
3
4.5, 31.4 (3C).
1
1a
3,5-Dimethylbiphenyl (3k: Table 2, Entry 11).
22:1). A colorless oil. Yield, 76.0 mg, 72 %. R
Eluent hexane/ethyl acetate
1
(
f
0.23（hexane）; H NMR （400
MHz, CDCl ）
δ
7.57 (d, 2H, J=7.3 Hz), 7.42 (t, 2H, J=7.7 Hz), 7.32 (t, 1H, J=7.3 Hz),
3
13
7
.21 (s, 2H), 7.00 (s, 1H), 2.38 (s, 6H, CH
3
). C-NMR (100 MHz , CDCl
3
)
δ
141.4,
1
41.2, 138.2 (2C), 128.9, 128.6 (2C), 127.2 (2C), 127.1, 125.1 (2C), 21.4 (2C).
1
7
2-Phenyl-pyridine (3l: Table 2, Entry 13).
Eluent hexane/ethyl acetate (3:1).
1
A colorless oil. Yield, 62.8 mg, 83 %. R
f
0.4 9（ hexane:AcOEt=3:1）; H NMR（400
MHz, CDCl ）
δ
8.73-8.68 (m, 1H), 8.02-7.97 (m, 2H), 7.77-7.70 (m, 2H), 7.52-7.45 (m,
3
13
H), 7.45-7.39 (m, 1H), 7.25-7.20 (m, 1H). C-NMR (100 MHz, CDCl
2
3
)
δ
157.4, 149.6,
139.4, 136.7, 128.9, 128.7 (2C), 126.9 (2C), 122.0, 120.5.
17
3-Phenyl-pyridine (3m: Table 2, Entry 14).
Eluent hexane/ethyl acetate (3:1).
1
A yellow oil. Yield, 41.0 mg, 53 %. R
MHz, CDCl ）
f
0.30（hexane:AcOEt=3:1）; H NMR （400
δ
8.86 (s, 1H), 8.60 (d, 1H, J=4.4 Hz), 7.78 (d, 1H, J=8.0 Hz), 7.59 (d, 2H,
3
J=7.1 Hz), 7.49 (t, 2H, J=7.4 Hz), 7.42 (d, 1H, J=7.1 Hz), 7.37 (dd, 1H, J=4.9, 7.8 Hz).
1
3
C-NMR (100 MHz , CDCl
3
)
δ
148.5, 148.3, 137.8, 136.6, 134.3, 129.1 (2C), 128.1,
1
27.1 (2C), 123.5.
1
7
4-Phenyl-pyridine (3n: Table 2, Entry 15).
Eluent hexane/ethyl acetate (3:1).
1
7
A brown solid. Yield, 38.5 mg, 50 %. Mp 72-74
℃
(lit. 73-74
8.67 (d, 2H, J=4.9 Hz), 7.65 (d,
150.3 (2C), 148.3,
℃); R 0.12
f
1
（
hexane:AcOEt=3:1）; H NMR（400 MHz, CDCl
13
3
）δ
2H, J=7.1 Hz), 7.56-7.40 (m, 5H). C-NMR (100 MHz , CDCl
3
)
δ
138.1, 129.1 (2C), 129.0, 127.0 (2C), 121.6 (2C).
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7
3-Phenyl-thiophene (3o: Table 2, Entry 16).
Eluent hexane/ethyl acetate (22:1).
17
A white solid. Yield, 55.5 mg, 69 %. Mp 81-84
℃ (lit. 89-91 ℃); R 0.26
f
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
（
hexane）; H NMR （400 MHz, CDCl
3
）δ
7.60 (d, 2H, J=7.8 Hz), 7.48-7.24 (m, 6H).
1
3
C-NMR (100 MHz , CDCl
26.2, 120.3.
3
)
δ
142.3, 135.8, 128.8 (2C), 127.1, 126.4 (2C), 126.3,
1
(
p-Methoxyphenyl)benzonitrile (a mixture of o:m:p = 56:17:23) (3p: Table 3,
10
Entry 1).
74 %. R
Eluent hexane/ethyl acetate (22:1). A pale yellow oil. Yield, 77.7 mg,
1
f
0.15（hexane:AcOEt=22:1）; H NMR （400 MHz, CDCl
3
）δ
7.84-7.81 (m,
0
.17H), 7.80-7.72 (m, 0.75H), 7.72-7.7.68 (m, 0.46H), 7.68-7.7.46 (m, 4.06H), 7.40 (dt,
13
0
.56H, J=1.2, 7.5 Hz), 7.05-6.98 (m, 1.93H), 3.87 (s, 3H, OCH ). C-NMR (100 MHz ,
3
CDCl
3
)
δ
160.0, 145.2, 133.7, 132.8, 132.6, 131.5, 131.0, 130.5, 130.2, 130.0, 130.0,
1
29.8, 129.5, 128.3, 128.1, 127.1, 127.0, 119.0, 114.5, 114.1, 111.0, 55.4, 55.3.
(
p-Methoxyphenyl)toluene (a mixture of o:m:p = 55:27:18) (3q: Table 3, Entry
11a
2
).
Eluent hexane/ethyl acetate (22:1). A pale yellow oil. Yield, 43.6 mg, 44 %.
1
R 0.1 3（ hexane）; H NMR（400 MHz, CDCl ）
δ
7.56-7.10 (m, 6H), 7.00-6.90 (m, 2H),
f
3
3
1
1
1
.84 (s, 3H, OCH ), 2.41 (s, 0.82H, o-CH C H ), 2.38 (s, 0.55H, p-CH C H ), 2.28 (s,
3 3 6 4 3 6 4
1
3
.65H, o-CH
3
C
6
H
4
). C-NMR (100 MHz , CDCl
3
)
δ
159.0, 158.5, 141.5, 140.8, 138.3,
36.3, 135.5, 134.3, 133.8, 133.7, 130.3, 130.2, 129.9, 129.4, 128.6, 128.1, 127.9, 127.5,
27.4, 126.9, 126.6, 125.7, 123.8, 114.1, 113.4, 55.3, 55.2, 21.5, 21.0, 20.5.
18
2,5-Difluoro-4’-methylbiphenyl (3r: Table 3, Entry 3).
Eluent hexane/ethyl
1
acetate (22:1). A colorless oil. Yield, 41.6 mg, 41 %. R
f
0.26（hexane）; H NMR
（
400 MHz, CDCl
3
）
δ
7.43 (d, 2H, J=6.3 Hz ), 7.26 (d, 2H, J= 8.3 Hz), 7.15-7.05 (m,
13
). C-NMR (100 MHz , CDCl
2
H), 7.00-6.93 (m, 1H), 2.40 (s, 3H, CH
3
3
)
δ
158.7 (dd,
J = 2.5, 241.7 Hz), 155.7 (dd, J = 2.5, 243.3 Hz), 138.1, 132.1, 130.3-130.5 (m), 129.3,
F
F
1
28.7 (d, J = 3.3 Hz), 126.8, 114.6-117.2(m), 21.2.
F
7
-Methoxyphenylpyrazine (3s: Table 3, Entry 4). Eluent hexane/ethyl acetate
4
7
1:1). A white solid. Yield, 65.4 mg, 70 %. Mp 87-89 ℃ (lit. 95 ℃); R 0.58
(
f
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2
2
2
2
2
2
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3
3
3
3
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3
3
3
3
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
（
hexane:AcOEt=1:1）; H NMR （400 MHz, CDCl
3
）δ 8.98 (d, 1H, J=1.5 Hz), 8.59 (t,
1H, J =2.0 Hz), 8.45 (d, 1H, J=2.4 Hz, ), 8.02-7.95 (m, 2H), 7.06-7.00 (m, 2H), 3.89 (s,
1
). C-NMR (100 MHz , CDCl
3
3H, OCH
3
3
)
δ
161.1, 152.5, 144.0, 142.1, 141.6, 128.8,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
28.3 (2C), 114.5 (2C), 55.4.
8
2
,3,4,5,6-Pentadeuterio-4'-methoxy-biphenyl (3a-d : Table 5, Entry 2) .
5
Eluent hexane/ethyl acetate (22:1). A white solid. Yield, 67.1 mg, 71 %. Mp
19
1
87-88
3
℃ (lit. 133-134 ℃); H NMR（400 MHz, CDCl ）δ 7.53 (d, 2H, J=8.5 Hz, Ar),
6
.98 (d, 2H, J=8.5 Hz, Ar), 3.86 (s, 3H, OCH3).
20
1
-Pentyl-4-phenylbenzene (10: Scheme 2).
4-Iodopentylbenzene (1.37 g, 5.0
mmol ) and L-proline (115.0 mg, 1.0 mmol, 20 mol%) were added to a 100 mL
oven-dried pressure-resistant tube with a tap. KOt-Bu (1.68 g, 15.0 mmol, 3.0 equiv.)
was then added. Benzene (30 mL) was added to the tubes using a syringe. The
mixture was then stirred in a sealed tube for 24 h under argon atmosphere. The
mixture was diluted with ethyl acetate, passed through a fritted glass filter and solvent
was removed with the aid of rotary evaporator. The residue was purified by column
chromatography on silica gel using hexane/ethyl acetate (22:1) as eluent to provide the
1
desired biaryl 10 (853.6 mg, 76 %) as a pale yellow oil. R
f
0.28（hexane）; H NMR
（400 MHz, CDCl
3
）
δ
7.59 (d, J= 8.05 Hz, 2H), 7.51 (d, J= 7.81 Hz, 2H), 7.43 (t, J=
7
2
.68 Hz, 2H), 7.36-7.28 (m, 1H), 7.26 (d, 2H), 2.64 (t, J= 7.81 Hz, 2H), 1.70-1.60 (m,
13
H), 1.40-1.30 (m, 4H), 0.91 (t, J= 6.71 Hz, 3H). C-NMR (100 MHz , CDCl
3
) δ 142.1,
141.2, 138.5, 128.8 (2C), 128.7 (2C), 127.0 (2C), 127.0 (2C), 126.9, 35.6, 31.5, 31.2,
2.6, 14.0.
2
Acknowledgment. We gratefully acknowledge Ms. Sachiko Asami, Mrs. Ikuhiro
Inada, Atsushi Isayama and Hiroaki Ohmukai for generous supports in some of the
experiments.
Supporting Information
1
Copies of H NMR spectra of products. This material is available free of charge via
the Internet at http://pubs.acs.org.
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References
(1) Evans, D. A., Dinsmore, C. J., Watson, P. S., Wood, M. R., Richardson, T. I., Trotter,
B. W. and Katz, J. L. Angew. Chem., Int. Ed. 1998, 37, 2704.
10
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