Synthesis of 2,5-furandicarboxylic acid by catalytic carbonylation of renewable furfural derived 5-bromofuroic acid

Article abstract of DOI:10.1016/j.mcat.2018.06.015

2,5-Furandicarboxylic acid (FDCA) is regarded as a biomass based alternative to p-phthalic acid in polymer industry. Although it can be produced through catalytic oxidation of hydroxylmethyl furfural (HMF), HMF itself is currently produced in limited volume from mono- and polysaccharides like glucose and fructose, thus blocking the utilization of FDCA to replace p-phthalic acid as monomer in industrial applications. This work presents an alternative route for FDCA synthesis originally from renewable furfural through carbonylation of 5-bromofuroic acid with common palladium catalysts, which achieves >99% yield of FDCA based on HPLC analysis, or above 90% isolated yield in gram scale. The influences of the palladium sources, phosphine ligands and bases were investigated in details, and a simplified mechanism was also presented.

Full text of DOI:10.1016/j.mcat.2018.06.015

MolecularCatalysis455(2018)204–209
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Synthesis of 2,5-furandicarboxylic acid by catalytic carbonylation of
renewable furfural derived 5-bromofuroic acid
Guanfei Shen, Sicheng Zhang, Yu Lei, Zhuqi Chen, Guochuan Yin⁎
School of Chemistry and Chemical Engineering, Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology),
Ministry of Education, Hubei Key Laboratory o Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan, 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
2,5-Furandicarboxylic acid (FDCA) is regarded as a biomass based alternative to p-phthalic acid in polymer
industry. Although it can be produced through catalytic oxidation of hydroxylmethyl furfural (HMF), HMF itself
is currently produced in limited volume from mono- and polysaccharides like glucose and fructose, thus blocking
the utilization of FDCA to replace p-phthalic acid as monomer in industrial applications. This work presents an
alternative route for FDCA synthesis originally from renewable furfural through carbonylation of 5-bromofuroic
acid with common palladium catalysts, which achieves > 99% yield of FDCA based on HPLC analysis, or above
90% isolated yield in gram scale. The influences of the palladium sources, phosphine ligands and bases were
investigated in details, and a simplified mechanism was also presented.
Biomass utilization
2,5-furandicarboxylic acid
5-bromofuroic acid
carbonylation
carbon monoxide
1. Introduction
transformation of furfural based platform molecules to HMF derivatives
like FDCA can offer a novel route for furfural utilization, and provide an
With the rapid depletion of fossil feedstock, exploring new catalytic
technologies to utilize sustainable biomass to partially replace the fossil
resources as the carbon source of chemical industry has been widely
recognized [1–3]. Among versatile biomass platform molecules, 5-hy-
dromethyl furfural (HMF) is among the most attractive ones, because its
sub-products having multiple functional groups can be potentially uti-
lized as monomers in polymer industry with huge market volume [4].
In particular, 2,5-furandicarboxylic acid (FDCA) has been recognized as
a biomass based alternative to p-phthalic acid, a monomer in polymer
industry consumed in amounts more than 50 million tons per year
[5–9]. The challenge is that, up to now, HMF is currently synthesized
from mono- and polysaccharides like glucose and fructose [10–13],
while these mono- and polysaccharides are not able to be largely pro-
duced from cellulose until now, which apparently blocks the production
of HMF in large scale, and makes using FDCA to replace p-phthalic acid
in polymer industry not immediately operative. Therefore, the alter-
native routes to HMF derivatives like FDCA are apparently attractive.
Unlike HMF, furfural is on-going industrially produced from bulky
C5 carbohydrates containing raw materials like corncobs, oat, wheat
bran and sawdust, etc., which do not compete with human food supply
[14]. However, due to the limited markets for furfural and its current
sub-products, its global industrial production is still less than 500 k tons
per year. Regarding the huge potential market volume of FDCA and the
current challenge of HMF production from cellulose in large scale,
alternative source for the sub-products of HMF. On the other side, as an
essential starting chemical of carbonylation, carbon monoxide (CO) is
not only derived from fossil feedstock based syngas, but also can be
obtained through biomass gasification [15–17]. Therefore, carbonyla-
tion of furfural based platform molecules to HMF derivatives simulta-
neously offers a bright future in utilizing potential biomass based CO
resources.
In fact, carbonylation is a very important process for CO utilization
in chemical industry, which provides a general protocol for bulky and
fine chemical synthesis [18]. In literature, various metal complexes
have been employed as catalysts in different carbonylation reactions
[19,20]. Oxidative carbonylation through C-H activation could be the
most attractive because of its environmentally benign merits [21],
however, the directly oxidative carbonylation of furfural through C-H
activation to FDCA was not very successful [22]. As the alternative
routes, the C-H carboxylations of furoic acid by CO2 to FDCA under
strong basic conditions or with carbonate promoter were explored, and
the disproportionation of furioc acid to FDCA under harsh conditions
was also reported [23–25]. Most recently, we first explored a four-step
synthesis of FDCA from furoic acid through bromination, esterification,
carbonylation and hydrolysis, which achieved 65% total yield of FDCA
under gentle conditions [26]. Here we report a new catalyst system for
the carbonylation of 5-bromofuroic acid, which directly offers FDCA
with > 99% yield in one step.
⁎
Corresponding author.
E-mail address: gyin@hust.edu.cn (G. Yin).
https://doi.org/10.1016/j.mcat.2018.06.015
Received 28 March 2018; Received in revised form 31 May 2018; Accepted 13 June 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

G. Shen et al.
MolecularCatalysis455(2018)204–209
Table 1
Catalyst screening for carbonylation of 5-bromofuroic acid to 2,5-furandicarboxylic acida.
Entry
[Pd] source
Conv./%b
Yield/%b
1
2
Pd(CF3COO)2
Pd(CH3COO)2
Pd(CH3CN)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
PdCl2
PdCl2
Pd2(dba)3
Pd2(dba)3
Pd(dppf)Cl2
40
67
70
38
40
trace
92
15
69
68
30
29
50
28
37
N.D.
64
trace
53
40
3
4c
5
6c
7
8c
9
10c
aReaction conditions: 5-bromofuroic acid (0.2 mmol), [Pd] source (5 mol%), Xantphos (5 mol%), H2O (25 equiv.), NaHCO3 (0.6 mmol), DMF (1 mL), 90 °C, CO
balloon, 24 h, pre-mixing of PdCl2 and Xantphos in DMF at 30 °C for 0.5 h prior to carbonylation. bConv. and yield were determined by HPLC analysis. cNo Xantphos.
N.D.= no detected.
2. Experimental
stirred at 30 °C for 0.5 h. Then, 5-bromofuroic acid (0.2 mmol), H2O
(10 mmol), K2CO3 (0.4 mmol) and DMF (0.5 mL) were added. The glass
tube containing the reaction mixture was next put into a 50 mL auto-
clave, evacuated and back-filled with CO (three times, 10 bar). Then, it
was heated in an oil bath to keep the reaction temperature at 90 °C with
magnetic stirring at 650 r/min for 24 h. After the reaction, the reaction
mixture was cooled to room temperature and vented to release the
excess CO in the ventilator. The product analysis was performed by
HPLC using the external standard method. The reactions were per-
formed in triplicate and the mean values obtained after product analysis
were further used.
Unless otherwise noted, all of the reagents are analytic purity grade
and were used without further purification. Pd(OAc)2 was purchased
from Strem Chemicals Inc. Palladium(II) trifluoroacetate was from
Energy Chemical. Pd(CH3CN)2Cl2 came from Shanghai Boka-chem Tech
Inc. Pd2(dba)3 was from Adamas Reagent Co., Ltd. PdCl2 and different
bases were purchased from Sinopharm Chemical Reagent Co., Ltd. 1H
and 13C NMR spectra were recorded on a Bruker AV-400 using TMS as
an internal reference. The product analysis was conducted with HPLC
(FL-2000) by the external standard method; the HPLC instrument was
equipped with a UV detector and a C18 column (250 mm × 4.6 mm).
The temperature of the column for analysis was 303 K, and the UV
wavelength of the detector was 247 nm. The mobile phase was com-
posed of H2O, MeOH and CH3CN by the ratio of 80% : 10% : 10% (v/v/
v), which was further adjusted to the pH value of 3.2 with concentrated
H2SO4, and the flowing rate was 1 mL min−1. The conversion and yield
were calculated using equations as follows:
2.3. Synthesis of 2,5-furandicarboxylic acid via carbonylation of 5-
bromofuroic acid in gram scale
In a general procedure, PdCl2 (4.9 mg, 27 μmol), PPh3 (7.1 mg,
27 μmol), Xantphos (15.6 mg, 27 μmol) and DMF (2 mL) were added to
a PTFE tube. The mixture was next stirred at 30 °C for 0.5 h. Then, 5-
bromofuroic acid (1.05 g, 5.5 mmol), H2O (2. 52 g, 140 mmol), K2CO3
(1.14 g, 8.3 mmol) and DMF (14 mL) were added. The PTFE tube con-
taining the reaction mixture was put into a 50 mL stainless autoclave.
The autoclave was next evacuated and back-filled with CO (three times,
10 bar), and then heated in an oil bath to keep the reaction temperature
at 90 °C with magnetic stirring at 650 r/min for 12 h. After the reaction,
the reaction mixture was cooled down to room temperature and vented
to release the excess CO in the ventilator. The reaction solution was
diluted with water and filtered to get a clean filtrate. Then, the solvent
was removed by vacuum distillation to obtain a residue which was
added with 20 mL of 3 M HCl aqueous solution. After filtration, the
filter cake was washed with ice-water and CH2Cl2 successively, then
dried under vacuum to obtain an off-white solid as FDCA product which
was next characterized by 1H and 13C NMR.
mol of consumed substrate
mol of added substrate
mol of generated FDCA
mol of added substrate
conversion =
× 100%
yield =
× 100%
2.1. Synthesis of 2,5-furandicarboxylic acid via carbonylation of 5-
bromofuroic acid under CO balloon atmosphere
In a general procedure, PdCl2 (10 μmol), PPh3 (10 μmol), Xantphos
(10 μmol) and DMF (0.5 mL) were added to a glass tube, which was next
stirred at 30 °C for 0.5 h. Then, 5-bromofuroic acid (0.2 mmol), H2O
(10 mmol), K2CO3 (0.4 mmol) and DMF (0.5 mL) were added. The glass
tube containing the reaction mixture was evacuated and back-filled
with CO (three times, balloon), then heated in a boil bath at 90 °C with
stirring for 24 h. After the reaction, the mixtures were cooled to room
temperature and vented to release the excess CO. The product analysis
was performed by HPLC using the external standard method. The re-
actions were performed in triplicate and the mean values obtained after
product analysis were further used.
3. Results and discussion
5-Bromofuroic acid was feasibly synthesized according to the pre-
vious procedures [26]. Since palladium catalysts have been convinced
to be very efficient in versatile carbonylation reactions [27–31], in
present studies, the carbonylation of 5-bromofuroic acid was focused on
palladium based catalysts. The reactions were first conducted in di-
methylformamide (DMF) by using different palladium sources as cata-
lyst, Xantphos as ligand, CO balloon as CO source, NaHCO3 as base,
water as nucleophile, and the results are summarized in Table 1. In the
2.2. Synthesis of 2,5-furandicarboxylic acid via carbonylation of 5-
bromofuroic acid under pressurized CO
In a general procedure, PdCl2 (10 μmol), PPh3 (10 μmol), Xantphos
(10 μmol) and DMF (0.5 mL) were added to a glass tube, which was next
205

G. Shen et al.
MolecularCatalysis455(2018)204–209
case of adding free ligand to the Pd(II) sources as catalyst, a pre-mixing
of the Pd(II) source with ligand in DMF for 0.5 h at 30 °C was performed
prior to the carbonylation reaction. As shown in Table 1, using the same
Pd(II) cation, its counter anions significantly affected the efficiency of
carbonylation. Among these palladium sources, PdCl2 showed the best
efficiency, offering 92% conversion with 64% yield of FDCA in 24 h at
90 °C (Table 1, entry 7). In the absence of phosphine ligand, the car-
bonylation reaction didn’t proceed by using PdCl2 alone as catalyst
(Table 1, entry 6). In the presence of Xantphos, using Pd(OTF)2, Pd
(OAc)2 and Pd(CH3CN)2Cl2 gave 40%, 67% and 70% conversion with
30%, 29% and 50% yield of FDCA, respectively (Table 1, entries 1–3).
Since Pd(PPh3)2Cl2 and Pd(dppf)Cl2 already contain phosphine ligand,
for their tests, in the absence of Xantphos, they provided 38% and 68%
conversion with 28% and 40% yield of FDCA, respectively (Table 1,
entries 4 and 10). Pd2(dba)3 was even less active, giving only 15%
conversion with trace FDCA formation (Table 1, entry 8). Even in the
presence of Xantphos, the activities of these palladium complexes were
still poorer than that of PdCl2, giving 40% and 69% conversion with
37% and 53% yield of FDCA, respectively (Table 1, entries 5 and 9).
Next, using PdCl2/Xantphos as the catalyst system, different solvents
were screened, and the results are summarized in Table S1. Among all
of the tested solvents, DMF exhibited the best efficiency than others
including NMP, DMAC and toluene, etc., which is possibly related to its
physicochemical properties including high boiling point, large polarity,
and good miscibility with water (Table S1, entries 1–8). When the re-
action temperature decreased to 70 °C, the catalytic efficiency also be-
came poor, giving 22% conversion with trace FDCA formation, in-
experiment, using OPPh3 in place of PPh3 as ligand was inactive for
carbonylation (Table 2, entry 3), indicating that the coordination of
phosphorous in PPh3, rather than the oxygen in OPPh3, to the palla-
dium species is critical for catalysis. Here, OPPh3 was generated in situ
reducing of the Pd(II) species to the active Pd(0) species by PPh3 before
carbonylation (vide infra). Adding certain bi-dentate ligands like dppf,
dppe or dppb to PdCl2 also disclosed very poor efficiency for carbo-
nylation, giving less than 10% yield of FDCA (Table 2, entries 4–6). It is
worth mentioning that using Pd(dppf)Cl2 as catalyst directly provided
68% conversion with 40% yield of FDCA (Table 1, entry 10), clearly
supporting that the coordination of phosphine ligand to PdCl2 is es-
sential for Pd-catalyzed carbonylation in present studies. In the case of
dppp and Xantphos, their additions to PdCl2 offered good results in the
preliminary ligand screening, giving 51% and 92% conversion with
47% and 64% yield of FDCA, respectively (Table 2, entries 7 and 8).
Clearly, the properties of phosphine ligand significantly affect the cat-
alytic efficiency of PdCl2 catalyst. A reasonable structure and rigidity of
ligand may facilitate the high activity of PdCl2 in present carbonylation
reaction as well as those in the literature [32,33]. Notably, a pre-mixing
procedure is essential to improve the catalytic efficiency when adding
free phosphine ligand to PdCl2 as catalyst. In the case of no pre-mixing,
adding Xantphos to PdCl2 offered a slightly lower activity, giving 82%
conversion with 52% yield of FDCA (Table 2, entry 9). Extending the
pre-mixing time to 1 h at 30 °C or pre-mixing at 45 °C for 0.5 h also led
to the lower efficiency, giving 66% and 50% conversion with 38% and
39% yield of FDCA, respectively (Table 2, entries 10 and 11), which can
be attributed to the slight loss of the active palladium species due to the
longer time pre-mixing as disclosed by UV–vis studies (vide infra).
In the literature, it has been generally believed that, in amination,
carbonylation and heck reactions etc., the key active species for C-X
activation (X = Cl, Br or I) is the Pd(0) species [34–36]. Therefore, the
added Pd(II) sources actually function as the pre-catalyst for in-situ
generating of the active Pd(0) species in the carbonylation reaction. In
this case, excess ligands or reductive solvents are essential for reducing
the Pd(II) species to the active Pd(0) species [37,38]. In present studies,
adding 1.5 equiv. of Xantphos (7.5 mol%) to PdCl2 apparently im-
proved the carbonylation efficiency, giving > 99% conversion with
80% yield of FDCA (Table 2, entry 8 vs 12). However, adding 2 equiv.
of Xantphos made the catalyst deactivated, giving only 8% conversion
with 5% yield of FDCA (Table 2, entry 13), which can be attributed to
the fully coordination of the palladium catalyst by the phosphine li-
gand, leaving no free site for the C-Br bond activation. Since the bi-
dentate ligands are generally expensive to be employed as the sacrificial
additives to reduce the Pd(II) pre-catalyst, here, we tested cheap,
monodentate PPh3 as the sacrificial additives in place of Xantphos. As
disclosed, adding 2.5 mol% PPh3 to the PdCl2/Xantphos catalyst im-
proved the catalytic efficiency significantly, giving 83% conversion
with 74% yield of FDCA (Table 2, entry 14), and adding 5 mol% PPh3
sharply improved the yield up to 90% with > 99% conversion (Table 2,
entry 15). However, further increasing the PPh3 loading, for example,
10 mol%, caused the activity of PdCl2/Xantphos catalyst decrease,
giving only 50% conversion with 40% yield of FDCA (Table 2, entry
16), indicating that over-loading of PPh3 led to its coordination to the
in-situ reduced Pd(Xantphos) species, thus blocking the C–X bond ac-
tivation for carbonylation. In the experiment of adding 5 mol% PPh3 to
the PdCl2/Xantphos catalyst, after pre-mixing at 30 °C for 0.5 h, there
was no free PPh3 detected with only trace OPPh3 formation observed by
HPLC analysis, indicating all of PPh3 were ligated to the PdCl2/Xant-
phos catalyst, and only trace of them were oxidized to OPPh3 with the
reduction of the Pd(II) species. The UV–vis spectrum changes of pre-
mixing PdCl2 with equally equiv. of PPh3 and Xantphos in DMF also
indicated the coordination of the phosphine ligands to generate the new
Pd(II) species (Fig. S1), and the extended pre-mixing time leads to the
absorbance decrease of this palladium(II) species. In addition, the mass
studies disclosed a dominant mass peak at 721.0650 which can be at-
tributed to the Pd(Xantphos)Cl+ species (Fig. S2). It is worth
dicating
a slightly elevated temperature is essential for efficient
carbonylation in present studies (Table S2).
The influences of different phosphine ligands on PdCl2-catalyzed
carbonylation of 5-bromofuroic acid were next investigated, and the
results are summarized in Table 2. It was found that adding P(t-butyl)3
to PdCl2 was inactive for carbonylation as well as using PdCl2 alone
(Table 2, entry 1), while adding PPh3 to PdCl2 gave 44% conversion
with 26% yield of FDCA (Table 2, entry 2), which is comparable to that
by using Pd(PPh3)2Cl2 directly as catalyst (Table 1, entry 4). In control
Table 2
The influence of ligand on catalytic carbonylation of 5-bromofuroic acid to 2,5-
furandicarboxylic acida.
Entry
Ligand
Conv./%b
Yield/%b
1c
P(t-butyl)3
PPh3
OPPh3
8
trace
26
trace
4
2c
44
11
8
3c
4
dppf
5
dppe
6
2
6
7
8
dppb
dppp
19
51
92
82
66
50
> 99
8
9
47
64
52
38
39
80
5
74
90
40
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
PPh3/Xantphos=0.5
PPh3/Xantphos = 1
PPh3/Xantphos=2
9d
10e
11f
12g
13c
14h
15h
16h
83
> 99
50
aReaction conditions: 5-bromofuroic acid (0.2 mmol), PdCl2 (5 mol%), ligand
(5 mol%), H2O (25 equiv.), NaHCO3 (0.6 mmol), DMF (1 mL), 90 °C, CO bal-
loon, 24 h, pre-mixing of PdCl2 and ligand in DMF at 30 °C for 0.5 h prior to
carbonylation. bConv. and yield were determined by HPLC analysis. c10 mol%
ligand added. dNo pre-mixing of PdCl2 and Xantphos in DMF prior to carbo-
nylation. ePre-mixing of PdCl2 and Xantphos in DMF at 30 °C for 1 h. fPre-
mixing of PdCl2 and Xantphos in DMF at 45 °C for 0.5 h. gXantphos (7.5 mol%).
hXantphos (5 mol%). P(t-butyl)3 = tributylphosphine, Xantphos = 4,5-bis(di-
phenylphosphino)-9,9-dimethylxanthene, dppe = 1,2-bis(diphenylphosphino)
ethane, dppp = 1,3-bis(diphenylphosphino)propane, dppb = 1,4-bis(diphenyl-
phosphino)butane, dppf = 1,1′-bis(diphenyphosphino)ferrocene.
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G. Shen et al.
MolecularCatalysis455(2018)204–209
Fig. 1. The time course of PPh3 oxidation to OPPh3 under carbonylation con-
ditions. Reaction conditions: 5-bromofuroic acid (0.2 mmol), PdCl2 (5 mol%),
PPh3=Xantphos (5 mol%), H2O (50 equiv.), K2CO3 (0.4 mmol), DMF (1 mL),
90 °C, CO balloon, pre-mixing of PdCl2 and ligand in DMF at 30 °C for 0.5 h
prior to carbonylation, yields were determined by HPLC analysis.
Fig. 2. The time course of the catalytic carbonylation of 5-bromofuroic acid to
2,5-furandicarboxylic acid. Reaction conditions: 5-bromofuroic acid
(0.2 mmol), PdCl2 (5 mol%), PPh3 (5 mol%), Xantphos (5 mol%), H2O (50
equiv.), K2CO3 (0.4 mmol), DMF (1 mL), 90 °C, CO balloon, pre-mixing of PdCl2
and ligand in DMF at 30 °C for 0.5 h prior to carbonylation, conv. and yield
were determined by HPLC analysis.
Table 3
The influence of bases on catalytic carbonylation of 5-bromofuroic acid to 2,5-
mentioning that no obvious mass peak can be attributed to the Pd
(Xantphos)(PPh3)2Cl+ species under the mass conditions, even that
HPLC analysis of pre-mixing solution of PdCl2 with Xantphos and PPh3
indicated no free PPh3 existing in the solution. The controversial results
between mass data and HPLC analysis can be tentatively attributed to
the dissociation of flexible PPh3 ligand from the palladium species
under the mass conditions. Remarkably, under the carbonylation con-
ditions at 90 °C, all of added PPh3 was oxidized to OPPh3 in 1 h (Fig. 1),
which led to the formation of the active Pd(0) species for carbonylation.
In activation of the CeX bond, versatile bases were generally em-
ployed to remove the generated HX for driving the reaction proceeding
[39–43]. As disclosed above, adding 3 equiv. of NaHCO3 to the catalyst
system containing 5 mol% PdCl2, 5 mol% PPh3, 5 mol% Xantphos, 25
equiv. of water with CO balloon as the CO source offered > 99% con-
version with 90% yield of FDCA. Next, different bases were tested to
improve the catalytic efficiency, and the results are summarized in
Table 3. As shown, adding inorganic bases generally offered higher
catalytic efficiency than those by using organic bases. For examples,
adding 3 equiv. of DNPA or TEA as base provided 63% conversion with
58% yield, or 84% conversion with 67% yield of FDCA, respectively,
while adding 3 equiv. of NaHCO3 offered 99% conversion with 90%
yield of FDCA (Table 3, entries 1–3). In the case of adding 3 equiv. of
KHCO3, Na2CO3, K2CO3 or Cs2CO3, they provided 95%, 90%, 96% and
80% conversion with 88%, 78%, 90% and 50% yield of FDCA, re-
spectively (Table 3, entries 4–7). Notably, in the case of adding 2 equiv.
of inorganic base, using K2CO3 demonstrated the highest carbonylation
efficiency, giving > 99% conversion with > 99% yield of FDCA
(Table 3, entry 10), while other 2 equiv. of inorganic bases offered
relatively lower yields (Table 3, entries 8, 9 and11). Further reducing
the K2CO3 amount to one equiv. led to a lower efficiency, pro-
viding > 99% conversion with only 66% yield of FDCA (Table 3, entry
12). Apparently, the amount and basicity of added inorganic base sig-
nificantly affected the carbonylation activity and FDCA selectivity, and
adding 2 equiv. of K2CO3 offered the best carbonylation efficiency
under current conditions. In literature, it was generally believed that
the role of a base in carbonylation is to remove the generated HX
[19,20], which can be correlated to the pH of the reaction medium.
Unfortunately, due to the organic solvent (DMF) employed in present
studies with only trace water added as nucleophile, the measurement of
the pH value is not operative here.
furandicarboxylic acida.
Entry
base
Conv./%b
Yield/%b
1
2
3
4
5
6
7
DNPA
TEA
63
84
> 99
95
90
96
80
82
94
58
67
90
88
78
90
50
52
89
> 99
56
66
NaHCO3
KHCO3
Na2CO3
K2CO3
Cs2CO3
NaHCO3
Na2CO3
K2CO3
Cs2CO3
K2CO3
8c
9c
10c
11c
12d
> 99
> 99
> 99
aReaction conditions: 5-bromofuroic acid (0.2 mmol), PdCl2 (5 mol%),
Xantphos (5 mol%), PPh3 (5 mol%), H2O (25 equiv.), base (0.6 mmol), DMF
(1 mL), 90 °C, CO balloon, 24 h, pre-mixing of PdCl2 and ligand in DMF at 30 °C
b
for 0.5 h prior to carbonylation. Conv. and yield were determined by HPLC
c
d
analysis.
Base (2 equiv.).
Base (one equiv.). DNPA = dipropylamine,
TEA = trimethylamine.
Table 4
The influence of water content on catalytic carbonylation of 5-bromofuroic acid
to 2,5-furandicarboxylic acida.
Entry
H2O
(equiv.)
CO pressure
(bar)
Conv./%b
Yield/%b
1
2
3
4
10
25
50
60
25
50
25
50
1
1
1
1
1
1
10
10
78
69
> 99
> 99
97
> 99
> 99
> 99
> 99
> 99
> 99
77
78
96
5c
6c
7c
8c
96
> 99
aReaction conditions: 5-bromofuroic acid (0.2 mmol), PdCl2 (5 mol%),
Xantphos (5 mol%), PPh3 (5 mol%), K2CO3 (0.4 mmol), 1 mL DMF, 90 °C, 24 h,
pre-mixing of PdCl2 and ligand in DMF at 30 °C for 0.5 h prior to carbonylation.
b
c
Conv. and yield were determined by HPLC analysis. Without 5 mol% PPh3.
The reaction conditions for this carbonylation were further
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G. Shen et al.
MolecularCatalysis455(2018)204–209
Scheme 1. Synthesis of 2,5-furandicarboxylic acid (2) by
carbonylation of 5-bromofuroic acid (1) in gram scale.
Scheme 2. Proposed mechanism for Pd-catalyzed carbonylation of 5-bromofuroic acid to FDCA.
optimized in terms of the amount of H2O which is the nucleophile in
this reaction. As shown in Table 4, using a CO balloon as the CO source,
adding 10 equiv. of water provided 78% conversion with 69% yield of
FDCA (Table 4, entry 1). Increasing the water content to 25 equiv.
achieved > 99% conversion with 99% yield of FDCA (Table 4, entry 2).
However, further increasing water content to 60 equiv. caused the ac-
tivity slightly decrease, giving 97% conversion with 77% yield of FDCA
(Table 4, entry 4). Very interestingly, even in the absence of PPh3 as the
sacrificial additives, adding 50 equiv. of water also achieved 99%
conversion with 96% yield of FDCA (Table 4, entry 6). Furthermore,
under this condition, using 10 bar of CO in place of the CO balloon as
the CO source also achieved 99% conversion with 99% yield of FDCA,
as well as adding PPh3 as the sacrificial additives with CO balloon
(Table 3, entry 8), implicating that pressurized CO can also promote the
reduction of the Pd(II) pre-catalyst to the active Pd(0) species, thus
offering multiple opportunities to improve the carbonylation selectivity
to FDCA in this reaction. It is worth mentioning that, comparing the
entry 3 with entry 6 in Table 4, it seems that adding PPh3 as the sa-
crificial additives did not apparently improve the catalytic efficiency.
However, when the experiments were conducted under the conditions
of entries 3 and 6 for only 12 h, it did show significant difference, giving
99% and 85% conversion with 99% and 72% yield, respectively, thus
supporting that adding PPh3 apparently improved the catalytic effi-
ciency. The extended time to 24 h had masked the improvements by
adding PPh3 as the sacrificial additives. Consistent with this, in-
vestigating the time course of carbonylation reaction further revealed
that > 99% yield of FDCA could be achieved in 2 h at 90 °C with CO
balloon under the optimized conditions (Fig. 2). In particular, Fig. 1
even disclosed that transforming the Pd(II) pre-catalyst to the active Pd
(0) species can be achieved in 1 h under the carbonylation conditions
with the complete oxidation of PPh3 to OPPh3 (vide supra); therefore,
achieving complete carbonylation of 5-bromofuroic acid to FDCA in 2 h
as shown in Fig. 2 further highlighted the high activity of this Pd(II)/
Xantphos catalyst for FDCA synthesis. Finally, in a gram scale synthesis
of FDCA under 10 bar of CO, 91% isolated yield of FDCA was achieved
with only 0.5 mol% catalyst loading (Scheme 1), which further high-
lighted the high activity of the catalyst in this carbonylation reaction.
A plausible mechanism for this carbonylation was proposed in
Scheme 2 which follows the general carbonylation mechanism in the
literature [44–47]. First, the reaction was initiated by the reduction of
the Pd(II) pre-catalyst with PPh3 to produce the active Pd(0) species (I),
and this process was indicated by the formation of OPPh3 as displayed
in Fig. 1. Next, the generated active Pd(0) species (I) attacked 5-bro-
mofuroic acid (1) through oxidative addition to generate the inter-
mediate II. Then, the CO insertion to the intermediate II formed the
acyl-Pd(II) intermediate (III). In the presence of base, the intermediate
III proceeded through the bromide exchange with OH− to generate the
intermediate IV with the release of Br−
. Finally, the reductive
208

G. Shen et al.
MolecularCatalysis455(2018)204–209
elimination of the intermediate IV gave 2,5-furandicarboxylic acid (2)
and regenerated the Pd(0) species to achieve the catalytic cycle. It is
worth mentioning that trace furoic acid and 2,2′-bifuran-5,5′-di-
carboxylic acid were even identified as byproducts in this carbonylation
reaction.
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This work was financially supported by the National Natural Science
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2018.06.015.
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