Torii-Type Electrosynthesis of α,β-Unsaturated Esters from Furfurylated Ethylene Glycols and Amino Alcohols

Article abstract of DOI:10.1002/ejoc.202100605

Electrosynthesis of unsaturated esters from furan derivatives, reported by Torii et al. in 1976, is an attractive method for the valorization of furanoic platform chemicals. Nevertheless, it has received practically no attention, presumably due to specific reaction conditions including the use of expensive Pt electrodes. With the aim of expanding the application of Torii-type ester electrosynthesis, we explored the electrochemical transformation of O-furfuryl ethylene glycols and N-furfuryl amino alcohols to esters 5. These can be obtained in two consecutive electrochemical steps: bis-alkoxylation of the furan derived substrates 3 to give spirocycles 4, followed by ring-opening involving oxidative fragmentation of the C?C bond. Both steps can be carried out at ambient conditions, using inexpensive graphite electrodes; however, each step required a different supporting electrolyte and acidic additive to achieve good yields of the product. Additionally, conditions were found for efficient one-pot transformation of N-furfuryl amino alcohols to esters 5 while O-furfuryl ethylene glycols under the same conditions gave esters 5 in moderate yields.

Full text of DOI:10.1002/ejoc.202100605

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Torii-Type Electrosynthesis of α,β-Unsaturated Esters from
Furfurylated Ethylene Glycols and Amino Alcohols
Madara Darzina,[a] Anna Lielpetere,[a] and Aigars Jirgensons*[a]
Electrosynthesis of unsaturated esters from furan derivatives,
reported by Torii et al. in 1976, is an attractive method for the
valorization of furanoic platform chemicals. Nevertheless, it has
received practically no attention, presumably due to specific
reaction conditions including the use of expensive Pt electro-
des. With the aim of expanding the application of Torii-type
ester electrosynthesis, we explored the electrochemical trans-
formation of O-furfuryl ethylene glycols and N-furfuryl amino
alcohols to esters 5. These can be obtained in two consecutive
electrochemical steps: bis-alkoxylation of the furan derived
substrates 3 to give spirocycles 4, followed by ring-opening
involving oxidative fragmentation of the CÀ C bond. Both steps
can be carried out at ambient conditions, using inexpensive
graphite electrodes; however, each step required a different
supporting electrolyte and acidic additive to achieve good
yields of the product. Additionally, conditions were found for
efficient one-pot transformation of N-furfuryl amino alcohols to
esters 5 while O-furfuryl ethylene glycols under the same
conditions gave esters 5 in moderate yields.
Introduction
The utilization of biomass has received increasing attention as
an alternative to replace the dwindling fossil resources for the
production of value-added products.[1] Biomass-derived plat-
form chemicals are central to this initiative. Among them,
furanoics, accessible in bulk amounts from lignocellulosic feed-
stocks, are versatile starting materials to achieve a range of
chemicals with an application in material science, drug
discovery, and agriculture.[2] Electrochemistry has been demon-
strated as a useful tool for valorization of biomass-derived
compounds.[3] Furanoics are particularly suitable substrates for
electrochemical functionalization due to the low oxidation
potential of the furan ring[4] (see also Supporting Information).
Notable examples include oxidative dihydroxylation[5] and
dialkoxylation[6] of furan derivatives. Anodic oxidative dialkox-
ylation was also employed in electrochemical synthesis of
unsaturated ester 2 from furfuryl alcohol 1a, furfural 1b, and 2-
furoic acid 1c in the presence of methanol, first demonstrated
by Torii et al (Figure 1).[7] According to their proposed
mechanism, oxidative dimethoxylation of the furan ring leads
to intermediate A. Further oxidation leads to cleavage of the
CÀ C bond resulting in oxonium ion B and subsequent ring
opening by methanol gives ester 2. Despite the high potential
value of the Torii ester electrosynthesis products, surprisingly
limited application of this transformation has been demon-
strated in the scientific literature.[8] Our work was focused on
the electrochemical oxidation of furylmethyl derivatives 3
bearing hydroxyl group as an internal nucleophile (Figure 1). In
Figure 1. Torii-type electrosynthesis of unsaturated esters.
this case, oxidative methoxylation should provide spirocyclic
derivatives 4 which would undergo fragmentation to give
products 5 with functionalized ester moiety. These products are
valuable building blocks for further chemical transformations,
including tailored polymer synthesis.
[a] M. Darzina, A. Lielpetere, Prof. Dr. A. Jirgensons
Latvian Institute of Organic Synthesis
Aizkraukles 21, Riga, LV-1006, Latvia
E-mail: aigars@osi.lv
Results and Discussion
https://osmg.osi.lv/
O-Furfuryl ethylene glycol (3a) was used as the model substrate
to find efficient conditions for the spirocycle (4a) formation by
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100605
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electrochemical oxidation of the furan ring (Table 1). The
was observed at increased charge. However, in this case, a
significant amount of ester 5a formed as a by-product (Table 1,
entry 8). If HFIP was used as an additive and LiClO4 as an
electrolyte, poor yield of spirocycle 4a was obtained due to
competing dimethoxylation of the furane (Table 1, entry 9).
Optimal conditions were suitable also for the oxidation of
alcohol 3a to spirocycle 4a in 0.5 g scale (Table 1, entry 10).
The scope for spirocycle 4b–h synthesis was investigated
for a wider range of ethylene glycol and amino ethanol
derivatives 3a–h bearing furfuryl substituent (Scheme 1).
Ethanol was also found to be a competent nucleophile to form
ethoxy-substituted product 4b, although in a lower yield than
4a formed with methanol as nucleophile. Moreover, electrolysis
in ethanol led to precipitate formation on the cathode.
Substitution at ethylene glycol linker in starting materials 3c–f
gave products 4c–f with good yields. Methyl substitution at
furan 5th position in the starting material did not significantly
affect the yield of the product 4g. N-substituted amino ethanol
derivative 3h was also subjected to electrochemical cyclization
to give spirocycle 4h in good yield.
reaction previously has been described using one-pot two-step
transformation which includes electrochemical bromination of
furan using NH4Br and a carbon anode/nickel cathode, followed
by addition of sodium methoxide.[9] Given the low oxidation
potential of furan, we explored direct spirocycle formation[6a]
with a simple electrochemical setup using undivided cell and
graphite electrodes. Methanol was used as a solvent and proton
reduction as the cathode reaction. We found that the addition
of PPTS (1 equiv.) was beneficial to achieve a good yield of the
product 4a (Table 1, entry 1). Decreasing or increasing the
amount of PPTS led to a reduced yield of the product 4a
(Table 1, entries 2–4). It can be hypothesized that the addition
of PPTS prevents formation of methoxide as the reduction
product of methanol which may decrease the dimethoxylation
of furane favoring formation of spirocycle 4a.[6a] A slight
increase of the total charge passed through the solution
(measured in faradays per mole of substrate (F/mol)) led to a
slightly higher yield (Table 1, entry 5), while significant increase
of the charge was detrimental to the product 4a formation
(Table 1, entry 6). LiClO4 as an electrolyte was less efficient
compared to TBABF4 (Table 1, entry 7). If HFIP was used as an
additive instead of PPTS, a relatively good yield of product 4a
With spirocycles 4 in hand, their electroxidative fragmenta-
tion was investigated with an aim to obtain esters 5. Spirocycle
4a was used as a model compound to establish the conditions
for this step (Table 2). PPTS as an additive and TBABF4 as an
electrolyte used for spirocycle 4a formation were not suitable
for obtaining ester 5a due to precipitation of the PPTS
decomposition products on the cathode during prolonged
electrolysis (Table 2, Entry 1).
Table 1. Oxidative cyclization of alcohol 3a to spirocycle 4a.
When acetic acid and HFIP were used as additives, the yield
of product 5a was considerably increased, however, the
formation of an inseparable side-product along ester 5a was
observed (Table 2, entry 2). To obtain ester 5a in a good yield,
AcOH (4 equiv.) as an additive and LiClO4 as electrolyte were
Entry
Conditions[a]
Yield of 4a[b]
1
2
3
4
5
6
7
8
none
no PPTS
0.5 equiv. PPTS
2.0 equiv. PPTS
2.5 F/mol
4.0 F/mol
LiClO4 instead of TBABF4
no PPTS, 20 equiv. HFIP, 4 F/mol
no PPTS, 20 equiv. HFIP, LiClO4 instead of TBABF4,
45 mA, 500 mg scale
70%
50%
58%
55%[c]
77%
59%
59%
52%[d]
38%[e]
71%
Table 2. Conditions for the oxidative fragmentation of spirocycle 4a.
9
10
[a] Deviation from the conditions given in the scheme. [b] Isolated yields
are given; [c] Precipitate deposition on cathode observed; [d] Formation
of 5a was observed, isolated yield 15%. [e] dimethoxylation product of
furane is a major by-product according to 1H-NMR of a crude mixture
Entry
1
Conditions[a]
Yield of 5a[b]
1 equiv. PPTS instead of AcOH
TBABF4 instead of LiClO4
Add 20 equiv. HFIP
TBABF4 instead of LiClO4
None
no AcOH
1 equiv. AcOH
2 equiv. AcOH
add. 20 equiv. HFIP
2.0 F/mol
9%[c]
2
58%[d]
3
4
5
6
7
8
9
68% (77%)[e]
11%
58%
55%
67%
(59%)[e,f]
(74%)[e,g]
3.0 F/mol
[a] Deviation from the conditions given in the scheme. [b] Isolated yields
are given if not indicated otherwise. [c] Unreacted starting material
(isolated yield: 55%). [d] Contains unidentified inseparable by-product
~15% [e] 1H-NMR-yield using 1,4-bis(trichloromethyl)benzene as an
internal standard. [f] Unreacted starting material (1H-NMR yield: 20%) [g]
Unreacted starting material (1H-NMR yield: 5%).
Scheme 1. Scope of spirocycle 4 synthesis.
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found to be crucial reaction components (Table 2, Entry 3).
Diol 3i derived from bis-hydroxymethylfuran was also
successfully transformed into the spirocycle 4i by anodic
oxidation (Scheme 3). However, electrochemical oxidative frag-
mentation of the spirocycle 4i gave the expected product 5i in
a relatively low yield (22%).
Decreasing the amount of AcOH reduced the yield of product
5a (Table 2, Entries 4–6). Addition of HFIP did not have an
impact on product 5a formation (Table 2, Entry 7). Notably,
4.0 F/mol of charge were needed to achieve complete con-
sumption of the starting material 4a (Table 2, Entries 8,9). This
indicated a parallel competitive oxidation process since in
theory only 2.0 F/mol are needed for the desired transformation
(vide infra). In all the experiments, the formation of ester 5a
with a Z-configuration double bond was observed while E-
isomer formation was not detected. Z-Configuration of ester 5a
was confirmed by characteristic coupling constant of double
bond protons (J=11.8 Hz) and their cross peaks in NOESY
spectra (see Supporting Information)
Spirocycles 4b–h were subjected to electrochemical oxida-
tive fragmentation to esters 5b–h using the optimized con-
ditions found for model substrate 4a (Scheme 2). Ethoxy
substituted spirocycle 4b gave product 5b in a slightly lower
yield compared to the methoxy analogue 5a. Noteworthy,
mixed acetal 5b formed exclusively, indicating that no trans-
acetalyzation with methanol takes place during the reaction.
Addition of HFIP to the reaction of substrate 4b improved the
yield of product 5b – such an effect was not observed in the
reaction of the model substrate 4a (Table 2, entry 8). Substrates
4c–f with a substituted ethylene linker gave the desired esters
5c–f in good yields using standard conditions with no HFIP
additive. Methyl substituted substrate 4g also gave the
expected product 5g, however, it was difficult to separate from
the unreacted starting material. In this case full conversion of
spirocycle 4g could not be achieved after 4.0 F/mol of charge
passed. Morpholine derivative 4h was efficiently transformed to
the O-acylated N-protected amino alcohol 5h.
One-pot synthesis of esters 5 from alcohols 3 was also
explored (Scheme 4). PPTS additive which was found beneficial
for the transformation of substrates 3 to spirocycles 4 could not
be applied for this purpose because the transformation of
intermediates 4a to ester 5a was low yielding in the presence
of this additive. Therefore, HFIP was used for the first step given
the good conversion of substrate 3a to the mixture of
compounds 4a and 5a using this additive (Table 1, entry 8).
After complete conversion of the starting material 3 at the first
stage (equal to 2.2 F/mol of passed charge), acetic acid was
added, and the electrolysis was continued for additional 4.0 F/
mol of passed charge to obtain products 5. This procedure led
to moderate yields of esters 5a,c,d from O-furfuryl ethylene
glycols 3a,c,d. Gratifyingly, this approach was more productive
in the case of synthesis of esters 5h–m containing protected
amine functionality from O-furfuryl amino alcohols 3h–m. It
should be noted that ester 5k synthesis was successfully
performed on 0.9 g scale.
Mechanisms of electrochemical bis-alkoxylation of furan and
(methoxy) anisole derivatives have been proposed previously
depending on the reaction conditions.[6a,10] For the oxidative
fragmentation of spirocycle 4a to ester 5a two possible
pathways are provided in Scheme 5. Path A involves reversible
SN type methanolysis of the acetal in the dihydrofuran part of
spirocycle 4a, leading to intermediate C. Electrochemical
oxidation of the hemi-acetal would give the O-centered radical
E which fragments to α-oxy-stabilized C-centered radical F.
Further oxidation of intermediate F would give oxonium ion G
which reacts with methanol giving ester 5a. The alternative
path B starts with electrochemical activation of acetal group in
spirocycle 4a to give radical cation D after which the ring opens
by methanolysis, leading to O-centered radical E. It cannot be
excluded that methanolysis of activated acetal D and fragmen-
tation occurs in a simultaneous fashion leading directly to
intermediate F.
Scheme 3. Transformation of alcohol 3i to spirocycle 4i and its fragmenta-
tion to ester 5i.
Scheme 2. Scope of oxidative fragmentation of spirocycles 4 to esters.
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Scheme 4. One-pot synthesis of esters 5 from alcohols 3.
Scheme 6. Deuterium labelling experiments supporting path B of ester 5a
formation mechanism.
Scheme 5. Proposed mechanistic pathways for ester 5a formation from
spirocycle 4a.
Conclusion
In summary, we have demonstrated an extended application of
Torii-type ester electrosynthesis from biomass-derived furan
conjugates with glycols and amino alcohols. The ester synthesis
from furanoics can be done in two steps or by using one-pot
protocol giving multifunctional building blocks and tailored
monomers for polymerization. Notably, the reactions can be
performed using undivided cell commercial electrochemical
set-up using inexpensive graphite electrodes.
To establish if path A is operational, spirocycle 4a was
subjected to the reaction conditions with no current passing
through the reaction mixture and using deuterated methanol
as the reaction solvent (Scheme 6). In this case, no deuterium
incorporation was observed by 1H-NMR even after 24 hours.
However, when the current was passed through the reaction
mixture, incorporation of deuterated methanol took place,
forming a mixture of the products d6-5a and d9-5a (ratio 1:0.3,
detected by 1H-NMR). These results clearly indicated the
necessity of electrochemical activation for the methanolysis of
spirocycle 4a and supported path B of the product 5a
formation mechanism.
Demonstration of the application for the reaction products
is planned as the next step of our research work.
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Optimized conditions for α,β-unsaturated ester formation: Sub-
strate (1.0 equiv.) and LiClO4 (1.0 equiv.) were transferred to an
undivided cell (10 mL) and dissolved in freshly distilled MeOH
(7 mL). AcOH (4.0 equiv.) was added. Graphite electrodes were
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added, and the reaction mixture was filtered through a silica plug.
The solvent was evaporated, and product was purified using
column chromatography.
Optimized conditions for one-pot transformation of alcohols to
α,β-unsaturated esters: Substrate (1.0 equiv.) and LiClO4 (1.0 equiv.)
were transferred to an undivided cell (10 mL) and dissolved in
freshly distilled MeOH (7 mL). HFIP (20.0 equiv.) was added. Graph-
ite electrodes were fitted to the cell and electrolysis was performed
using constant current (20–30 mA) conditions until 2.2 F/mol of
charge were passed through the cell. Then AcOH (4.0 equiv.) was
added, and electrolysis was continued for another 4.0 F/mol. After
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mixture was filtered through a silica plug. The solvent was
evaporated, and the product was purified using column chroma-
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The authors declare no conflict of interest.
Keywords: Anodic oxidation · Biomass · Electrochemistry ·
Furan · α,β-Unsaturated ester
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Manuscript received: May 18, 2021
Revised manuscript received: July 9, 2021
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