Palladium-catalyzed tandem reaction of 2-hydroxyarylacetonitriles with sodium sulfinates: One-pot synthesis of 2-arylbenzofurans

Article abstract of DOI:10.1039/c4ob00575a

The first example of the palladium-catalyzed one-pot synthesis of 2-arylbenzofurans in moderate to excellent yields via a tandem reaction of 2-hydroxyarylacetonitriles with sodium sulfinates is reported. A plausible mechanism for the formation of 2-arylbenzofurans involving desulfinative addition and intramolecular annulation reactions is proposed. Moreover, the present synthetic route to benzofurans could be readily scaled up to the gram quantity without any difficulty. Thus, the method represents a convenient and practical strategy for synthesis of benzofuran derivatives. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00575a

Organic &
Biomolecular Chemistry
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Palladium-catalyzed tandem reaction of
2
-hydroxyarylacetonitriles with sodium sulfinates:
Cite this: Org. Biomol. Chem., 2014,
2, 4078
1
one-pot synthesis of 2-arylbenzofurans†
Received 17th March 2014,
Accepted 23rd April 2014
a,b
a
a
Jiuxi Chen, Jianjun Li and Weike Su*
DOI: 10.1039/c4ob00575a
www.rsc.org/obc
The first example of the palladium-catalyzed one-pot synthesis of Whereas the other predominant type involves the assembly of
-arylbenzofurans in moderate to excellent yields via a tandem the functionalized furan nucleus on a benzenoid scaffold,
2
1
4
reaction of 2-hydroxyarylacetonitriles with sodium sulfinates is which mainly includes annulation of o-alkynylphenols, cycli-
reported. A plausible mechanism for the formation of 2-arylbenzo- zation of o-halobenzyl ketones, cyclodehydration of o-hydroxy-
furans involving desulfinative addition and intramolecular annula- benzyl ketones, Suzuki coupling/arylation, decarboxylative/
tion reactions is proposed. Moreover, the present synthetic route coupling of 3-arylcoumarins, C–H activation/oxidative cycli-
to benzofurans could be readily scaled up to the gram quantity zation of 2-(1-arylvinyl)phenols with iodobenzenes, oxidative
without any difficulty. Thus, the method represents a convenient cyclization of o-vinylphenols, Wittig reaction, and [3,3]-
1
5
1
6
17
1
8
1
9
2
0
21
2
2
and practical strategy for synthesis of benzofuran derivatives.
sigmatropic rearrangement of oxime ethers. Recently, tran-
sition-metal-catalyzed direct tandem reactions using phenols
The benzofuran ring is among the most prevalent heterocyclic
structural motifs that occur in a wide variety of isolated
natural products and is extremely important in medicinal
chemistry and functional materials. Among the numerous
benzofuran derivatives known, 2-arylbenzofurans have recently
attracted considerable attention due to their versatile pharma-
ceutical activities, such as S1P
antitumor, antiviral, antioxidative, and antifungal pro-
perties. Recently, Ono and Saji reported that F or
23
as starting materials have been reported.
Transformations of nitriles play an important role in both
the laboratory and industry due to their well-recognized chemi-
cal versatility. However, the nitrile group is generally inert in
organometallic reactions, and thus acetonitriles or benzo-
nitriles usually participate as solvents or ligands in metal-cata-
lyzed reactions. The addition of arylpalladium species to the
cyano group, pioneered by the Larock group and elegantly
employed in recent years, provided a conceptual basis for
our approach. Lu’s group and our group have also develo-
ped the palladium-catalyzed one-pot synthesis of benzofurans
by addition of organoboron reagents to functionalized nitriles.
To the best of our knowledge, synthesis of benzofurans
with the use of sodium sulfinates as coupling partners has
even though sodium sulfinates are
generally used as the aryl source in transition-metal-catalyzed
1
2
3
24
25
4
1
receptor agonists, and their
5
6
7
26
8
18
99m
Tc-
27
labeled benzofuran derivatives were investigated by positron
emission tomography (PET) and single photon emission com-
puted tomography (SPECT) imaging for β-amyloid plaques in
28
29
9
Alzheimer’s brains. The importance of 2-arylbenzofurans has
resulted in the development of two types of synthetic methods.
One type involves the introduction of substituents by direct
arylation of a preexisting benzofuran ring via the palladium-
catalyzed cross-couplings of 2-halobenzofurans with arylmetal-
12b,30
rarely been reported,
31
desulfinative reactions. As part of the continuing efforts in
our laboratory toward the development of novel transition
metal-catalyzed coupling reactions with arylation reagents,
herein we report a simple and efficient protocol for the syn-
thesis of 2-arylbenzofurans by palladium-catalyzed tandem
reaction of 2-hydroxyarylacetonitriles with sodium sulfinates
1
0
lic reagents,
or 2-benzofuryl organometallics with aryl
1
1
12
13
halides. Recently, Wang and Rao independently reported
palladium-catalyzed direct tandem reaction of 2-(gem-dibromo-
vinyl)phenols with arylation reagents such as phenyl(trialkoxy)-
32
1
2a
12b
13
silanes,
sodium arylsulfinates,
and triarylbismuth.
(
Scheme 1).
a
Collaborative Innovation Center of Yangtze River Delta Region Green
Pharmaceuticals, College of Pharmaceutical Sciences, Zhejiang University of
Technology, Hangzhou 310014, China. E-mail: pharmlab@zjut.edu.cn
Our preliminary studies focused on the reaction between
2-hydroxyphenylacetonitrile (1a) and sodium benzenesulfinate
2a) to obtain the optimal reaction conditions (Table 1). On
the basis of the previous addition protocol of organoborons to
(
b
College of Chemistry & Materials Engineering, Wenzhou University,
Wenzhou 325035, PR China
†
2
9a
nitriles, a test reaction with Pd(O CCF ) and 2,2′-bipyridine
(L1) as the catalytic system was performed under an air
Electronic supplementary information (ESI) available: Analytical data and
2
3 2
1
13
spectra ( H, C NMR) for all products. See DOI: 10.1039/c4ob00575a
4078 | Org. Biomol. Chem., 2014, 12, 4078–4083
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a
Table 1 Optimization of the reaction conditions
Scheme 1 Palladium-catalyzed one-pot synthesis of 2-arylbenzofurans.
b
Entry Pd source
Ligand Additive
Solvent
Yield (%)
1
2
3
4
5
6
7
8
9
1
Pd(CF
PdCl
Pd(OAc)
Pd(OH)
3
CO
2
)
2
L1
L1
L1
L1
L1
L1
L1
L1
L1
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
3
3
3
3
3
3
3
3
3
3
3
3
3
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
2
2
2
2
2
2
2
2
2
2
2
2
2
H
H
H
H
H
H
H
H
H
H
H
H
H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2-MeTHF–H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
22
19
31
29
12
17
Trace
26
Trace
0
0
18
Trace
Trace
74
79
63
52
22
Trace
Trace
62
61
31
Trace
68
51
17
71
14
37
34
51
Trace
73
34
2
atmosphere. To our delight, the desired product 2-phenylben-
zofuran (3a) was isolated in 22% yield (entry 1). Encouraged by
this promising result, reaction parameters including palla-
dium sources, ligands, additives, and solvents were adjusted.
2
2
PdCl
PdCl
PdCl
2
(PPh
2
(Py)
2
(NH
3
)
2
2
3
)
2
Pd(acac)
Pd(PPh
2
Among the palladium sources used, Pd(OAc)
2
exhibited the
3
)
4
highest catalytic reactivity in 31% yield (entries 1–12). Consi-
0
PdCl
PdCl
2
(dppe) L1
(cod)
dering that additives always played important roles in organic 11
2
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L1
L1
L1
L1
L1
L1
L1
L1
1
1
1
2
3
4
Pd(dba)
3
reactions, subsequently various additives were examined for
this transformation (entries 3, 13–21). The reaction yield
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
PhCOc
2
H
improved to 74% when p-toluenesulfonic acid (p-TSA) was 15
p-TSA
d
1
1
1
6
7
8
p-NBSA
MeSO
CF SO
SO
used (entry 15). However, the best yield was obtained with
p-nitrobenzenesulfonic acid (p-NBSA) as the additive (79%, entry
3
H
H
3
3
1
6). While MeSO H, CF SO H, and H SO were less efficient, 19
H
2
4
3
3
3
2
4
2
2
2
0
1
2
HCl
HNO
3 2 2 3
CH CO H, PhCO H, HCl and HNO were unsuitable additives
and prohibited the reaction. Replacement of 2,2′-bipyridine
(
lower yields (entries 16 and 22–30). The influence of the
solvent on the reaction was also noteworthy. Screening
3
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
p-NBSA
L1) with other ligands, such as L2–L10, resulted in slightly 23
2
2
2
4
5
6
revealed that the use of THF or 2-MeTHF greatly increased the 27
3
3
28
yield of 3a (entries 16 and 31–37). 2-MeTHF offers both eco-
2
3
9
0
nomical and environmentally friendly advantages over THF.
The yield of 3a improved to 92% when the model reaction was 31
Toluene–H
Xylene–H
Dioxane–H
DMF–H
THF–H
2
O
32
33
34
2
O
O
performed under a N
2
atmosphere (entry 38).
2
With the optimized reaction conditions in hand, the
2
O
2
O
tandem reactions between 2-hydroxyphenylacetonitrile (1a) 35
i
36
37
38
2
PrOH–H O
and various sodium arylsulfinates (2a–2j) were investigated
Table 2). The steric effects of substituents had an obvious
2
H O
25
92
e
(
2-MeTHF–H
2
O
impact on the yield of the reaction. For example, reaction of 1a
with para- and ortho-tolylsulfinate (2b–2c) provided 89% of 3b,
while the yield of 3c was decreased to 62% (entries 2 and 3). (2 mL), H O (1 mL), 80 °C, 36 h, air. Isolated yield. p-TSA =
The same phenomenon was observed in the reaction of 1a
with para- and ortho-chlorobenzenesulfinates (2g–2h) (entries
a
Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol), indicated Pd
source (10 mol%), ligand (20 mol%), additive (10 equiv.), solvent
b
c
2
d
p-toluenesulfonic acid.
p-NBSA = p-nitrobenzenesulfonic acid.
e
2
Under a N atmosphere.
7
and 8). The electronic properties of the substituents on the
phenyl ring of the sodium arylsulfinates affected the yields of
the reaction to some extent. In general, the sodium arylsulfi- furan (3i) in slightly lower yield (entry 9). To specially mention,
nates bearing an electron-donating substituent (e.g., –OMe only trace amounts of 3i were observed by GC/MS analysis
t
and – Bu) produced slightly higher yields than those analogues when arylboronic reagents, such as (4-(trifluoromethyl)phenyl)-
bearing an electron-withdrawing substituent (e.g., –F and –Cl) boronic acid, were used as the substrates under the same reac-
(entries 4–7). However, sodium arylsulfinates bearing a strong tion conditions (see Scheme S1 in ESI†). It is noteworthy that
electron-withdrawing substituent (e.g., –CF ) at the para posi- an excellent yield of 2-(naphthalen-2-yl)benzofuran (3j) was
3
tion, such as sodium 4-(trifluoromethyl)benzenesulfinate (2i), obtained when sodium naphthalene-2-sulfinate (2j) was used
led to the corresponding 2-(4-(trifluoromethyl)phenyl)benzo- as the substrate (entry 10).
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a
a
Table 2 Reaction of 1a with various sodium sulfinates
Table 3 Reaction of 2a with various 2-hydroxyarylacetonitriles
b
Entry
1
2
ArSO Na (2)
Product (3)
Yield (%)
3a
92
2
3
3b
3c
89
62
a
Reaction conditions: 1 (0.3 mmol), 2a (0.6 mmol), Pd(OAc)
2
(10 mol%),
4
5
6
7
8
3d
3e
3f
85
84
76
81
41
L1 (20 mol%), p-NBSA (10 equiv.), 2-MeTHF (2 mL), H
6 h, N . The isolated yield is given in parentheses.
2
O (1 mL), 80 °C,
3
2
products 3f–3h (Table 2, entries 6–8) and 3n–3o (Table 3, runs
and 6) in moderate to good yields, making further elabor-
5
ations of the corresponding biaryl products possible.
3g
3h
Finally, the present synthetic route to 2-arylbenzofurans
could be readily scaled up to the gram quantity without any
difficulty. For instance, the reaction at the 20 mmol scale
afforded the corresponding product 2-phenylbenzofuran (3a)
in 86% yield (Scheme 2).
To elucidate the mechanism of formation of 2-arylbenzofur-
ans, we performed control experiments (see Schemes S2–S4 in
ESI†). Reaction of 2-phenylacetonitrile (4) with sodium ben-
zenesulfinate (2a) was examined under the standard conditions,
affording the corresponding product 1,2-diphenylethanone (5)
in 89% yield. However, no desired product 4 was observed in
the absence of the palladium catalyst and the ligand (see
9
3i
3j
56
94
10
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), Pd(OAc)
2
(10 mol%),
L1 (20 mol%), p-NBSA (10 equiv.), 2-MeTHF (2 mL), H
2
O (1 mL), 80 °C, Scheme S2 in ESI†). The desired product 3a could not be
detected and when 2-hydroxyphenylacetonitrile (1a) was
treated with 2a in the absence of the palladium catalyst and
the ligand, almost 90% of 1a was recovered (see Scheme S3 in
b
3
6 h, N
2
.
Isolated yield.
We next turned our attention to the effect of the reactions
ESI†). We found that 3a was obtained in 89% yield when the
intramolecular annulation of 2-(2-hydroxyphenyl)-1-phenyl-
ethanone (6) was performed in the absence of the palladium
catalyst and the ligand, while a trace yield of the desired
product 3a was observed in the absence of p-NBSA (see
Scheme S4 in ESI†). These results showed that the addition
reaction depends on the palladium catalyst and the ligand,
whereas the intramolecular annulation is independent of the
addition reaction and does not depend on the palladium cata-
lyst and the ligand, but it depends on the additive.
of sodium benzenesulfinate (2a) with various 2-hydroxyarylace-
tonitriles (1a–1f) under the optimized conditions (Table 3). As
expected, the groups on the phenyl ring of 2-hydroxyarylaceto-
nitriles, such as methyl, methoxy, chloro, and bromo, were
quite compatible. The electronic properties of the groups on
the phenyl ring moiety of 2-hydroxyarylacetonitriles had some
effects on the reaction. Generally, 2-hydroxyarylacetonitriles
with an electron-donating substituent on the phenyl group
gave slightly higher yields. Substrates 1b and 1c bearing a
methoxy group, for example, were treated with 2a to afford
9
0% and 93% yields of 3k and 3l, respectively (runs 2 and 3).
While the yields of 3n and 3o were decreased to 75% and
1% from substrates 1e and 1f possessing a halogen group,
7
respectively (runs 5 and 6).
It is noteworthy that the chloro, fluoro, and bromo moieties
(commonly used for cross-coupling reactions) in substrates
were all tolerated and afforded several halogen-containing Scheme 2 Gram-scale synthesis of 3a.
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Scheme 3 Plausible mechanism.
On the basis of the above experimental results, a plausible
mechanism for the formation of 2-arylbenzofurans is proposed
in Scheme 3. The following key steps are included in the cataly-
tic pathway: (i) coordination of Pd(OAc)
2
with arylsulfinic acid
(or sodium arylsulfinate 2) to afford a palladium species A; (ii)
the desulfination of the arylsulfinic acid to give aryl-palladium
species B, which was followed by (iii) the coordination of
4
2
7; (c) B. L. Flynn, E. Hamel and M. K. Jung, J. Med. Chem.,
002, 45, 2670.
2
-hydroxyarylacetonitriles 1 to generate intermediate C; (iv) car-
6
7
(a) S. A. Galal, A. S. Abd El-All, M. M. Abdallah and H. I. El
Diwani, Bioorg. Med. Chem. Lett., 2009, 19, 2420;
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corresponding ketimine complex D; (v) protonation of the keti-
mine complex D by p-NBSA to afford the free ketimine, which
undergoes hydrolysis to form the corresponding 2-(2-hydroxy-
phenyl)-1-arylethanones E under acidic conditions and regen-
erates an active palladium species. Finally, intramolecular
annulation of 2-(2-hydroxyphenyl)-1-arylethanones E under
acidic conditions readily delivers 2-arylbenzofurans 3 as the
desired products.
In summary, we have developed a new strategy for con-
structing 2-arylbenzofurans in moderate to excellent yields
from the palladium-catalyzed tandem reaction of 2-hydroxyaryl-
acetonitriles with sodium sulfinates. Further efforts to extend
this catalytic system to the preparation of other useful hetero-
cycles are currently underway in our laboratories.
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Financial support was provided by the National Natural
Science Foundation of China (no. 21102105) and the Zhejiang
Provincial Natural Science Foundation (no. LY13B020015).
10 For selected examples, see: (a) M. L. N. Rao, D. K. Awasthi
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