Nickel‐Electrocatalyzed Synthesis of Bifuran‐Based Monomers

Abstract

Bifuran motifs can be accessed with nickel‐bipyridine electrocatalyzed homocouplings of bromine‐substituted methyl furancarboxylates, which, in turn, can be prepared from hemicellulose‐derived furfural. The described protocol uses sustainable carbon‐based graphite electrodes in the simplest setup – an undivided cell with constant current electrolysis. The reported method avoids using a sacrificial anode by employing triethanolamine as an electron donor.

Full text

Nickel-Electrocatalyzed Synthesis of Bifuran-Based
Monomers
Valtteri Oksanen,[a] Sari Rautiainen,[a] and Tom Wirtanen*[a]
Bifuran motifs can be accessed with nickel-bipyridine electro-
catalyzed homocouplings of bromine-substituted methyl fur-
ancarboxylates, which, in turn, can be prepared from hemi-
cellulose-derived furfural. The described protocol uses
sustainable carbon-based graphite electrodes in the simplest
setup – an undivided cell with constant current electrolysis. The
reported method avoids using a sacrificial anode by employing
triethanolamine as an electron donor.
Introduction
The upcoming end of the fossil era has led chemists to search
for new biogenous alternatives for existing materials. This
development has been particularly pronounced in high-volume
products, such as polymers.[1–3] At one of the most advanced
states are the furan-based materials, particularly polyethylene
furan-2,5-dicarboxylate (PEF), which has been envisioned as a
replacement for polyethylene terephthalate (PET).[4] The furan
monomer of PEF, 2,5-furandicarboxylic acid (FDCA), is typically
manufactured from lignocellulosic biomass,[4] for example, from
galactaric acids (Figure 1, top left),[5–8] which contain the
required six carbons for polycondensations. Another approach
has been to convert furfural, furoic acid or alkyl furoates, all
derived from the second most abundant polysaccharide, hemi-
cellulose, into suitable monomers.[9] For example, Henkel-type
carboxylation of furoic acids has been developed by several
research groups.[10–16] In addition, hydroxymethylation[17–22] and
chloromethylation[23,24] have been used to convert furfural into
suitable C6-platforms. One particularly appealing approach has
been the Pd-catalyzed carbonylative functionalization of
5-bromo derivatives (Figure 1, bottom left).[25–27] Interestingly,
suitable monomers can also be accessed when the furfural
derivatives are directly dimerized (Figure 1, bottom right). In
fact, bifuran polymers or bifuran-furan co-polymers exhibit
complementary properties to PEF. For example, they demon-
strate increased resistance to UV-A,[28,29] can elevate the glass
transition temperature,[29] and display remarkable O2barrier
properties.[30] However, in comparison to FDCA synthesis, the
synthesis of bifuran monomers is less established. A typical
strategy involves using Pd-catalysis in either BrBr
homocoupling,[30–34] Mizoroki–Heck,[28,35] or direct CH-
functionalization.[36,37] Other approaches include oxidative ho-
mocoupling of Grignard reagents[38] and nickel-catalyzed homo-
coupling of ethyl 5-bromofuroate.[36] Some common hindrances
in these protocols are the use of expensive palladium
catalysts,[28,30–37] poisonous CO,[34,39] or metal powders[33,36] as
reductants.
Electrosynthesis has recently been highlighted as the 21st
century technique for organic synthesis,[40] particularly suitable
for converting renewables into value-added products.[41] One of
the beauties of the method lies in its use of massless electrons
or electron-holes as reductants and oxidants, which can lead to
ecological and economic benefits.[42–46] These advantages have
also been harnessed in transition-metal electrocatalysis in
organic synthesis.[44–47] Within this domain, the nickel-catalyzed
electrosynthesis of bi(hetero)aryls from (pseudo)halogen-func-
tionalized substrates has been demonstrated as a powerful
synthetic tool for the homocoupling of C(sp2)X bonds.[47–59] To
date, it has been applied to the synthesis biaryls,[48–50,56–59] and
N-heterocyclic compounds such as bipyridines,[51–55,57]
biazines,[57] biindoles,[49,50] and biquinolines.[50,57] A few examples
have also been reported for the synthesis of bithiophenes, but
we are unaware of this methodology being applied to the
synthesis of bifurans.[50,57] Typical electrochemical set-ups for
these reactions use an undivided cell with a constant-current
electrolysis,[48,49,51–54,56–59] while other setups, such as divided cells
and potentiostatic conditions, have been less frequently
applied.[50,56] Commonly, electrolysis has been conducted using
nickel,[48,49,51–54,56–59] gold,[50] and graphite/carbon nanotube[55]
[a] V. Oksanen, Dr. S. Rautiainen, Dr. T. Wirtanen
Industrial Synthesis & Catalysis
VTT Technical Research Centre of Finland Ltd.
Box 1000, FI-02044 Espoo (Finland)
E-mail: tom.wirtanen@vtt.fi
tom.wirtanen@iki.fi
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202302572
© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH
GmbH. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and re-
production in any medium, provided the original work is properly cited.
Figure 1. Application of different biogenous starting materials into the
synthesis of 2,5-furandicarboxylic acid and [2,2’-bifuran]-5,5’-dicarboxylic
acid motifs.
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cathodes with sacrificial anodes such as iron,[49,51,54,58]
duralumin,[49] stainless steel,[48,59] magnesium,[50] zinc,[51–54]
nickel,[56] or Fe64Ni36.[57] While sacrificial anodes offer some
benefits in organic electrosynthesis,[60,61] the dissolution of the
anode can complicate electrolysis, altering the inter-electrode
gap as reaction progresses,[62] and the generation of stoichio-
metric metal waste can complicate waste disposal on a larger
scale.[63] A few methods in the literature have avoided the use
of sacrificial anodes in nickel-mediated electrohomocouplings
by utilizing water[56] and urea oxidations[64] as counter-reactions
in a divided cell. However, the use of cell separators increases
cell resistivity and adds complexity to the system, therefore,
undivided electrosynthesis without a sacrificial anode in nickel-
catalyzed electrohomocouplings would be desirable.
Results and Discussion
As furoic acid is currently produced as a side product in the
synthesis of FDCA from galactaric acids developed at VTT
(Figure 1),[5] we embarked upon an effort to design an electro-
catalytic synthesis of both the 2,2’- and the 3,3’-bifuran core
due to their interesting properties in polymer applications.[28–30]
We chose nickel-electrocatalyzed homocouplings as our syn-
thetic strategy since both 5-bromo and 4-bromo furoic acids
can be readily accessed from 2-furoic acid. In electrosynthesis,
our key guidelines were to avert using a sacrificial anode by
finding an anodic counter-reaction compatible with carbon-
based electrodes. Our initial trials in converting methyl 5-
bromofuran-2-carboxylate 1 a into 2,2’-bifuran-5,5’-dicarboxylic
acid dimethyl ester 2 a using the oxidation of potassium
formate as the anodic reaction with glassy carbon electrodes
were generally promising. However, the faradaic efficiencies,
yields and selectivities varied too much from reaction to
reaction and despite our best efforts, we could not achieve
reproducible synthesis. This led us to explore other options.
Particularly, the oxidation of tertiary amines seemed promising
given its successful application in various electrochemical
reactions.[63,65–68] Interestingly, using 2 equiv. of triethanolamine
(TEOA), the reaction furnished 2 a in 89 % NMR-yield (82 %
isolated) with graphite electrodes (Sigrafine® V2100, SGL
Carbon), a current density of 7 mA/cm2, a temperature of 50°C,
tetrabutylammonium tetrafluoroborate (TBABF4, 0.02 M) as an
electrolyte, and 10 mol-% Ni(dmbpy)Br2as a catalyst (Table 1,
Entry 1). Along with the desired 2 a, dehalogenated methyl 2-
furoate 3 a was received as a side product. Changing the
electrolyte cation from TBA+to TEA+or Na+and the anion
from BF4to PF6or OTsresulted in lower yields of 2 a,
although it still remained as the main product. (Entries 2–4). In
addition, a larger fraction of 3 a was recovered. Interestingly,
using TEA or N,N-diisopropylethylamine (DIPEA) instead of
TEOA led to lower yields and a more pronounced formation of
the dehalogenated side product 3 a (Entries 5–6). In the absence
of TEOA, only 3 a was obtained with a yield of 15 % (Entry 7).
Without electricity, no products were formed, and 100 % of 1 a
was recovered unchanged (Entry 8). If no catalyst was used,
only the dehalogenation of 1 a into 3 a was observed (Entry 9).
Both 2,2’-bipyridine (bpy) and 4,4’-di-tert-butyl-2,2’-dipyridine
(dtbbpy) ligands gave slightly lower yields of 76 and 71 % for
2 a, respectively (Entries 10–11). In addition, the bpy ligand
resulted in a higher fraction of dehalogenated 3 a compared to
the dmbpy and dtbbpy ligands. The current density seems to
be somewhat critical and attenuating or accentuating it from
7 mA/cm2by 1 mA/cm2led to lower yields of 2 a (Entries 12–
13). Alternative solvents to DMF such as MeOH or MeCN led to
diminished yields (Entries 14–15), while using N-butylpyrroli-
done (NBP) or 3-methoxy-N,N-dimethylpropanamide (3-MDMP)
resulted in the formation of 3 a as the main product (Entries 16–
17). Interestingly, when DMF was replaced with DMSO, it
resulted in 51% yield of 2 a, along with an increased amount of
dehalogenated 3 a (Entry 18). This suggests that DMSO can
serve as an alternative solvent to DMF in the reaction.
After optimizing the reaction conditions for the yield of 2 a,
we proceeded to explore the scope of the homocoupling by
focusing on other potential monomers (Table 2). We found that
1 b is readily dimerized to 2,2’-bifuran-5,5’-dicarboxylic acid
di(2-chloroethyl) ester 2 b in a 73 % yield (Entry 2). Interestingly,
we also discovered that 3,3’-bifuran-5,5’-dicarboxylic acid
dimethyl ester can be synthesized using this methodology with
a 55 % isolated yield (Entry 3). In this case, increasing the
Table 1. Optimization of the reaction conditions.
Entry Variation from optimal
conditions
Yield of 2 a
(%)[a]
Yield of 3 a
(%)[a]
1 – 89 10
2 TBAPF6instead of TBABF474 18
3 NaBF4instead of TBABF466 14
4 TEAOTs instead of TBABF473 16
5 TEA instead of TEOA 15 32
6 DIPEA instead of TEOA 18 38
7 no TEOA 0 15
8 no electricity 0 0
9 no catalyst 0 20
10 Ni(bpy) as catalyst 76 20
11 Ni(dtbbpy) as catalyst 71 13
12 8 mA/cm2instead of 7 mA/
cm2
49 16
13 6 mA/cm2instead of 7 mA/
cm2
77 18
14 MeOH instead of DMF 15 10
15 MeCN instead of DMF trace 0
16 NBP instead of DMF 8 60
17 3-MDMP instead of DMF 8 65
18 DMSO instead of DMF 51 29
[a] Yields determined by 1H NMR using 1,3,5-trimethoxybenzene (TMB) as
an internal standard,
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catalyst loading from 10 to 20 mol-%, and changing the ligand
from dmbpy to bpy proved to be beneficial. With 10 mol-%
loading of dmbpy, we obtained a 32 % isolated yield of 2 c. In
addition, methyl 4,5-dibromofuran-2-carboxylate 1 d could be
coupled to 2 d with a 43 % yield (Entry 4). The reaction proceeds
regioselectively through the homocoupling of the two 5-Br sites
(Supporting Information). Extending the scope to thiophene
and aryl monomers proved also possible and we received 2,2’-
bithiophene-5,5’-dicarboxylic acid dimethyl ester 2 e at a 49 %
yield (Entry 5), and dimethyl biphenyl-4,4’-dicarboxylate 2 f at a
30% yield (Entry 6). For the latter substrate, we also explored
the replacement of Br with Cl and I to facilitate the reaction.
However, the best result was obtained with Br, while Cl
furnished 2 f with a 16 % yield, and only traces could be
detected with I. We performed the reaction of 1 e to 2 e in the
presence of 20 mol-% of Zn(OAc)2, Fe(NO3)3, and Ni(NO3)2
additives, based on the following hypothesis that dissolved
metal ions could help the reaction, given their presence in
various literature examples involving sacrificial anodes. How-
ever, these additives did not increase the yield of 2 e.
During trials with 2 e, we observed the formation of a charry
deposit on the cathode surface (Supporting Information). While
the formation of dehalogenated side-products explains only
part of the loss of starting material, we conducted a detailed
examination of the deposit using scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDS). When
comparing a pristine cathode to one after reaction with 1 e,
SEM images reveal some morphological changes (Supporting
Information). More interestingly, EDS analysis reveals the
presence of both sulfur and nickel on the latter cathode surface,
suggesting that an intermediate derived from 1 e and the
catalyst are absorbed into the surface, either separately or
together. Notably, a cathode used in the electrolysis of 1 a also
shows some absorbed nickel in EDS, although considerably less
than with 1 e (Supporting Information). Therefore, it is plausible
that both cathode surface passivation and the consumption of
both the catalyst and substrate may affect the yield with certain
starting materials. For example, a cathode after electrolysis with
1 c shows similar visual darkening than with 1 e.
The reaction mechanism for the nickel-electrocatalyzed
homocouplings of aryl halides has been extensively discussed
in the literature.[49,50,57,58] First, nickel(II)-precatalyst is reduced to
a [Ni]0-complex (Figure 2, a), which then undergoes oxidative
addition with the heteroaryl compound (b), forming an Ar[Ni]IIX
complex as an intermediate (c). The reduction of this species by
a one electron generates an Ar[Ni]Icomplex that can further
react through a second oxidative addition to produce Ar2[Ni]IIIX
complex (d). The reductive elimination of this intermediate
results in the formation of bi(hetero)aryl and NiIX (e), which
then undergoes a one-electron reduction to [Ni]0(f). At the
anode, the oxidation of triethanolamine supplies the necessary
electrons. Following the initial one-electron oxidation, the N-
centered TEOA*+is likely to be deprotonated, generating
carbon-centered aminoalkyl radical. This radical intermediate
can then be oxidized to generate an iminium cation, which can
Table 2. Studied substrates.
Entry Substrate Product Yield of
2[a] (%)
Yield of
3[b]
182[c]
82
10[c]
10
1 a 2 a
273[d] 0
1 b 2b
355[c]
32
27[c]
n.d.
1 c 2 c
439[c]
43
0[c]
0
1 d 2d
549[c]
34
35[c]
n.d.
1 e 2 e
6
30[c]
(X=Br)
16[c]
(X=Cl)
traces[c]
(X=I)
33[c]
(X=Br)
1 f 2 f
[a] Isolated yields. [b] Yields determined by 1H NMR using 1,3,5-
trimethoxybenzene (TMB) as an internal standard. [c] with 20 mol-%
Ni(bpy)Br2catalyst loading [d] with 20 mol-% Ni(dmbpy)Br2catalyst
loading.
Figure 2. Plausible cathodic reaction mechanism and postulated follow-up
reactions of TEOA based on the literature.
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further hydrolyze or intramolecularly cyclize.[69,70] The formation
of dehalogenated 3 a can be rationalized by either the direct
reduction of 1 a at the cathode or by halogen-atom transfer
from 1 a to α-aminoalkyl radicals,[71,72] which are plausible
intermediates in the anodic oxidation of tertiary amines.[69,70]
As both 2 a and 2 c are highly interesting bifuran-based
monomers, we decided to scale-up the reaction from 5 mL PTFE
screening cells[73] to a larger glass cells (Table 3). The isolated
yields of 2 a were slightly lower, ranging from 72 to 73 %
(Entries 2–3), compared to the yield obtained in the PTFE
screening cell (82 %, Entry 1). Similarly, the yield of 2 c was
slightly lower, decreasing from 55 to 49 % (Entries 4–5) in the
larger cell. One interesting observation is that some of the 2 a
starts precipitating already during the electrolysis, while the rest
of the product can be precipitated by adding water after the
reaction. This could prove beneficial for product isolation in
continuous processes and/or simplify downstream processing.
Conclusions
In this report, we demonstrated that furans are viable substrates
in nickel-catalyzed electrohomocouplings, despite their high
reactivity in oxidative processes such as in anodic meth-
oxylations, cyanations, and polymerizations.[74–82] The developed
method could be applied to the synthesis of both 2,2’-bifuran-
5,5’-dicarboxylic acid dimethyl ester and 3,3’-bifuran-5,5’-dicar-
boxylic acid dimethyl ester, which are key constituents in highly
potential bio-based bifuran polymers and bifuran-furan co-
polymers.[28–30] Importantly, the electrolysis can be performed in
the absence of sacrificial anodes, and the products can be
recovered without chromatography by precipitation, which
might facilitate the upscaling of the reaction.
Experimental
General procedure for nickel-electrocatalyzed synthesis of bifuran,
bithiophene and biaryl monomers: Bromo(hetero)aryl (0.5 mmol),
nickel-catalyst (0.05–0.1 mmol), TBABF4(0.1 mmol, 33 mg) and
triethanolamine (TEOA, 1.0 mmol, 149 mg) were dissolved to 5 mL
DMF in one compartment PTFE screening cell.[73] Cell was preheated
for 15 minutes at 50°C, after which 7 mA/cm2constant current was
passed through graphite electrodes (Sigrafine® V2100, SGL Carbon)
for 2.1 F. After the electrolysis, the mixture was poured into 15 mL
water and the resulting precipitate was collected through vacuum
filtration. The filtrate was either collected directly or dissolved in
DCM and dried with Na2SO4. In the latter case, drying agent was
filtered out and the volatiles were removed under vacuum to
obtain the desired compounds.
Supporting Information
The authors have cited additional references within the
Supporting Information.[83,84] Full experimental details and
characterization of the compounds are described in the
Supporting Information.
Acknowledgements
Kiia Malinen from Aalto University is acknowledged for her help
preparing this manuscript. T. W. is grateful for financial support
from the Research Council of Finland (Grant no. 348889).
Conflict of Interests
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
Keywords: Electrochemistry ·Catalysis ·Molecular
Electrochemistry
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Manuscript received: September 8, 2023
Accepted manuscript online: September 21, 2023
Version of record online: November 2, 2023
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