Cobalt-catalyzed direct arylation of unactivated arenes with aryl halides

Article abstract of DOI:10.1002/chem.201002290

Inexpensive and simple: The direct arylation of unactivated arenes with aryl halides has been carried out with [Co(acac)₃] as the catalyst and LiHMDS as the base. The corresponding biaryl compounds have been prepared in good to excellent yields and at a relatively low reaction temperature (see scheme; acac=acetylacetonate, LiHMDS=lithium bis(trimethylsilyl)amide). An intramolecular direct arylation has also been achieved under the same conditions.

Full text of DOI:10.1002/chem.201002290

DOI: 10.1002/chem.201002290
Cobalt-Catalyzed Direct Arylation of Unactivated Arenes with Aryl Halides
[
a]
[a]
[a]
[a]
[a, b]
Wei Liu, Hao Cao, Jie Xin, Liqun Jin, and Aiwen Lei*
Over the past four decades, the development of transi-
tion-metal-catalyzed cross-coupling reactions has revolution-
ized the existing techniques for the construction of CÀC
2
2
bonds, especially those C
A
H
U
G
R
N
U
(sp )ÀC
ACHTUNGERTNNUNG( sp ) bonds which are in-
[
1]
volved in biaryl syntheses. Among the various transition
metals applied, cobalt—one of the “life” elements—exists in
many metalloenzymes and exhibits versatile functions in
various chemical transformations occurring in nature.
However, compared with the most widely applied transition
metals in cross-coupling reactions, such as Pd, Ni, and Cu,
cobalt as catalyst has received much less attention, although
it is less toxic than the others and inexpensive. Recently,
cobalt-catalyzed cross-coupling reactions have been realized
[2]
[1]
Scheme 1. Transition-metal-catalyzed direct arylation of arenes with aryl
halides.
[3]
as a fascinating and developing area in synthetic chemistry.
Because of its environmentally friendly character and its
potential economic values, the direct arylation of various
arenes with aryl halides or other electrophiles to produce
biaryls has become one of the “hottest” areas during recent
years. The substrate classes of the arenes could be mainly
divided into three different categories: 1) arenes containing
The coupling between 4-bromoanisole (1a) and benzene
was initially selected as the model reaction for the optimiza-
tion of the reaction conditions (Table 1).The reactions have
been carried out with CoBr2 (10 mol%) combined with
TMEDA (20 mol%) in benzene (4 mL) and different bases
(3 equiv) have been used. LiOtBu, NaOtBu, and KOtBu
showed no effects to promote the arylations, while the reac-
tion with LiHMDS (lithium bis(trimeth AHCTUNGTERNNUNyG lsilyl)amide) as the
base afforded direct arylated product 3a in 18% yield
(Table 1, entries 1–4). Then more ligands, such as bipy, dppf,
[4]
[
5]
[6]
directing groups, 2) electron-deficient arenes, and 3) het-
[4b,g,7]
erocycles.
zene, have been scarcely employed in these fascinating syn-
However, unactivated arenes, such as ben-
[
8]
thetic methods. In fact, the exploration of different metal
catalysts has always been the key for the promotion of new
[5c,g,7a,f,j,k,8a,9]
reactions. Many transition metals, such as Pd,
[
7c,8c]
[4b,7l,n,o,8b]
[5f,10]
[7p,q,8f]
[5h,6a,7b,h,i,m]
Ir,
Rh,
Ru,
Ni,
Cu,
and
dppe, dppm, PPh , and l-proline have been further investi-
3
[
8d,e]
Fe,
have been successfully applied in the direct arylation
of arenes, whereas cobalt catalysis has been rarely studied
gated in the presence of the LiHMDS base, however, no ob-
vious improvements have been observed (Table 1, entries 5–
10). Without extra addition of ligands, the reaction with
CoBr2 (10 mol%) led to a 37% yield of 3a (Table 1,
entry 11). The use of DMEDA as ligand led to slightly
higher yields (54% and 55% respectively; Table 1, en-
tries 12 and 13). We were pleased to note that the reactions
[11]
in CÀH activation/direct arylation (Scheme 1). Herein, we
describe a novel and efficient method for the direct aryla-
tion of unactivated benzene and its derivatives with various
aryl halides to construct biaryl molecules, in which the reac-
tion is promoted by a readily available cobalt salts without
extra ligands.
that employed a lower amount of [Co
ACTHUNGTRENUNN(G acac) ] or [Co ACHTUNGTRENNUNG( acac)
2 3
(
10 mol%) as catalysts, could produce 3a in 71% or 72%
yields, respectively (Table 1, entries 15 and 16). CoCl result-
ed less effective as a catalyst and produced a 16% yield of
2
[
a] W. Liu, H. Cao, J. Xin, L. Jin, Prof. Dr. A. Lei
College of Chemistry and Molecular Sciences, Wuhan University
Wuhan, Hubei, 430072 (P. R. China)
Fax : (+86)27-68754067
E-mail: aiwenlei@whu.edu.cn
3
a (Table 1, entry 14). If the amount of benzene was in-
creased to 6 mL in the presence of [Co(acac) ] or [Co-
(acac) ] (15 mol%), the reaction efficiently afforded the
ACHTUNGTRENNUNG
2
ACHTUNGTRENNUNG
3
[
b] Prof. Dr. A. Lei
direct arylation product 3a in 84% or 94% yield, respec-
tively (Table 1, entries 19 and 20). Decreasing the catalyst
loading led to lower conversions and yields. Furthermore,
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
compared with [Co
as [Fe(acac) ], FeCl , [Ni
cient under the same reaction conditions (Table 1, en-
ACHTUNGTNERNUNG( acac) ], other inexpensive catalysts, such
3
54 Fenglin Lu, Shanghai, 200032 (P. R. China)
3
A
H
U
G
R
N
N
ACHTUNGTNRENUNG( acac) ], and MnCl were less effi-
2 2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002290.
3
3
3588
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3588 – 3592

COMMUNICATION
Table 1. Cobalt-catalyzed CÀH bond activation: screening of reaction
Table 2. Cobalt-catalyzed direct arylation of unactivated benzene with
aryl bromides.
[
a]
[a]
conditions for coupling of 1a and unactivated benzene.
[
b]
[b]
Entry Cat. [mol%] Ligand [mol%] Base
Benzene Yield [%]
Entry
1
Aryl halides
Product
Yield [%]
85
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
CoBr
2
2
2
2
2
2
2
2
2
2
2
2
2
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
TMEDA (20)
TMEDA (20)
TMEDA (20)
TMEDA (20)
LiOtBu
NaOtBu 4 mL
KOtBu 4 mL
LiHMDS 4 mL
4 mL
0
0
1a
1b
1c
1d
3a
3b
3c
3d
0
18
32
7
2
3
4
82
80
86
Bipyridine (20) LiHMDS 4 mL
Dppf (10)
Dppe (10)
Dppm (10)
PPh (20)
3
l-proline (20)
LiHMDS 4 mL
LiHMDS 4 mL
LiHMDS 4 mL
LiHMDS 4 mL
LiHMDS 4 mL
LiHMDS 4 mL
16
trace
trace
25
37
54
55
16
71
0
1
2
3
4
5
5
1e
3e
88
–
DMEDA (10) LiHMDS 4 mL
DMEDA (20) LiHMDS 4 mL
–
–
6
7
8
1 f
1g
1h
3 f <5
CoCl
[Co(acac)
10)
[Co(acac)
10)
[Co(acac)
15)
[Co(acac)
15)
[Co(acac)
15)
[Co(acac)
15)
[Co(acac)
15)
[Fe(acac)
15)
FeCl
[Ni(acac)
15)
2
LiHMDS 4 mL
LiHMDS 4 mL
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
]
3
]
2
]
2
]
2
]
3
]
3
]
(
3g
3h
82
80
16
17
18
19
20
21
22
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
–
–
–
–
–
–
–
LiHMDS 4 mL
LiHMDS 4 mL
LiHMDS 5 mL
LiHMDS 6 mL
LiHMDS 6 mL
72
74
(
ACHTUNGTRENNUNG
(
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
80
9
1i
3i
65
(
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
84
(
10
11
1j
3j
79
89
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
94
(
1k
3k
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
KOtBu
6 mL
trace
15
(
1
2
1l
3l
92
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
]
LiHMDS 6 mL
(
2
2
3
4
3
(15)
DMEDA (30) LiHMDS 6 mL
–
82
5
13
14
15
1m
1n
1o
1p
1q
3m 82
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
]
LiHMDS 6 mL
(
25
26
27
MnCl
MnCl
–
2
(15)
(15)
–
LiHMDS 6 mL
0
0
0
[c]
3n
3o
3p
3q
49(75)
2
DMEDA (30) LiHMDS 6 mL
LiHMDS 6 mL
–
77
[
a] Reactions were carried out with 1a (0.5 mmol) and base (3.0 equiv) in
benzene (4 mL, 45 mmol; 5 mL, 56 mmol; 6 mL, 68 mmol) at 808C for
8 h; TMEDA=N ,N ,N ,N -tetramethylethane-1,2-diamine. DMEDA=
N ,N -dimethylethane-1,2-diamine. [b] Yields were determined by gas
1
1
2
2
4
16
55
1
2
chromatography.
[
d]
1
7
10(62)
tries 22–26). In the absence of catalyst, no reaction occurred
[a] Reactions were carried out with
(
(
1
(0.5 mmol) and LiHMDS
3.0 equiv) in benzene (6 mL, 68 mmol) in the presence of [Co(acac)
15 mol%). All reactions were performed at 808C unless otherwise indi-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
]
(
Table 1, entry 27).
With the optimized reaction conditions in hand, the scope
cated; acac=acetylacetonate. [b] Yield of isolated product. [c] Isolated
of the direct arylation of unactivated benzene with various
aryl bromides was investigated (Table 2). A wide range of
electron-rich aryl bromides resulted to be suitable electro-
philes, and afforded the corresponding products in good
yields (80–88%, Table 2, entries 1–8). When aryl bromides
contained the chlorine group in the 2-, 3-, or 4-positions,
only the CÀBr bonds were reactive, while the CÀCl bonds
yield in parentheses was obtained in the presence of 30 mol% [Co-
AHCTUNGTRNENUG( acac) ]. [d] Isolated yield in parentheses was obtained when the reaction
3
was performed at 1208C.
group led to the products of the direct arylation in moderate
yields (49% for 1n, 55% for 1p; Table 2, entries 14 and 16).
Notably, 2-F-C H Br (1o) underwent direct arylation
6
4
were well-tolerated and afforded the directly arylated prod-
ucts in excellent yields (3k 89%, 3l 92%, and 3m 82%, re-
spectively; Table 2, entries 11–13). Also, 4-Ph-C H Br af-
smoothly and generated 3o in 77% yield (Table 2, entry 15).
Moreover, the yield of 3n could be increased to 75% when
30 mol% of the catalyst [Co ACHTUNGTRENN(UNG acac) ] was employed (Table 2,
3
6
4
forded the direct arylated product 3j in 79% yield (Table 2,
entry 14). However, the aryl bromides with ortho-substitu-
entry 10). The electrophiles with electron-withdrawing
ents (Me, OMe) led to only sluggishly to the direct arylation
Chem. Eur. J. 2011, 17, 3588 – 3592
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3589

A. Lei et al.
products (Table 2, entries 6 and 17). Importantly, when the
reaction temperature was increased from 808C to 1208C,
the direct arylation of 2-Me-C H Br (1q) produced 3q in a
ligand could promote this transformation (see the Support-
ing Information for details). Moderate yields (37-57%) were
obtained in the direct arylation of benzene with various aryl
chlorides (Scheme 2).
6
4
moderate yield (62%;Table 2, entry 17).
In the presence of a catalytic amount of readily available
Notably, no homocoupling products from the correspond-
ing aryl halides (chlorides, bromides, and iodides) were ob-
served in the transformation, and a trace amount of biphen-
yl generated from benzene was identified.
[
Co ACHTUNGTRENNUNG( acac) ], the direct arylation of unactivated benzene with
3
various aryl iodides proceeded smoothly and excellent
yields of corresponding products have been achieved
(
Table 3). When aryl iodides containing a chlorine group
The direct arylation of other unactivated arenes with aryl
bromides was also investigated using [Co ACHTUNGTNERNUN(G acac) ] as the cata-
3
lyst. The direct arylation of naphthalene with 4-OMeC H Br
proceeded smoothly and led to 82% combined yield, which
6
4
Table 3. Cobalt-catalyzed direct arylation of unactivated benzene with
aryl iodides.
[
a]
contained two isomers in the ratio of 78:22 (a/b)
[
Eq. (1)].
[
b]
Entry Aryl iodides
1
Product
Yield [%]
3a 86
4a
4d
4k
2
3
3d 72
3k 94
4
5
4l
3l 87
3m 80
Besides intermolecular direct arylation, the capability of
our catalytic system in intramolecular reactions was also
attempted (Scheme 3). Using simple readily available [Co-
[11]
4m
[
a] Reactions were carried out with 4 (0.5 mmol) and LiHMDS
(
(
3
3.0 equiv) in benzene (6 mL, 68 mmol) in the presence of [Co ACHTNUGTNERNUNG( acac) ]
15 mol%) at 808C for 48 h. [b] Yield of isolated product.
(
4k, 4l, and 4m) were introduced as electrophiles, the CÀCl
bonds were well-tolerated and afforded the direct arylated
products in excellent yields (94%, 87%, and 80% respec-
tively; Table 3, entries 3–5).
The comparatively cheaper and unactivated aryl chlorides
as electrophiles were investigated as well and the results are
reported in Scheme 2. The [Co ACHTUNGRETNN(UNG acac) ] complex resulted less
3
efficient in the activation of various aryl chlorides than aryl
bromides and iodides in the direct arylation of benzene. It
was noteworthy that CoBr2 combined with DMEDA as
Scheme 3. Cobalt-catalyzed intramolecular direct arylation. Reaction
conditions: aryl bromide (0.5 mmol), LiHMDS (3.0 equiv), [Co
15 mol%) in benzene (3.0 mL) at 808C for 48 h.
3
ACHTUNGTRENNUNG( acac) ]
(
AHCTUNGTNERNUG( acac) ] as catalyst and benzene as solvent, the reactions
3
took place at a relatively mild temperature (808C). Note
that no direct arylation of benzene with 1r or 1s was ob-
AHCTUNGTRENNUNG
served. Importantly, the direct arylation of 1r proceeded
smoothly with 100% conversion, and provided the desired
[12]
cyclization product 3r in 86% yield, while the direct ary-
lation of 1s showed less efficiency under the same reaction
conditions.
Finally, the mechanism of this transformation was studied
preliminarily. Under our conditions, no regioisomers were
observed when substituted aryl halides coupled with ben-
zene (Table 2 and Table 3), which unambiguously ruled out
[7h]
Scheme 2. Cobalt-catalyzed direct arylation of unactivated benzene
the benzyne-type mechanism. Indeed, we were inspired to
consider the possibility that the reaction could follow a radi-
(
6.0 mL) with aryl chlorides (0.5 mmol).
3590
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3588 – 3592

Cobalt-Catalyzed Arylation of Unactivated Arenes
COMMUNICATION
[
8f,13,14]
cal-type pathway.
the reaction mechanism, a radical scavenger, TEMPO
1.0 equiv), was added to the reaction mixture, and the
direct arylation reaction was completely shut down
Eq. (2)]. Moreover, isotopic labeling experiments were also
In order to gain further insight into
the support from “the Fundamental Research Funds for the Central Uni-
versities”, and Program for New Century Excellent Talents in University
(
NCET).
(
[
Keywords: aryl halides · biaryls · cobalt · cross-coupling ·
direct arylation
carried out (see the Supporting Information for details). The
resulting k /k value of 1.0 strongly suggests that the aro-
H
D
matic CÀH bond cleavage is not the rate-determining step
for this process. The above mentioned results indicate that
an aryl radical species could indeed be involved in this reac-
tion.
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In summary, we have successfully developed the novel
cobalt catalyzed direct arylation of unactivated arenes, such
as benzene, with a broad range of aryl halides, including aryl
iodides, aryl bromides, and aryl chlorides at a relatively mild
reaction temperature, to prepare biaryl compounds. Com-
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1
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[8e]
[8d]
and co-workers and by our working group, the inexpen-
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AHCTUNGTRENNUNG
3
2
4
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efficiency, in most cases even without extra ligands, and led
to various biaryl products in excellent yields. In addition, an
intramolecular direct arylation was also achieved using
simple cobalt catalysis at 808C. To explain the mechanism of
this transformation, a radical pathway has been proposed.
The detailed mechanistic studies are currently under way.
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Experimental Section
1
19, 5650–5654; Angew. Chem. Int. Ed. 2007, 46, 5554–5558.
General procedure:
26.7 mg, 0.075 mmol) and LiHMDS (250.0 mg, 1.5 mmol) under an
argon atmosphere at room temperature, and then benzene (6.0 mL) and
-bromoanisole (93.5 mg, 0.5 mmol) were added. The resulting mixture
was stirred at 808C for 48 h. After cooling to room temperature, the reac-
tion mixture was quenched and extracted with ethyl acetate (3 times with
3
A Schlenk tube was charged with [Co ACHTUNRTEGNNUNG( acac) ]
[
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(
4
1
2 4
0 mL). The organic layers were combined, dried over Na SO , and con-
1
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centrated under reduced pressure, and then purified by silica gel chroma-
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product as a white solid (78.2 mg, 85% yield).
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Acknowledgements
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Received: August 10, 2010
Revised: December 20, 2010
Published online: February 23, 2011
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