Electrochemical esterification via oxidative coupling of aldehydes and alcohols

Article abstract of DOI:10.1016/j.tetlet.2021.152898

An electrolytic method for the direct oxidative coupling of aldehydes with alcohols to produce esters is described. Our method involves anodic oxidation in presence of TBAF as supporting electrolyte in an undivided electrochemical cell equipped with graphite electrodes. This method successfully couples a wide range of alcohols to benzaldehydes with yields ranging from 70 to 90%. The protocol is easy to perform at a constant voltage conditions and offers a sustainable alternative over conventional methods.

Full text of DOI:10.1016/j.tetlet.2021.152898

Tetrahedron Letters 68 (2021) 152898
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electrochemical esterification via oxidative coupling of aldehydes
and alcohols
⇑
Dylan Smeyne, Katherine Verboom, Maria Bryan, James LoBue, Abid Shaikh
Department of Chemistry, Georgia Southern University, 521 College of Education Drive Statesboro, GA 30458, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
An electrolytic method for the direct oxidative coupling of aldehydes with alcohols to produce esters is
described. Our method involves anodic oxidation in presence of TBAF as supporting electrolyte in an
undivided electrochemical cell equipped with graphite electrodes. This method successfully couples a
wide range of alcohols to benzaldehydes with yields ranging from 70 to 90%. The protocol is easy to per-
form at a constant voltage conditions and offers a sustainable alternative over conventional methods.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 28 December 2020
Revised 29 January 2021
Accepted 1 February 2021
Available online 12 February 2021
Keywords:
Esterification
Electrolysis
Aldehydes
Alcohols
Oxidation
Introduction
of esters, we have developed an electrochemical synthesis for the
direct coupling of aldehydes to alcohols. Electrochemistry has a
Esters represent one of the most valuable classes of compounds
that have widespread applications in pharmaceuticals, food, per-
fumes, flavors and fine chemicals [1]. Considering the wide spread
prevalence of esters, over the years, a large number of synthetic
protocols have been reported [2] and the development of novel
methods is still in the forefront of organic synthesis [3]. Tradition-
ally, the Fischer esterification method is employed starting from
carboxylic acids and alcohols under acid-catalyzed conditions [4].
To date, this remains the most efficient method for the synthesis
of esters. Apart from this, various chemical transformations involv-
ing use of coupling reagents that can be applied to a diverse pool of
carboxylic acids and alcohols have also been reported [5]. As an
alternate approach, oxidative direct coupling of aldehydes with
alcohols to form esters have also gained significant attention in
recent years [6]. A few known methods involve use of Pd, Cu, V,
and Au based heterogeneous as well as homogeneous catalysis
[7]. Additionally, hypervalent iodine reagents with suitable addi-
tives have also been reported as an oxidant combined with a metal
or in metal-free esterification [8]. Despite the development of a
number of catalytic processes, discovery of a mild catalyst-free
approach is highly desirable as the current known methods involve
toxic transition metals or complicated oxidant-additive combina-
tion [9]. In search of a catalyst-free approach for rapid production
long history for its application in organic synthesis [10], but it
has not yet been adopted into the chemical industry as a main-
⇑
Corresponding author.
E-mail address: malnu@georgiasouthern.edu (A. Shaikh).
Scheme 1. Literature overview of relevant electrocatalytic oxidative coupling of
aldehydes with alcohols.
https://doi.org/10.1016/j.tetlet.2021.152898
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

D. Smeyne, K. Verboom, M. Bryan et al.
Tetrahedron Letters 68 (2021) 152898
Table 1
Screening of reaction conditions for electrocatalytic coupling of benzaldehyde and methanol.a
Entry
Solvent
Electrolyte
Voltage (V)
% Conv.b
1
2
3
4
5
6
7
8
9
CH3OH
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
–
2
2
4
5
6
8
5
5
5
10
18
60
88
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
CH3OH:CH3CN
88
86c
trace
48
NaCl
TBAI
53
a
b
c
Reaction conditions: Benzaldehyde (2 mmol), solvent (4 mL), CH3OH:CH3CN (50:50 mixture, 4 mL), electrolyte (0.2 mmol), reaction time (5 h).
Determined by 1H NMR using residual starting material.
TBAF (0.4 mmol) was used.
stream method for esterification. The benefits of electrochemical
methods are innumerous, the most prominent of which include
them as green and sustainable [11]. A recent example of electro-
chemical synthesis of esters involves the use of N-heterocyclic car-
bene (NHC) catalysts to generate the Breslow intermediate form of
an aldehyde (Scheme 1) [12], which on further electrochemical
oxidation and coupling with an alcohol yields a corresponding
ester. The formation of the Breslow intermediate substantially low-
ers the oxidation potential and the reaction occurs at the very low
voltage of 0.1 V.
Herein, we present an electrochemical method for ester synthe-
sis utilizing commercially available aldehydes and alcohols under
catalyst-free conditions. This method produced the expected esters
electrolytically without external chemical oxidant. Since, the pro-
cess does not involve a catalyst, the reaction must be carried out
at a higher voltage (5 V) to overcome the high oxidation energy
barrier.
To further optimize the reaction conditions, we investigated the
progress of the reaction at different time intervals. Small aliquots
of the reaction mixture were sampled at various time intervals
and the conversion was monitored using 1H NMR spectroscopy.
To our surprise, reaction rate was decided by a non-linear exhibit-
ing a sigmoidal time dependence with a rapid increase in yield
observed after 5 h with 88% conversion. No further improvement
was seen even after extending the reaction time to 8 h (Figure 1).
After a systematic screening of solvent, electrolyte, current density
and time, the optimal reaction conditions were determined at CH3-
OH: CH3CN (50:50 mixture), 0.2 mmol of TBAF, 5 V and 5 h reac-
tion time resulting in an 88% conversion.
With the optimized conditions in hand, we further investigated
the compatibility of oxidative esterification protocol with various
alcohols keeping benzaldehyde as a common substrate. The results
showed that aliphatic primary alcohols such as methanol, ethanol,
propanol, butanol, pentanol, hexanol, heptanol, undecanol and
isobutanol delivered the desired products in 73–90% isolated yields
(Table 2, entries 1–9). Gratifyingly, the reaction yield was consis-
tent with a secondary alcohol such as isopropanol (entry 10),
although, tert-butanol provided the relatively low yield of 71% (en-
try 11). Moreover, aliphatic alcohols including those with cyclo-
pentyl and cyclohexyl rings could also be employed to afford the
target compounds in good but somewhat lower yields (entries 12
and 13). Aliphatic alcohol with oxygen heteroatom also produced
Results and discussion
To test the above hypothesis, we began our study by optimizing
the electrolysis conditions for the synthesis of esters using ben-
zaldehyde as the model substrate along with tetrabutylammonium
fluoride (TBAF) as an electrolyte and methanol as solvent at room
temperature. The reaction was conducted at a constant voltage in
an undivided cell equipped with graphite electrodes. The expected
ester product was obtained in 88% NMR yields. Considering the
inexpensive and environmentally benign reaction medium,
together with the catalyst-free condition, this process is highly
desirable and environmentally valuable for direct synthesis of
esters. The electrolysis at various current densities largely affected
the product formation. The desired product was observed using
methanol both as a reactant and solvent, albeit with trace yields
at a lower current density (Table 1, entry 1). Introduction of ace-
tonitrile as solvent boosted the product yield to 18% at relatively
low voltage of 2 V (entry 2). However, performing electrolysis at
a higher voltage of 4 V jumped the product yield to 60% (entry
3). Further increase in voltage to 5 V, produced the product in
88% yield (entry 4). The higher current potential 6 V (entry 5)
and 8 V along with higher electrolyte concentration (0.4 mmol)
did not improve the reaction yield (entry 6). When the reaction
was performed without an electrolyte, a trace amount of product
was observed (entry 7). Introduction of electrolytes such as, NaCl
and TBAI in place of TBAF provided lower yields of the desired pro-
duct (entries 8, 9).
Figure 1. Reaction progress at various time intervals. Benzladehyde (2 mmol), TBAF
(0.2 mmol), CH3OH:CH3CN (50:50 mixture, 4 mL), Voltage (5 V). Conversion deter-
mined by 1H NMR.
2

D. Smeyne, K. Verboom, M. Bryan et al.
Tetrahedron Letters 68 (2021) 152898
Table 2
Entry
1
Alcohol
Product, Yield (%)b
Entry
9
Alcohol
Product, Yield (%)b
H3C–OH
3a, 85
3i, 85
2
3
3b, 91
3c, 85
10
11
3j, 83
3k, 71
4
5
3d, 90
3e, 84
12
13
3l, 79
3m, 75
6
7
8
3f, 77
3g, 81
3h, 73
14
15
16
3n, 78
3o, 86
3p, 70
aReaction conditions: Benzaldehyde 1 (2 mmol), TBAF (0.2 mmol), ROH:CH3CN (50:50 mixture, 4 mL).
b
Isolated yeilds after column chromatography.
Table 3
Entry
Product, Yield (%)b
Entry
Product, Yield (%)b
1
2
3
4a, 83
4b, 91
4c, 85
7
8
9
4g, 86
4h, 90
4i, 81
4
5
4d, 78
4e, 88
10
11
4j, 78
4k, 80
6
4f, 77
12
4l, 82
aReaction conditions: Aldehyde 1 (2 mmol), TBAF (0.2 mmol), CH3OH:CH3CN (50:50 mixture, 4 mL).
b
Isolated yeilds after column chromatography.
3

D. Smeyne, K. Verboom, M. Bryan et al.
Tetrahedron Letters 68 (2021) 152898
a successful transformation (entry 14). In addition, allyl alcohol
was also found suitable for this reaction (entry 15). Especially, an
aryl alcohol such as phenol was also been well tolerated, giving
the desired product in 70% yield (entry 16).
of aldehyde 9 via a single electron transfer produced a radical
cation species 10. Subsequently, loss of hydrogen atom produced
a stable acylium cation 11, which on further nucleophilic addition
with an alcohol could form an intermediate cation 12. Upon further
deprotonation of the acidic proton would afford a stable ester
product. Formation of H2 gas was observed at the cathode. Also,
during anodic oxidation, generation of the radical cation is facili-
tated by the stable carbonyl, making it attractive for nucleophilic
attack to form an intermediate product 12 which on loss of hydro-
gen provide the expected product 13. To gain further mechanistic
insight of the reaction pathway, a control experiment using radical
scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was car-
ried out. When TEMPO was added to the electrochemical cell, we
did not observe the formation of the desired product 13, which
confirms that the reaction probably involves a single electron
transfer (SET) pathway.
With a broad scope of alcohols established, we were particu-
larly interested in extending the study with various aliphatic and
aromatic aldehydes (Table 3). We have shown that a variety of sub-
stituted aldehydes react similarly to benzaldehyde, affording the
corresponding esters in good to excellent yields. Aryl aldehydes
bearing electron-withdrawing groups, such as 4-Cl, 3-Br and 4-
CN, or electron-donating groups, such as 4–CH3 and 4-OCH3 were
subjected to esterification with methanol, and proceeded smoothly
(entries 1–5). Bulky aryl aldehydes such as 2-naphthaldehyde and
4-phenyl benzaldehyde are proved to be effective to obtain good
yields (entries 6 and 7). Surprisingly, heteroaryl aldehyde such
as, indole-5-carboxyldehyde was also found to undergo esterifica-
tion to form the corresponding product in 81% yield (entry 9).
Moreover, aliphatic aldehydes including those with hexyl, cyclo-
hexyl and isobutyl could also be employed to afford the target
compounds in good yields (entries 10–12).
Conclusion
To further demonstrate the potential of this method, we per-
formed the reaction with phthalaldehyde 5, with methanol under
standard reaction conditions to obtain dimethyl phthalate 6, in
77% yeild (Scheme 2a). Interestingly, when ethylenediol 7 was sub-
jected to the oxidative coupling with benzaldehyde 1, predomi-
nantly a dibenzoate 8, product formed in 89% yield. The use of
excess ethylenediol 7 provided a mixture of mono and di-ester
products. In order to obtain the dibenzoate product 8 exclusively,
the reaction conditions were modified and excess of benzladyhyde
1 (2.2 eqv.) was used as compared to ethylenediol 7.
In summary, we have demonstrated that the electrocatalytic
oxidative coupling can be used to prepare esters in good to excel-
lent yields starting from aldehydes and alcohols at room tempera-
ture under mild conditions. The protocol has been shown to be
useful for accessing esters form alkyl/aryl aldehydes and alcohols.
Compared with the existing procedures for the synthesis of esters,
this protocol has several advantages including, the use of environ-
mental benign solvent, simple operation and performed under an
air atmosphere. Also, our method does not make use of toxic
metal-based catalysts, costly coupling reagents and chemical oxi-
dants; instead, electrical energy is used as an oxidant.
According to the above results and previous literature [13] a
plausible mechanism is depicted in Scheme 3. Anodic oxidation
Scheme 2. (a) Oxidative coupling of phthalaldehyde 5 with methanol. (b) Formation of dibenzoate 8, via coupling of benzaldehyde 1 (233 mg, 2.2 eqv.) and ethylene glycol 7
(62 mg, 1 mmol).
Scheme 3. A plausible mechanism for the electrocatalytic oxidative coupling of aldehydes with alcohols.
4

D. Smeyne, K. Verboom, M. Bryan et al.
Tetrahedron Letters 68 (2021) 152898
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Declaration of Competing Interest
(c) A.B. Powell, S.S. Stahl, Org. Lett. 15 (2013) 5072–5075;
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
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Acknowledgments
Financial support provided by Georgia Southern University, Col-
lege Office of Undergraduate Research (COUR) award and GSU-
honors program are gratefully acknowledged.
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Appendix A. Supplementary data
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Supplementary data (detailed experimental procedure and
characterization of all the products) to this article can be found
online at https://doi.org/10.1016/j.tetlet.2021.152898.
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