Integrated Flow Synthesis of α-Amino Acids byIn SituGeneration of Aldimines and Subsequent Electrochemical Carboxylation

Article abstract of DOI:10.1021/acs.joc.1c00821

The synthesis of α-amino acids was carried out in a continuous flow system. In this system, aldimines were efficiently generatedin situvia the dehydration-condensation of aldehydes with anilines in a desiccant bed column filled with 4 ? molecular sieves d

Full text of DOI:10.1021/acs.joc.1c00821

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Article
Integrated Flow Synthesis of α‑Amino Acids by In Situ Generation of
Aldimines and Subsequent Electrochemical Carboxylation
Yuki Naito, Yuto Nakamura, Naoki Shida, Hisanori Senboku, Kenta Tanaka,* and Mahito Atobe*
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ABSTRACT: The synthesis of α-amino acids was carried out in a
continuous flow system. In this system, aldimines were efficiently
generated in situ via the dehydration−condensation of aldehydes
with anilines in a desiccant bed column filled with 4 Å molecular
sieves desiccant, followed by reaction with CO2 in an electro-
chemical flow microreactor to afford the α-amino acids in high to
moderate yields. The present system can provide α-amino acids
without using stoichiometric amounts of metal reagents or highly
toxic cyanide reagents.
amount of reagent waste is usually generated after the reaction.
In this context, electrochemical carboxylation represents an
attractive alternative to conventional chemical carboxylation
methods (Figure 1C).7
INTRODUCTION
■
The organic synthesis of valuable products has thus far been
based on reactions involving low-cost and readily available raw
materials. However, in the multistep synthesis, the step-by-step
synthetic method leads to some problems such as large
amounts of organic solvent waste and time-consuming
isolation procedures.1 By contrast, a flow reactor enables
simple reactions to be integrated with complete molecular
transformations that comprise numerous steps without
isolating intermediates.2 Therefore, integrated synthesis in a
single operation in a flow reactor system can minimize solvent
waste and time consumption. Consequently, numerous
conventional step-by-step syntheses have recently been
replaced with integrated synthetic methods in a flow reactor.3
α-Amino acids are important and essential compounds in
foods, drugs, cosmetics, and other products.4 Therefore, the
synthesis of α-amino acids has long been an important research
topic. Some natural α-amino acids can be prepared on an
industrial scale using a conventional fermentation process;5
however, synthesizing unnatural α-amino acids using this
process is difficult. Therefore, chemical synthetic routes to
unnatural α-amino acids need to be developed. Although many
chemical synthetic routes to α-amino acids have been
developed, the α-carboxylation of amine derivatives with
CO2 is regarded as a reasonable and economical route because
the carboxy unit can be derived directly from inexpensive and
easily handled CO2.4c,6 For a carboxylation reaction with
poorly reactive CO2, the generation of highly reactive carbon
nucleophiles from amine derivatives is necessary. Hence,
stoichiometric quantities of basic organolithium reagents
(Figure 1A)6a−g and difficult-to-handle metal reductants
(Figure 1B)6g have been traditionally used; moreover, a large
In electrochemical carboxylation, highly reactive carbon
nucleophiles are readily generated at cathodes and then react
with CO2 immediately to provide the corresponding α-amino
acids without handling sensitive, expensive, and toxic reagents.
To date, several groups have demonstrated the synthesis of α-
amino acids via the electrochemical carboxylation of
aldimines.7a−c Moreover, in our previous work, we reported
the electrochemical carboxylation of several aldimines in a flow
microreactor to afford the corresponding α-amino acids in
good to moderate yields.8 However, numerous aldimines are
moisture sensitive and not sufficiently stable for isolation.9 In
addition, some aldimines are readily tautomerized to the
corresponding enamines. Therefore, the use of aldimines as
substrates for electrochemical carboxylation appears to be less
versatile, warranting the development of an alternative
approach. To address this challenge, we developed an
integrated synthesis in a single operation in a flow reactor
system. We expected the integration of aldimine generation via
condensation of the aldehyde and amine substrates and
Special Issue: Electrochemistry in Synthetic Organic
Chemistry
Received: April 13, 2021
https://doi.org/10.1021/acs.joc.1c00821
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© XXXX American Chemical Society
A

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Figure 1. Approaches for carboxylation of amine derivatives with CO2.
Figure 2. Schematic representation of the electrochemical carboxylation of in situ generated aldimines in a continuous flow system.
subsequent electrochemical carboxylation with CO2 to
represent an elegant solution to the aforementioned challenge.
Herein, we report the first in situ generation of aldimines and
subsequent electrochemical carboxylation for the efficient
synthesis of α-amino acids in a continuous flow system.
Factors such as the desiccant material, supporting electrolyte,
and current density were optimized to establish a highly
efficient and green α-amino acid synthesis system.
reaction mixture is a serious drawback. In sharp contrast,
because of the rapid acidification of unstable carboxylate ions
B in the flow operation, the electrochemical carboxylation
would be completed without decomposition, even in the
absence of a sacrificial anode.
Prior to demonstrating the integrated synthetic reaction, we
examined the dehydration−condensation of benzaldehyde 1a
with aniline 2a in two types of desiccant-bed columns with
different dimensions to determine which column was better
suited for the present reaction system. In these experiments, 4
Å molecular sieves (4 Å MS) were used as a desiccant and
were filled into the columns (Table 1). As shown in entry 1,
the use of a column 10 cm long and with a 1.5 cm inner
diameter gave the desired aldimine 3a in 87% yield. In this
case, the corresponding amount of unreacted aldehyde 1a and
aniline 2a were also recovered. By contrast, aldimine 3a was
obtained in 98% yield when a column 20 cm long and with a
1.0 cm inner diameter (entry 2) was used. Although the
volumes of both columns were approximately the same and the
amount of desiccant filled was also approximately the same, the
reaction using the longer column proceeded more efficiently.
In addition, such an excellent yield was maintained even when
the concentration of the reaction substrates was increased by
RESULTS AND DISCUSSION
■
To realize our objective, as shown in Figure 2, we fabricated a
flow microreactor for a model integrated synthetic reaction
comprising three stages: the aldimine A formation stage
involving the dehydration−condensation of aldehydes with
anilines in a desiccant bed column, the electrochemical
carboxylation stage for the generation of carboxylate ions B
in an electrochemical flow microreactor, and the acidification
stage for the rapid protonation of B with 1 M HCl aq. For
electrochemical carboxylation to proceed in a conventional
batch-type reactor, metal ions generated at sacrificial anodes
(e.g., Mg and Al anodes) are generally required in order to
stabilize the unstable carboxylate ions.7b−d However, from a
green chemistry perspective, metal-ion contamination in the
B
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column dimensions, the use of 4 Å MS, which has larger pores
than 3 Å MS, further increased the yield of 3a (98% yield,
entry 4). Therefore, 4 Å MS appears to be a promising
desiccant material for dehydration−condensation; 4 Å MS was
selected as the desiccant for this reaction system and was used
in the subsequent experiments.
Table 1. Effect of Column Type and Concentration of
Benzaldehyde (1a) on the Yield of Benzylideneaniline (3a)
in the Dehydration−Condensation of 1a with Aniline (2a)
a
We next investigated the model integrated synthetic reaction
in the flow microreactor system illustrated in Figure 2. In this
demonstration, the previous dehydration−condensation reac-
tion of 1a with 2a was carried out using the longer column (20
cm length and 1.0 cm inner diameter) filled with 4 Å MS, and
the subsequent electrochemical carboxylation of 3a was
conducted with Bu4NClO4 electrolyte in THF at different
current densities (Table 3, entries 1−4). Since THF is a
reduction-resistant solvent, it is expected not to interfere with
the reduction of 3a. On the other hand, it is relatively easy to
be oxidized, so sacrificial anodic oxidation of THF is expected
to prevent the oxidation of the product 5a. The electrolysis at
6.3 mA cm−2 smoothly proceeded to afford the desired α-
amino acid 5a in 65% yield (entry 1). In this case, N-
phenylbenzylamine was also obtained as a side product in
∼24% yield, and unreacted aldimine 3a was recovered to some
extent. The yield of 5a reached 74% at 12.7 mA cm−2 (entry 2)
but decreased at current densities greater than 19 mA cm−2
(entries 3 and 4). This decrease is attributed to a competing
CO2 reduction reaction in the electrochemical carboxylation
step.
The rate of the nucleophilic reaction between the radical−
anion intermediate generated reductively from aldimine 3a and
CO2 can be accelerated by using a supporting electrolyte with
larger cation because this leads to fewer ion-pair interactions
with the radical−anion intermediate. To confirm this
conjecture, we carried out electrochemical carboxylation
using a different supporting electrolyte with larger cations:
tetrahexylammonium (Hex4N+) (Table 3, entry 5). As
expected, the use of Hex4NClO4 further increased the yield
of the desired product 5a.
column length
(cm)
diameter
(cm)
concentration of 1a
yield of 3a
b
entry
(M)
(%)
1
2
3
10
20
20
1.5
1.0
1.0
0.12
0.12
0.18
87
98
95
a
Experimental conditions: flow rate, 7.5 mL h−1; flow solution, 0.14
M Bu4NClO4 in THF; substrate, benzaldehyde 1a (1 equiv) and
aniline 2a (1.2 equiv); desiccant, 11 g 4 Å MS. Determined by
b
HPLC.
1.5 times (entry 3). This result is attributed to the larger
theoretical plate number exhibited by the longer column.
The dehydration−condensation reaction of 1a with 2a was
subsequently carried out using the longer column (20 cm long
and 1.0 cm inner diameter) filled with different desiccants
(Table 2). When the reaction was carried out with MgSO4 as
Table 2. Effect of Desiccant Type on Yield of
Benzylideneaniline (3a) in the Dehydration−Condensation
of Benzaldehyde (1a) with Aniline (2a)
a
b
entry
desiccant type
yield of 3a (%)
1
2
3
4
MgSO4
Na2SO4
3 Å MS
4 Å MS
19
21
83
98
a
Experimental conditions: flow rate, 7.5 mL h−1; flow solution, 0.14
M Bu4NClO4 in THF; substrate, 0.12 M benzaldehyde (1a) and
0.144 M aniline (2a). Determined by HPLC.
b
Finally, to demonstrate the general applicability of the
proposed continuous flow system, we investigated the use of
various aldehydes and anilines as starting substrates (Table 4).
In parallel with a demonstration of the integrated synthesis, the
yield of nonenolizable and isolable aryl-substituted aldimine
intermediates formed after the dehydration−condensation
the desiccant, the desired aldimine 3a was obtained in a 19%
yield (entry 1). The use of Na2SO4 did not improve the yield
of 3a (entry 2). By contrast, the reaction proceeded smoothly
with molecular sieves 3A (3 Å MS) to give 3a in 83% yield
(entry 3). As shown in the data from the investigation of
Table 3. Effect of the Supporting Electrolyte and Current Density on the Yield of α-Amino Acid 5a in the Integrated
a
Synthesis
b
b
entry
supporting electrolyte
current density (mA cm−2
)
yield of 3a (%)
yield of 5a (%)
1
2
3
4
5
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Hex4NClO4
6.30
12.7
19.0
25.0
12.7
98
98
98
98
95
65
74
71
62
81
a
Experimental conditions: cathode, glassy carbon plate; anode, Pt plate; charge passed, 8 F mol−1 (25 mA cm−2), 6 F mol−1 (19 mA cm−2), 4 F
mol−1 (12.7 mA cm−2), 2 F mol−1 (6.3 mA cm−2); electrode distance, 20 μm, desiccant; 4 Å MS, 11 g. Inlet 1: solvent, THF; substrate, 0.12 M
benzaldehyde and 0.144 M aniline; supporting electrolyte, 0.14 M; flow rate, 3.3 mL h−1. Inlet 2: solution, THF saturated with CO2; supporting
b
electrolyte, 0.14 M; flow rate, 7.5 mL h−1. Determined by HPLC.
C
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a
Table 4. Integrated Synthetic Reaction Using Aryl-Substituted Aldehydes with Anilines in a Continuous Flow System
a
Experimental conditions: cathode, glassy carbon plate; anode, Pt plate; charge passed, 4 F mol−1; current density, 12.7 mA cm−2; electrode
distance, 20 μm; desiccant, 4 Å MS, 11 g. Inlet 1: solvent, THF; substrate, 0.12 M aldehyde (1) and 0.144 M aniline (2); supporting electrolyte,
0.14 M Hex4NClO4; flow rate, 3.3 mL h−1. Inlet 2: solution, THF saturated with CO2; supporting electrolyte, 0.14 M Hex4NClO4; flow rate, 7.5
b
mL h−1. Determined by HPLC.
reaction was also confirmed by collecting samples of the
reaction solutions at the outlet of the desiccant column and
analyzing them (Table 5). The reaction of benzaldehyde 1a
with aniline 2a provided the desired α-amino acid 5a in 81%
yield (Table 4, entry 1) via the generation of aldimine 3a in
98% yield (Table 5, entry 1). The dehydration−condensation
of benzaldehyde bearing a methoxy group (1b) with aniline 2a
gave the corresponding aldimine 3b in excellent yield (92%,
Table 5, entry 2); the subsequent carboxylation of 3b afforded
the α-amino acid 5b in 89% yield (Table 4, entry 2). In
general, the increase in electron density on the carbon atom of
the CN bond enhances the nucleophilic reactivity of the
D
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a
Table 5. Dehydration−Condensation of Various Benzaldehydes and Anilines in a Desiccant Bed Column Filled with 4 Å MS
a
Experimental conditions: flow rate, 7.5 mL h−1; flow solution, 0.14 M Hex4NClO4 in THF; substrate, 0.12 M aldehyde (1) and 0.144 M aniline
b
c
(2). Determined by HPLC. Determined by GC.
radical−anion intermediates. Therefore, the carboxylation of
3b bearing an electron-donating group (CH3O group) would
proceed very smoothly. The aniline bearing a methoxy group
(2b) was also a good substrate for the formation of amino acid
5c as a final product (65%, Table 4, entry 3). In our previous
work, we carried out the electrochemical carboxylation of 3c as
a single reaction.8 At that time, almost no product 5c was
obtained. However, in the present work, before the integrated
reaction, the supporting electrolyte was sufficiently dried to
prevent the deactivation of the radical−anion intermediate by a
small amount of water dissolved in the medium. In addition,
because the desiccant column in the former process not only
drives the dehydration−condensation reaction but also further
dehydrates the reaction medium, it positively affects the latter
carboxylation reaction. The dehydration−condensation of
benzaldehyde bearing a CF3 group (1c) with aniline 2a
provided the corresponding aldimine 3d in good yield (76%,
Table 5, entry 3). However, the subsequent carboxylation of 3c
resulted in a lower yield of the corresponding amino acid 5d
(43%, Table 4, entry 3). This lower yield is attributed to a
decrease in electron density on the carbon atom of the CN
bond, which diminished the nucleophilic reactivity of the
radical−anion intermediate toward CO2. When the reaction
was carried out with aniline bearing a CF3 functionality (2c),
the corresponding amino acid 5e was not obtained at all
(Table 4, entry 5). In this case, aldimine 3e formed from the
former dehydration−condensation of benzaldehyde 1a with
aniline 2c was hardly obtained (9%, Table 5, entry 5).
In the aforementioned investigations, nonenolizable and
isolable aryl-substituted aldimine intermediates were involved
in the integrated synthesis reactions. However, in the case of
entries 6−8, we carried out integrated synthesis reactions that
involve more demanding alkyl-substituted aldimine intermedi-
ates derived from alkyl-substituted aldehydes. Alkyl-substituted
aldimines are a class of intermediates known to not be readily
isolated and purified and are typically prone to decom-
position.10 Therefore, these investigations are also important to
confirm the versatility of the integrated synthesis system. The
reaction of alkyl-substituted aldehyde 1d with aniline 2a
provided the desired α-amino acid 5f in 73% yield (Table 4,
entry 6). Similarly, the integrated reaction of alkyl-substituted
aldehyde 1e with aniline 2a proceeded to afford the desired α-
amino acid 5g in reasonable yield (41%) (Table 4, entry 7).
However, when the alkyl-substituted aldehyde 1f was used as
the starting substrate, the corresponding α-amino acid 5h was
obtained in low yield (9%) (Table 4, entry 8). This result is
ascribed to poor reactivity in the dehydration−condensation
because a large amount of unreacted aldehyde 1f and aniline
2a were recovered from the solution that passed through the
desiccant bed column.
These generality experiments show that this integrated
synthesis system is extremely useful in controlling reactions
involving unstable aldimines and their rapid use in subsequent
E
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(Figure S1). A spacer (20 μm thickness double-faced adhesive type)
was used to leave a rectangular channel exposed, and the two
electrodes were simply sandwiched together (area of the two
electrodes: 10 × 30 mm2). After Teflon tubing was connected to
the inlets and outlet, the reactor was sealed with epoxy resin (Figure
S2).
electrochemical carboxylation, although there are a few
exceptions.
CONCLUSIONS
■
We demonstrated the synthesis of α-amino acids in a
continuous flow system. In the present system, aldimines
were generated in situ from the dehydration−condensation of
aldehydes with anilines in a desiccant bed column filled with a
desiccant and subsequently reacted with CO2 in an electro-
chemical flow microreactor to afford the α-amino acids. By
examining various desiccant materials, 4 Å molecular sieves (4
Å MS) were found to be the most efficient in providing
aldimines. In addition, the structure of a desiccant bed column
was also optimized. For the electrochemical carboxylation step,
a supporting electrolyte with a large cation such as Hex4N+ was
also found to be effective. From the generality experiments, it
was also found that this integrated synthesis system was
extremely useful in controlling reactions involving in situ
generated aldimines and their rapid use in subsequent
electrochemical carboxylation, although there are a few
exceptions. This process does not require the handling of
stoichiometric amounts of metal reagents or highly toxic
cyanide reagents. Therefore, in order to further improve this
reaction system, we will continue to optimize the structure of
the electrochemical flow microreactor. In addition, we plan to
investigate ortho/meta-substituents and electrophiles other
than CO2 in order to expand the scope of the reaction.
Continuous-Flow Microreactor System. The continuous-flow
microreactor system for the model integrated synthetic reaction was
fabricated as illustrated in Figure 2. Dehydration−condensation of
aldehydes with anilines was performed with two types of desiccant
bed columns with different dimensions (glass column 178170 (20 cm
long with 1.0 cm inner diameter) and glass column 129450 (10 cm
long with 1.5 cm inner diameter), EYELA Glass). The dehydration−
condensation reaction was carried out by introducing a solution (0.14
M Bu4NClO4 or Hex4NClO4 in THF) containing aldehydes and
anilines into the column via a syringe pump (KdScientific KDS100,
Muromachi Kikai). Electrolyte (Bu4NClO4 or Hex4NClO4) was also
added to the THF solution in advance for subsequent electrochemical
carboxylation. CO2 solution was prepared by bubbling analytical
grade CO2 in 0.14 M Bu4NClO4 or Hex4NClO4 in THF for 30 min.
As a micromixer in which the aldimines A formed by the
dehydration−condensation reaction and CO2 solution were mixed,
a T-type micromixer (SUS316L ⌀1.0) manufactured by Sankoh Seiki
Co. was used. Thus, a solution of aldimines A and a solution of CO2
were introduced into the T-type micromixer via a syringe pumping
technique (the KDS100 syringe pump was also used to flow CO2
solution). The solution mixture was then introduced into the
electrochemical flow microreactor, where the electrochemical
carboxylation of aldimines A was carried out. The electrochemical
carboxylation was conducted in constant-current mode using a
galvanostat (HABF-501A, Hokuto Denko).
General Procedure for the Synthesis of Aldimines (3c−e) as
Authentic Samples for HPLC Analysis. Aldimines (3c−e) were
synthesized according to a method reported in the literature.11 To a
solution of the corresponding aromatic aldehyde (5.0 mmol, 1.0
equiv) in dry toluene (5.0 mL) and 4 Å MS (1.0 g) was added the
corresponding aromatic amine (5.0 mmol, 1.0 equiv). The reaction
mixture was heated at reflux for 24 h, cooled to room temperature
(rt), and filtered through Celite. The solvent was removed under
reduced pressure. The residue was purified by column chromatog-
raphy using silica gel (hexane/Et2O 50:1) to afford the products (3c−
e).
EXPERIMENTAL SECTION
■
General Methods. Commercial reagents and solvents were used
without further purification. Benzylideneaniline (3a) and N-(4-
methoxybenzylidene)aniline (3b) were purchased from Tokyo
Chemical Industry. A silicon oil bath was used as a heat source for
the reactions. High-performance liquid chromatography (HPLC)
analyses of 3a, 3b, 3d, and 3e were performed on an apparatus with a
liquid chromatography (LC) pump (LC-20AD, Shimadzu), a UV
detector (SPD-10A, Shimadzu), and a column (Mightysil RP-18 GP
Aqua 250−4.6 (5 μm), Kanto Kagaku) using a mixture of n-hexane/
isopropyl alcohol/diethylamine as a mobile phase. Gas chromatog-
raphy (GC) analysis of 3c was performed using a Shimadzu gas
chromatograph (GC2014) equipped with a capillary column (0.50
μm, 30.0 m, 0.25 mm ID (CP-Sil 8 CB for amines, Agilent
Technologies)). HPLC analyses for 5a, 5b, 5c, 5d, and 5e were
performed with a column (Inertsil ODS-4, GL Sciences) using a
mixture of H2O/MeCN/H3PO4 as a mobile phase. HPLC analyses of
5f, 5g, and 5h were performed with a column (Inertsil ODS-4, GL
Sciences) using a mixture of H2O/tetrahydrofuran (THF)/H3PO4 as
a mobile phase. All chromatograms were recorded using an LC or GC
workstation (LabSolutions DB, Shimadzu). Infrared (IR) spectra were
N-Benzylidene-4-methoxyaniline (3c).12 Yellow solid, 0.99 g, 79%
yield. 1H NMR (500 MHz; CDCl3): δ 8.49 (s, 1H), 7.89 (dt, J = 3.94,
2.92 Hz, 2H), 7.48−7.45 (m, 3H), 7.23−7.26 (m, 2H), 6.96−6.92
(m, 2H), 3.84 (s, 3H).
N-(4-(Trifluoromethyl)benzylidene)aniline (3d).13 Yellow solid,
0.85 g, 81% yield. 1H NMR (500 MHz; CDCl3): δ 8.52 (s, 1H), 8.03
(d, J = 7.88 Hz, 2H), 7.74 (d, J = 7.88 Hz, 2H), 7.44−7.40 (m, 2H),
7.29−7.20 (m, 1H).
N-Benzylidene-4-trifluoromethylaniline (3e).12 Yellow solid, 0.73
g, 59% yield. 1H NMR (500 MHz; CDCl3): δ 8.43 (s, 1H), 7.93−7.90
(m, 2H), 7.65 (dd, J = 8.83, 0.63 Hz, 2H), 7.53−7.48 (m, 3H), 7.25
(s, 2H).
1
recorded using an IRAffinity-1 (Shimadzu). H NMR spectra were
recorded on a Bruker DRX-500 (500 MHz) spectrometer using
tetramethylsilane (TMS) as an internal standard with the solvent
resonance (CDCl3: δ 7.26, DMSO: δ 2.50). Data are reported as
follows: chemical shift, multiplicity (s = singlet, d = doublet, t =
triplet, q = quartet, m = multiplet), coupling constants, integration.
13C NMR spectra were recorded on a Bruker DRX-500 (126 MHz)
spectrometer with complete proton decoupling. Chemical shifts were
reported in ppm from the TMS internal standard with the solvent
resonance (DMSO: δ 39.5). 19F NMR spectra were recorded on a
JEOL Resonance ECA-500 (471 MHz) spectrometer with complete
proton decoupling. Chemical shifts were reported in ppm from a
fluorobenzene internal standard (δ −113.15) in dimethyl sulfoxide
(DMSO).
General Procedure for the Synthesis of α-Amino Acids (5a,
5b, 5d, 5e) as Authentic Samples for HPLC Analysis. α-Amino
acids (5a, 5b, 5d, and 5e) were synthesized according to a method
reported in the literature.14 A solution of the corresponding aromatic
aldehyde (5.0 mmol, 1.0 equiv) and aromatic amine (5.0 mmol, 1.0
equiv) in MeOH (20 mL) was stirred for 10 min at rt. After the
reaction, a mixture of pyruvic acid (5.0 mmol, 1.0 equiv) and
cyclohexyl isocyanide (5.0 mmol, 1.0 equiv) was added, and the
resultant mixture was stirred at rt for 48 h. The reaction mixture was
then treated with a solution of KOH (6.0 mmol, 1.2 equiv) in MeOH
(4.0 mL). After the mixture was stirred for 2 h at room temperature,
the solvent was removed under reduced pressure, and the residue was
partitioned between H2O (30 mL) and CHCl3 (30 mL). The layers
were separated, and the aqueous phase was acidified to pH 4 by the
addition of formic acid. The resultant suspension was filtered, and the
Electrochemical Flow Microreactor. The reactor was con-
structed from an anode plate (Pt, 30 mm width, 30 mm length) and a
cathode plate (glassy carbon (GC), 30 mm width, 30 mm length)
F
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collected solid product was washed with water and dried to give the α-
amino acids (5a, 5b, 5d, and 5e).
pH was adjusted to 6−7. The acidified solution was extracted with
EtOAc. The organic layer was separated, and the aqueous layer was
acidified with hydrochloric acid to adjust the pH to 5−6. The aqueous
layer was extracted with EtOAc again. The organic layer was
separated, and the aqueous layer was adjusted to a pH of 4−5. The
aqueous layer was extracted with EtOAc again. The combined organic
layers were washed with brine and water, and the solvent was
removed by evaporation. The crude product was purified by column
chromatography (hexane/EtOAc = 5:1 to 1:1) to give the α-amino
acids (5g and 5h).
α-(Phenylamino)benzeneacetic Acid (5a).14 Yellow solid, 0.30 g,
1
26% yield. H NMR (500 MHz, DMSO-d6): δ 7.50 (d, J = 7.30 Hz,
2H), 7.35 (t, J = 7.41 Hz, 2H), 7.31−7.26 (m, 1H), 6.64 (d, J = 7.57
Hz, 2H), 6.53 (t, J = 7.02 Hz, 1 H), 5.06 (s, 1 H).
4-Methoxy-α-(phenylamino)benzeneacetic Acid (5b).14 Yellow
solid, 0.18 g, 14% yield. 1H NMR (500 MHz; DMSO-d6): δ 6.91 (d, J
= 8.51 Hz, 2H), 6.63 (d, J = 8.20 Hz, 2H), 6.53 (t, J = 7.09 Hz, 1H),
4.99 (s, 1H), 3.76−3.70 (m, 3H).
N-Phenylleucine (5g).17 Brown solid, 0.68 g, 22% yield. H NMR
1
α-(Phenylamino)-4-(trifluoromethyl)benzeneacetic Acid (5d).
1
(500 MHz, DMSO-d6): δ 7.06 (t, J = 7.15 Hz, 2H), 6.58−6.50 (q,
3H), 3.84 (dd, J = 8.67, 5.52 Hz, 1H), 1.85−1.73 (m, 1H), 1.66−1.53
(m, 2H), 0.95 (d, J = 6.62 Hz, 3H), 0.88 (d, J = 6.62 Hz, 3H).
2-(Phenylamino)butyric Acid (5h).15 Brown solid, 1.9 g, 69%
Yellow solid, 0.33 g, 22% yield. H NMR (500 MHz; DMSO-d6): δ
7.77−7.72 (m, 4H), 7.72−7.63 (m, 1H), 7.04 (t, J = 8.04 Hz, 2H),
6.66 (d, J = 7.88 Hz, 2H), 6.56 (t, J = 7.25 Hz, 1H), 5.26 (s, 1H).
13C{1H} NMR (126 MHz; DMSO-d6): δ 172.0, 146.6, 128.7, 128.2
(q, J = 31.46 Hz), 128.2, 125.2 (q, J = 3.94 Hz), 124.2 (q, J = 271.9
Hz), 116.7, 113.1, 72.1, 59.42; 19F NMR (471 MHz; DMSO-d6): δ
−61.1 (s, 3F). IR (KBr): 2934, 1717, 1589, 1496, 1420, 1379, 1328,
1259, 1167, 1126, 1070, 760 cm−1. HRMS (ESI): m/z calcd for
C15H13NO2F3 [M + H]+, 296.0893; found, 296.0917.
1
yield. H NMR (500 MHz, DMSO-d6): δ 7.06 (t, J = 7.41, 2H),
6.59−6.52 (m, 3H), 3.79 (t, J = 6.62 Hz, 1 H), 1.83−1.68 (m, 2 H),
0.97 (t, J = 7.25 Hz, 3H).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00821.
■
α-[(4-Trifluoromethylphenyl)amino]benzeneacetic Acid (5e).
sı
1
Yellow solid, 0.65 g, 44% yield. H NMR (500 MHz; DMSO-d6): δ
7.51 (d, J = 7.88 Hz, 2H), 7.41−7.30 (m, 6H), 6.98 (d, J = 6.62 Hz,
1H), 6.79 (d, J = 8.20 Hz, 2H), 5.20 (d, J = 6.94 Hz, 1H). 13C{1H}
NMR (126 MHz; DMSO-d6): δ 172.3, 150.0, 137.8, 128.5, 127.9,
127.4, 126.0 (q, J = 270.02 Hz), 125.1 (q, J = 3.97 Hz), 116.3 (q, J =
32.07 Hz), 112.4, 59.3; 19F NMR (471 MHz; DMSO-d6): δ −59.4 (s,
3F). IR (KBr): 3422, 3273, 1719, 1618, 1533, 1329, 1267, 1219,
1182, 1125, 1067 cm−1. HRMS (ESI): m/z calcd for C15H13NO2F3
[M + H]+, 296.0893; found, 296.0898.
Procedure for the Synthesis of α-Amino Acid (5c) as
Authentic Samples for HPLC Analysis. α-Amino acid 5c was
synthesized according to a method reported in the literature.15 α-
Bromophenylacetic acid (1.1 g, 5.0 mmol), NaHCO3 (0.42 g, 5.0
mmol), and p-anisidine (0.74 g, 6.0 mmol) in ethanol (5.0 mL) were
added to a solution of NaOH (0.20 g, 5.0 mmol) in water (20 mL) at
0 °C. After the mixture was heated at 90 °C for 12 h, the solution was
concentrated, and 1 M hydrochloric acid was added at 0 °C until the
pH was 4. The precipitate was filtered off, dried in vacuo, and
recrystallized (Et2O/hexane = 1:9) to afford the α-amino acid 5c.
α-[(4-Methoxyphenyl)amino]benzeneacetic Acid (5c).16 Yellow
solid, 0.15 g, 12% yield. 1H NMR (500 MHz; DMSO-d6): δ 7.48 (d, J
= 7.57 Hz, 2H), 7.35−7.29 (m, 1H), 7.28−7.23 (m, 1H), 6.67−6.63
(m, 2H), 6.59−6.55 (m, 2H), 4.89 (s, 1H).
General Procedure for the Synthesis of α-Amino Acids (5f)
as an Authentic Sample for HPLC Analysis. α-Amino acid 5f was
synthesized according to a method reported in the literature.15 2-
Bromopropionic acid (1.8 g, 10 mmol), NaHCO3 (0.84 g, 10 mmol),
and a solution of aniline (1.1 mL, 12 mmol) in ethanol (5.0 mL) were
sequentially added with cooling to a solution of NaOH (0.4 g, 10
mmol) in water (20 mL). The resultant mixture was heated for 24 h,
the solution was concentrated to a volume of 150 mL, dilute
hydrochloric acid was added until the pH was 4−5, and the solution
was then cooled to 0 °C. The precipitate was filtered, dried in the air,
and recrystallized from absolute ethanol to afford α-amino acid 5f.
N-Phenylvaline (5f).15 Brown solid, 0.36 g, 19% yield. (500 MHz;
DMSO-d6): δ 7.06 (t, J = 7.17 Hz, 2H), 6.62 (d, J = 7.68 Hz, 2H),
6.54 (t, J = 7.52 Hz, 1H), 3.62 (d, J = 6.62 Hz, 1H), 2.05 (dq, J =
13.56 Hz, 1H), 1.07−0.93 (m, 6H).
General Procedure for the Synthesis of α-Amino Acids (5g
and 5h) as Authentic Samples for HPLC Analysis. α-Amino
acids (5g and 5h) were synthesized according to a method reported
in the literature.15 Iodobenzene (1.1 mL, 10 mmol), CuI (190 mg, 1.0
mmol, 10 mol %), an aliphatic amino acid (15 mmol), potassium
phosphate monohydrate (6.4 g, 30 mmol), decanol (3.0 mL), and
water (10 mL) were added to a 50 mL round-bottom flask. The air
was evacuated, and the flask was filled with N2. The reaction mixture
was vigorously stirred at 90 °C for 48 h. The reaction was cooled, and
the reaction mixture was poured onto ice. Hydrochloric acid was
added dropwise with stirring until precipitation was observed, and the
Schematic of the electrochemical flow microreactor,
schematic of the construction procedure for the
electrochemical flow microreactor, and 1H, 13C, and
19F NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Kenta Tanaka − Faculty of Pharmaceutical Sciences, Tokyo
University of Science, Chiba 278−8510, Japan; orcid.org/
0000-0001-8253-3561; Email: ktanaka@rs.tus.ac.jp
Mahito Atobe − Graduate School of Science and Engineering,
Yokohama National University, Yokohama, Kanagawa 240-
8501, Japan; orcid.org/0000-0002-3173-3608;
Email: atobe@ynu.ac.jp
Authors
Yuki Naito − Graduate School of Science and Engineering,
Yokohama National University, Yokohama, Kanagawa 240-
8501, Japan
Yuto Nakamura − Graduate School of Science and
Engineering, Yokohama National University, Yokohama,
Kanagawa 240-8501, Japan
Naoki Shida − Graduate School of Science and Engineering,
Yokohama National University, Yokohama, Kanagawa 240-
8501, Japan
Hisanori Senboku − Faculty of Engineering, Hokkaido
University, Sapporo, Hokkaido 060-8628, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00821
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by CREST (JST grant no.
JPMJCR18R1) and by a Grant-in-Aid for Scientific Research
(JSPS grant nos. 20H02513 and 20K21106), Japan.
REFERENCES
■
(1) Hayashi, Y. Pot economy and one-pot synthesis. Chem. Sci. 2016,
7, 866−880.
G
https://doi.org/10.1021/acs.joc.1c00821
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pubs.acs.org/joc
Article
(2) (a) Yoshida, J.-i; Shimizu, A.; Ashikari, Y.; Morofuji, T.; Hayashi,
R.; Nokami, T.; Nagaki, A. Reaction Integration Using Electro-
generated Cationic Intermediates. Bull. Chem. Soc. Jpn. 2015, 88,
763−775. (b) Yoshida, J.-i.; Suga, S. Basic Concepts of “Cation Pool”
and “Cation Flow” Methods and Their Applications in Conventional
and Combinatorial Organic Synthesis. Chem. - Eur. J. 2002, 8, 2650−
2658.
(3) (a) Fanelli, F.; Parisi, G.; Degennaro, L.; Luisi, R. Contribution
of Microreactor Technology and Flow Chemistry to the Development
of Green and Sustainable Synthesis. Beilstein J. Org. Chem. 2017, 13,
520−542. (b) Wiles, C.; Watts, P. Continuous flow reactors: a
perspective. Green Chem. 2012, 14, 38−54. (c) Yoshida, J.-i.; Kim, H.;
Nagaki, A. Green and Sustainable Chemical Synthesis Using Flow
Microreactors. ChemSusChem 2011, 4, 331−340.
(4) (a) Zuend, S. J.; Coughlin, M. P.; Lalonde, M. P.; Jacobsen, E. N.
Scaleable catalytic asymmetric Strecker syntheses of unnatural αamino
acids. Nature 2009, 461, 968−970. (b) Wang, L.; Schultz, P. G.
Expanding the Genetic Code. Angew. Chem., Int. Ed. 2005, 44, 34−66.
(c) Angele, M. K.; Nitsch, S. M.; Hatz, R. A.; Angele, P.; Hernandez-
Richter, T.; Wichmann, M. W.; Chaudry, I. H.; Schildberg, F. W. L-
Arginine: A Unique Amino Acid for Improving Depressed Wound
Immune Function following Hemorrhage. Eur. Surg. Res. 2002, 34,
53−60. (d) Jarvo, E. R.; Miller, S. J. Tetrahedron 2002, 58, 2481.
(d1) Jarvo, E. R.; Miller, S. J. Amino acids and peptides as asymmetric
organocatalysts. Tetrahedron 2002, 58, 2481−2495.
(5) (a) Leuchtenberger, W.; Huthmacher, K.; Drauz, K. Biotechno-
logical production of amino acids and derivatives: current status and
prospects. Appl. Microbiol. Biotechnol. 2005, 69, 1−8. (b) Lutke-
Eversloh, T.; Stephanopoulos, G. L-tyrosine production by deregu-
lated strains of Escherichia coli. Appl. Microbiol. Biotechnol. 2007, 75,
103−110.
(9) Movassagh, B.; Alapour, S. P-Dodecylbenzenesulfonic Acid: A
Highly Efficient Catalyst for One-Pot Synthesis of α-Amino-
phosphonates in Aqueous Media. Heteroat. Chem. 2013, 24, 174−178.
(10) Mandai, H.; Mandai, K.; Snapper, M. L.; Hoveyda, A. H.
Three-Component Ag-Catalyzed Enantioselective Vinylogous Man-
nich and Aza-Diels−Alder Reactions with Alkyl-Substituted Alde-
hydes. J. Am. Chem. Soc. 2008, 130, 17961−17969.
(11) Arroyo, Y.; Sanz-Tejedor, M. A.; Alonso, I.; García-Ruano, J. L.
Synthesis of optically pure vic-sulfanyl amines mediated by a remote
sulfinyl group. Org. Lett. 2011, 13, 4534−4537.
(12) Junor, G. P.; Romero, E. A.; Chen, X.; Jazzar, R.; Bertrand, G.
Readily Available Primary Aminoboranes as Powerful Reagents for
Aldimine Synthesis. Angew. Chem., Int. Ed. 2019, 58, 2875−2878.
(13) Durán-Galván, M.; Connell, B. T. Synthesis of 2-amino-1,3-
dienes by chromium-catalyzed addition of silylated propargyl
bromides to imines. Tetrahedron 2011, 67, 7901.
(14) Faggi, C.; Neo, A. N.; Marcaccini, S.; Menchi, G.; Revuelta, J.
Ugi Four-Component Condensation with Two Cleavable Compo-
nents: the Easiest Synthesis of 2,N-Diarylglycines. Tetrahedron Lett.
2008, 49, 2099−2102.
(15) (a) Kurkin, A. V.; Golantsov, N. E.; Karchava, A. V.;
Yurovskaya, M. A. Synthesis of Derivatives of 2-(Indol-1-yl)propionic
Acids. Chem. Heterocycl. Compd. 2003, 39, 74−82. (b) Lu, Z.; Twieg,
R. J. Copper-catalyzed aryl amination in aqueous media with 2-
dimethylaminoethanol ligand. Tetrahedron Lett. 2005, 46, 2997−
3001.
(16) Sathe, A. A.; Hartline, D. R.; Radosevich, A. T. A synthesis of α-
amino acids via direct reductive carboxylation of imines with carbon
dioxide. Chem. Commun. 2013, 49, 5040−5042.
(17) Röttger, S.; Sjöberg, P. J. R.; Larhed, M. Microwave-Enhanced
Copper-Catalyzed N-Arylation of Free and Protected Amino Acids in
Water. J. Comb. Chem. 2007, 9, 204−209.
(6) (a) Katsoulos, G.; Schlosser, M. Site Selective Substitution of
Carbamate and Carbamoyl Protected Benzylamines. Tetrahedron Lett.
1993, 34, 6263−6264. (b) Mita, T.; Chen, J.; Sugawara, M.; Sato, Y.
One-Pot Synthesis of α-Amino Acids from Imines through CO2
Incorporation: An Method for Strecker Synthesis. Angew. Chem., Int.
Ed. 2011, 50, 1393−1396. (c) Mita, T.; Chen, J.; Sato, Y. Synthesis of
Arylglycines from CO2 through α-Amino Organomanganese Species.
Org. Lett. 2014, 16, 2200−2203. (d) Mita, T.; Michigami, K.; Sato, Y.
Sequential Protocol for C(sp3)−H Carboxylation with CO2:
Transition-Metal-Catalyzed Benzylic C−H Silylation and Fluoride-
Mediated Carboxylation. Org. Lett. 2012, 14, 3462−3465. (e) Mita,
T.; Sugawara, M.; Saito, K.; Sato, Y. Catalytic Enantioselective
Silylation of N-Sulfonylimines: Asymmetric Synthesis of α-Amino
Acids from CO2 via Stereospecific Carboxylation of α-Amino Silanes.
Org. Lett. 2014, 16, 3028−3031. (f) Mita, T.; Higuchi, Y.; Sato, Y.
One-Step Synthesis of Racemic α-Amino Acids from Aldehydes,
Amine Components, and Gaseous CO2 by the Aid of a Bismetal
Reagent. Chem. - Eur. J. 2013, 19, 1123−1128. (g) Mita, T.; Sato, Y.
Syntheses of α-Amino Acids by Using CO2 as a C1 Source. Chem. -
Asian J. 2019, 14, 2038−2047.
(7) (a) Weinberg, N. L.; Hoffmann, A. K.; Reddy, T. B. The
electrochemical reductive carboxylation of benzalaniline in molten
tetraethyl ammonium p-toluenesulfonate. Tetrahedron Lett. 1971, 12,
2271−2274. (b) Root, D. K.; Smith, W. H. Electrochemical Behavior
of Selected Imine Derivatives, Reductive Carboxylation, α-Amino
Acid Synthesis. J. Electrochem. Soc. 1982, 129, 1231. (c) Koshechko,
V. G.; Titov, V. E.; Bondarenko, V. N.; Pokhodenko, V. D.
Electrochemical carboxylation of fluorocontaining imines with
preparation of fluorinated N-phenylphenylglycines. J. Fluorine Chem.
2008, 129, 701−706. (d) Li, C.-H.; Song, X.-Z.; Tao, L.-M.; Li, Q.-G.;
Xie, J.-Q.; Peng, M.- N.; Pan, L.; Jiang, C.; Peng, Z.-Y.; Xu, M.-F.
Electrogenerated-Bases Promoted Electrochemical Synthesis of N-
Bromoamino Acids From Imines and Carbon Dioxide. Tetrahedron
2014, 70, 1855−1860.
(8) Qu, Y.; Tsuneishi, C.; Tateno, H.; Matsumura, Y.; Atobe, M.
Green synthesis of α-amino acids by electrochemical carboxylation of
imines in a flow microreactor. React. Chem. Eng. 2017, 2, 871−875.
H
https://doi.org/10.1021/acs.joc.1c00821
J. Org. Chem. XXXX, XXX, XXX−XXX