Iron-catalyzed direct arylation of unactivated arenes with aryl halides

Article abstract of DOI:10.1002/anie.200906870

(Chemical Equation Presented) Iron tough: Various aryl halides were treated with unactivated arenes to form biaryl compounds in moderate to good yields. The reactions were carried out at relatively low temperature in the presence of a catalytic amount of FeCl3, with DMEDA as the ligand and LiHMDS as the base (see scheme; DMEDA=N,N'-dimethylethane-1,2-diamine, HDMS=hexamethyldisilazanide) .

Full text of DOI:10.1002/anie.200906870

Communications
DOI: 10.1002/anie.200906870
À
C H Activation
Iron-Catalyzed Direct Arylation of Unactivated Arenes with Aryl
Halides**
Wei Liu, Hao Cao, and Aiwen Lei*
Cross-coupling reactions used to construct biaryl compounds
mainly involve Ar1X as an electrophile and Ar2M as a
nucleophile.[1] Recently, C H bond activation has been used,
Scheme 1. Iron-catalyzed direct arylation of arenes with aryl halides.
À
where Ar2H served as the nucleophile to react with Ar1X
(direct arylation of arenes).[2,3] This strategy efficiently avoids
the disposal of stoichiometric amounts of metal waste
generated from the organometallic reagent, ArM, in the
traditional coupling manner.[2,3] Various transition metals
have been reported as efficient catalysts for this transforma-
tion, for example, Pd,[4–11] Rh,[12–15] Ru,[16–18] Ir,[19,20] Cu,[21–27]
and other transition metals.[28–30]
philic Ar1X (X = I, Br, Cl) and unactivated Ar2H coupling
partners.
Initially, we chose 4-bromoanisole and unactivated ben-
zene as the model substrates for surveying reaction param-
eters in the model reaction (Table 1). Commonly used
inorganic bases, such as Li2CO3, Na2CO3, K2CO3, Cs2CO3,
and K3PO4 were employed in the reaction, however, no
conversion was observed at 808C for 48 hours in the presence
of the iron salt. When NaOtBu was employed, a trace amount
of the direct arylation product was observed by GC/MS
analysis. This outcome encouraged us to further examine the
feasibility of this catalysis.
The application of inexpensive, non-toxic, commercially
available, and environmentally benign iron complexes as
catalysts in chemical syntheses has attracted much atten-
tion.[31,32] Recently, iron has been utilized extensively as a
catalyst to promote the “traditional” cross-coupling between
R1X and R2M.[33–41] Iron catalysts are also involved in many
important transformations, such as Friedel–Crafts benzyla-
tion,[42,43] carbonylation,[44] oxidation[45,46] and other process-
es.[47–54]
When LiHMDS (3.0 equiv) was added as a base, the
desired product 3a (16% yield) was obtained in the presence
Of great interest are the recently developed oxidative
À
1
2
1
2
Table 1: Iron-catalyzed C H bond activation: Screening of reaction
À
coupling reactions of Ar M with Ar H to generate Ar Ar
conditions.[a]
products by employing Fe complexes as the catalysts.
Nakamura and co-workers reported an elegant Fe-catalyzed
oxidative coupling reaction between Ar2H, which contain
directing groups, and diaryl zinc reagents.[55,56] Yu and co-
workers explored the oxidative reaction between Ar2H and
Ar1B(OH)2 using a stoichiometric amount of iron reagent.[57]
Entry [Fe]
(mol%)
Ligand
Base
Benzene Yield
[mL]
[%][b]
À
Many other oxidative coupling reactions, which involve C H
1
2
3
4
5
6
7
8
FeCl2 (20) none
FeBr2 (20) none
FeCl3 (20) none
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
3
3
3
3
3
3
3
3
3
4
5
4
4
4
4
4
4
4
4
4
9
16
18
8
14
21
6
66
75
66
79
66
76
–
activation using iron catalysts in the presence of stoichiomet-
ric amounts of oxidants, have also been reported.[58–61]
However, to the best of our knowledge, no example of Fe-
catalyzed cross-coupling between Ar1X and Ar2H has been
reported (Scheme 1). Herein, we report the development of
novel Fe-catalyzed cross-coupling reactions between electro-
FeCl3 (20) bipy
FeCl3 (20) TMEDA
FeCl3 (20) NH2CH2CH2NH2
FeCl3 (20) l-proline
FeCl3 (20) CH3O(CH2)2OCH3
FeCl3 (20) DMEDA
FeCl3 (20) DMEDA
FeCl3 (20) DMEDA
FeCl3 (15) DMEDA
FeCl3 (10) DMEDA
9
10
11
12
13
14
15
16
17
18
19
[*] W. Liu, H. Cao, Prof. A. Lei
College of Chemistry and Molecular Sciences,
Wuhan University, Wuhan, Hubei, 430072 (China)
Fax: (+86)27-6875-4067
FeCl3 (15) tBuNH(CH2)2NHtBu LiHMDS
E-mail: aiwenlei@whu.edu.cn
FeCl3 (15) DMEDA
FeCl3 (15) DMEDA
FeCl3 (15) DMEDA
LiOtBu
NaOtBu
KOtBu
LiHMDS
LiHMDS
trace
61
–
Prof. A. Lei
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Lu, Shanghai, 200032 (China)
none
none
none
DMEDA
–
[**] This work was supported by the National Natural Science
Foundation of China (20772093, 20972118, and 20832003), and the
Doctoral Fund of the Ministry of Education of China (20060486005).
[a] Reactions were carried out with 1a (0.5 mmol) and base (3.0 equiv) in
benzene (3 mL, 34 mmol; 4 mL, 45 mmol; 5 mL, 56 mmol). [b] Yields
were determined by GC analysis. TMEDA=N,N,N’,N’-tetramethyl-
ethane-1,2-diamine.
HMDS=hexamethyldisilazane, bipy= 2,2‘-bipyridine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906870.
DMEDA=N,N’-dimethylethane-1,2-diamine,
2004
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Chemie
Table 2: Different aryl bromides coupling with benzene.[a]
of FeCl3 (20 mol%; Table 1, entry 3). Meanwhile, FeCl2 and
FeBr2 were less efficient catalysts under the same reaction
conditions (Table 1, entries 1 and 2). The nature of the base
and the ligand was essential to influence the yield of the
product (Table 1, entries 4–14). In the presence of DMEDA
(40 mol%), the direct arylation of benzene was achieved in
66% yield (Table 1, entry 9), and this result showed that this
particular diamine ligand was more efficient than others.
Notably, the amount of benzene has some effect on this
reaction. The best result (75% yield) was obtained when the
reaction was carried out in the presence of 4 mL of benzene
(90 equiv; compare Table 1, entries 9–11). The effects of
catalyst loading was also examined, and 15 mol% was the
best and yielded 3a in 79% yield (compare Table 1,
entries 10, 12, and 13). When LiHMDS was replaced with
NaOtBu or LiOtBu, the reactions only produced trace
amounts or no desired product, respectively (Table 1,
entries 15 and 16). When KOtBu was employed as the base,
the desired product 3a was obtained in 61% yield (Table 1,
entry 17). Moreover, the direct arylation of benzene was not
observed at room temperature, and an increase to 608C led to
poor conversion.
Entry Aryl halides
Product
Yield
[%][b]
1
2
3
1a
1b
1c
3a
3b
3c
81
77
73
4
5
1d
1e
3d
3e
45
73
6
1 f
3 f
37
7
1g
1h
1i
3g
3h
3i
72
70
72
62
Control experiments revealed that no reaction was
observed in the absence of FeCl3 (Table 1, entries 18 and
19). To eliminate the contaminants which might potentially
affect the catalysis, a different batch of Fe complex was
employed. When high purity FeCl3 (> 99.99%, from Aldrich)
was used under the standard conditions (15 mol% of FeCl3,
30 mol% of DMEDA, 2.0 equiv of base), the yield remained
unchanged (see the Supporting Information).[62] This result
8
9
10
1j
3j
11
12
13
1k
1l
3k
3l
63
À
suggested that the C H activation reaction was solely
51
45
catalyzed by the Fe complex. When CuCl, CuBr, or CuI
were tested as catalysts under the standard conditions, both
conversion and yield became relatively low, thus indicating
that Cu salts were much less effective catalysts in this reaction
(for more details see the Supporting Information). These
experiments further indicate that the Fe catalyst plays a
1m
3m
[a] Reactions were carried out with
1
(0.5 mmol) and LiHMDS
(2.0 equiv) in benzene (4.0 mL, 45 mmol). [b] Yield of isolated product.
À
crucial role in this aromatic C H transformation.
With our optimized reaction conditions in hand, the scope
of this direct arylation of benzene with various aryl halides
was investigated (Table 2, Scheme 2, and Scheme 3). In
general, the yields of the reactions with electron-rich aryl
bromides were good (72%–81% yield; Table 2, entries 1–3, 5,
7, and 9) and were higher than those with the electron-
deficient aryl bromides (Table 2, entries 11–13). 2-Naphthyl
bromides 1h and 1j also underwent direct arylation smoothly
and afforded the desired products in 70% and 62% yield,
respectively (Table 2, entries 8 and 10). Aryl bromides with
ortho-substituted groups led to lower yields (Table 2, entries 4
and 6).
Aryl iodides containing electron-donating and electron-
withdrawing groups reacted smoothly with benzene and gave
the corresponding products in the range of yields (62%–82%;
Scheme 2). Interestingly, in the direct arylation of benzene
with aryl iodides reactions which employed KOtBu as the
base gave better yields compared with LiHMDS (Scheme 2).
Notably, no homo-coupling products from the corresponding
aryl halides were observed when using either aryl bromides or
aryl iodides.
Scheme 2. Different aryl iodides coupling with benzene. Reactions
were carried out using aryl iodide (0.5 mmol) and KOtBu (3.0 equiv) in
benzene (4.0 mL, 45 mmol). In parentheses, reactions were carried out
using LiHMDS (2.0 equiv) in benzene (4.0 mL, 45 mmol).
In addition to aryl bromides and iodides, the applicability
of more challenging aryl chlorides in this reaction was
examined. The reaction of benzene chloride gave the desired
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coupling product (52% yield), and the reactions of
p-MeC6H4Cl and p-MeOC6H4Cl gave 34% and 44% yields,
respectively (Scheme 3). Under the standard reaction con-
ditions, ArCl bearing electron-withdrawing groups resulted in
poor yields (< 5%; Scheme 3). Moreover, no reaction was
observed using fluorobenzene as the electrophile under the
same conditions.
À
Scheme 4. Iron-catalyzed two-fold C H bond activations. Reaction
conditions: aryl bromide (0.5 mmol) or aryl iodide (0.5 mmol),
LiHMDS (3.0 equiv) or KOtBu (3.0 equiv) in benzene (4.0 mL) at 808C
for 48 h.
arylated with bromobenzene, and gave the corresponding
monoarylated product (54% yield; Table 3, entry 5).
If the reaction proceeded through a benzyne pathway,
when substituted aryl bromide such as 1a was employed, two
products would be generated. However, the reactions of
various substituted aryl bromides with benzene all produced
the sole corresponding direct arylation products (Table 2),
which clearly ruled out the benzyne mechanism. The isotope
effect of the reaction was examined. The reaction was
conducted under the standard conditions using 4-bromoani-
sole (1a) coupling with equimolar amounts of benzene and
[D6]benzene (see the Supporting Information). The labeling
experiment yielded 3a and deuterated 3a in the ratio of 5:3
(kH/kD = 1.7).
Scheme 3. Iron-catalyzed direct arylation of benzene with aryl chlor-
ides. Reaction conditions: aryl chloride (0.5 mmol), LiHMDS
(2.0 equiv), FeCl3 (15 mol%), and DMEDA (30 mol%) in benzene
(4.0 mL) at 808C for 48 h.
Competitive experiments were carried out to probe the
reactivity between the chloro and the bromo group. Aryl
bromide was preferentially transformed, while the aryl
chloride was tolerated at either the para or ortho position
under the reaction conditions [Eq. (1) and (2)]. Both the
reaction of p-ClC6H4Br and o-ClC6H4Br gave good yields,
72% and 73%, respectively.
In summary, we have successfully developed the first
novel Fe-catalyzed direct arylation of unactivated arenes with
a broad range of aryl halides, including aryl chlorides,
À
bromides, and iodides, through C H bond activation to
prepare biaryl compounds. The choice of the base and ligand
are essential to the success of this Fe-catalyzed direct
arylation of unactivated arenes. Further studies on the
mechanism are actively underway, and will be reported in
due course.
Experimental Section
General procedure: Benzene (4.0 mL), DMEDA (13.2 mg,
0.15 mmol), and 4-bromoanisole (93.5 mg, 0.5 mmol) were added to
a Schlenk tube charged with FeCl3 (12.1 mg, 0.075 mmol) and
LiHMDS (167 mg, 1.0 mmol) under an argon atmosphere at RT.
The resulting reaction mixture was stirred at 808C for 48 h. After
cooling to RT, the mixture was quenched and extracted with ethyl
acetate (10 mL ꢀ 3). The organic layers were combined, dried over
Na2SO4, concentrated under reduced pressure, and then purified by
column chromatography on silica gel (ethyl acetate/petroleum
ether= 1:100) to yield the desired product as a white solid (74.5 mg,
81% yield).
When 1,4-dibromobenzene or 1,4-diiodobenzene was
treated with benzene in the presence of base (3 equiv), 1,4-
diphenylbenzene was produced in 43% and 53% yields,
respectively (Scheme 4).
The Fe-catalyzed direct arylation of substituted arenes
with aryl bromides was investigated, and the results are listed
in Table 3. The reaction between anisole and bromobenzene
gave 69% combined yield of direct arylation product (three
isomers, with a ratio of 64:24:12; Table 3, entry 2). Naphtha-
lene could be arylated in moderate yields (65% and 68%,
respectively; Table 3, entries 3 and 4), and produced a
mixture of 1- and 2-substituted naphthalene derivatives.
Direct arylation of toluene with 4-MeOC6H4Br afforded the
direct arylated product in 39% yield. Significantly, in the
presence of Fe catalyst (15 mol%), ferrocene was successfully
Received: December 6, 2009
Published online: February 16, 2010
À
Keywords: aryl halides · biaryls · C H activation ·
cross-coupling · iron
.
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