Ferrous salt-promoted homocoupling of arenediazonium tetrafluoroborates under mild conditions

Article abstract of DOI:10.1016/j.tetlet.2012.09.028

A simple and efficient protocol has been developed for the synthesis of various symmetrical biaryls in good to excellent yields from the homocoupling reactions of arenediazonium salts with ferrous salt in carbon tetrachloride solution under mild reaction conditions. Moreover, the novel homocoupling has been demonstrated to proceed via an electron transfer reaction mechanism.

Full text of DOI:10.1016/j.tetlet.2012.09.028

Tetrahedron Letters 53 (2012) 6269–6272
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ferrous salt-promoted homocoupling of arenediazonium tetrafluoroborates
under mild conditions
Yiyuan Ding , Kai Cheng , Chenze Qi , Qingbao Song a,
a
b
b,
⇑
⇑
a
College of Chemical & Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
Institute of Applied Chemistry and Department of Chemistry, University of Shaoxing, Shaoxing, Zhejiang Province 312000, People’s Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 April 2012
Revised 10 July 2012
Accepted 7 September 2012
Available online 15 September 2012
A simple and efficient protocol has been developed for the synthesis of various symmetrical biaryls in
good to excellent yields from the homocoupling reactions of arenediazonium salts with ferrous salt in
carbon tetrachloride solution under mild reaction conditions. Moreover, the novel homocoupling has
been demonstrated to proceed via an electron transfer reaction mechanism.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Homocoupling
Ferrous salt-promoted
Arenediazonium tetrafluoroborates
Electron transfer reaction
Introduction
biaryl moieties. Also, no additional base is required for these reac-
tions. Thus, it is not surprising that arenediazonium salts have
Biaryls are not only the key chemical intermediates for a wide
range of organic processes, but are also very important structural
motif for many natural and synthetic organic compounds. Thus,
been used as electrophilic partners for the palladium-catalyzed
11
cross-coupling reactions including the Mizoroki–Heck, Suzuki–
1
12
13
Miyaura, and Stille reactions.
it is interesting to develop new straightforward and environmen-
tally friendly reactions for the biaryl synthesis. Symmetrical biaryls
are traditionally prepared from the reductive homocoupling of aryl
halides and the Ullmann reaction is the first practical and efficient
process for the synthesis of symmetrical biaryls.2 However, the
Ullmann reaction generally requires a stoichiometric amount of
However, much less attention has been paid to the synthetic
utility of the homocoupling of arenediazonium salts for the sym-
1
4
metrical biaryls. Hanna co-workers reported palladium-cata-
lyzed homocoupling of arenediazonium salts for the synthesis of
symmetrical biaryls in methanol, but the anisole and benzene
by-products were formed in a significant amount. Anisole was be-
lieved to be derived from the reaction of arenediazonium salts with
methanol. The copper (I)-mediated reductive homocoupling of
aryldiazonium salts was first discovered as the side reaction for
the Gattermann synthesis of aryl halides from arenediazonium
copper and occurs only at
a relatively high temperature
(
ꢀ200 °C). The harsh reaction conditions limit its applications for
the synthesis of any delicate compounds, which also spawn the
development of much milder reactions such as catalyzed by the
palladium, rhodium, and gold5 catalysts for the biaryl synthesis
from aryl halides or aryl sulfonates. More recently, much attention
has been paid to the developments using catalysts based on rela-
tive cheaper transition metals such as nickel,6 copper,7 and iron
for the carbon–carbon bond formation coupling reactions.
3
4
salts. Cepanec et al. reported that copper (I) triflate acts as an effi-
15
cient stoichiometric reagent for the homocoupling of arenediazoni-
um salt to yield symmetrical biaryl. However, the copper (I) salts
are instable, and it require to be in situ prepared for the homocou-
pling reaction. In the present Letter, we would like to present our
results using the much cheaper and environmentally benign fer-
rous salts to mediate the reductive homocoupling of arenediazoni-
um salts for the synthesis of various symmetrical biaryls under
mild reaction conditions.
8
Arenediazonium salts are attractive alternative to the common
9
electrophiles, aryl halides, or triflates, for the carbon–carbon bond
formation coupling reactions because of their ready availability
from anilines, superior reactivity, and environmental friendli-
ness.1 Their insensitivity to the substituent electronic and steric
effects allows the introduction of various substituents into the
0
Results and discussion
⇑
Corresponding authors. Tel./fax: +86 575 88345682.
E-mail addresses: qichenze@usx.edu.cn (C. Qi), qbsong@zjut.edu.cn (Q. Song).
No homocoupling biphenyl products were observed for the
reaction of benzenediazonium salt with ferrous chloride (FeCl )
2
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.028

6
270
Y. Ding et al. / Tetrahedron Letters 53 (2012) 6269–6272
in CH
2
Cl
2
, CHCl3, and acetone (Table 1, entries 3–5). A mixture of
Table 2
a
The screening of various arenediazonium salts
anisole (25%) and biphenyl (44%) was obtained when carried out
in methanol (Table 1, entry 1). Essentially identical biphenyl yields
(
54.5 ± 1.5%) were obtained when carried out in THF, 1,4-dioxane,
FeCl2
N BF
and MTBE (methyl tert-butyl ether) (Table 1, entries 6–8). Carbon
tetrachloride proved to be the most suitable medium for the homo-
coupling, leading to a yield of 85% for benzenediazonium salt
2
4
R
o
R
R
CCl4, 60 C
Entry Arenediazonium salts
Products
Yieldb
%)
(Table 1, entry 2). On the other hand, the aromatic solvents such
(
as toluene and xylene seem to be not suitable for the homocou-
pling of arenediazonium salts (Table 1, entries 9 and 10). Other
1
2
3
4
5
6
7
N BF
2
4
85
77
80
76
77
80
82
iron (II) salts such as Fe(NO
3
)
2
2
, FeBr2, and Fe(acac) can afford
biphenyl with the yield of 77%, 76%, and 81% in CCl
4
, respectively
.
N BF
4
F
F
F
2
The scope of the homocoupling reaction of arenediazonium
salts has been examined with respect to the substituents on the
aromatic ring (Table 2). The biaryl yields were found to be essen-
tially identical (80 ± 5%) for the seven p-substituted (R = H, F, Cl,
Cl
Br
I
N BF
2
4
Cl
Cl
N BF
2
4
Br
I
Br
2
Br, I, CF3, and NO ) arenediazonium salts (Table 2, entries 1–7),
suggesting that the polarity of the p-substituents has negligible ef-
fects on the homocoupling reaction. Most interestingly, the steric
effects of the ortho- and meta-substituents of the arenediazonium
salts seem to be also negligible for the homocoupling reaction
2
N BF
I
4
F
F3C
N
2
BF
4
3
C
CF
NO2
3
(Table 2, entries 8–10). It is interesting to note that the homocou-
O N
N
2
BF
4
O N
2
2
pling of 4-bromo and 4-iodobenzenediazonium tetrafluoroborate
F
F
F
0
0
leads to the exclusive formation 4,4 -dibromo and 4,4 -diiodobi-
phenyl in 76% and 77%, respectively, (Table 2, entries 4 and 5), indi-
cating the excellent chemoselectivity in contrast to the traditional
Ullmann homocoupling of aryl bromides and aryl iodides. Our
investigation revealed that depressions of yields were obtained in
8
9
79
82
N BF
4
2
Cl
Cl
2
N BF
4
4
5
2–71% for the alkyl or alkoxy substituent. Some by-product mix-
tures via radical mechanism have been detected by GC–MS which
is in accord with the reaction applied in toluene and xylene. The re-
sult maybe gave an explanation for the reductive cross-coupling
yields (Table 2, entries 11–15). Notably, the aryl rings structure
such as thiophene and benzofuran had also been tested and the
desired homocoupling biaryls were synthesized in moderate yield
Cl
NO
2
NO
2
10
2
N BF
86
2
O N
(
Table 2, entries 16 and 17).
The ferrous salt-promoted homocoupling of arenediazonium
1
1
1
2
H C
N
2
BF
4
H
3
C
CH
3
71
64
3
salts could be terminated by the addition of radical scavengers such
as hydroquinone or 2,6-di-tert-butylphenol, suggesting that the
homocoupling reactions proceed via a radical mechanism as shown
in Scheme 1. Quantitative analysis by formation of blue potassium
N
2
BF
4
CH
3
CH3
2
+
ferricyanide [K
3
Fe(CN)
6
] solution indicates that over 95% Fe has
2 4
N BF
1
3
70
3
+
been oxidized into Fe , indicating that the reductive homocoupling
of arenediazonium salt with the oxidation of ferrous ion is
stoichiometrically coupled. Reduction of arenediazonium cation by
ferrous ion results in the cleavage of the carbon–nitrogen bond,
H C
3
H
3
CO
H
3
CO
OCH
3
1
4
5
69
52
2 4
N BF
OCH
3
OCH
3
Table 1
a
Solvent effect on the homocoupling reaction in the presence of iron (II) chloride
1
2 4
N BF
3
H CO
FeCl2
N BF
S
2
4
S
S
o
16
N
2
BF
4
56
59
Solvent, 60 C
O
O
O
17
2 4
N BF
a
Entry
Solvent
Yieldb (%)
Entry
Solvent
Yieldb (%)
Reaction conditions: arenediazonium tetrafluoroborate salt (1 mmol), FeCl
1 mmol), CCl (2 mL), 60 °C, 3 h.
Isolated yields.
2
(
4
1
2
3
4
5
Methanol
44 (25c)
85
—
—
—
6
7
8
9
10
THF
1,4-Dioxane
MTBE
Toluene
Xylene
56
53
55
Complex
Complex
b
CCl
CH
CHCl
Acetone
4
Cl
2 2
3
accompanied with the release of nitrogen gas and aryl radical forma-
tion. Recombination of the resultant aryl radicals leads to the forma-
tion of the desired homocoupling biaryl product (Scheme 1).
a
Reaction conditions: benzenediazonium tetrafluoroborate salt (1 mmol), FeCl
1 mmol), solvent (2 mL), 60 °C, 3 h.
Isolated yields.
Yield of anisole is shown in brackets.
2
(
b
Obviously, CCl
4
plays an important role in the reaction since it,
c
acting as a good solvent for free radicals reaction, stabilizes the

Y. Ding et al. / Tetrahedron Letters 53 (2012) 6269–6272
6271
2+
3+
Fe
N
N BF
4
Fe
N2
BF4
(Eq 1)
(Eq 2)
Scheme 1.
0
1
intermediate aryl-radicals. Solvents which are not suitable for
radical reaction cannot facilitate the reaction efficiently (THF,
4,4 -Dimethylbiphenyl (T2-11): H NMR (400 MHz, CDCl
3
, TMS):
d 7.66 (d, 4H, J = 8.0 Hz), 7.41 (d, 4H, J = 8.0 Hz), 2.56 (s, 6H) ppm.
MS (EI) m/z (%): 182 (100), 167 (55), 152 (12), 89 (11), 76 (3).
1
2 2 3
,4-dioxane, or MTBE), suppress the reaction (CH Cl , CHCl or
0
1
acetone), or cause the reduction of aryldiazonium salt to form
the parent arene (methanol). Otherwise, homo-coupling reactions
conducted in toluene or xylene afforded complex products
3
4,4 -Diisopropylbiphenyl (T2-12): H NMR (400 MHz, CDCl ,
TMS): d 7.51 (d, 4H, J = 8.4 Hz), 7.29 (d, 4H, J = 8.4 Hz), 2.90–3.00
(m, 2H), 1.29 (d, 12H, J = 6.8 Hz) ppm. MS (EI) m/z (%): 238 (100),
195 (55), 165 (12), 119 (33), 82 (5), 64 (11).
1
6
revealed that radicals may be trapped by aromatic solvents.
0
1
Herein, we report a simple and efficient ferrous salt-promoted
reductive homocoupling of arenediazonium salts to synthesize
various symmetrical biaryls in moderate to good yields. In contrast
to the traditional protocols, the homocoupling of arenediazonium
salts can be performed directly in air without using additional
additives and it will be an alternative for the traditional Ullmann
reaction.
3
2,2 -Dimethyl-biphenyl (T2-13): H NMR (400 MHz, CDCl , TMS):
d 7.27–7.20 (m, 6H), 7.11–7.09 (d, 2H, J = 7.2 Hz), 2.05 (s, 6H) ppm.
MS (EI) m/z (%): 182 (70), 167 (100), 152 (20), 89 (11).
0
1
3,3 -Dimethoxybiphenyl (T2-14):
3
H NMR (400 MHz, CDCl ,
TMS): d 3.86 (s, 6H), 6.90 (dd, 2H, J = 8.2, 2.6 Hz), 7.12 (d, 2H,
J = 1.6 Hz), 7.17 (d, 2H, J = 7.6 Hz), 7.34 (d, 2H, J = 8.0 Hz) ppm.
MS (EI) m/z (%): 214 (100), 171 (16), 128 (10).
0
1
2
,2 -Dimethoxybiphenyl (T2-15):
3
H NMR (400 MHz, CDCl ,
TMS): d 7.33 (t, 2H, J = 8.0 Hz), 7.24 (s, 2H), 6.97–7.02 (m, 4H),
General procedure for homocoupling reactions
3
8
.77 (s, 6H) ppm. MS (EI) m/z (%): 214 (100), 183 (44), 107 (39),
6 (16), 65 (9).
A mixture of arenediazonium tetrafluoroborate salt (1 mmol),
FeCl (1 mmol) was stirred in CCl (2 mL) at 60 °C for 3 h. After-
2 4
ward, the reaction solution was cooled to rt and filtered through
a filter paper. The organic phase was evaporated under reduced
0
1
3
3
,3 -Dithiophene (T2-16): H NMR (400 MHz, CDCl , TMS): d 7.38
(
(
s, 2H), 7.33–7.36 (m, 4H) ppm. MS (EI) m/z (%): 166 (100), 137
40), 83 (42), 69 (8), 41 (5).
0
,2 -Dibenzofuran (T2-17): 1H NMR (400 MHz, CDCl
2
3
, TMS): d
pressure. The residue was purified on a SiO
2
column to afford the
7
.63 (d, 2H, J = 7.2 Hz), 7.54 (d, 2H, J = 8.4 Hz), 7.33 (t, 2H,
desired product.
1
J = 8.0 Hz), 7.263 (t, 2H, J = 7.6 Hz), 7.16 (s, 2H) ppm. MS (EI) m/z
%): 234 (100), 169 (24), 117 (63), 79 (19), 45 (6).
Biphenyl (T2-1): H NMR (400 MHz, CDCl
3
, TMS): d 7.60 (d, 4H,
(
J = 7.4 Hz), 7.44 (d, 4H, J = 7.8 Hz), 7.35 (d, 2H, J = 7.3 Hz) ppm.
MS (EI) m/z (%): 154 (100), 76 (7).
0
,4 -Difluorobiphenyl (T2-2): 1H NMR (400 MHz, CDCl
Uncited references
4
3
, TMS): d
7
.48 (m, 4H), 7.09 (t, 4H, J = 8.6 Hz) ppm. MS (EI) m/z (%): 190
AUTHOR PLEASE CITE REFS. 5,11 IN THE TEXT.
Acknowledgments
(
100), 170 (20).
4
,4 -Dichlorobiphenyl (T2-3): 1H NMR (400 MHz, CDCl
0
3
, TMS): d
7
.45 (d, 4H, J = 10.8 Hz), 7.40 (d, 4H, J = 10.8 Hz) ppm. MS (EI)
m/z (%): 222 (100), 186 (15), 152 (51), 111 (5), 93 (7), 75 (14).
0
1
This study was supported by the Science Technology Depart-
ment of Zhejiang Province (Grant # 2012R10014-15) and the Na-
tional Science Foundation of China (Grant # 21172149).
3
4,4 -Dibromobiphenyl (T2-4): H NMR (400 MHz, CDCl , TMS): d
7
3
.41 (d, 4H, J = 8.8 Hz), 7.21 (d, 4H, J = 8.8 Hz) ppm. MS (EI) m/z (%):
12 (55), 311 (100), 309 (47), 157 (10), 119 (12), 65 (7).
0
1
4
,4 -Diiodobiphenyl (T2-5): H NMR (400 MHz, CDCl
3
, TMS): d
7
.76 (d, 4H, J = 8.0 Hz), 7.28 (d, 4H, J = 8.4 Hz) ppm. MS (EI) m/z
Supplementary data
(
%): 406 (100), 280 (35), 203 (53), 156 (10), 117 (8), 74 (9).
0
1
4,4 -Trifluoromethylbiphenyl (T2-6): H NMR (400 MHz, CDCl
3
,
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.09.
028.
TMS): d 7.72 (q, 8H, J = 16.0 Hz) ppm. MS (EI) m/z (%): 290 (100),
1
8
72 (50), 85 (10).
4
,4 -Dinitrobiphenyl (T2-7): 1H NMR (400 MHz, CDCl
0
, TMS): d
3
Reference and notes
.19 (d, 4H, J = 9.2 Hz), 7.53 (d, 4H, J = 9.2 Hz) ppm. MS (EI) m/z
(
%): 244 (100), 198 (46), 122 (60), 88 (28).
3
.05 (tdd, 2H, J = 8.0, 2.4, 0.4 Hz), 7.24 (t, 1H, J = 2.0 Hz), 7.27 (t,
H, J = 2.0 Hz), 7.32 (t, 1H, J = 1.2 Hz), 7.34 (t, 1H, J = 1.6 Hz),
.36–7.42 (m, 2H) ppm. MS (EI) m/z (%): 190 (100), 189 (24), 170
1
.
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