Electrochemical synthesis of nitriles from aldehydes using TEMPO as a mediator

Article abstract of DOI:10.1016/j.elecom.2016.01.011

A novel electrochemical route to generate nitriles from aldehydes under mild conditions using a catalytic amount of TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) as the mediator and hexamethyldisilazane as the nitrogen source in the presence of acetic acid has been developed. A variety of aromatic, heteroaromatic and aliphatic aldehydes have been converted to their corresponding nitriles in good to excellent yields. A plausible reaction mechanism is proposed based on the cyclic voltammetry, in situ FTIR and the identification of intermediates.

Full text of DOI:10.1016/j.elecom.2016.01.011

Electrochemical synthesis of nitriles from aldehydes using TEMPO as a
mediator
Qiguo Chen, Chaojie Fang, Zhenlu Shen, Meichao Li
PII:
DOI:
Reference:
S1388-2481(16)00021-7
doi: 10.1016/j.elecom.2016.01.011
ELECOM 5623
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
30 November 2015
20 January 2016
21 January 2016
Please cite this article as: Qiguo Chen, Chaojie Fang, Zhenlu Shen, Meichao Li, Electro-
chemical synthesis of nitriles from aldehydes using TEMPO as a mediator, Electrochem-
istry Communications (2016), doi: 10.1016/j.elecom.2016.01.011
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ACCEPTED MANUSCRIPT
Electrochemical synthesis of nitriles from aldehydes
using TEMPO as a mediator
Qiguo Chen, Chaojie Fang, Zhenlu Shen*, Meichao Li*
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
E-mail address: zhenlushen@zjut.edu.cn; limc@zjut.edu.cn
Tel.: +86-571-88320568, fax: +86-571-88320961
Abstract
A novel electrochemical route to generate nitriles from aldehydes under mild conditions using a
catalytic amount of TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) as the mediator and
hexamethyldisilazane as the nitrogen source in the presence of acetic acid has been developed. A
variety of aromatic, heteroaromatic and aliphatic aldehydes have been converted to their
corresponding nitriles in good to excellent yields. A plausible reaction mechanism is proposed based
on the cyclic voltammetry, in situ FTIR and the identification of intermediates.
Keywords: Electrosynthsis; Nitriles; TEMPO; Aldehydes; Hexamethyldisilazane
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1. Introduction
Nitriles are widely present in dyes, herbicides, agrochemicals and electronic materials. The
nitrile group serves as an important intermediate in the organic synthesis of many pharmaceuticals
[1-2]. Representative methods for the preparation of nitriles, such as Rosenmund–von Braun reaction
[3], Sandmeyer reaction [4], and other approaches [5-9] involve reactions of halide/CN exchange.
Dehydration of amides is another commonly used method. However, different dehydrating agents
and high temperature are needed [10,11], and sometimes the scope of reaction is limited [12]. The
synthesis of nitriles from aldoximes has also received much attention during the past decades and
several methods have been reported [13,14]. Again, these methods suffer from the disadvantages of
low yields and excess use of toxic reagents. Considering the accessibility of starting materials, the
oxidations to nitriles from aldehydes, alcohols or acids have emerged as alternative synthetic
strategies. Various nitrogen sources such as urea, ammonium and hydroxylamine have been
employed [15-19]. Yet in most existing methods, transition metals are always needed. Therefore,
there is a growing demand for a mild, efficient and versatile method for synthesis of nitriles from
cheap substrates with commercially available oxidants or catalysts. During the compilation of the
present work, Jinho Kim et al reported a nitroxyl/NOx catalyst system for the aerobic oxidative
conversion of aromatic aldehydes to nitriles at 50℃ [20].
Very recently, a successful attempt has been made to generate nitriles from aldehydes and
hexamethyldisilazane
(HMDS)
mediated
by
an
oxoammonium
salt
(4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate) [21]. However, 2.5
equiv. of oxoammonium salt are used as the oxidant and a stoichiometric amount of pyridine is
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needed in this route. Additionally, the reaction is carried out in the solvent dichloromethane, which is
considered to be environment-unfriendly [22]. 2,2,6,6-Tetramethylpiperidinyl-l-oxy (TEMPO),
which can be easily oxidized to TEMPO+ (oxoammonium ion) via one-electron oxidation, is widely
utilized in the oxidation of alcohols into their corresponding carbonyl compounds [23,24]. In our
previous studies of alcohols oxidation, TEMPO was employed as the catalyst with oxygen [25,26] or
electric current [27-30] as the terminal oxidant. Especially, electrochemical routes offer remarkable
advantages such as good atom economy and non-pollution [31,32]. Chiba and Okimoto prepared
nitriles from aldehydes by means of indirect electrooxidation using potassium iodide as the redox
catalyst [33]. Unfortunately, when some aldehydes such as nitrobenzaldehydes and
furan-2-carbaldehyde were used as substrates, no desired products could be obtained. Herein we
report a novel electrochemical route to generate nitriles using a catalytic amount of TEMPO as the
mediator and HMDS as the nitrogen source without toxic reagents under mild conditions (Scheme
1).
Scheme 1. Synthesis of nitriles from aldehydes
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2. Experimental
Adamantane-1-carbaldehyde and benzofuran-2-carbaldehyde were prepared in our laboratory.
Other chemicals were purchased from supplier and used without purification. Cyclic voltammetry
was carried out using CHI6208 electrochemical workstation (CH Instrument, Inc. USA.) on an
L-type platinum electrode in 0.1 M NaClO4-CH3CN at room temperature. A platinum sheet (3.0 cm2)
was employed as the counter electrode and Ag/Ag+ electrode (0.1 M AgNO3 in acetonitrile) as the
reference one.
Preparative electrolysis experiments were performed using 263A Potentiostat/Galvanostat
(Princeton Applied Research). 0.1 M NaClO4-CH3CN solution (10 mL) containing aldehydes (1
mmol), TEMPO (0.1 mmol), HMDS (2.5 mmol) and AcOH (2.5 mmol) was electrolyzed with
stirring in an undivided cell (30 mL) equipped with two platinum sheets as anode (1.5 cm2) and
cathode (3.0 cm2) respectively at a constant potential of 1.5 V vs Ag/Ag+ (0.1 M AgNO3 in
acetonitrile). The electrode separation was 1 cm. When the reaction was finished, 10 mL of saturated
Na2SO3 solution was added into the reaction mixture and stirred for 15 minutes. Then the mixture
was extracted with CH2Cl2 (20 mL x 3). The organic layer was dried with anhydrous Na2SO4 and
concentrated in a rotary evaporator. The products were obtained via purification of column
1
13
chromatography and their structures were confirmed by H NMR, C 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.
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In situ FTIR spectroscopic experiments were carried out on a Nicolet 670 FTIR spectrometer
equipped with a MCT-A detector cooled by liquid nitrogen. The disk electrode of Pt (5 mm in
diameter) was used as the working electrode. Two hundred interferograms with the resolution of 8
cm−1 were collected and co-added into a single-beam spectrum.
N,N'-dibenzylidene-1-phenylmethanediamine
(A)
and
N,N'-bis(4-fluorobenzylidene)-1-(4-fluorophenyl)methanediamine (B) were prepared according to
the literature [34]. A suspension of benzaldehydes (20 mmol), HMDS (40 mmol) and dried ZnCl2
(10 mmol) in 60 mL CH2Cl2 was stirred for 12 h at room temperature. The reaction mixture was
poured into 50 mL H2O and extracted with CH2Cl2 (30 mL x 3). The extract was dried by anhydrous
Na2SO4 and concentrated under reduced pressure. The resulting mixture was recrystallized with 20
mL hexane to give the desired products. The characterization data of A were in agreement with the
reported values [34]. B was obtained as a white solid, mp: 75-76 oC. 1H NMR (500 MHz, CDCl3):
13
δ=8.52 (2H, s), 7.85-7.88 (4H, m), 7.48-7.51 (2H, m), 7.06-7.15 (6H, m), 5.92 (1H, s). C NMR
(125 MHz, CDCl3): 164.6 (d, J=250.3 Hz), 162.5 (d, J=244.8 Hz), 159.4, 137.5 (d, J=2.4 Hz), 132.1
(d, J=3.2 Hz), 130.7 (d, J=9.0 Hz), 128.8 (d, J=8.1 Hz), 115.8 (d, J=21.8 Hz), 115.4 (d, J=21.1 Hz),
91.8. HRMS (ESI), m/z, 353.1259, [MH+], calcd for C21H16F3N2, 353.1266.
3. Results and discussion
3.1. Cyclic voltammetry
Fig.1a shows the cyclic voltammogram of 0.01 M TEMPO dissolved in 0.1 M NaClO4-CH3CN
solution, in which a typical redox couple is observed at 0.337 V and 0.180 V vs. Ag/Ag+. It
corresponds to the one-electron redox reaction of nitroxyl radical and oxoammonium ion [31]. When
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benzaldehyde was added to the solution, almost the same peak currents were observed in Fig. 1b
which revealed benzaldehyde showed only a little voltammtric response on the bare Pt. When
HMDS and AcOH were added, the oxoammonium ion reduction current disappeared and the anodic
peak current was greatly enhanced as shown in Fig.1c. It demonstrated that TEMPO+ has reacted
with benzaldehyde/HMDS/AcOH and been reduced to the hydroxylamine (TEMPOH) which could
be oxidized in the following cycle.
0.5
c
0.4
0.3
0.2
0.1
0.0
-0.1
b
a
0.0
0.5
1.0
E / V / Ag/Ag+
Fig.1. Cyclic voltammograms of TEMPO (0.01 M) recorded at 50 mV s-1 in 0.1M NaClO4-CH3CN solution (a) in
the presence of (b) 0.1 M benzaldehyde and (c) 0.1 M benzaldehyde+ 0.25 M HMDS +0.25 M AcOH.
3.2. Preparative electrolysis
Inspired by cyclic voltammetric experiments, we were interested in developing an
electrochemical method for the synthesis of benzonitrile from benzaldehyde. The optimized loading
of HMDS was 2.5 equiv. according to a series of experiments. The requirement for excess HMDS
might be due to the hydrolysis of HMDS. To avoid the unwanted oxidation of aldehydes to
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carboxylic acids and the decomposition of acetonitrile on the bare Pt electrode [35], electrolysis
experiments were carried out at different low potentials in our initial investigation. But a long
reaction time was required. Thus the electrolysis was performed at a moderate potential (1.5 V vs
Ag/Ag+) to promote the reaction rate. Although the amount of TEMPO employed in the reaction
should be as less as possible, the increase of the loading of TEMPO led to a shorter reaction time. It
was proved that 0.1 equiv. of TEMPO could provide a reasonable conversion and an acceptable
reaction rate.
As shown in Table 1, the scope of the electrochemical reaction was surveyed. Aromatic
aldehydes could be converted smoothly to aromatic nitriles. Benzaldehyde readily reacted to form
benzonitrile, and the GC yield of benzonitrile was high (99%, entry 1). The isolated yield of
benzonitrile was relatively low (78%) because, due to its volatility, part of it was lost during the
work up. Indeed, benzonitrile was detected in the solvent removed by rotary evaporation. The
same applies to o- and p-fluorobenzonitriles (entries 8 and 9). In agreement with this observation,
the isolated yield of non-volatile solid nitriles was very high (92-94%, entries 2, 14, and 17).
Benzaldehydes bearing electron-withdrawing substituents such as 4-nitrobenzaldehyde,
chlorobenzaldehyde (o-, m- and p-) and fluorobenzonitrile (o- and p-) underwent oxidative
conversion to produce the corresponding nitriles in good to excellent yields (entries 2-5, 8 and 9),
while it was difficult to fully convert bromobenzaldehydes (o- and m-) into bromobenzonitriles with
10 mol% of TEMPO (entrie 6 and 7). When a variety of tolualdehydes (o-, m- and p-) were
submitted to this electrochemical transformation, the conversions were higher than 99%, and yields
of methylbenzonitriles were higher than 83% (entries 10-12). The reaction of
4-(tert-butyl)benzaldehyde required a longer reaction time (15 h), and 4-(tert-butyl)benzonitrile
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could be obtained in 87% yield (entry 13). However, 4-methoxybenzaldehyde needed an increased
TEMPO loading and extended reaction time. A 98% conversion of 4-methoxybenzaldehyde with
92% isolated yield of 4-methoxybenzonitrile was achieved in 25 h by increasing the loading of
TEMPO to 20 mol% (entry 14).
The successful conversion of 5-bromofuran-2-carbaldehyde and benzofuran-2-carbaldehyde to
5-bromofuran-2-carbonitrile and benzofuran-2-carbonitrile showed that this reaction could tolerate
heteroaromatic substrates (entries 15 and 16). Aliphatic aldehydes were also investigated. The
oxidation of adamantane-1-carbaldehyde generated adamantane-1-carbonitrile in 94% isolated yield
(entry 17). When undecanal was used as the substrate, acceptable conversion could be achieved
under our initial reaction conditions, but the yield of undecanenitrile was very poor. It was due to the
self-aldol reaction of undecanal in the presence of HMDS [36]. Adding HMDS dropwise slowly into
the reaction solution actually led to a higher yield, though the self-aldol reaction could not be
completely avoided (entriey 18). For allylic aldehydes, such as cinnamaldehyde, this electrocatalytic
oxidation system could provide 94% conversion with 64% isolated yield of cinnamonitrile (entry
19).
Table 1. Electrochemical synthesis of nitriles from aldehydes[a]
Entry
1
Substrate
Product
Time/h Conversion/% Yield[b]/%
15
>99
78 (99)
12
>99
92
2
3
12
96
86
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12
12
>99
86
87
4
5
>99
86
12
12
12
12
12
79
79
6
7
92
>99
>99
>99
78 (92)
77 (90)
92
8
9
10
12
12
>99
>99
83
90
11
12
15
25
95
98
87
92
13
14[c]
15
15
10
15
15
90
91
97
90
92
82
80
94
59
64
15
16
17
18[d]
19[d]
[a] Reaction conditions: substrates (1 mmol), HMDS (2.5 equiv.), AcOH (2.5 equiv.), TEMPO (10 mol%) and
NaClO4 (0.1 M) in CH3CN (10 mL). [b] Isolated yield. Values in parentheses were determined by GC internal
standard method. [e]TEMPO (20 mol%). [d] HMDS was slowly added into the solution.
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3.3. In situ FTIR investigation
Fig. 2 shows the in situ FTIR spectra of 0.1 M NaClO4-CH3CN solution containing 0.1 M
benzaldehyde, 0.25 M HMDS and 0.01 M TEMPO. The reference potential was set at 0 mV and the
potential was varied stepwise from 0 mV to 800 mV. At potentials of 200 mV and higher, positive
and negative bands can be clearly seen.
Fig.2. In situ FTIR spectra collected on Pt electrode during the oxidation of 0.1 M benzaldehyde with
0.01 M TEMPO and 0.25 M HMDS in 0.1 M NaClO4-CH3CN solution.
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The positive bands at 1703 cm-1 and 1204 cm-1 were ascribed to the C=O and C-C stretching
vibrations of benzaldehyde respectively thus showing the consumption of benzaldehyde [37]. The
negative band at 1626 cm-1 was attributed to the N+=O stretching vibration of TEMPO+ [38].
-
Moreover, the C≡N vibration of acetonitrile was found at 2260 cm-1. ClO4 exhibited a characteristic
absorption peak at 1119 cm-1. In addition, another important signal, the negative-band at 2226 cm-1,
was detected as the C≡N vibration of the major product benzonitrile. The negative bands at 1076
cm-1 and 1261 cm-1 were ascribed to asymmetric stretching of Si-O-Si and Si-CH3 respectively
[39,40].
3.4. Mechanism investigation
In an effort to gain insight into the plausible reaction mechanism, the reaction mixture was
1
directly subjected to H NMR analysis. It was found that besides protons of benzaldehyde and
benzonitrile, several protons belonging to another compound could be observed within an hour. After
comparison with the authentic product synthesized by reaction of benzaldehyde with HMDS
catalysed
by
ZnCl2
[34],
we
confirmed
that
this
compound
was
N,N'-dibenzylidene-1-phenylmethanediamine (A). In addition, when 4-fluorobenzaldehyde was used
as the substrate, 1H NMR analysis showed it was almost completely converted to
N,N'-bis(4-fluorobenzylidene)-1-(4-fluorophenyl)methanediamine (B) in an hour (Scheme 2a). B
was also synthesized (see Experimental section) and its electrooxidation did afford
4-fluorobenzonitrile and 4-fluorobenzaldehyde. Thus, we infer that the N,N'-disubstituted
methanediamines are the crucial intermediates for the current synthesis of benzonitriles.
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With benzaldehyde as the model molecule, the overall reaction mechanism for the
electrochemical synthesis of benzonitrile from benzaldehyde using TEMPO as the mediator is shown
in Scheme 2b. Three benzaldehyde molecules can react with two HMDS molecules to give a
N,N'-dibenzylidene-1-phenylmethanediamine molecule, which is oxidized by two TEMPO+ ions to
give a benzaldehyde and two benzonitrile molecules in the presence of H2O. At the same time
TEMPO+ is reduced to TEMPOH, which can be regenerated by electro-oxidation.
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Scheme 2. (a) 1H NMR spectra of (1) 4-fluorobenzaldehyde, (2)
N,N'-bis(4-fluorobenzylidene)-1-(4-fluorophenyl)methanediamine and (3) the reaction mixture of oxidative
conversion of 4-fluorobenzaldehyde after reaction time of 1 h.
(b) Plausible reaction mechanism.
4. Conclusion
In summary, we have reported an efficient electrochemical method to generate nitriles from
aldehydes under mild conditions using a catalytic amount of TEMPO as the mediator and HMDS as
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the nitrogen source. Both aromatic aldehydes and aliphatic aldehydes give very good yields of
nitriles. According to the results of in situ FTIR and the identification of
N,N'-dibenzylidene-1-arylmethanediamine intermediates, a plausible reaction mechanism is
proposed.
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science
Foundations of China (21206147, 21376224).
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Highlights
(1) A novel electrochemical route to generate nitriles from aldehydes under mild conditions using a
catalytic amount of TEMPO as the mediator and hexamethyldisilazane as the nitrogen source
in the presence of acetic acid has been developed.
(2) A plausible reaction mechanism is proposed based on the cyclic voltammetry, in situ FTIR and
the confirmed intermediates.
18