Electrochemical Synthesis of α-Ketoamides under Catalyst-, Oxidant-, and Electrolyte-Free Conditions

Article abstract of DOI:10.1021/acs.orglett.0c00387

A catalyst-, oxidant-, electrolyte-free method for the preparation of α-ketoamides through the direct electrochemical amidation of α-ketoaldehydes and amines with innocuous hydrogen as the sole byproduct at ambient temperature was developed. The present reaction features clean and mild conditions, excellent functional-group tolerance, and high atom economy and scalability, enabling facile applications in pharmaceutical chemistry.

Full text of DOI:10.1021/acs.orglett.0c00387

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Letter
Electrochemical Synthesis of α‑Ketoamides under Catalyst‑,
Oxidant‑, and Electrolyte-Free Conditions
Jin-Yang Chen, Hong-Yu Wu, Qing-Wen Gui, Xiao-Ran Han, Yan Wu, Kui Du, Zhong Cao,
Ying-Wu Lin, and Wei-Min He*
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ABSTRACT: A catalyst-, oxidant-, electrolyte-free method for the
preparation of α-ketoamides through the direct electrochemical
amidation of α-ketoaldehydes and amines with innocuous hydrogen
as the sole byproduct at ambient temperature was developed. The
present reaction features clean and mild conditions, excellent
functional-group tolerance, and high atom economy and scalability,
enabling facile applications in pharmaceutical chemistry.
he continuous understanding of sustainable development
has prompted chemists to minimize the generation of
Scheme 1. Amidation of α-Ketoaldehydes with Amines
T
chemical waste in order to comply with the 12 principles of
green chemistry.1 Traditional redox reactions usually require
the use of transition metal/organic catalysts (and/or relevant
promoters) and a stoichiometric amount of oxidants to achieve
an overall redox-neutral process, during which environmental
hazardous waste is usually generated. Electrochemical syn-
thesis, which solely uses sustainable electric current as a green
and traceless reagent for driving the redox process, can
efficiently avoid the usage of stoichiometric amounts of often
harmful oxidants or reducing agents.2 Therefore, electro-
chemical synthesis is considered to be sustainable, eco-friendly,
and safe.3
α-Ketoamides not only are valuable structural motifs present
in a broad range of natural products and synthetic
pharmaceuticals but also are used as versatile intermediates
and building blocks in organic synthesis.4 In recent years,
much effort has been dedicated to constructing such scaffolds.5
The cross-dehydrogenation coupling (CDC) of α-keto
aldehydes with amines is regarded as one of the most efficient
and straightforward protocols.5o−v However, the practicability
and atomic economy of these CDC reactions are impaired by
the required catalysts, cocatalysts, and stoichiometric oxidants,
because they inevitably lead to undesirable catalyst residues
and chemical wastes (Scheme 1a). Moreover, most of these
approaches do not work with primary amines, regardless if they
are aliphatic or (hetero)aromatic ones.
base- and electrolyte-free conditions has never been reported.
With our continuing interest in green organic synthesis,7
herein, we disclose a clean and practical electrochemical cross-
Very recently, Ke and co-workers reported a novel method
for the electrochemical synthesis of amide bond through
radical amidation of carboxylic acids with amines.6 However,
both a strong base (Cs2CO3) and supporting electrolyte
(tetrabutylammonium bromide) were needed for the electro-
chemical reaction to proceed. To the best of our knowledge,
the direct electrochemical synthesis of α-ketoamides under
Received: January 30, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00387
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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dehydrogenative coupling reaction of α-ketoaldehydes and
amines at ambient temperature, by which a series of α-
ketoamides could be synthesized under catalyst-, oxidant-,
electrolyte-free and mild conditions (Scheme 1b). In addition,
the reaction proceeded via an ionic mechanism and only
generated innocuous hydrogen as the sole side product.
At the outset of the investigation, the reaction between 2-
oxo-2-phenylacetaldehyde (1a) and 1-methylpiperazine (2a)
was taken as the template reaction to optimize the reaction
conditions (Table 1). The initial coupling reaction was
respect to both amines and α-ketoaldehydes (Scheme 2). A
variety of aliphatic and aromatic amines were allowed, and the
a b
,
Scheme 2. Reaction Scope
a
Table 1. Optimization of Reaction Conditions
b
Entry
Variation from the standard reaction conditions
Yield (%)
1
None
92
2
3
4
5
6
7
8
DMSO as the solvent
H2O as the solvent
32
trace
68
36
85
C(+)|Ni(−) instead of C(+)|Pt(−)
C(+)|C(−) instead of C(+)|Pt(−)
Pt(+)|Pt(−) instead of C(+)|Pt(−)
RVC(+)|RVC(−) instead of C(+)|Pt(−)
n-Bu4NBr
64
19
9
n-Bu4NBF4
16
10
11
12
13
14
15
LiClO4
18
75
92
92
92
N.R.
4 mA instead of 6 mA
8 mA instead of 6 mA
1.5 mmol of 2a was used
50 °C instead of rt
a
Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), MeCN (8.0 mL),
Pt plate (15 mm × 15 mm × 0.3 mm) cathode, graphite rod (6 mm)
b
No electric current
anode, I = 6.0 mA, in air, rt, 23 h. Isolated yields.
a
Conditions: Pt plate (15 mm × 15 mm × 0.3 mm) cathode, graphite
rod anode (Φ 6 mm), constant current = 6 mA, 1a (0.5 mmol), 2a
(0.5 mmol), MeCN (8 mL), in air, rt, 23 h, undivided cell. Estimated
isolated yields were mostly ≥80%. A series of bulky
heterocyclic cyclic secondary amines, including N-substituted
piperazines (3aa−3ae), morpholine (3af), and pyrrolidine
(3ag), were well compatible in the present reaction and
provided the target products in good to excellent yields. In the
reported method, it was hard to synthesize α-ketoamides from
primary aliphatic amines, likely due to the fact that the primary
amines are less reactive than the secondary amines. To our
delight, the aliphatic primary amines containing diverse chain
lengths and isomeric alkyl, phenyl, and cyclic alkyl substituents
underwent the amidation smoothly, delivering the desired
products in 79−90% yields (3ah−3ao). In addition, both the
aniline and substituted 2-aminopyridines are suitable substrates
for this transformation (3ap−3as), which are incompatible
with the conventional thermal method and the visible light
induced protocol. Next, various aromatic α-ketoaldehydes were
investigated under the optimal conditions to react with 1-
methyl piperazine. 2-Oxo-2-phenylacetaldehydes, whether the
phenyl ring was substituted with an electron-donating, an
electron-withdrawing, or a sterically hindered group, provided
the desired products in high yields (3ba−3ja). Among these,
the resulting products bearing halogens such as F, Cl, Br, and I
could be further utilized for the synthesis of more complex
organic compounds. The structure of α-ketoamides 3ea
(CCDC: 1958206) was confirmed using single crystal X-ray
diffraction as shown in Scheme 2.
b
by GC-MS. NR: no reaction.
conducted in an undivided cell with a graphite rod as the
anode and a platinum plate as the cathode under 6 mA
constant current at ambient temperature with MeCN as the
solvent. Delightedly, the targeted product 3aa was formed in
92% GC yield (Table 1, entry 1). When other solvents, such as
DMSO or water, were used instead of MeCN, a low or trace
yield of 3aa was observed (entries 2 and 3). The influence of
electrode effect was next investigated (entries 4−7), and the
results revealed that the graphite rod anode and platinum plate
cathode were the best choice for the electrode materials. When
supporting electrolytes such as n-Bu4NBr, n-Bu4NBF4, or
LiClO4 were added to the reaction system, the yield of 3aa was
severely reduced (entries 8−10). Decreasing the constant
current to 4 mA resulted in the formation of 3aa in a lower
yield (entry 11). No improvement in the transformation was
obtained when 8 mA constant current was employed (entry
12). Increasing the loading of 2a as well as reaction
temperature did not provide improved reaction efficiency
(entries 13 and 14). Changing the air atmosphere to a nitrogen
atmosphere had virtually no effect on the yield of 3aa. No
reaction occurred without a constant current, and the substrate
1a was quantitatively recovered (entry 15).
With the optimal conditions in hand, the substrate scope of
the electrochemical amidation reaction was explored with
The applicability of the present electrochemical reaction was
further tested (Scheme 3). In the small-scale synthesis, a
B
https://dx.doi.org/10.1021/acs.orglett.0c00387
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Scheme 3. Large-scale Synthesis of 3aa
Scheme 5. Proposed Mechanism
concentration of 0.0625 M of the α-ketoaldehydes was
required, and the increase of reaction substrate in concen-
tration would reduce the yield of the target product. Pleasingly,
the amidation reaction of 1a with a 10-fold increase in the
concentration of 1a gave the target product 3aa in 79% yield.
To understand the reaction mechanism, the radical capture
experiments were conducted. The addition of 2 equiv of radical
scavenger (TEMPO or BHT) had virtually no effect on the
reaction efficiency (Scheme 4), suggesting that a free-radical
Scheme 4. Radical-Capturing Experiments
gram scale with excellent efficiency. The present method has
favorable characteristics over the previous synthetic methods:
(1) the well-matched reactivity of α-ketoaldehydes and amines
avoids the usage of supporting electrolyte and traditional
heating; (2) the use of current as a green and traceless oxidant
leads to simple and clean conditions (catalyst- and oxidant-free
conditions) and harmless H2 as the sole side product; (3) in
comparison with the previous methodologies, both aliphatic
and aromatic primary amines, as well as heteroaromatic
amines, were suitable for cross-dehydrogenative coupling
reaction.
process might not be involved in the transformation.
Moreover, the redox potential of the substrates and reaction
mixture were investigated. As shown in Figure 1, the oxidation
ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge at https://pubs.acs.org/doi/10.1021/
acs.orglett.0c00387.
(PDF)
Accession Codes
CCDC 1958206 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 1. Cyclic voltammetry experiments.
peak of N-methylpiperazine (2a) (1.095 V, vs Ag/AgCl) was
lower than that of 2-oxo-2-phenylacetaldehyde (1a) (2.533 V
vs Ag/AgCl), which indicated that the oxidation of 1-
methylpiperazine (2a) should occur first under the galvano-
static mode. In addition, the oxidation peak (1.09 V, vs Ag/
AgCl) was also observed in the cyclic voltammetry experiment
of mixture of 1a and 2a.
On the basis of the mechanistic studies above and the
literature,2e,5p,8 a plausible mechanism is outlined in Scheme 5.
Initially, 2-oxo-2-phenylacetaldehyde (1a) reacted with 1-
methylpiperazine (2a) to form an iminium intermediate A,
which underwent water addition to generate a hemiaminal
intermediate B. Next, the intermediate B was oxidized to target
product 3aa via Shono oxidation by the anode. On the
cathode, the proton was reduced to the hydrogen gas.
In summary, we have developed a general and efficient
protocol for the clean preparation of various α-ketoamides (28
examples, 71−91%) through the direct electrochemical
amidation of α-ketoaldehydes and amines at ambient temper-
ature. Moreover, the amidation reaction can be conducted on a
AUTHOR INFORMATION
Corresponding Author
■
Wei-Min He − Hunan Provincial Key Laboratory of Materials
Protection for Electric Power and Transportation, Changsha
University of Science and Technology, Changsha 410114,
China; orcid.org/0000-0002-9481-6697;
Email: weiminhe2016@yeah.net
Authors
Jin-Yang Chen − College of Chemistry and Chemical
Engineering, Yangtze Normal University, Chongqing 408000,
China
Hong-Yu Wu − College of Chemistry and Chemical Engineering,
Yangtze Normal University, Chongqing 408000, China
Qing-Wen Gui − School of Chemistry and Chemical
Engineering, Hunan University of Science and Technology,
C
https://dx.doi.org/10.1021/acs.orglett.0c00387
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Xiao-Ran Han − College of Chemistry and Chemical
Engineering, Yangtze Normal University, Chongqing 408000,
China
Yan Wu − College of Chemistry and Chemical Engineering,
Yangtze Normal University, Chongqing 408000, China
Kui Du − College of Chemistry and Chemical Engineering,
Yangtze Normal University, Chongqing 408000, China
Zhong Cao − Hunan Provincial Key Laboratory of Materials
Protection for Electric Power and Transportation, Changsha
University of Science and Technology, Changsha 410114, China
Ying-Wu Lin − School of Chemistry and Chemical Engineering,
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (No. 21902014), the Basic and
Frontier Research Project of Chongqing (No. Cstc2018jcyj-
AX0051), and Hunan Provincial Natural Science Foundation
of China (No. 2019JJ20008).
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