Electrochemical N -Formylation of Amines via Decarboxylation of Glyoxylic Acid

Article abstract of DOI:10.1021/acs.orglett.8b00698

A new method for the synthesis of formamides has been developed through electrochemical decarboxylative N-formylation of amines with glyoxylic acid. This protocol provides an efficient approach to formamides with a broad range of functional group tolerance under ambient conditions.

Full text of DOI:10.1021/acs.orglett.8b00698

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Electrochemical N‑Formylation of Amines via Decarboxylation of
Glyoxylic Acid
Dian-Zhao Lin and Jing-Mei Huang*
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South
China University of Technology, Guangzhou, Guangdong 510640, China
S
* Supporting Information
ABSTRACT: A new method for the synthesis of formamides
has been developed through electrochemical decarboxylative
N-formylation of amines with glyoxylic acid. This protocol
provides an efficient approach to formamides with a broad
range of functional group tolerance under ambient conditions.
ormamides are one group of valuable intermediates in
glyoxylic acid is still unknown. Inspired by our previous studies
of electrochemical synthesis,19 herein, we report the first highly
efficient N-formylation of amines by electrochemical decarbox-
ylative C−N bond formation with glyoxylic acid as a formyl
source.
1
Forganic synthesis and pharmaceutical chemistry.
The
synthesis of many N-heterocycles, such as imidazole,
isocyanides, as well as formamidines, is achieved via
formamides as intermediates.2 Various pharmaceutically
important compounds, for instance, leucovorin,3 formoterol4
and orlistat,5 contain formamides blocks. Furthermore, the
application of formamides in Vilsmeier−Haack formylation
reaction has been well documented.6
To start our investigation, N-methylaniline (1a) and
glyoxylic acid (2) were chosen as model substrates. Under
constant-current at 5 mA in an undivided cell, the reaction of
1a and 2 with 10 mol % Cu(OAc)2·2H2O and 20 mol % NiCl2·
6H2O in the presence of cesium carbonate (1.5 equiv) gave
95% formylated product 3a (Table 1, entry 1). The yield
decreased when the reaction was performed in the absence of
Cu(OAc)2·2H2O or NiCl2·6H2O (Table 1, entries 2 and 3).
When NiCl2·6H2O was replaced with some other Lewis acids,
such as InCl3 and BiCl3, comparable activities were shown. The
results implied that the nickel salt might act as a Lewis acid in
this transformation (see Supporting Information (SI) for
details). CuCl gave an inferior result to Cu(OAc)2·2H2O
(Table 1, entry 4).20 When the reaction was carried out in the
absence of cesium carbonate, only a trace of product was
obtained (Table 1, entry 5). Both NaOAc and DBU led to a
lower yield (Table 1, entries 6 and 7). Next, the effect of
electrolyte was explored. Sodium perchlorate was found to be
optimal among the electrolytes tested (Table 1, entries 1, 8−
10).20 The choice of solvent also had substantial impact on the
result of this reaction. Use of N,N-dimethylformamide resulted
in a low yield, while acetonitrile had proven ineffective (Table
1, entries 11 and 12).20 The electrode material was also tested.
When graphite rod was used as anode or cathode, the reaction
efficiency decreased (Table 1, entries 13 and 14). Nickel foam
could serve as cathode with a reduced productivity (Table 1,
entry 15). Increase or decrease of the electric current caused
lower yields (Table 1, entries 16 and 17). Predictably, no
desired product could be detected without an electric current
(Table 1, entry 18). It is worth noting that only 1 equiv of
Various approaches have been developed for N-formylation
of amines to access formamides. Besides the use of formic acid
and its derivatives,7 other formylating reagents, including
methanol,8 aldehydes,9 cyanides,10 carbon monoxide,11 and
carbon dioxide,12 have also been focused on. Nevertheless,
these methodologies suffer from one or more disadvantages,
such as unwanted side reactions, the toxicity of the reagents, the
necessity for the preactivation of the formylating agents, high
temperatures, and anhydrous conditions.
Nowadays, decarboxylative cross-coupling reaction has
developed as an efficient method for the construction of
carbon−carbon or carbon−heteroatom bonds.13 Formylation
through decarboxylative cross-coupling of a glyoxylic acid is
envisioned as a promising alternative protocol, since glyoxylic
acid is stable and readily available. However, it was not explored
until very recently; Wang and Xu et al. have developed several
formylation protocols by employing diethoxyacetic acid14 or
glyoxylic acid15 as a formyl equivalent. Nevertheless, C−C
bond formations are targeted for these works.
Since Kolbe electrolysis was discovered, anodic oxidative
decarboxylation of a carboxylic acid has become a significant
method allowing one-step C−C or C−heteroatom bond
formations in electrooganic synthesis, which is difficult to be
achieved through other routes.16 In spite of the convenience
and the green aspects that electrochemistry contributes to
contemporary organic synthesis,17,18 its utilizations in decar-
boxylative cross-coupling reactions remain challenging, and the
present processes are yet limited. To the best of our knowledge,
the electrochemical formylation through decarboxylation of
Received: March 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00698
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a,b
Table 1. Optimization of Reaction Conditions
Scheme 1. Scope of the N-Formylation of Amines
b
entry
variation from the standard conditions
yield (%)
1
none
95
2
in the absence of Cu(OAc)2·2H2O
in the absence of NiCl2·6H2O
CuCl instead of Cu(OAc)2·2H2O
in the absence of Cs2CO3
NaOAc instead of Cs2CO3
DBU instead of Cs2CO3
NaBF4 instead of NaClO4
LiClO4 instead of NaClO4
n-Bu4NBF4 instead of NaClO4
DMF instead of DMSO
MeCN instead of DMSO
graphite rods as anode
65
3
79
4
63
5
trace
78
6
7
64
8
69
9
32
10
11
12
73
72
trace
65
c
13
c
14
graphite rods as cathode
Ni foam as cathode
61
d
15
76
16
17
18
10 mA instead of 5 mA, 5 h
2.5 mA instead of 5 mA, 20 h
no electric current
55
82
N.R.
a
Standard conditions: 1a (0.3 mmol), 2 (0.9 mmol), Cu(OAc)2·2H2O
(10 mol %), NiCl2·6H2O (20 mol %), Cs2CO3 (0.45 mmol), DMSO
(5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as
anode and cathode, undivided cell, constant current = 5 mA, 10 h,
a
Standard conditions: 1 or 4 (0.3 mmol), 2 (0.9 mmol), Cu(OAc)2·
2H2O (10 mol %), NiCl2·6H2O (20 mol %), Cs2CO3 (for secondary
amines: 0.45 mmol; for primary amines: 0.3 mmol), DMSO (5 mL)
with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode
and cathode, undivided cell, constant current = 5 mA, 10 h, room
b
1
room temperature. The yield of the product was determined by H
c
NMR spectroscopy; N.R. = no reaction. Graphite rods (diameter: 0.5
d
cm, height: 1.78 cm). Ni foam electrode (length, 1.5 cm; width, 1.0
b
cm; thickness, 0.01 cm).
temperature (see the Supporting Information for details). Isolated
yield.
Cs2CO3 was necessary for the N-formylation of primary amines
(Table S5, entry 3; see SI for details).
Scheme 2. Control Experimental Studies
With the optimized conditions in hand, we examined the
scope of the N-formylation reactions by testing a series of
primary and secondary amines containing different substituents
on the phenyl ring (Scheme 1). The reactions of secondary
amines with both an electron-releasing group (ERG) and an
electron-withdrawing group (EWG) could gain the desired
products in satisfying yields. It is worthwhile to note that cyclic
amines, such as indoline, piperazine, and tetrahydroisoquino-
line, as well as morpholine could transform into the formylation
products in good to excellent yields (3g−3j). For the N-
isopropylaniline, unfortunately, no reaction occurred under the
optimized conditions, which indicated that a bulky substituent
on the nitrogen atom might play a negative impact. When it
turned to primary amines, a variety of functionalities were
tolerated, including methyl (5b), methoxyl (5c), trifluorome-
thoxyl (5d), trifluoromethyl (5e−5f), nitro (5g), cyano (5h),
ester (5i), ketone (5j), and halogen groups (5k−5o).
Heterocyclic amines, for instance, 2-aminopyridine and methyl
3-aminothiophene-2-carboxylate, were also compatible with the
current protocol and afforded the products in good yields (5p−
5q). It is good to see that this protocol is also applicable to the
aliphatic amines (5r).
control experiment was carried out (Scheme 2B). When 100 μL
of H218O was added to the reaction system in N2 atmosphere,
two products were obtained in a total yield of 95% (3a and 3a′,
in the ratio of 5:6; see SI). This result demonstrated that the
oxygen source in the final products was the water in the system.
Next, when a radical scavenger (RS), TEMPO or 1,1-
diphenylethene, was added into the reaction mixture under
the standard conditions, the desired product was obtained in
64% and 75% yields, respectively (Scheme 2C). Neither 1a nor
any radical trapping adducts were detected by TLC, GC−MS,
To understand the mechanism for this reaction, some control
experiments were implemented. A reaction was conducted
under N2 atmosphere, and no change was observed in terms of
yield (Scheme 2A). This result indicated that the molecular
oxygen was not crucial for this transformation. Then, a H218O
B
DOI: 10.1021/acs.orglett.8b00698
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and NMR analysis. At the current stage, the radical mechanism
should not be ruled out (see SI). Cyclic voltammetry (CV)
experiments were also implemented (Figure 1; for more details,
further removed rapidly due to the electron-donating
substituent located at the α position. The proton released
subsequently to produce the desired formylation product 10a.
Meanwhile, the Cu(II) was regenerated by anodic oxidation.
NiCl2 was proposed to act as a Lewis acid to promote the imine
condensation. On the cathode, the proton was reduced into
hydrogen.
In conclusion, we have developed a highly efficient N-
formylation of amines by electrochemical decarboxylation of
glyoxylic acid. The method demonstrated a broad substrate
scope, including primary/secondary amines and aliphatic/
aromatic amines, with excellent functional group tolerance.
This electrochemical protocol, which proceeded smoothly
under ambient conditions, provides a new and sustainable
alternative for the N-formylating reaction. Further investiga-
tions into the mechanistic details and synthetic applications are
currently underway in our laboratory.
Figure 1. Cyclic voltammograms of 0.1 M NaClO4 solution in DMSO
at room temperature. (a) None; (b) N-methylaniline (0.03 M); (c)
glyoxylic acid (0.09 M) + Cs2CO3 (0.045 M); (d) b + c; Cu(OAc)2
(0.003 M); reaction mixture: glyoxylic acid (0.01 M) + N-
methylaniline (0.01 M) + Cs2CO3 (0.01 M) in DMSO. The
voltammogram was obtained with Pt wire as auxiliary electrode and
a saturated calomel electrode (SCE) as a reference electrode. The scan
rate was 0.1 V/s on a platinum disk electrode (d = 2 mm).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00698.
see the SI). It was observed that the oxidation potential of the
glyoxylic acid in the presence of cesium carbonate is Ep = 0.80
V vs SCE (Figure 1A, curve b), while the oxidation potential of
N-methylaniline is Ep = 0.67 V vs SCE (Figure 1A, curve c). It
is interesting to find that when the glyoxylic acid, aniline, and
cesium carbonate were introduced into the solution, a new
oxidation peak at Ep = 0.76 V vs SCE appeared (Figure 1A,
curve d). This peak could be attributed to a condensation
product of 1a and 2, which was oxidized at a relatively lower
potential than glyoxylate to facilitate the process of
decarboxylation.21 Meanwhile, the catalytic current of Cu-
(OAc)2 was observed (Figure 1B). This result indicated that
the Cu(OAc)2 might act as an active oxidant and that the high
valent copper is regenerated by anodic oxidation.
Based on the mechanistic investigations above and the
reported works,22 a plausible pathway for this decarboxylative
formylation process is proposed (Scheme 3). Glyoxylic acid 2
was first transformed into carboxylate ion 6a by cesium
carbonate and then condensed with the aniline to form 7a. The
intermediate 7a was oxidized by cupric acetate, followed by the
decarboxylation to generate 9a. Another electron of 9a was
Experimental procedures and spectroscopic data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chehjm@scut.edu.cn.
ORCID
Jing-Mei Huang: 0000-0003-2861-3856
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21672074 and 21372089) for financial
support.
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