Room-temperature arylation of arenes and heteroarenes with diaryliodonium salts by photoredox catalysis

Article abstract of DOI:10.1055/s-0032-1318155

Aryl radicals produced by irradiation of diaryliodonium salts with visible light under the catalysis of [Ru(bpy)₃]²⁺ undergo coupling with a wide range of arenes and heteroarenes, affording various biaryls through direct C-H arylation at room temperature. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1318155

LETTER
▌507
lRettoerom-Temperature Arylation of Arenes and Heteroarenes with Diaryl-
iodonium Salts by Photoredox Catalysis
Room-Temperature Arylation of Arenes and Heteroarenes with Diaryliodonium Salts
Yu-Xia Liu,a Dong Xue,*a Jia-Di Wang,a Cong-Jun Zhao,a Qing-Zhu Zou,a Chao Wang,a Jianliang Xiao*a,b
a
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an, 710062, P. R. of China
Fax +86(29)81530727; E-mail: xuedong_welcome@snnu.edu.cn
b
Department of Chemistry, Liverpool Centre for Materials and Catalysis, University of Liverpool, Liverpool, L69 7ZD, UK
E-mail: J.Xiao@liverpool.ac.uk
Received: 10.12.2012; Accepted: 10.01.2013
[Ru(bpy)3][Cl]2·6H2O (maximum absorption at λ = 452
nm) as photocatalyst, the expected product 2-phenyl-N-
methylpyrrol (3a) was formed with 87% isolated yield af-
ter 12 hours reaction time in MeCN at room temperature
(Table 1, entry 4). Notably, the RuII photocatalyst and vis-
ible light are all essential for the reaction. In the absence
of any of these components, no reaction occurred (Table
1, entries 1–3). The amount of 2a and the reaction temper-
ature also play an important role; reducing the usage of 2a
led to lower yield and deviating from room temperature
does not benefit the reaction either (Table 1, entries 5–8).
The counteranion of diphenyliodonium appears to impact
on the product yield as well, with the trifluoromethanesul-
fonate giving the most promising results (Table 1, entries
4, 9–12). This may not be surprising, as counteranions are
known to affect the product distribution of diaryliodoni-
um photolysis.9a The RuII catalyst appears highly effective
for the reaction. At 0.1 mol% or even 0.05 mol% loading,
3a was still isolated in over 70% yield, showing the poten-
tial of such a photocatalyst in promoting visible-light-
driven coupling reactions (Table 1, entries 13 and 14).
Abstract: Aryl radicals produced by irradiation of diaryliodonium
salts with visible light under the catalysis of [Ru(bpy)3]2+ undergo
coupling with a wide range of arenes and heteroarenes, affording
various biaryls through direct C–H arylation at room temperature.
Key words: arenes, heteroarenes, visible light, aryl radicals, ruthe-
nium
Photoredox catalysis promoted by visible light has arisen
as a growing field in organic synthesis, which provides ef-
ficient synthetic methods and sustainable catalytic pro-
cesses.1 The group of MacMillan,2 Yoon,3 Stephenson,4
Akita,5 and others6 have demonstrated the successful ap-
plication of this concept in chemical synthesis. Despite
these advances, the synthesis of biaryls under visible-light
photoredox catalysis is still a challenge for chemists. Re-
cently, Sanford7a and König7b reported the construction of
aryl–aryl bonds by visible-light-driven catalysis. Using
aryldiazonium salts as the source of the aryl moiety is of
particular interest, as it provides good solutions for the
preparation of compounds containing the biaryl motif.
With the optimal conditions in hand, the substrate scope
was subsequently investigated. First, the coupling of 2a
with a wide range of diaryliodonium salts 1 with the phe-
nyl ring bearing various substituents, including OMe and
NO2 as well as CO2Me and Br, was attempted at ambient
temperature.16 As can be seen from Table 2, the desired
products were isolated in better yields with electron-with-
drawing substituents. This may stem from easier electron
transfer from the excited RuII* to such iodonium species
(vide infra). In particular, the diaryliodonium salts con-
taining halides afforded excellent yields regardless of the
substitution position (3c–i). The C–X (X = F, Cl, and Br)
bonds survived the mild reaction conditions, leaving
space for further reactions.
Diaryliodonium salts8 are another source of aryl radicals,9
which have led to their widespread use as one of the most
important photoinitiators for cationic polymerization.10
Although they have also been widely used as important
arylating reagents in organic synthesis,11–13 their substan-
tial applications catalyzed by visible light in organic syn-
thesis are rare. In continuing our work in aryl–aryl
coupling,14 herein we report the direct C–H bond arylation
of arenes and heteroarenes with diaryliodonium salts,
which requires visible light and Ru(bpy)3Cl2 as a catalyst
and which provides a valuable alternative method for the
synthesis of biaryls.
Our initial investigation focused on the direct arylation of
N-methylpyrrole (2a) with diphenyliodonium salt 1a. Fol-
lowing the screening of a wide range of conditions (Sup-
porting Information, Table S1), we were delighted to find
that using a low-energy, 3W blue LED bulb (emitting at
460 ± 15 nm) as the source of visible light and
Unsymmetrical diaryliodonium salts are also viable, and
it was found that it is the sterically less hindered aromatic
moiety that is preferentially transferred. As can be seen
from Table 3, a range of arylmesityliodonium salts under-
went coupling with 2a under the photocatalytic conditions
above, affording arylated N-methylpyrrols 3 in good
yield, albeit lower than those obtained with the symmetric
iodonium salts in Table 2.16 Again, electron-withdrawing
substituents in the transferring aryl group afforded the de-
SYNLETT 2013, 24, 0507–0513
Advanced online publication: 31.01.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1318155; Art ID: ST-2012-W1056-L
© Georg Thieme Verlag Stuttgart · New York

508
Y.-X. Liu et al.
LETTER
Table 2 Direct Arylation of N-Methyl Pyrrole (2a) with Symmetric
Diaryliodonium Salts 1a (continued)
Table 1 Optimization of Conditions for the Photocatalytic Cross-
Coupling of Diphenyliodonium Salt 1a with N-Methyl Pyrrole (2a)a
–OTf
I
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
R
H
+
R
[Ph2I]X
+
N
H
N
N
MeCN, Ar, blue LED
25 °C, 12 h
Me
Me
1
2a
Me
2a
1a
3a
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
Entry
Reaction conditions
X
Time (h) Yield (%)b
N
MeCN, Ar, blue LED
25 °C, 12 h
R
Me
1
no catalyst, no light
catalyst, no light
no catalyst, light
catalyst, light
catalyst, lightc
catalyst, lightd
OTf
OTf
OTf
OTf
OTf
OTf
OTf
24
24
24
12
12
12
12
12
12
12
0
3
2
0
Entry
2
Product
Yield (%)
91
3
0
F
4
87
37
59
54
37
84
71
70
69
78
77
N
5
Me
6
3b
7
catalyst, light, 0 °C
F
8
catalyst, light, 40 °C OTf
9
catalyst, light
catalyst, light
catalyst, light
catalyst, lighte
catalyst, lightf
catalyst, lightg
PF6
BF4
3
4
5
6
91
85
92
95
N
10
11
12
13
14
Me
OTFAc 12
3c
Br
12
24
36
F
OTf
OTf
N
Me
a All the reactions were carried out at the scale of 0.4 mmol of 1 and
20 mmol of 2a in 3 mL MeCN.
3d
b Isolated yields.
F
c Conditions: 4 mmol of 2a.
d Conditions: 8 mmol of 2a.
F
e The solvent was DMSO.
N
f Conditions: 0.1 mol% RuII catalyst were used.
g Conditions: 0.05 mol% RuII catalyst were used.
Me
3e
Cl
sired products in better yields, and C–X bonds were toler-
ated. The flexibility of the arylating reagents enhances the
applicability of this method in the preparation of biaryl
compounds.
N
Me
Table 2 Direct Arylation of N-Methyl Pyrrole (2a) with Symmetric
Diaryliodonium Salts 1a
3f
Cl
Br
–OTf
I
N
7
8
9
94
89
89
R
H
+
R
N
Me
Me
3g
1
2a
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
N
N
Me
MeCN, Ar, blue LED
25 °C, 12 h
R
Me
3
3h
3i
Entry
1
Product
Yield (%)
87
CF3
N
Me
N
Me
NO2
3a
10
85
N
Me
3j
Synlett 2013, 24, 507–513
© Georg Thieme Verlag Stuttgart · New York

LETTER
Room-Temperature Arylation of Arenes and Heteroarenes with Diaryliodonium Salts
509
Table 2 Direct Arylation of N-Methyl Pyrrole (2a) with Symmetric
Diaryliodonium Salts 1a (continued)
arenes demonstrate higher reactivity than their electron-
poor analogues. For instance, the yield of 4n is signifi-
cantly higher than that of 4j, even though the former is ste-
rically more demanding, and little reaction was observed
when nitrobenzene was used. It is also worth noting that
the reactions of pyrroles, furan, thiophenes, 2,3-benzo-
thiophene, and 2,3-benzofuran with diarylionium salts all
took place primarily at the 2-position of the heteroarenes.
This regio-preference is probably a result of resonance
stabilization of intermediate radical adduct generated
during the photocatalysis (vide infra).
–OTf
I
R
H
+
R
N
Me
1
2a
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
N
MeCN, Ar, blue LED
25 °C, 12 h
R
Me
3
Entry
11
Product
Yield (%)
78
Table 3 Direct Arylation of N-Methyl Pyrrole (2a) with Un-
OMe
O
symmetrical Diaryliodonium Saltsa
–OTf
N
I
Me
+
H
R
N
3k
Me
2a
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
N
12
13
73
65
N
MeCN, Ar, blue LED
25 °C, 12 h
Me
Me
R
3
3l
Entry
1
Product
Yield (%)
60
t-Bu
N
F
Me
N
3m
Me
3b
F
N
14
15
75
40
Me
2
3
4
60
64
65
N
3n
Me
OMe
3c
N
F
Me
3o
N
Me
3d
16
75
F
N
Me
F
3p
N
a All the reactions were carried out at the scale of 0.4 mmol of 1 and
20 mmol of 2a in 3 mL MeCN. Isolated yields.
Me
3e
Cl
Br
Of particular interest is that the visible-light-promoted
arylation can be extended into arenes and heteroarenes
of diverse properties, including substituted benzenes,
pyrroles, furans, and thiophenes. A broad range of aryl–
heteroaryl and diaryl products 4a–p were formed in good
to excellent yield, as shown in Table 4. In many cases, the
quantity of the arenes or heteroarenes can be reduced
without adversely affecting the yield. Moreover, no pro-
tection was required for the arylation of pyrrole, where
only monoarylation occurred. In general, electron-rich
N
5
6
64
70
Me
3g
N
Me
3h
© Georg Thieme Verlag Stuttgart · New York
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510
Y.-X. Liu et al.
LETTER
Table 3 Direct Arylation of N-Methyl Pyrrole (2a) with Un-
Table 4 Direct Arylation of Arenes and Heteroarenes with Diaryl-
iodonium Salt 1ka (continued)
symmetrical Diaryliodonium Saltsa (continued)
–OTf
Entry
Product
Yield (%)
I
+
H
R
CO2Me
N
Me
2a
Et
[Ru(bpy)3][Cl]2⋅6H2O
3
90b
NH
(1 mol%)
N
MeCN, Ar, blue LED
25 °C, 12 h
Me
R
4c
3
CO2Me
Entry
7
Product
Yield (%)
55
OMe
O
4
63b
N
Ph
N
Me
4d
4e
4f
CO2Me
3k
Me
5
6
60
82
O
S
8
N
35
68
35
72
Me
CO2Me
3n
Br
9
N
Me
CO2Me
3p
7
8
66b
S
N
10
11
Me
4g
4h
CO2Me
3q
Cl
Cl
56b
O
N
Me
3r
CO2Me
a The conditions were the same as those given in Table 2.
N
N
9
45b
Table 4 Direct Arylation of Arenes and Heteroarenes with Diaryl-
iodonium Salt 1ka
4i
4j
Entry
Product
Yield (%)
CO2Me
CO2Me
10
50
1
81
NH
4a
CO2Me
CO2Me
11
55
2
85b
NH
4k
4b
Synlett 2013, 24, 507–513
© Georg Thieme Verlag Stuttgart · New York

LETTER
Room-Temperature Arylation of Arenes and Heteroarenes with Diaryliodonium Salts
511
Table 4 Direct Arylation of Arenes and Heteroarenes with Diaryl-
iodonium Salt 1ka (continued)
The C–H arylation described is likely to proceed through
a radical mechanism, and this is supported by preliminary
mechanistic investigations. Thus, when the diphenyliodo-
nium salt 1a was irradiated with the 3W blue LED bulb in
the presence of a radical trap, 2,2,6,6-tetramethylpiperid-
inoxyl (TEMPO), while in the absence of N-methyl pyr-
role (2a), compound 5a was formed and isolated in 26%
yield (Scheme 1). Furthermore, addition of TEMPO to the
photocatalytic reaction of 1a with 2a suppressed the ary-
lation process significantly, with 3a obtained only in 22%
yield (Scheme 1). The TEMPO-trapped intermediate 5a
was isolated in 17% yield. The formation of 5a and the
suppression of the arylation reaction in the presence of
TEMPO lend support to radical being involved in the
photocatalytic arylation.
Entry
Product
Yield (%)
CO2Me
12
70
4l
CO2Me
OMe
13
51b
OMe
A plausible mechanism is shown in Scheme 2 on the basis
of the above experiments and literature reports.9,10 There
are three key steps: (1) irradiation of the catalyst at the
ground state A, generating excited [Ru(bpy)3]II* B; (2) sin-
gle electron transfer from the excited catalyst B (–0.86 V
vs. SCE)1 to the diaryliodonium salt 1a (E1/2 = –0.2 V vs.
SCE, X = PF6),15 releasing a phenyl radical while oxidiz-
ing the catalyst to [Ru(bpy)3]III C; (3) addition of the phe-
nyl radical to the arene to give a new radical intermediate,
which is subsequently transformed into a carbocation in-
termediate by two possible pathways: oxidation by the
strongly oxidizing [Ru(bpy)3]III (pathway a) or by the dia-
ryliodonium salt (pathway b). The latter would lead to a
radical-chain transfer reaction and appears to be less like-
ly, as judged by the reduction potentials of the two oxi-
dants (1.3 against –0.2 V, vs. SCE)1,15 and by the
observation that switching off the light stopped the aryla-
tion. Finally, deprotonation of the carbocation intermedi-
ate regenerates the aromatic system, furnishing the
desired coupling adducts.
4m
CO2Me
OMe
14
15
79b
MeO
OMe
4n
CO2Me
65b
4o
CO2Me
16
52b
4p
In summary, a highly effective visible-light-promoted
coupling of arenes and heteroarenes with diaryliodonium
salts has been developed. The reaction proceeds at room
temperature and displays a broad substrate scope, thus
providing a mild alternative method for the direct aryla-
tion of aromatic C–H bonds. The reaction mechanism and
further application of diaryliodoniumsalts in other reac-
tions are being actively explored in our group.
a The reactions were carried out on a scale of 0.4 mmol of 1k and 20
mmol of arene or heteroarene in 3 mL MeCN. Isolated yields are
given.
b Conditions: 0.4 mmol of 1a and 4 mmol of arene or heteroarene in
3 mL MeCN.
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
+
Ph2IOTf
(1)
N
N
O
MeCN, Ar
blue LED
O
.
1a
25 °C, 12 h
Ph
(2 equiv)
5a 26% yield
[Ru(bpy)3][Cl]2⋅6H2O
(1 mol%)
+
+
Ph2IOTf
1a
(2)
+
Ph
N
N
N
N
O
MeCN, Ar
blue LED
O
.
Me
Me
25 °C, 12 h
Ph
(2 equiv)
(50 equiv)
5a 17% yield
3a 22% yield
Scheme 1 Mechanistic investigations
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 507–513

512
Y.-X. Liu et al.
LETTER
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+
R
A
hν
a
b
Ar
Ar
R
[Ru(bpy)3]2+*
[Ru(bpy)3]3+
B
C
R
[Ar–I–Ar]OTf
Ar
ArI + TfO–
[Ar–I–Ar] OTf
Scheme 2 A plausible mechanisms made possible by photocatalysis
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We are grateful for the financial support of the National Science
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Synlett 2013, 24, 507–513
© Georg Thieme Verlag Stuttgart · New York

LETTER
Room-Temperature Arylation of Arenes and Heteroarenes with Diaryliodonium Salts
513
M.; Gaunt, M. J. Angew. Chem. Int. Ed. 2011, 50, 458.
solid, mp 43–44 °C; Rf = 0.2 (PE–CH2Cl2 = 10:1); yield 87%
(63 mg). 1H NMR (400 MHz, CDCl3): δ = 7.43–7.40 (m, 4
H), 7.34–7.31 (m, 1 H), 6.75 (s, 1 H), 6.27–6.24 (m, 2 H),
3.69 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 134.7, 133.5,
128.8, 128.4, 126.7, 123.7, 108.7, 107.8, 35.1. MS (EI): m/z
= 158.1 [M + H]+.
(j) Duong, H. A.; Gilligan, R. E.; Cooke, M. L.; Phipps, R.
J.; Gaunt, M. J. Angew. Chem. Int. Ed. 2011, 50, 463.
(14) Xue, D.; Li, J.; Liu, Y.-X.; Han, W.-Y.; Zhang, Z.-T.; Wang,
C.; Xiao, J. Synlett 2012, 23, 1941.
(15) Bachofner, H. E.; Beringer, F. M.; Meites, L. J. Am. Chem.
Soc. 1958, 80, 4274.
1-Methyl-2-(4-Fluorophenyl)-1H-pyrrole (3b)
(16) General Procedure for the Direct Arylation of Arenes
and Heteroarenes with Diaryliodonium Salts
Compound 3b was obtained following the method A. White
solid, mp 64 °C; Rf = 0.2 (PE–CH2Cl2 = 20:1); yield 91% (64
mg). 1H NMR (400 MHz, CDCl3): δ = 7.39–7.36 (m, 2 H),
7.13–7.09 (m, 2 H), 6.73 (d, J = 2.0, 1 H), 6.23–6.22 (m, 2
H), 3.65 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 162.0 (d,
JC–F = 245.8 Hz), 133.6, 130.3 (d, JC–F = 8.8 Hz), 129.5 (d,
Method A
A flame dried 10 mL Schlenk tube was equipped with a
rubber septum and magnetic stir bar and was charged with
tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate (3
mg, 0.004 mmol, 0.01 equiv), diarylionium salts (0.4 mmol,
1.0 equiv), arenes or heteroarenes (20 mmol, 50 equiv), and
MeCN (3 mL). The mixture was degassed by the freeze-
pump-thaw procedure with Ar, and then the Schlenk tube
was placed at a distance of 10 cm from a 3w Blue LED bulb.
After stirring at 25 °C for 12 h, the solvent was removed
under reduced pressure, and the residue purified by flash
chromatography to afford the product. The identity and
JC–F = 3.3 Hz), 123.6, 115.3 (d, JC–F = 21.3 Hz), 108.7,
107.8, 34.9. IR (KBr, plate): ν = 3429, 2950, 2923, 1719,
1609, 1494, 1437, 1279, 859, 779, 705 cm–1. ESI-HRMS:
m/z calcd for C11H11FN [M + H]+: 176.0876; found:
176.0866.
1-Methyl-2-(2-Fluorophenyl)-1H-pyrrole (3c)
Compound 3c was obtained following the method A.
Colorless oil; Rf = 0.2 (PE–CH2Cl2 = 20:1); yield 91% (64
mg). 1H NMR (400 MHz, CDCl3): δ = 7.38–7.32 (m, 2 H),
7.19–7.12 (m, 2 H), 6.78 (t, J = 2.0 Hz, 1 H), 6.25 (d, J = 2.0
Hz, 2 H), 3.58 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ =
160.0 (d, JC–F = 245.0 Hz), 132.1 (d, JC–F = 3.1 Hz), 129.2 (d,
purity of the product was confirmed by 1H NMR and 13
NMR spectroscopy and MS or HRMS.
Method B
C
This method is the same as method A except that a smaller
amount of arene or heteroarene (4 mmol, 10 equiv) was
used. The method was used mainly for substrates that did not
fully dissolve under the conditions in method A.
JC–F = 8.1 Hz), 128.2, 124.1 (d, JC–F = 3.4 Hz), 123.5, 121.2
(d, JC–F = 15.3 Hz), 115.7 (d, JC–F = 22.5 Hz), 109.9, 107.9,
34.7. ESI-HRMS: m/z calcd for C11H11FN [M + H]+:
176.0876; found: 176.0871.
1-Methyl-2-Phenyl-1H-pyrrole (3a)
Compound 3a was obtained following the method A. White
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 507–513

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