Synthesis of [2,2’]Bifuranyl-5,5’-dicarboxylic Acid Esters via Reductive Homocoupling of 5-Bromofuran-2-carboxylates Using Alcohols as Reductants?

Article abstract of DOI:10.1002/cjoc.202000303

Herein, we describe an environmentally benign and cost-effective protocol for the synthesis of valuable bifuranyl dicarboxylates, starting with α-bromination of readily accessible furan-2-carboxylates by LiBr and K₂S₂O₈. Furthermore, the bromination intermediate product 5-bromofuran-2-carboxylates were then conducted in a palladium-catalyzed reductive homocoupling reactions in the presence of alcohols to afford bifuranyl dicarboxylates. One of the final products in this protocol, [2,2’]bifuran-5,5’-dicarboxylic acid esters, are essential monomers of poly(ethylene bifuranoate), which can be served as an green and versatile alternative polymer for traditional poly(ethylene terephthalate) that is currently common in technical plastics.

Full text of DOI:10.1002/cjoc.202000303

Chinese Journal
of Chemistry
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中 国 化 学 - An International Journal
Accepted Article
Title: Synthesis of [2,2’] Bifuranyl-5,5’-dicarboxylic Acid Esters via Reductive
Homocoupling of 5-Bromofuran-2-carboxylates Using Alcohols as Reductants
Authors: Yi Xie, Bin Yu, Jiajun Luo, Biaolin Yin* and Huanfeng Jiang*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2020, 38, 10.1002/cjoc.202000303.
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published online in Early View: http://dx.doi.org/10.1002/cjoc.202000303.
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Chinese Journal
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Keyword 1, Keyword 2, Keyword 3, Keyword 4, Keyword 5, Keyword 6
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国 化 学 - An International Journal
Synthesis of [2,2’] Bifuranyl-5,5’-dicarboxylic Acid Esters via Reductive
Homocoupling of 5-Bromofuran-2-carboxylates Using Alcohols as
Reductants
Yi Xie, Bin Yu, Jiajun Luo, Biaolin Yin* and Huanfeng Jiang*
a Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510640, P. R. China.
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Herein, we describe an environmentally benign and cost-effective protocol for the synthesis of valuable bifuranyl dicarboxylates, starting with
α-bromination of readily accessible furan-2-carboxylates by LiBr and K2S2O8. Furthermore, the bromination intermediate product
5-bromofuran-2-carboxylates were then conducted in a palladium-catalyzed reductive homocoupling reactions in the presence of alcohols to afford
bifuranyl dicarboxylates. One of the final products in this protocol, [2,2’]bifuran-5,5’-dicarboxylic acid esters, are essential monomers of poly(ethylene
bifuranoate), which can be served as an green and versatile alternative polymer for traditional poly(ethylene terephthalate) that is currently common in
technical plastics.
(Scheme 2): (1) C–H homocoupling of furan-2-carboxylates with
Background and Originality Content
palladium catalysis under oxidative conditions,27-30 (2)
Producing valuable chemicals from biomass and its derivatives
palladium-catalyzed coupling reactions between 5-bromofuran-
furan-2-carboxylates,31-37
and
(3)
is a focus of research in order to reduce the consumption of fossil
resources.1-4 Furan derivatives such as furfural and
5-(hydroxymethyl)furfural are extensively used in the preparation
of value-added products, as their wide and readily available
sources from the degradation of biomass feedstocks. On the
other hands, the low aromaticity featuring with highly reactive
functional groups (e.g. –CHO, –CH2OH) enable furans to act as
masked alkenes, dienes, enol ethers and carbonyl compounds in
various transformation. Therefore, the development of methods
for value-added chemical with furans as starting materials is of
both practical and academic significance.5-12
2-carboxylates
and
NiCl2-catalyzed reductive homocoupling of 5-bromofuran-
2-carboxylates with zinc powder as a reductant.38-42 It should be
noted that the first two methods require harsh conditions (e.g.,
high reaction temperature) with unappealing yields, while the
third method consumes over equivalent zinc powder. Herein, we
newly develops a mild and cost-effective route to 1, in which the
alcohol is used as reductant within the palladium-catalyzed
dimerization of 5-bromofuran-2-carboxylates 3 prepared by
α-bromination of readily accessible furan-2-carboxylates 2 with
LiBr/K2S2O8 as the green brominating agent (Scheme 3).
Recently, 2,5-FDCA (furan-2,5-dicarboxylic acid, Scheme 1) is
being frequently investigated in cutting-edge researches as it is
the requisite precursor of PEF (polyethylene furanoate), which
have been widely acknowledged as the promising alternative for
traditional and industrially important polymer relying on the fossil
fuels, i.e. PET (polyethylene terephthalate).13-19 Similarly, BFDCA
(2,2’-bifuran-5,5’- dicarboxylic acid), the preparation of which was
first reported in 1966,20 has aroused much interest because of the
potential utility of bifuran moieties in organic electronics21-24 and
coordination polymers.25 More importantly, PEBF (polyethylene
bifuranoate), a polymer produced by polycondensation of
dimethyl 2,2’-bifuran-5,5’-dicarboxylate and ethylene glycol, has
been revealed to have intriguing properties, including high tensile
modulus, impermeability to oxygen and water, and the ability to
block ultraviolet radiation.26 Accordingly, the development of
efficient and environmentally benign methods for access to
BFDCAs is highly desirable.
Scheme 1 Structures of terephthalic acid, furan-2,5-dicarboxylic acid
(2,5-FDCA), 2,2’-bifuran-5,5’-dicarboxylic acid (BFDCA), and related
polyesters.
onomers:
M
O
O
O
O
O
O
O
OH
HO
OH
HO
O
HO
OH
O
ere
a
c ac
-
T
phth li
id
,
2 5 FDCA
BFDCA
o es ers:
P ly t
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
n
n
n
PEBF
PEF
PET
To date, three methods for the synthesis of
2,2’-bifuran-5,5’-dicarboxylic acid esters 1 have been reported
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202000303
This article is protected by copyright. All rights reserved.

First author et al.
Report
Scheme 2 Conventional synthetic methods for BFDCA esters 1.
8
9
10c
11
12
60
100
80
DCE
DCE
DCE
DCE
DCE
3
3
3
3
4
1.6
86
31
36
61
71
1.6
1.6
3
Pd / O
[ ] [ ]
R
O
OR
RO
H
O
OR
H
O
O
O
1
O
O
O
2
O
80
ase
Pd /b
80
1.6
[
]
OR
OR
RO
RO
1
r
H
B
B
O
O
O
a Reaction conditions, unless otherwise noted: methyl furan-2-carboxylate
(0.5 mmol), solvent (2 mL), N2 b Isolated yields are reported. NR = no
reaction. c Under air.
3
O
O
O
O
Pd
[
]
mer z n re uc ve cou
n
pli g
di
i i g
d
ti
r
r
B
O
1
me a as e re uc an
t l th t
d
t
We further explored the scope of 2-acylated furans 2 for
bromination, including corresponding carboxylic esters, carboxylic
acid, carbaldehyde, and carboxamides (Table 2). Firstly,
alkoxycarbonyl-substituted furans are analyzed. Except for 2h
and 2q, furans substrates containing either primary or secondary
2-alkoxycarbonyl groups gave the corresponding brominated
products (3a–3p) in moderate to good yields. The poor yield of 3h
may be due to the overoxidation of the methylene of benzyl
group. The failure of 2q to afford desired product 3q under the
standard conditions may be due to the decomposition of the
tert-butyl group. Secondly, we evaluated 2-formyl-, 2-carboxy-,
and 2-amide-substituted furans. 2-Furfural and 2-furoic acid were
brominated to give 3r and 3s respectively, albeit in low yields.
However, when 2-furamides were used as substrates,
bromination proceeded well, giving the corresponding
compounds (3t–3y) in 59-80% yields.
Scheme 3 This work for our newly developed synthesis of BFDCA esters 1.
re uc ve omocou
n
d
it
h
pli g
K
2S2O8
3
1
2
a co o as e re uc an
h l
th t
r
l
d
t
LiB
Results and Discussion
Conventionally, α-bromination of furan rings is achieved by
using bromine or N-bromosuccinimide as brominating agents.43-49
In this study, we envisaged that using LiBr/K2S2O8 instead could be
more environmentally benign and industrially accessible (Scheme
3). Methyl furan-2-carboxylate have been employed as the model
substrate (Table 1). We began by evaluating a variety of solvents
in the presence of LiBr (3 equiv) and K2S2O8 (1.6 equiv) at 60 °C
under N2 atmosphere (entries 1–7). DCE was found to be the
optimal solvent, providing 3a in 68% yield. Gratifyingly, when the
reaction temperature was elevated to 80 °C, the yield of 3a was
increased to 86% (entry 8). However, further elevation of the
reaction temperature, for example to 100 °C, dramatically
decreased the yield (to 31%, entry 9). When the reaction was
performed under air, the yield was only 36% (entry 10). Increasing
the amount of K2S2O8 to 3 equiv (entry 11) or the amount of LiBr
to 4 equiv (entry 12) did not favor the yield of 3a. Thus, the
optimal reaction conditions were determined to be those shown
in entry 8 of Table 1.
Table 2 Scope of the bromination.a,b
,
80 oC N2
X
CO
X
CO
O
O
+
+
r
LiB
K
2S2O8
r
B
D
C
E
.
e u v
q i
e u v
1 6 q i
e u v
q i
3
2
3
1
(
)
Table
1
Reaction conditions optimization for the
bromination.a
O
O
,
o ven
S l
O
O
T
t
e
e
OM
+
+
r
OM
LiB
K2S2O8
e u v
r
B
x e u v
q i
a
2
u v
q i
)
a
3
e
y
q i
1
(
Entry
T (oC)
60
Solvent
DCE
x
3
3
3
3
3
3
3
y
Yield (%)b
68
1
2
3
4
5
6
7
1.6
1.6
1.6
1.6
1.6
1.6
1.6
60
DCM
CHCl3
DMF
15
60
Trace
NR
60
60
MeCN
THF
17
60
Trace
21
60
1,4-dioxane
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© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.

Running title
Chin. J. Chem.
O
Table 3 Reaction conditions optimization for the reductive coupling.a
O
O
O
O
O
,
,
=
n
a
3
0
86%
3a, n = 0, 86%
O
O
,
,
,
,
=
O
O
O
O
e u v
EtOH 2
q i
( )
[ ]
[ ]
3
3
n
n
n
O
O
r
B
Br
3b
1
3
7
9%
O
3b,
n = 1, 79%
O
O
n
O
r
r
B
Br
O
O
O
n
B
Br
=
=
=
c
,
,
b
3
7
8%
3c, n = 3, 78%
an s a zer ase
Pd Lig d/ t bili
O
[
]
,
,
,
,
7
t
OE
3d
5
8
5
%
O
O
3d,
,
r
B
n = 5, 85%
t
OE
EtO
,
3g
2
%
8
110 o
C
t
3g, 82%
,
,
o
N2
S
e
n
3f
80
80%
v
en
l
%
3f,
3
7
3%
3e,
n = 7, 73%
3b
1b
O
O
O
O
O
O
c
n
OA
O
O
r
B
Br
B
O
OAc
O
O
O
O
Bn
O
O
O
O
O
O
O
O
r
O
r
B
r
B
B
Br
Br
Br
O
O
Ligand/
stabilizer
Solvent Yield (%)b
,
,
,
70
c
b
b
Entry
[Pd]
Base
c
%
3
i
3
3h
3h,
22
83
83%
0%
%
%
3i,
3j,
0% (22%)
j
70%
(
)
O
O
O
O
O
O
n
)
n
( )
(
1
2
Pd(PPh3)2Cl2 DPEPHOS
KOAc
KOAc
toluene
toluene
36
57
O
O
O
O
O
O
O
O
r
O
O
r
B
B
Br
O
O
Br
6
6
r
B
Br
Pd(PPh3)2Cl2 (2-furyl)3P
,
,
80%
=
n
,
o
3k
3l
3l,
3
1
,
3k, n = 1, 80%
n
3
1
%
3o, 91%
9
3
7
8
%
3n,
87%
,
,
=
n
3
90%
n = 3, 90%
PHDAVE-
Pd(PPh3)2Cl2
,
,
=
m
n
5
89%
3m, n = 5, 89%
3
KOAc
toluene
0
PHOS
O
O
O
O
O
O
O
O
4
5
Pd(PPh3)2Cl2 S-PHOS
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
Cs2CO3
DBU
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
DMF
40
74
52
0
O
O
O
O
r
B
Br
H
H
O
O
r
B
Br
O
O
r
B
Br
Pd(PPh3)2Cl2
PdCl2
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
PVP
,
,
,
r
3q
0%
0%
3q,
3
25
%
25%
3p
7
8%
3r,
3p, 78%
6
O
O
O
O
O
O
7
Pd(PPh3)4
O
O
,
,
=
6
1 2
O
O
u
n
N
N
3
3
%
3u,
N
N
r
n
n = 1, 62%
O
O
r
B
n
B
Br
Br
,
v
,
OH
OH
=
7
r
n
B
71
%
Br
3v,
n = 7, 71%
8
Pd(MeCN)2Cl2
PdBr2
28
12
11
48
0
n
n
,
,
s
3t
5
9%
59%
3
38%
38%
3t,
3s,
9
O
O
O
O
O
O
O
O
O
O
O
O
Cy
Cy
10
11
12
13
14
15c
16d
17e
18
19
20
21
22
23f
24g
Pd(OAc)2
O
O
N
N
r
N
N
B
Br
r
N
B
Br
r
N
B
Br
Cy
Cy
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
,
3y
7
3y,
,
,
x
3
80%
3x,
80%
w
3
6%
76%
71
%
3w, 71%
a
EA
22
0
Reaction conditions, unless otherwise noted: 2 (0.5 mmol), LiBr (1.5
mmol), K2S2O8 (0.8 mmol), solvent (2 mL), N2, 80 °C. Isolated yields are
provided. b Reactions were carried out at 60 °C. c LiBr (3 mmol), K2S2O8 (1.6
mmol), DCE (4 mL).
DCE
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
58
54
40
12
56
0
With brominated products 3 in hand, we explored the
conditions of next reductive homocoupling reactions to BFDCA
esters 1 using ethanol as green and cost-effective reductants
(Table 3).50-53 Ligands and stabilizers such as DPEPHOS, (2-furyl)3P,
PHDAVE-PHOS, S-PHOS, and poly(vinylpyrrolidone) [PVP, MW =
30,000]) have been firstly investigated with Pd(PPh3)2Cl2 as the
catalyst, KOAc as a base, and toluene as the solvent (entries 1–5).
PVP was found to be the optimal, giving 1b in 74% yield. It is
worth noting that although PVP-stabilized Pd is widely used to
catalyze organic transformations such as Heck54-60 and Suzuki
coupling reactions,61-64 it has not, to our knowledge, been used
for reductive homocoupling reactions yet. With PVP as the
stabilizer, PdCl2, Pd(PPh3)4, Pd(MeCN)2Cl2, PdBr2, and Pd(OAc)2
gave lower yields than Pd(PPh3)2Cl2 (entries 6–10). Screening of
solvents showed that toluene was the optimal (entries 11–14).
Neither increasing nor decreasing the PVP/[Pd] ratio improved
the outcome (entries 15 and 16). Notably, in the absence of PVP,
the yield of 1b was only 40% (entry 17). Other weak bases such as
K2CO3, NaOAc, and NEt3 (entries 18, 19, and 22, respectively)
were inferior to KOAc; and the strong bases Cs2CO3 (entry 20) and
DBU (entry 21) were unsuitable, giving none of the desired
product. Increasing or reducing the amount of KOAc did not
improve the yield of 1b(entries 23 and 24).
0
NEt3
53
72
53
KOAc
KOAc
a Reaction conditions, unless otherwise noted: 3b (0.5 mmol), base (0.75
mmol), [Pd] (3 mol %), PVP/[Pd] = 35/1 (w/w), EtOH (1.0 mmol), solvent (2
mL), N2, 110 °C. DPEPHOS = Oxydi-2,1-phenylene)bis(diphenylphosphine);
PHDAVE-PHOS
= 2-diphenylphosphino-2'-(N,N-dimethylamino)biphenyl;
S-PHOS 2-Dicyclohexylphosphino-2',6'-dimethoxybiphenyl; DBU
=
=
1,8-diazabicyclo[5.4.0]undec-7-ene. b Isolated yields are provided. PVP/[Pd]
= 35/1 (w/w). c PVP/[Pd] = 20/1 (w/w). d PVP/[Pd] = 45/1 (w/w). e PVP was
omitted. f The amount of KOAc was increased to 1 mmol. g The amount of
KOAc was decreased to 0.5 mmol.
Having optimized the conditions for the reductive
homocoupling, we tested them on various 5-bromofurans 3 (Table
4) with the corresponding ROH (instead of EtOH to avoid of
side-reaction of transesterification) as the reductant. When R was
a linear primary alkoxy group, the homocoupling products (1a–1e)
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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This article is protected by copyright. All rights reserved.

First author et al.
Report
were obtained in moderate to good yields (54–74%).
Branched-chain primary alkoxy groups also gave the desired
products in fair yields (1f–1g, 62–70%). While substrate bearing a
PhCH2 group, corresponding substrate gave the highest yield (1h,
89%). However, when was R was PhCH2CH2CH2, the yield of the
corresponding product (1i’) was only 54%. Several cyclic and
acyclic secondary alkyl R groups were also tested and resulted in
corresponding products (1j–1n’) with yields ranging from 31% to
71%. Notably, when R was t-Bu or tolyl, the coupled products (1q
and 1r’, respectively) formed in moderate yields in the presence
of 2 equiv of EtOH as the reductant (the corresponding tert-butyl
alcohol and p-cresol cannot be the reductant).
HO
HO
O
O
O
O
O
O
O
O
O
O
OH
OH
O
O
H
H
O
O
H
H
O
O
,
,
0
%
r
s
1
1
0%
1r, 0%
1s,
0%
O
O
O
O
O
O
O
O
N
N
N
N
O
O
O
O
N
N
N
N
O
O
Ph
Ph
O
O
Ph
Ph
,
u
'
,
1
38%
1u, 38%
1t
1t',
65%
65%
O
O
O
O
O
O
O
O
N
N
N
N
e
OM
OMe
e
M O
MeO
,
1y
34%
1y, 34%
Table 4 Scope of the reductive coupling.a,b
a Reaction conditions, unless otherwise noted: 3 (0.5 mmol), KOAc (0.75
mmol), ROH (1.0 mmol), Pd(PPh3)2Cl2 (3 mol %), PVP/[Pd] = 35/1 (w/w),
toluene (2 mL), N2, 110 °C. Isolated yields are provided. b EtOH was used as
the reductant.
mo
l
%
O
O
O
Pd PPh 2Cl2
3
(
)
3
(
)
.
,
c
e u v
PVP KOA 1 5 q i
O
(
)
OR
O
O
+
(
OR
OR
r
B
ROH
,
o uene
C
N2 t l
,
110 o
e u v
q i
2
)
3
1
Pd PPh
l
3 2C 2
(
)
O
O
,
,
54%
=
n
n
O
a
O
1
0
1a, n = 0, 54%
,
,
,
7
4
=
O
( )
( )33
O
n
( ) 1b
1
( )n
%
O
O
1b,
n
a re uc on
n = 1, 74%
O
O
i iti l
d
ti
O
O
,
=
3 61%
c
n
n
n
O
O
1
1c, n = 3, 61%
O
O
O
O
by E OH
t
,
66
( )
( )33
,
,
=
=
O
O
1d
5
n
( )
( )n
%
1d,
n = 5, 66%
,
O
O
,
69
e
1
7
R
O
66%
erm na
l
1f
1e,
O
O
n = 7, 66%
%
1f, 69%
t
i
r
B
O
re uc on
Pd 0
( )
d
ti
O
O
O
O
O
3
by EtOH
O
O
O
O
Ph
Ph
O
O
O
O
Ph
Ph
O
O
O
O
O
O
O
O
,
h
89%
89%
O
O
1
,
1h,
O
O
1g
62%
62%
1g,
,
,
=
n
n
O
O
1
1
52
64
%
%
1j,
j
n = 1, 52%
R
O
'
,
,
=
r
1k
2
r
B
PdB
( )
( )33
1k', n = 2, 64%
O
O
II Pd
( )
O
2
O
O
,
,
=
n
3
Ph
Ph
O
O
O
O
O
O
O
O
1l
1l,
68%
n = 3, 68%
Ph
Ph
O
O
A
'
,
,
56
=
n
O
m
O
O
1
1
4
( )
( )33
%
1m',
n = 4, 56%
O
O
O
O
'
,
,
=
n
n
12 45
'
%
,
1n', n = 12, 45%
O
O
1i
54
%
1i', 54%
n
( )
( )n
( )
n
( )n
O
O
O
O
O
O
O
O
1
re uc
v
e
d
ti
O
O
O
O
( )5
( )5
O
O
O
O
( )
( )55
e m na on
li
ti
i
O
O
O
O
R
O
,
o
1
65%
1o, 65%
O
O
,
7
1
O
O
r
IV Pd
( )
2
n
B
1
1n,
%
O
71%
R
O
O
r
B
O
O
O
O
O
O
O
O
B
O
O
O
3
O
O
O
O
t
t
t
t
u
O
O
O
O
u
B
O
O B
BuO
OBu
O
O
O
O
,
b
60%b
1q
1q
, 60%
,
1p
53
%
1p, 53%
Scheme 4 Catalytic cycle for the Pd(0)-catalyzed homocoupling of 3
O
O
O
O
o
T
l
o
Tol
T
l
Tol
O
O
O
O
O
O
O
O
We also evaluated 5-bromofurans bearing with other
electron-withdrawing substituents (Table 5). Although
5-bromofuran-2-carbaldehyde and 5-bromofuran-2-carboxylic
acid failed to afford the corresponding coupled products (1r and
1s, respectively), 5-bromofuran-2-carboxamides were confirmed
to be available substrates, giving 1t’, 1u and 1y in moderate yields
(34%-65%).
On the basis of the previous studies on the reaction
mechanism of the homocoupling of aryl halides,66-68 a plausible
catalytic cycle is provided in Scheme 4. Pd(PPh3)2Cl2 is initially
reduced by EtOH to form Pd(0) species. An oxidative addition of 3
with Pd(0) gives the palladium(II) species A, which then
undergoes a second oxidative addition with 3 providing the
palladium(IV) species B. The resulting Pd(IV) species is very
unstable and readily undergoes reductive elimination to form
PdBr2 and 1. A terminal reducing agent regenerates Pd(0) from
Pd(II) to complete the catalytic cycle. The role of PVP serves as a
',
b
r
60%b
1
1r', 60%
a Reaction conditions, unless otherwise noted: 3 (0.5 mmol), KOAc (0.75
mmol), ROH (1.0 mmol), Pd(PPh3)2Cl2 (3 mol %), PVP/[Pd] = 35/1 (w/w),
toluene (2 mL), N2, 110 °C. Isolated yields are provided. b EtOH was used as
the reductant.
Table 5 Scope of the reductive coupling.a,b
mo
l
%
O
Pd PPh3 2Cl2
3
O
O
(
)
(
)
.
,
c
e u v
PVP KOA 1 5 q i
O
(
)
+
O
O
R
R
R
r
B
EtOH
e u v
110 o
,
,
o uene
C
N2 t l
2
(
q i
)
3
1
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© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.

Running title
Chin. J. Chem.
stabilizer to prevent the aggregation of palladium species.
then was quenched with saturated aq. NH4Cl solution (10 mL).
The organic layer was separated and the aqueous layer was
extracted with acetyl acetate (15 mL×3). The combined organic
layers were washed with saturated NH4Cl solution, dried with
anhydrous sodium sulfate, filtered, concentrated and finally
purified by chromatography (using petroleum ether/ethyl acetate
= 30:1 as the eluent) to provide 2i (1.25 g, 80%).
Conclusions
In summary, we have exploited an environmentally benign
and economical protocol for the synthesis of BFDCA esters that
are essential monomers for valuable BPEF in polymer or industrial
Synthesis of 2q and 3q.66 To a solution of 2-furoic acid (560
mg, 5 mmol) [5-bromofuran-2-carboxylic acid (960 mg, 5 mmol)]
in DCM (5 mL), t-butyl alcohol (5.25 mmol), DCC (5.5 mmol) and
DMAP (0.5 mmol) were added at room temperature. The reaction
mixture was stirred at room temperature overnight then filtered
by a pad of celite, and the residue was washed with ethyl acetate
(3×10 mL). Solvents were removed under vacuum and the crude
product was purified by chromatography on silica gel (using
petroleum ether/ethyl acetate = 100:1 as the eluent) to give the
product 2q (507 mg, 60%) [3q (0.641 g, 52% )].
General Procedure for the Synthesis of 3. To a Schlenk tube
with a magnetic bar under nitrogen atmosphere, LiBr (130.3 mg,
1.5 mmol), DCE (2 mL), K2S2O8 (216 mg, 0.8 mmol) and the
substrates 2 (0.5 mmol) were successively added. The mixture
was stirred at 80 °C until the starting materials was completely
consumed (monitored by TLC). The resulting reaction mixture was
quenched by saturated aq. Na2S2O3 (15 mL). The organic lay was
separated and the aqueous layer was extracted with ethyl acetate
(3×20 mL). The combined organic layers were concentrated in
vacuo and purified by column chromatography on silica gel (using
petroleum ether/ethyl acetate = 30:1 as the eluent) to afford the
desired product 3.
Synthesis of 3i’, 3k’, 3m’, 3n’, 3r’ and 3t’. To a solution of
alcohol (or secondary amine) (5 mmol) and Et3N (1.2 mL) in DCM
(15 mL), 5-bromofuran-2-carbonyl chloride (1.0 g) was added
dropwise at 0 °C. The resulting mixture was vigorously stirred at
room temperature. After the reaction was completed monitored
by TLC, the mixture was concentrated in vacuo. The residue was
dissolved in ethyl acetate and filtered by a short column of silica
gel. After evaporation of the solvent, the crude product was
purified by chromatography on silica gel (using petroleum
ether/ethyl acetate = 30:1 as the eluent) to give the product.
General Procedure for the Synthesis of 1. To a Schlenk tube
with a magnetic bar under nitrogen atmosphere, Pd(PPh)3Cl2 (3
mol %, 10.5 mg), PVP (55.9 mg) , KOAc (73.5 mg, 0.75 mmol), the
specified alcohol (1 mmol) in toluene (2.5 mL) and the substrates
3 (0.5 mmol) was added. The mixture was stirred at 110 °C until
the starting materials was completely consumed (monitored by
TLC). The corresponding reaction mixture was filtered through a
pad of celite, washed with ethyl acetate and concentrated under
reduced pressure. The residue was purified by flash
chromatography on silica gel (using petroleum ether/ethyl
acetate = 10:1 as eluent) to afford the desired product 1.
chemistry.
furan-2-carboxylates using LiBr/K2S2O8 as green and mild
brominating agents, following with palladium-catalyzed
The
protocol
involves
α-bromination
of
a
reductive homocoupling of the resulting 5-bromofuran-2-
carboxylates with alcohols instead of metallic reductants.
Experimental
General Information. All reactions were carried out in
flame-dried sealed tubes with magnetic stirrer. Unless otherwise
noted, all experiments were performed under N2 atmosphere.
Solvents were treated with 4 Å molecular sieves or sodium and
distilled prior to use. Purifications of reaction products were
carried out by flash chromatography using Qingdao Haiyang
1
Chemical Co. Ltd silica gel (200-300 mesh). H, 13C NMR spectra
1
were recorded on a Bruker AVANCE 400 (400 MHz for H; 100
MHz for 13C), 1H NMR and 13C NMR chemical shifts were
determined relative to internal standard TMS at δ 0.0 Chemical
shifts (δ) are reported in ppm, and coupling constants (J) are in
Hertz (Hz). The following abbreviations were used to explain the
multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet, br = broad. Mass spectra (MS) were obtained using ESI,
DART mass spectrometer. Melting points were determined using
a hot stage apparatus. All reagents were used as received from
commercial sources, unless specified otherwise, or prepared as
described in the literature.
General Procedure for the Synthesis of 2c-2h, 2j-2p, 2t-2y.
To a solution of alcohol (or secondary amine) (5 mmol), Et3N (10
mmol, 1.2 mL) in DCM (15 mL), furan-2-carbonyl chloride (650 mg,
5 mmol) was added dropwise at 0 °C. The resulting mixture was
vigorously stirred at room temperature until the disappearance of
the starting material monitored by TLC. The reaction mixture was
concentrated in vacuo and the residue was dissolved in ethyl
acetate and filtered by a short column of silica gel. After
evaporation of the solvent, the crude product was purified by
chromatography on silica gel (using petroleum ether/ethyl
acetate = 30:1 as the eluent) to give the product.
Synthesis of 2i.65 To a solution of ethylene glycol (0.50 g, 8
mmol), Et3N (1.8 mL) in DCM (20 mL), furan-2-carbonyl chloride
(1.0 g, 8 mmol) was added dropwise at 0 °C. The resulting mixture
was vigorously stirred at room temperature for 2 h. After the
reaction was completed, the mixture was concentrated in vacuo.
The residue was dissolved in ethyl acetate and filtered by a short
column of silica gel. Evaporation of the solvent provided the
crude product which was submitted to the next step without
further purification.
Supporting Information
1
Copies of H and 13C NMR spectra of all the new compounds.
The mixture of the crude product with DCM (25 mL), Et3N (1.8
mL) and DMAP (96 mg, 0.8 mmol) were stirred for 10 min before
being cooled to 0 °C, then acetic anhydride (1.1 g, 10 mmol) was
added. The reaction was stirred at room temperature for 4 h,
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
This article is protected by copyright. All rights reserved.

First author et al.
Report
temperatures of polyamides. J. Appl. Polym. Sci. 2017, 134, 1－12.
Acknowledgement
[15] Long, Y.; Zhang, R.; Huang, J.; Wang, J.; Jiang, Y.; Hu, G.-h.; Yang, J.; Zhu, J.
Tensile Property Balanced and Gas Barrier Improved Poly(lactic acid) by
Blending with Biobased Poly(butylene 2,5-furan dicarboxylate). ACS
Sustainable Chem. Eng. 2017, 5, 9244－9253.
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This work was supported by grants from the National Program
on Key Research Project (No. 2016YFA0602900), the National
Natural Science Foundation of China (No. 21871094), the Science
and Technology Program of Guangzhou, China (No.
201707010057), Guangdong Natural Science Foundation (No.
2017A030312005), and the Science and Technology Planning
Project of Guangdong Province, China (No. 2017A020216021).
[17] Kainulainen, T. P.; Sirviö, J. A.; Sethi, J.; Hukka, T. I.; Heiskanen, J. P.
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Running title
Chin. J. Chem.
Entry for the Table of Contents
Page No.
Synthesis of [2,2’] Bifuranyl-5,5’-dicarboxylic
Acid Esters via Reductive Homocoupling of
5-Bromofuran-2-carboxylates Using Alcohols as
the Reductants
mo
l
%
Pd PPh
Pd(PPh ) Cl (3 mol %)
)
l
3
(
2
C
(
)
3 2
3 2 c 2
1 5 q i
PVP, KOAc (1.5 equiv)
e u
,
.
,
r
X
CO
COX
O
O
LiB
K
S O
v
LiBr, K22S22O88
PVP K
A
N
O
(
)
,
,
110 ooC
o uene
t l
110 C, N22, toluene
r
B
o
,
,
X
CO
80 oC N DCE
COX
Br
X
O
O
O
O
CO
X
OC
COX
80 C, N22, DCE
XOC
O
O
a co o as e re uc an
h l th t
alcohol as the reductant
l
d
t
x m
e a
x m
e a
es
pl
25 examples
es
pl
24% examples
25
24
%
e
s u
o
e
y
i ld
s u
o
t 74
p
t
1
%
yi ld
p
9
%
yields up to 91%
yields up to 74%
We developed an environmentally benign protocol for α-bromination of furan-2-carboxylates with
LiBr and K2S2O8 in good yields. The resulting 5-bromofuran-2-carboxylates further underwent
palladium-catalyzed reductive homocoupling reactions in alcohols using as reductants to afford
[2,2’]bifuran-5,5’-dicarboxylic acid esters as the monomers of promising polymer.
Yi Xie, Bin Yu, Jiajun Luo，Biaolin Yin* and
Huanfeng Jiang*
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
This article is protected by copyright. All rights reserved.