Amine-catalyzed direct photoarylation of unactivated arenes

Article abstract of DOI:10.1002/asia.201301371

Constructing biaryls through direct aromatic C-H functionalization of unactivated arenes has become a popular topic in organic chemistry. Many efficient methods have been developed. In this Communication, a direct arylation of unactivated arenes with a broad range of aryl iodides is reported. This reaction proceeds through a new type of amine-catalyzed single electron transfer initiated radical coupling procedure to form biaryls in high yields under UV irradiation at room temperature. Only 20 mol% of TMEDA is used as the catalyst. No other additives are required for this transformation, thus avoiding the use of toxic transition metal catalysts, strong bases, or large amounts of other organic additives. This greener protocol provides a new strategy to achieve direct aromatic C-H functionalization and offers a new example of cost-effective and environmentally benign access to biaryls. Copyright

Full text of DOI:10.1002/asia.201301371

COMMUNICATION
DOI: 10.1002/asia.201301371
Amine-Catalyzed Direct Photoarylation of Unactivated Arenes
Xingliang Zheng,*[b] Luo Yang,[b] Weiyuan Du,[a] Aishun Ding,[a] and Hao Guo*[a]
À
Abstract: Constructing biaryls through direct aromatic C H
functionalization of unactivated arenes has become a popular
topic in organic chemistry. Many efficient methods have
been developed. In this Communication, a direct arylation
of unactivated arenes with a broad range of aryl iodides is
reported. This reaction proceeds through a new type of
amine-catalyzed single electron transfer initiated radical
coupling procedure to form biaryls in high yields under UV
irradiation at room temperature. Only 20 mol% of TMEDA
is used as the catalyst. No other additives are required for
this transformation, thus avoiding the use of toxic transition
metal catalysts, strong bases, or large amounts of other or-
ganic additives. This greener protocol provides a new strat-
several drawbacks. First of all, expensive late transition
metal catalysts are required. Secondly, organometallic re-
agents are normally unstable and need to be initially synthe-
sized. Thirdly, stoichiometric amounts of toxic metal waste
are generated, which is not environmentally friendly. Later,
the direct arylation of arenes with aryl halides has—as an
atom economical and more environmentally friendly alter-
native to traditional cross-coupling—attracted the attention
of synthetic chemists since only one or none of the cross-
coupling partners needs to be prefunctionalized. Many tran-
sition metals, such as Pd,[11] Rh,[12] Ru,[13] Ir,[14] Cu,[15] Ni,[16]
Fe,[17] and Co[18] have been reported as efficient catalysts for
this transformation. However, in most cases, directing
À
À
egy to achieve direct aromatic C H functionalization and
offers a new example of cost-effective and environmentally
benign access to biaryls.
groups are required for successful C H bond functionaliza-
tion to achieve high reactivity and/or good regioselectivity.
Furthermore, these methods still require a transition metal
catalyst, which sometimes leads to transition metal impuri-
ties in the final product and requires an additional purifica-
tion step, especially in drug industry. Recently, a few re-
search groups[19] have reported organocatalyzed transition-
metal-free processes for the synthesis of biaryl targets by
Biaryls are one of the most important classes of organic
compounds that are frequently found in natural products,
pharmaceutically active reagents, agrochemicals, dyes, etc.,
and have attracted much interest in advanced materials and
supramolecular sciences.[1,2] Consequently, much effort has
been devoted to their preparation and a lot of efficient syn-
thetic methods have been developed in this field.
Since Ullmann reported the first aryl–aryl bond formation
using aryl halides and a stoichiometric amount of elemental
copper,[3] cross-coupling reactions, such as Suzuki,[4,5] Ne-
gishi,[6,7] Kumada,[8] Stille,[9] and Hiyama[10] coupling reac-
tions, have emerged as textbook methods for the synthesis
of both symmetrical and asymmetrical biaryls from aryl hal-
ides (or pseudohalides) and aryl organometallics. Despite
their remarkable effectiveness, these reactions suffer from
À
direct C H arylation of unactivated arenes with aryl halides,
which have the unique advantage that no toxic and expen-
sive transition metals are required. However, excess strong
bases like tert-butoxides were employed, limiting the appli-
cation to base-stable substrates. There are also several ex-
amples of synthetic transformations based on intermolecular
addition of aryl radicals to unactivated arenes, proceeding
smoothly without transition metals or tert-BuOK/tert-
BuONa; however, to generate aryl radicals from aryl hal-
ides, excess radical sources such as (TMS)3SiH or AIBN are
necessary.[20] Photoinduced biaryl formation from unactivat-
ed arenes with aryl halides via a radical process has been
studied for a long time.[21–23] Under simple irradiation with
UV light, such reactions proceed easily without any additive,
thus providing
a straightforward and environmentally
[a] W. Du, A. Ding, Prof. Dr. H. Guo
Department of Chemistry
friendly protocol for the synthesis of this type of important
molecules. However, the reactions usually suffer from low
reaction rates and/or low chemical yields. Very recently, visi-
ble light-induced direct photoarylation has been reported,
but the use of an excessive amount of tert-BuOK is still nec-
Fudan University
220 Handan Road, Shanghai 200433 (P. R. China)
Fax : (+86)21-55664361
E-mail: Hao_Guo@fudan.edu.cn
essary.[24] In this regard, more efficient C H bond arylation
[b] Prof. X. Zheng, L. Yang
À
Institute of Chemistry & Biological Engineering
Changsha University of Science and Technology
Changsha, 410114 (P. R. China)
reactions that can be performed under ambient conditions
are still in urgent need considering increasing environmental
concerns (Scheme 1). Herein, we wish to report our recent
observation on amine-catalyzed direct photoarylation of un-
activated arenes with aryl iodides at ambient temperature.
E-mail: xingliangzheng@163.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301371.
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Hao Guo et al.
yield dropped to 82% (Table 1, entry 7). Both 2,2’-bipyri-
dine (bipy) and N,N,N’,N’-tetramethyl-ethane-1,2-diamine
(TMEDA) resulted in excellent yields (Table 1, entries 8
and 9). Next, some test reactions using inorganic bases as
additives were carried out (Table 1, entries 10–13); however,
the results were even worse than those in the absence of ad-
ditive (Table 1, entry 1). Since the yield with TMEDA was
slightly higher than in all other cases (Table 1, entry 9),
TMEDA was chosen as the amine catalyst for further stud-
ies. We then examined the influence of the amount of
TMEDA. When 5 or 10 mol% of TMEDA were applied,
the yield significantly decreased (Table 1, entries 14 and 15).
The use of 50 or 100 mol% of TMEDA also led to similar
low yields (Table 1, entries 17 and 18). Considering that the
reactions using 25 or 20 mol% of TMEDA gave the same
results (Table 1, entries 9 and 16), we chose the lower load-
ing as the standard condition (Table 1, entry 16). Thus, we
applied conditions A (c=20 mm, l=254 nm, 20 mol% of
TMEDA, under air atmosphere, and rt) for the amine-cata-
lyzed direct photoarylation of unactivated arenes with aryl
iodides.
Scheme 1. Approaches for arylation of unactivated arenes with aryl hal-
ides.
Initially, 1-iodo-4-methoxybenzene (1a) and benzene (2a)
were chosen as model substrates. The direct arylation oc-
curred readily by simply irradiating a solution of 1a in ben-
zene at l=254 nm under argon atmosphere without any ad-
ditive, consistent with previous literature reports.[21a,22] How-
ever, the reaction was slow and only 69% of the coupling
product 3aa was formed (Table 1, entry 1). When 50 mol%
Table 1. Optimization of the reaction conditions.[a]
With the optimized reaction conditions in hand, we next
investigated the reaction of different aryl iodides under con-
ditions A (Table 2). The cross-coupling proceeded in high
efficiency with either electron-rich or electron-deficient aryl
iodides to form the desired products in good to excellent
yields. No clear electronic effect on the aryl ring was ob-
served. The relative position of the substituents showed
some influences. While 3- or 4-substituted aryl iodides af-
Entry Catalyst [mol%] Wavelength [nm] t [h] Isolated yield [%]
1[b]
–
254
365
313
254
185
254
254
254
254
254
254
254
254
254
254
254
254
254
25
65
51
7
11
7
7.5
7.5
7
27
27
27
27
11
8
69
2[b]
iPr2NEt (50)
iPr2NEt (50)
iPr2NEt (50)
iPr2NEt (50)
iPr2NEt (50)
Et3N (50)
31 (46)[d]
75
3[b]
4[b]
92
81
93
82
92
95
5[b]
6[c]
Table 2. The photoarylation of benzene with 1.[a]
7[c]
8[c]
bipy (25)
9[c]
TMEDA (25)
Na2CO3 (20)
Na2CO3 (100)
K2CO3 (20)
K2CO3 (100)
TMEDA (5)
TMEDA (10)
TMEDA (20)
TMEDA (50)
TMEDA (100)
10[c]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
17[c]
18[c]
54 (2)[d]
57 (1)[d]
38 (6)[d]
54 (1)[d]
79
76
95
79
73
Entry
1
R
t [h]
3
Isolated yield [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
4-MeO
3-MeO
2-MeO
2,6-diMeO
4-EtO
4-nBuO
4-OH
4-Me
3-Me
2-Me
2,6-diMe
4-Et
4-But
4-Ph
4-H
4-C(O)Me
4-COOMe
4-COOEt
4-CN
4-NO2
4-CF3
7
9
3aa
3ba
3ca
3da
3ea
3 fa
3ga
3ha
3ia
95
76
57
29
83
71
76
66
62
6
7
7.5
7.5
13
30
6
10
20
5
7
7
32
5
6
19
5
11
11
6
4
31
5
[a] All reactions were carried out using a Matrix-10 reactor with sixteen
ultraviolet lamps (10 W per lamp) as the irradiation source in deaerated
benzene (c=20 mm) at rt. [b] The reaction was carried out under argon
atmosphere. [c] The reaction was carried out in air. [d] Recovered yield
of 1a.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
3ja
3ka
3la
0
77
82
74
82
77
81
90
94
91
73
76
45
3ma
3na
3oa
3pa
3qa
3ra
3sa
3ta
of iPr2NEt were used as the catalyst, the reaction rate was
dramatically increased and the yield increased up to 92%
(Table 1, entry 4). An examination of the influence of irradi-
ation energy did not result in improved yields (Table 1, en-
tries 2, 3, and 5). Notably, almost the same yield was ob-
tained when the reaction was carried out in air (Table 1,
entry 6), thus showing that strictly anhydrous and anaerobic
operation is not necessary for this protocol. Subsequently,
the catalytic activities of other amines were examined.
When Et3N was used as the catalyst in this reaction, the
1s
1t
1u
1v
1w
3ua
3va
3wa
3-CF3
2-CF3
7
7
[a] All reactions were carried out under conditions A.
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Hao Guo et al.
forded the corresponding biaryls in similar yields (Table 2,
entries 1, 2, 8, 9, 21, and 22), the yields were much lower for
2-substituted reactants (Table 2, entries 3, 10, and 23).
Under the optimized conditions, sterically hindered sub-
strates hardly underwent this photoarylation. The yields
were very low (Table 2, entries 4 and 11). Notably, sub-
strates containing strong base-sensitive groups such as hy-
droxy and nitro groups were also well tolerated, and the cor-
responding products were isolated in good to excellent
yields (Table 2, entries 7 and 20).
To extend the scope of this process and obtain some
mechanistic information, the coupling reactions of 1p with
substituted arene derivatives were then examined. Anisole
2b was easily arylated to form a mixture of regioisomers
3pb (o/m/p=5.1:1.0:1.0) in 94% total yield [Eq. (1)]. With
a decreased electron density in the arene, the reaction
became less effective. In case of benzonitrile 2c, under stan-
dard conditions, the coupling product 3pc was formed only
in a very low overall yield of 11% (o/m/p=3.1:1.6:1.0)
[Eq. (2)]. In both cases, ortho-selectivity was favored, there-
by demonstrating that the electronic effect of the substituent
barely changed the regioselectivity. The observed regioselec-
tivity suggests that radical intermediates are involved in the
pathway for this arylation reaction.[11d,24,25]
Scheme 3. Effect of the radical scavenger.
Furthermore, the addition of the radical scavenger
(2,2,6,6-tetramethyl-1-piperidinyl)oxidanyl (TEMPO) led to
a dramatic retardation of the model reaction. Both the rate
and yield decreased significantly, thus indicating a radical
process is in operation in this reaction[17a,26] (Scheme 3).
Scheme 4. The proposed mechanism.
On the basis of the above data and literature prece-
dents,[22] a radical mechanism is proposed as shown in
Scheme 4. Under photoirradiation, a single electron transfer
from the amine to the aryl iodide 1 leads to an amine radi-
cal cation (A) and an aryl iodide radical anion (B).[26] The
À
following dissociation of the C I bond in B and release of
an iodide anion result in the aryl radical C, which subse-
quently adds to arene 2 to generate the aryl-substituted cy-
clohexadienyl radical intermediate D.[26] Finally, single elec-
tron oxidation by A and deprotonation affords the biaryl
product 3 and completes the catalytic cycle. However, the
above proposed mechanism may not be the only reaction
pathway. The possibility of a route via a normal photoin-
duced aryl radical formation process[21,22] may be also con-
sidered, since irradiation without the amine also gives the
desired product in an isolated yield of 69% (Table 1,
entry 1). The amine may first enhance the initial radical for-
mation and then act as a base to neutralize the formed HI.
In summary, we have successfully developed an amine-
catalyzed photoarylation of unactivated arenes with a broad
range of aryl iodides, which provides a novel catalytic proto-
col for the synthesis of biaryls under mild conditions. Nota-
bly, this method avoids the use of toxic transition metal cat-
alysts, strong bases, or large amounts of other organic addi-
tives. Thus, this reaction offers a new insight into the realiza-
tion of a cost-effective and environmentally benign access to
biaryls. A radical pathway has been proposed to explain the
mechanism of this transformation. Further studies on the
To gain further mechanistic insights, a competition reac-
tion using an equimolar mixture of benzene and
[D6]benzene with 1a was conducted. The observed primary
kinetic isotopic effect (kH/kD =1.0) was low, which implied
À
that the C H bond cleavage in benzene was not the rate-
limiting step[11h,18a,19g,25] (Scheme 2).
Scheme 2. Kinetic isotopic effect experiment.
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Hao Guo et al.
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Acknowledgements
We greatly acknowledge the financial support from the National Basic
Research Program of China (973 Program, 2012CB720300), National
Nature Science Foundation of China (21102016 and 21274023), Shanghai
Pujiang Program (11PJD004), the Specialized Research Fund for the
Doctoral Program of Higher Education of China (20110071120004), Sci-
entific Research Fund of Hunan Provincial Science & Technology De-
partment (2012J3003), and Scientific Research Fund of Hunan Changsha
Municipal Science & Technology Department (K101022-11).
À
Keywords: arylation · biaryls · C H functionalization ·
photochemistry · radical reactions
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