Electrocatalytic synthesis of nitriles from aldehydes with ammonium acetate as the nitrogen source

Article abstract of DOI:10.1016/j.electacta.2016.12.168

A simple synthesis method of nitriles from corresponding aldehydes by electrochemical oxidation was developed with ammonium acetate as the nitrogen source and 4-acetamido- 2,2,6,6-tetramethylpiperidinyl-l-oxy (4-AcNH-TEMPO) as the catalyst. Cyclic voltammetry was performed to investigate the electrocatalytic activity of 4-AcNH-TEMPO for the conversion of benzaldehyde to benzonitrile. On the basis of in situ FTIR data and cyclic voltammetry experiments, a reaction mechanism, involving the redox of 4-AcNH-TEMPO and the generation of intermediate imine during the reaction, was proposed. This electrocatalytic reaction system provided an efficient protocol for synthesis of aromatic nitriles at room temperature with moderate to high yields.

Full text of DOI:10.1016/j.electacta.2016.12.168

Electrochimica Acta 226 (2017) 53–59
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrocatalytic synthesis of nitriles from aldehydes with ammonium
acetate as the nitrogen source
a,b,
a,**
*
Xianjing Yanga,b, Zhongquan Fana,b, Zhenlu Shen , Meichao Li
a
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, PR China
Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310032, PR China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple synthesis method of nitriles from corresponding aldehydes by electrochemical oxidation was
developed with ammonium acetate as the nitrogen source and 4-acetamido- 2,2,6,6-tetramethylpiper-
idinyl-l-oxy (4-AcNH-TEMPO) as the catalyst. Cyclic voltammetry was performed to investigate the
electrocatalytic activity of 4-AcNH-TEMPO for the conversion of benzaldehyde to benzonitrile. On the
basis of in situ FTIR data and cyclic voltammetry experiments, a reaction mechanism, involving the redox
of 4-AcNH-TEMPO and the generation of intermediate imine during the reaction, was proposed. This
electrocatalytic reaction system provided an efficient protocol for synthesis of aromatic nitriles at room
temperature with moderate to high yields.
Received 17 November 2016
Received in revised form 19 December 2016
Accepted 26 December 2016
Available online 27 December 2016
Keywords:
nitriles
aldehydes
4-AcNH-TEMPO
electrooxidation
in situ FTIR
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
transformation. It was successfully employed as the catalyst in
our previous studies of aerobic oxidation [24,25] and electro-
Nitriles are known to be an important key intermediate and are
widely used in synthesis of various organic compounds including
amines [1], amides [2–4], ketones [5,6], carboxylic acids [7,8] and
heterocyclic compounds [9]. The traditional methods to synthesize
nitriles, such as halides/CN exchange [10–12], Sandmeyer reaction
[13], dehydration of amides [14,15] and direct functionalizations of
CꢀꢀH bonds [16], usually suffer from severe reaction conditions,
limited substrate scope, or use of hazardous reagents and
expensive catalysts. Because aldehydes are readily accessed and
easy to handle, synthesis of nitriles from corresponding aldehydes
is one of the most recognized strategies in organic chemistry. Over
the years, different nitrogen sources and oxidants have been
applied in this transformation [17–23]. However, some drawbacks
could not be avoided in these reported methods: (i) stoichiometric
oxidants, like tert-butyl hydroperoxide and NaICl2, should be used;
(ii) substrate scope was limited. Thus, from both economic and
environmental viewpoints, there is an urgent need for more
efficient and greener methods.
oxidation [26–30] of alcohol. In recent years, TEMPO and its
derivatives were introduced into the oxidative conversion of
amines or aldehydes into nitriles. Muldoon et al. reported that
copper/TEMPO catalysts could be used to prepare nitriles from
aldehydes using aqueous ammonia as the nitrogen source and 2,2-
bipyridine as the ligand with air as the oxidant [31]. Bailey et al.
reported metal-free oxidation protocols to convert amines into
nitriles using a stoichiometric quantity of 4-acetamido- 2,2,6,6-
tetramethylpiperidine-1-oxoammonium tetrafluoroborate (AcNH-
TEMPO+BF4ꢀ) [32] or a catalytic quantity of 4-acetamido- 2,2,6,6-
tetramethylpiperidinyl-l-oxy (4-AcNH-TEMPO) [33]. Nitriles could
also be prepared from aldehydes mediated by 2.5 equiv. of AcNH-
TEMPO+BF4 with hexamethyldisilazane (HMDS) as the nitrogen
ꢀ
source in the presence of 1.1 equiv. of pyridine [34]. Then we
developed an efficient methodology for the oxidative conversion of
aldehydes into nitriles with HMDS as the nitrogen source and
oxygen as the terminal oxidant using TEMPO as the catalyst, NaNO2
or tert-butyl nitrite as the co-catalyst [35]. Almost at the same
time, Kim et al. reported a similar methodology with ammonium
acetate (NH4OAc) as the nitrogen source using 4-AcNH-TEMPO as
the catalyst, NaNO2 and HNO3 as the co-catalyst [36].
2,2,6,6-Tetramethylpiperidinyl-l-oxy (TEMPO) and its deriva-
tives have been widely utilized in organic functional group
It is well-known that electrochemical syntheses possess
significant advantages such as high atom economy and less
pollution [37,38]. Chiba et al. reported that aldehydes could be
electrooxidated indirectly into nitriles using KI as the redox
catalyst and ammonia as the nitrogen source [39]. Unfortunately,
* Corresponding author at: College of Chemical Engineering, Zhejiang University
of Technology, Hangzhou 310032, PR China.
** Corresponding author.
E-mail addresses: zhenlushen@zjut.edu.cn (Z. Shen), limc@zjut.edu.cn (M. Li).
http://dx.doi.org/10.1016/j.electacta.2016.12.168
0013-4686/© 2016 Elsevier Ltd. All rights reserved.

54
X. Yang et al. / Electrochimica Acta 226 (2017) 53–59
no desired products could be obtained when nitrobenzaldehyde
and furan-2-carbaldehyde were used as substrates. Very recently,
our group reported a TEMPO-catalyzed method for electrochemi-
cal oxidation of aldehydes to nitriles with HMDS as the nitrogen
source [40]. In continuation of our work on the development of
electrochemical synthetic methodologies under different reaction
conditions, herein we developed another method for electro-
oxidative conversion of aldehydes into nitriles using 4-AcNH-
TEMPO as the catalyst and employing NH4OAc as the nitrogen
source, which is cheaper, safer and more stable than HMDS. The
redox properties of the reaction were studied by cyclic voltam-
metry and further mechanistic insight was investigated by in situ
FTIR.
0.12
0.08
0.04
0.00
-0.04
A
c
b
a
0.0
0.2
0.4
0.6
0.8
+
E / V (vs. Ag / Ag)
B
0.16
0.12
0.08
0.04
0.00
-0.04
d
c
2. Experimental
b
a
2.1. Reagents
Benzofuran-2-carbaldehyde was prepared in our laboratory and
was fully characterized. Other reagents and solvents were
purchased from commercial suppliers and used without further
purification.
0.0
0.2
0.4
0.6
0.8
E / V (vs. Ag+ / Ag)
2.2. Cyclic voltammetry of aldehyde oxidation
Cyclic voltammetry experiments were carried out in an
undivided cell in 15 mL MeCN solution containing 0.1 M NaClO4
as the supporting electrolyte and 0.1 mmol 4-AcNH-TEMPO as the
catalyst. The reactions were performed using Vertex Potentiostat/
Galvanostat with an “L” type Pt electrode as the working electrode
at the scan rate of 50 mV/s. The reference electrode and counter
electrode referred in this paper were Ag/Ag+ (0.1 M AgNO3 in
acetonitrile) and Pt sheet (2.25 cm2), respectively.
0.12
0.08
0.04
0.00
-0.04
C
d
c
b
a
2.3. In situ FTIR experiments of aldehyde oxidation
In situ FTIR spectra were obtained on a Nicolet 670 FTIR
spectrometer equipped with a liquid nitrogen cooled MCT-A
detector. A three-electrode spectro-electrochemical cell which was
fitted at the bottom with CaF2 window was used. The working
electrode was a Pt disk (5 mm in diameter).
0.0
0.2
0.4
0.6
0.8
E / V (vs. Ag+ / Ag)
Fig. 1. A. Cyclic voltammograms recorded in 0.1 M NaClO4/MeCN with (a) 4-AcNH-
TEMPO (0.1 mmol); (b) 4-AcNH-TEMPO (0.1 mmol) and benzaldehyde (1 mmol); (c)
4-AcNH-TEMPO (0.1 mmol), benzaldehyde (1 mmol) and NH4OAc (2.5 mmol).
Fig. 1B. Cyclic voltammograms of 4-AcNH-TEMPO (0.1 mmol) and NH4OAc
(2.5 mmol) in 0.1 M NaClO4/MeCN solution with benzaldehyde of (a) 1.0 mmol;
(b) 1.5 mmol; (c) 2.0 mmol; (d) 5.0 mmol. Fig.1C. Cyclic voltammograms of 4-AcNH-
TEMPO (0.1 mmol), NH4OAc (2.5 mmol) and benzaldehyde (1 mmol) in 0.1 M
NaClO4/MeCN solution at pH of (a) 4.6; (b) 5.6; (c) 6.6; (d) 7.6.
2.4. Preparative electrolysis experiments
The electrolysis experiments were performed in 15 mL 0.1 M
NaClO4/MeCN solution containing 1.0 mmol aldehyde (66.7 mM),
0.1 mmol 4-AcNH-TEMPO and 2.5 mmol NH4OAc using Pt sheet
(2.25 cm2) as the working electrode in an undivided cell on Vertex
Potentiostat/Galvanostat at the potential about 1.5 V. The products
were confirmed by 1H NMR and MS. NMR was performed on a
Bruker Avance III spectrometer. GC-MS was performed on Thermo
Trace ISQ instrument with TG 5MS capillary column.
which corresponded to one electron transfer between nitroxyl
radical and oxoammonium ion of 4-AcNH-TEMPO (4-AcNH-
TEMPO+) (curve a) [41]. In order to investigate the electrocatalytic
ability of 4-AcNH-TEMPO for oxidation of benzaldehyde, 1.0 mmol
benzaldehyde was added into the solution (curve b). As compared
with curve a, it could be found that the peak current at curve b
didn’t change obviously, which implied that almost no voltam-
metric response of benzaldehyde was detected at the electrode.
Then with the addition of the nitrogen source NH4OAc, the
oxidation peak of 4-AcNH-TEMPO increased dramatically (curve c).
It could be ascribed to the reaction between 4-AcNH-TEMPO+ and
benzaldehyde/NH4OAc, which greatly promoted the electrochem-
ical oxidation of 4-AcNH-TEMPO to 4-AcNH-TEMPO+.
3. Results and Discussion
3.1. Cyclic voltammetric study of benzaldehyde oxidation
Cyclic voltammograms were recorded for benzaldehyde in
15 mL MeCN containing 0.1 M NaClO4 and 0.1 mmol 4-AcNH-
TEMPO. As shown in Fig. 1A, a pair of redox potentials at 0.41 V and
0.29 V vs Ag/Ag+ (0.1 M AgNO3 in acetonitrile) were displayed,

X. Yang et al. / Electrochimica Acta 226 (2017) 53–59
55
Fig. 2. In situ FTIR spectra collected during oxidation of benzaldehyde (1 mmol) in the presence of 4-AcNH-TEMPO (0.1 mmol) and NH4OAc (2.5 mmol) in 15 mL 0.1 M NaClO4/
MeCN solution at potentials varied from 100 mV to 800 mV.
As for comparison, another experiment in 0.1 M NaClO4/MeCN
with benzaldehyde and NH4OAc in the absence of the mediator 4-
AcNH-TEMPO has also been carried out. Almost no voltammetric
response of the working electrode in the solution was observed. It
suggested that there was almost no electrochemical reaction
occurred without 4-AcNH-TEMPO. Therefore, 4-AcNH-TEMPO
acted as the electrocatalyst for benzaldehyde oxidation in the
presence of NH4OAc in this system.
To analyze the cyclic voltammograms more clearly, the
relationship between the peak current and the concentration of
benzaldehyde was also discussed, as shown in Fig. 1B. With the
increase of the concentration of benzaldehyde, the oxidation
current increased, and the reduction current decreased and even
disappeared. It indicated that the transformation of 4-AcNH-
TEMPO to 4-AcNH-TEMPO+ was increased and the electrocatalytic
oxidation became faster with increase of the benzaldehyde
concentration [42,43]. Therefore, 4-AcNH-TEMPO exhibited high
electrocatalytic activity for oxidation of benzaldehyde to benzoni-
trile. However, with the increase of the benzaldehyde concentra-
tion, the conductivity of the solution would decrease and the
oxidation peak potential shifted positively. Therefore, 1.0 mmol
benzaldehyde was used in 15 mL acetonitrile solution for all the
follow-up electrochemical experiments.
indicated that benzaldehyde was successfully converted to
benzonitrile. In addition, a downward peak could be observed at
1624 cmꢀ1, which was related to N+
¼
O stretching vibration of 4-
AcNH-TEMPO oxoammonium ion [26]. The corresponding upward
peak of NꢀꢀO
ꢁ
stretching vibration of 4-AcNH-TEMPO was detected
at 1354 cmꢀ1 [26,47]. Thus, 4-AcNH-TEMPO was oxidized to its
oxoammonium ion (4-AcNH-TEMPO+). The above results could
lead to the conclusion that 4-AcNH-TEMPO+ served as oxidizing
agent for benzaldehyde to form benzonitrile.
Other three downward bands at 1754, 1725 and 1218 cmꢀ1
could also be observed. The former two were ascribed to C O
¼
stretching vibration of acetic acid and acetate dimer respectively,
while 1218 cmꢀ1 was related to CꢀꢀO stretching vibration of acetic
acid [48]. It showed that NH4OAc had participated in the reaction
and AcOH was generaꢀted. The negative band located at 1122 cmꢀ1
was assigned to ClO4 [26] and it might be due to the excess of
anions migrating continually to the thin layer between CaF2 and
the working electrode. The CRN vibration of acetonitrile was
detected at 2262 cmꢀ1 [49].
Meanwhile, two important downward peaks at 1685 and
1655 cmꢀ1 were observed which might be attributed to C
N
¼
stretching vibration of benzenemethanimine [50]. In order to
verify the two bands, an additional in situ FTIR experiment was
carried out in the absence of 4-AcNH-TEMPO under the similar
conditions, as shown in Fig. 3. The downward bands at 1685 and
1655 cmꢀ1 could still be detected. The upward peak at 1703 cmꢀ1 of
The voltammetric study of the influence of pH value was also
performed, as shown in Fig. 1C. The concentration of H+ was
regulated by the addition of AcOH into the solution. It could be
seen that with the concentration of H+ increased, the oxidation
peak current decreased and the reduction peak current increased.
C O stretching vibration was also observed and demonstrated the
¼
consumption of benzaldehyde [46]. The result revealed that
benzenemethanimine was generated with the reaction of benzal-
dehyde and NH4OAc without the mediator 4-AcNH-TEMPO. In
addition, the CRN stretching vibration of benzonitrile at
2229 cmꢀ1 was not observed in Fig. 3 [45], which suggested that
the target product benzonitrile could hardly be obtained in the
absence of 4-AcNH-TEMPO.
3.2. In situ FTIR study of benzaldehyde oxidation
In order to gain mechanistic insight, the benzaldehyde
oxidation reaction in NaClO4/MeCN was examined using in situ
FTIR. As shown in Fig. 2, the downward bands referred to the
generation of species and the upward bands to their consumption
[44]. The bands at 2229 cmꢀ1 was assigned to the CRN stretching
vibration of benzonitrile [45]. The upward peaks at 1703 cmꢀ1 and
On the basis of in situ FTIR study and cyclic voltammetry
experiments, a possible mechanism for the present oxidative
conversion of benzaldehyde into benzonitrile was described in
Scheme 1. Firstly, benzaldehyde reacted with ammonium acetate
to give benzenemethanimine which could be converted into
1315 cmꢀ1 were attributed to C
O)ꢀꢀH in-plane bending of benzaldehyde, respectively [46]. It
¼O stretching vibration and C
(¼

56
X. Yang et al. / Electrochimica Acta 226 (2017) 53–59
Fig. 3. In situ FTIR spectra collected during oxidation of benzaldehyde (1 mmol) in the presence of NH4OAc (2.5 mmol) in 0.1 M NaClO4/MeCN solution (15 mL) at potentials
varied from 100 mV to 800 mV.
results showed that the high potential could promote the reaction
rate for most aldehydes. In addition, the byproducts of most
electrochemical reactions didn’t increase with the potential
increasing. Therefore, 1.5 V was applied for all substrates in this
paper for uniformity. Some initial optimization experiments were
performed with o-fluorobenzaldehyde as the model substrate.
When 4-AcNH-TEMPO was used as the catalyst, the conversion of
o-fluorobenzaldehyde was increased dramatically to 99%. Howev-
er, the conversion of o-fluorobenzaldehyde in the absence of 4-
AcNH-TEMPO was quite low which might be due to the direct
electrooxidation of aldehyde on Pt electrode (entry 1). It indicated
that 4-AcNH-TEMPO exhibited good catalytic ability for the
transformation of o-fluorobenzaldehyde to o-fluorobenzonitrile
Scheme 1. A proposed reaction mechanism.
(entry 2). To investigate the effect of electrode, preparative
electrolysis experiment was then carried out using graphite as
benzonitrile by oxidation reaction with 4-AcNH-TEMPO+. At the
same time, 4-AcNH-TEMPO+ was reduced to 4-AcNH-TEMPOH
Table 2
[36,51,52]. The hydroxylamine would be oxidized to 4-AcNH-
TEMPO through electrochemical step at the anode. Meanwhile,
oxoammonium ion would react with the hydroxylamine to
regenerate 4-AcNH-TEMPO, which was re-oxidized electrochemi-
cally to 4-AcNH-TEMPO+ to complete the catalytic cycle [42,43].
The electrolysis conversion of aldehydes mediated by TEMPO or 4-AcNH-TEMPOa.
b
Entry
1
Substrate
Time/h
20
mediator
Conversion
/%
TEMPO
82
99
4-AcNH-TEMPO
3.3. Optimization of electrolytic conditions
2
7
TEMPO
4-AcNH-TEMPO
84
99
Encouraged by the above results, we attempted to develop an
electrochemical method for synthesis of nitriles from aldehydes in
an undivided cell. Different potentials between 0.8 V and 1.5 V
were applied in the electrochemical oxidation reaction and the
3
7
TEMPO
4-AcNH-TEMPO
98
98
Table 1
Optimization of electrolytic conditionsa.
4c
20
TEMPO
4-AcNH-TEMPO
55
76
Entry
Catalyst (mmol)
Anode
Conversion b/%
1
2
3
–
Pt
Pt
13
99
93
4-AcNH-TEMPO (0.1)
4-AcNH-TEMPO (0.1)
Graphite
a
Electrolytic conditions: aldehyde (1 mmol), NH4OAc (2.5 mmol), 4-AcNH-
TEMPO or TEMPO (0.1 mmol), 0.1 M NaClO4/MeCN solution (15 mL), undivided
cell, constant potential (1.5 V), room temperature.
a
Electrolytic conditions: o-fluorobenzaldehyde (1 mmol), NH4OAc (2.5 mmol),
0.1 M NaClO4/MeCN solution (15 mL), undivided cell, constant potential (1.5 V), 7 h,
room temperature.
b
Conversions were determined by GC with peak area normalization method.
TEMPO (0.3 mmol), 4-AcNH-TEMPO (0.3 mmol).
b
c
Conversions were determined by GC with peak area normalization method.

X. Yang et al. / Electrochimica Acta 226 (2017) 53–59
57
Table 3
Electrochemical synthesis of nitriles from aldehydesa.
Entry
1
Substrate
Product
Time/h
20
Conversion b/%
99
Yield c/%
58(95)
2
7
99
86
3
4
5
7
96
88
85
84
20
7
97
>99
6
7
7
7
>99
96
81
85
8
7
>99
68(83)
9
20
7
95
98
70(88)
95
10
11
20
20
7
98
76
96
94
71
90
12d
13e
14
20
20
>99
>99
85
84
15f
16
15
96
63
a
Electrolytic conditions: substrate (1 mmol), 4-AcNH-TEMPO (0.1 mmol), NH4OAc (2.5 mmol), 0.1 M NaClO4/MeCN solution (15 mL), undivided cell, constant potential
(1.5 V), room temperature.
b
Conversions were determined by GC with peak area normalization method.
Isolated yield. Values in parentheses were determined by GC internal standard method.
4-AcNH-TEMPO (0.3 mmol).
4-AcNH-TEMPO (0.2 mmol), NH4OAc (5 mmol).
c
d
e
f
4-AcNH-TEMPO (0.2 mmol).

58
X. Yang et al. / Electrochimica Acta 226 (2017) 53–59
the working electrode. A 93% conversion of o-fluorobenzaldehyde
was obtained, which was slightly lower than that with Pt sheet as
the working electrode (entry 3).
important intermediate. This electrochemical synthesis method
is suitable for a variety of aromatic aldehydes and affords moderate
to high yields of nitriles at room temperature. The present method
possessed significant advantages as it is performed without
transition metal catalysts and stoichiometric amounts of reactive
oxidants.
4-AcNH-TEMPO bearing an electron-withdrawing substituent
in the 4-position might be a more active mediator than TEMPO in
the electrolysis reactions [53]. In order to compare the electro-
catalytic ability of 4-AcNH-TEMPO with TEMPO for oxidation of
aldehydes to nitriles, additional electrolysis experiments were also
performed with the model substrates of benzaldehyde, o-
fluorobenzaldehyde, 4-nitrobenzaldehyde and 4-methoxybenzal-
dehyde, as shown in Table 2. Except for 4-nitrobenzaldehyde, the
conversions of the aldehydes especially for that with electron-
donating group using the mediator 4-AcNH-TEMPO were higher
than TEMPO. Thus, 4-AcNH-TEMPO was applied for all electrolysis
experiments in the paper.
Acknowledgements
The authors gratefully acknowledge the financial support of the
National Natural Science Foundations of China (21206147,
21376224).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
electacta.2016.12.168.
3.4. Scope of oxidative conversion of aldehydes into nitriles
With the optimized condition in hand (entry 2 of Table 1), the
general applicability of the catalytic system for the oxidation of
diverse aromatic aldehydes as well as heteroaromatic aldehydes
and cinnamaldehyde was tested, and the results were summarized
in Table 3. Benzonitrile could be obtained with 58% isolated yield,
but the GC yield using an internal standard method was up to 95%
(entry 1). We attributed the result to the volatility of benzonitrile,
which led to some loss of the mass balance during product
isolation. This finding could also be applied to o- and p-fluoro-
benzaldehyde (entries 8 and 9). It was found that aromatic
aldehydes with either electron-deficient or electron-rich groups
could be transformed into corresponding nitriles with good yields
(Table 2, entries 2-11). Halide substituents such as chloro-
benzaldehyde (o-, m- and p-) and fluoro-benzaldehyde (o-, p-)
were well tolerated with this conversion, which underwent
oxidative conversion to produce corresponding nitriles in good
yields (entries 5–9). Furthermore, 4-nitrobenzonitrile and 4-tert-
butylbenzonitrile were also obtained in high yields with the
standard conditions (entries 10-11). Notably, it appeared that the
reaction with an electron-donating group became sluggish and
needed higher loading of catalyst. For example, 4-methoxyben-
zaldehyde was unable to be converted into 4-methoxybenzonitrile
under the optimized conditions until the loading of 4-AcNH-
TEMPO added to 0.3 mmol (entry 12). The result was in accordance
with our former observation [40]. The oxidation of terephtha-
laldehyde, which has two aldehyde groups, generated terephtha-
lonitrile almost quantitatively in the presence of 5 equiv. of
NH4OAc and 0.2 mmol 4-AcNH-TEMPO (entry 13). In the case of
heteroaromatic aldehydes, 5-bromo-2-furaldehyde and benzo-
furan-2-carbaldehyde had been smoothly converted into desired
nitriles with good yields, which indicated that this method could
tolerate heteroaromatic substrates (entries 14-15). Finally, we
enabled extension of this protocol to cinnamaldehyde. The
transformation of cinnamaldehyde proceeded to afford cinnamo-
nitrile in a moderate yield of 63% (entry16).
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