Copper (II)-catalysed direct conversion of aldehydes into nitriles in acetonitrile

Article abstract of DOI:10.3184/174751917X15005550651302

A mild one-pot method for the direct conversion of aryl, heteroaryl and alkyl aldehydes into nitriles was achieved by forming the corresponding oximes in situ with NH₂OH and allowing them to react with CuO and acetonitrile. Yields of the 13 nitriles prepared were moderate to very good (62–91%).

Full text of DOI:10.3184/174751917X15005550651302

JOURNAL OF CHEMICAL RESEARCH 2017
VOL. 41 AUGUST, 465–468
RESEARCH PAPER 465
Copper (II)-catalysed direct conversion of aldehydes into nitriles in
acetonitrile
Xiaoyun Ma*, Jun Ao, Zhengjian Chen and Yi Liu
School of Chemistry and Life Sciences, Guizhou Education University, Guiyang 550018, P.R. China
A mild one-pot method for the direct conversion of aryl, heteroaryl and alkyl aldehydes into nitriles was achieved by forming the
corresponding oximes in situ with NH OH and allowing them to react with CuO and acetonitrile. Yields of the 13 nitriles prepared were
2
moderate to very good (62–91%).
Keywords: nitrile, copper salt, aldehyde, hydroxylamine hydrochloride, acetonitrile
2
1,25–27
Nitriles are useful organic synthetic intermediates, and play
salts has been reported.
With certain copper salts as
1,2
an important role in organic chemistry. Nitriles have been
widely utilised as synthetic intermediates for pharmaceuticals,
agricultural chemicals, dyes and material sciences, and some
nitriles have seen direct use as pesticides, perfumes, metal
catalysts, the transformation of aldoximes to nitriles proceeds
However, the direct conversion of aldehydes into
nitriles has been found to be relatively difficult when other
2
5–27
smoothly.
2
1
21
copper salts are used as catalysts. According to this report,
3
–5
inhibitors and liquid crystal materials. For these reasons,
much attention has been paid to the development of efficient and
practical methods for their synthesis. There are diverse methods
for the synthesis of nitrile groups from different organic
functional groups. Among these methods, the conversion of an
aldehyde into a nitrile using hydroxylamine hydrochloride as the
nitrogen source is considered to be one of the most important.
Usually, this conversion is achieved via aldoxime dehydration
in a two-step process. Direct conversion of an aldehyde into a
nitrile has also been reported, either by using high-temperature
the direct conversion of an aldehyde into a nitrile was achieved
by use of a commercially unavailable copper salt catalyst with
the help of microwave irradiation. Therefore, there is still a
need for a mild and convenient method for this conversion.
In this paper, we report the development of a mild method
for the direct conversion of aldehydes into nitriles by use of a
commercially available copper catalyst in acetonitrile.
Results and discussion
For our optimisation studies, we selected 4-(dimethylamino)
6
–14
conditions,
reagents
or by treatment with various dehydrating
benzaldehyde as
a
model substrate to investigate
15–19
or transition metal catalysts such as red mud (Fe O ,
the influence of ratio of reactant, reaction temperature, type of
base, various copper salts and solvent on the yields of the
reaction. The results are listed in Table 1.
As shown in Table 1, no nitrile product was obtained in the
absence of catalyst (entry 1). In this case, the aldehyde was
completely converted into the aldoxime. When a catalytic
2
3
21
2
0
Al O , SiO , Na O, TiO , MgO and CaO), CuO nanoparticles,
2
3
2
2
2
Fe O -CTAB
(cetyltrimethyl
ammonium
bromide)
3
4
2
2
2
23,24
nanoparticles, W-Sn hydroxide or zinc compounds.
At the
same time, we have particularly noticed that the conversion of
aldoximes or aldehydes into nitriles catalysed by other copper
Table 1 Optimisation of the reaction conditions (catalyst, solvent, temperature, duration of reaction) for the conversion of 4-(dimethylamino)benzaldehyde
a
to 4-(dimethylamino)benzonitrile
O
N
H
N
N
b
Entry
Conditions
Yield (%)
1
2
3
4
5
6
7
8
9
NH OH·HCl (1.0 equiv.), NaOAc (1.0 equiv.), CH CN, reflux, 4 h
0
70
91
23
0
55
16
12
0
2
3
CuO (0.05 equiv.), NH OH·HCl (1.0 equiv.), NaOAc (1.0 equiv.), CH CN, reflux, 4 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, reflux, 4 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, 50°C, 4 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, 25 °C, 8 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOH (1.5 equiv.), CH CN, reflux, 4 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), Na CO (0.75 equiv.), CH CN, reflux, 4 h
2
2
3
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), pyridine (1.5 equiv.), CH CN, reflux, 4 h
2
3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), triethylamine (1.5 equiv.), CH CN, reflux, 4 h
2
3
1
0
Cu(OAc) (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, reflux, 4 h
CuSO ·5H O (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, reflux, 4 h
4 2 2 3
86
74
25
0
2
2
3
1
1
1
2
CuCl ·2H O (0.05 equiv.), NH OH·HCl (1.5 equiv.), NaOAc (1.5 equiv.), CH CN, reflux, 4 h
2 2 2 3
CuO (0.05 equiv.), NH OH·HCl (1.5 equiv.), triethylamine (1.5 equiv.), EtOH, reflux, 4 h
2
13
a
Reaction conditions: a stirred mixture of 4-(dimethylamino)-benzaldehyde (2.0 equiv.), hydroxylamine hydrochloride (1.0 or 1.5 equiv.), base (0.75–1.5 equiv.), and a Cu(II) salt (0.05
equiv.) in a solvent (6 mL) was reacted at various temperatures and for various times.
b
Isolated yield.
*
Correspondent. E-mail: maxiaoyun821215@163.com

4
66 JOURNAL OF CHEMICAL RESEARCH 2017
amount of copper oxide was added into the reaction system, the
yield of 4-(dimethylamino)benzonitrile drastically increased to
cupric acetate (Cu(OAc) ). Cupric acetate has similar catalytic
2
activity to copper oxide, but copper oxide features higher
catalytic selectivity and less by-product (amide). Use of ethanol
in place of acetonitrile resulted in no product. Therefore, we
thought that acetonitrile may be involved in the reaction.
On the basis of the above results, we can conclude that
increasing the amounts of hydroxylamine hydrochloride and
base will improve the yield of the product and using CuO,
70% (entry 2). It is worth noting that some 4-(dimethylamino)
benzaldehyde was recovered unchanged. When we increased
the amounts of NH OH·HCl and NaOAc to 1.5 equiv. each,
2
the yield went up to 91% (entry 3). Decreasing the reaction
temperature to 50 °C resulted in a decreased yield, while
at 25 °C the reaction was hampered (entries 4 and 5). The
influence of bases such as NaOAc, NaOH, Na CO , pyridine
NH OH·HCl and NaOAc in acetonitrile at refluxing temperature
2
2
3
is the optimum conditions for the conversion of an aldehyde into
a nitrile. This procedure was subsequently applied to various
aromatic aliphatic and heterocyclic aldoximes and the results
are shown in Table 2.
and triethylamine was examined. It is clear that NaOAc gave the
best result (entry 3); a lower yield was obtained using NaOH,
Na CO3 or pyridine (entries 6–8). No 4-(dimethylamino)
2
benzonitrile was detected in the case of triethylamine (entry 9).
The various copper salts listed in Table 1 appear to efficiently
catalyse the reaction (entries 3, 10 and 11), and it was shown that
Various aldehydes including aromatic aldehydes (entries
1–8), an aliphatic aldehyde (entry 9) and heterocyclic aldehydes
(entries 10–12) were converted into the corresponding nitriles
copper oxide (CuO) and cupric acetate (Cu(OAc) ) were much
2
in moderate to good yields. Aldehydes bearing an electron-
donating group reacted more effectively than substrates with
an electron-withdrawing group (entries 1 and 5) and gave the
corresponding nitriles in good yields and in a relatively shorter
time (entries 1–3). In contrast, electron-withdrawing groups
on the aromatic rings decreased the speed of the reaction and
nitriles were obtained in slightly lower yields (entries 5 and
6). The sterically hindered o-substituted aldehydes were slow
to react and the reaction time needed to be increased to 10 h
more efficient than copper sulfate pentahydrate (CuSO ·5H O)
4
2
and copper(II) chloride dihydrate (CuCl ·2H O). We also
2
2
compared the catalytic activity of copper oxide (CuO) and
Table 2 Yields of a series of nitriles (R = various) prepared by reaction of
a
an aldehyde (R = various) with hydroxylamine and CuO in MeCN
O
CuO, CH CN
N
3
R
R
H
NH OH HCl, CH
3
COONa
2
(entries 7 and 8). In the case of an aliphatic aldehyde (entry 9),
b
Entry
1
R
Time (h)
4
Yield (%)
90
there was a smooth conversion into the corresponding nitrile in
a good yield. Heterocyclic aldehydes with a heteroatom lone pair
positioned ortho to the aldehyde group showed high reactivity
and the reactions were completed after a relatively short
reaction time of 3 h. However, in these cases, the corresponding
nitriles were obtained in relatively low yields and the yield of
the corresponding amide was appreciable (26–34%; entries 11
and 12).
As can be seen from Table 1, no nitrile product was detected
when ethanol was used in place of acetonitrile as solvent (entry
13); therefore, we thought that acetonitrile not only serves
as a solvent but also participates in the reaction. Indeed, we
thought that the mechanism of the conversion of aldoximes to
4-MeO-C6H4
2
3
4
5
6
7
8
9
4-OH-C6H4
3-MeO-4-MeO-5-MeO-C6H
4
5
5
6
88
86
87
78
83
87
88
85
75
2
Ph
4-NO -C H
2
6 4
4-Cl-C6H
4
6
2-Cl-4-Cl-C6H
3
10
10
6
6
3
2-Cl-C6H
4
n-C H
1 23
1
1
0
4-pyridyl
2-furyl
c
11
62(34)
c
12
2-thienyl
3
69(26)
nitriles was likely to be similar to the mechanism reported in
a
26,27
Reaction conditions: a stirred mixture of an aldehyde (2 mmol), NH OH·HCl (3 mmol),
the literature.
A possible reaction pathway is presented in
2
NaOAc (3 mmol), CuO (0.1 mmol) in MeCN (6 mL) was refluxed for various times.
Scheme 1. The coordination of acetonitrile to copper ion results
in an enhanced electrophilicity of the nitrile carbon facilitating
the nucleophilic addition of 4-(dimethylamino)benzaldehyde
b
Isolated yield.
c
Yield of the corresponding amide is given in brackets.
O
H
NOH
N
O
NH OH HCl
2
Cu(II), CH CN
3
N
H C
NH2
Base
3
N
N
Cu(II), CH CN
3
N
N
Cu(II)
H C
3
N
HO
C
HO
N
H
H
N
H C
C
N
3
Cu(II)
Scheme 1 Proposed mechanism for nitrile synthesis reaction.

JOURNAL OF CHEMICAL RESEARCH 2017 467
oxime. The reaction proceeds smoothly via a six-membered
intermediate to yield the hydrated product, acetamide and the
dehydrated product, 4-(dimethylamino)benzonitrile. The yield
of the by-product amide depends on the hydrolytic lability of
the product nitrile. The by-product of this reaction, acetamide,
was identified in the crude by extract by LC-MS.
In conclusion, we disclose a simple method for the direct
conversion of aldehydes into nitriles using an inexpensive copper
salt catalyst in acetonitrile. Both the catalyst and the reagents are
commercially available and the reactions were carried out under
relatively mild reaction conditions. In this method, aromatic,
heterocyclic and aliphatic aldehydes were converted into the
corresponding nitriles in moderate to good yields.
to give 4-hydroxybenzonitrile as: White solid; yield 209 mg (88%);
14
1
m.p. 109–111 °C (lit. 108–110 °C); H NMR (500 MHz, DMSO-d ):
6
δ 6.89 (dd, J = 8.7, 1.9 Hz, 2H), 7.61 (m, 2H), 10.64 (s, 1H). Data are in
3
0
good agreement with the literature.
3
,4,5-Trimethoxylbenzonitrile (Table 2, entry 3): The residue was
purified by column chromatography on silica gel (ethyl acetate/n-
hexane = 1:3) to give 3,4,5-trimethoxybenzonitrile as: White solid;
8
1
yield 323 mg (86%); m.p. 92–94 °C (lit. 93 °C); H NMR (500 MHz,
CDCl ): δ 3.87 (s, 6H), 3.89 (s, 3H), 6.86 (s, 2H). Data are in good
3
31
agreement with the literature.
Benzonitrile (Table 2, entry 4): The residue was purified by column
chromatography on silica gel (ethyl acetate/n-hexane = 1:15) to give
1
benzonitrile as: Colourless liquid; yield 179 mg (87%); H NMR
(
500 MHz, CDCl ): δ 7.49 (m, 2H), 7.63 (m, 1H), 7.67 (m, 2H). Data
3
32
are in good agreement with the literature.
Experimental
Reagent grade chemicals were purchased from Aladdin Reagent
4
-Nitrobenzonitrile (Table 2, entry 5): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:5)
(
Shanghai, China) and were used without further purification.
to give 4-nitrobenzonitrile as: White solid; yield 231 mg (78%); m.p.
1
H NMR spectra were obtained on a Bruker DPX-500 or a Bruker
20
1
1
47–148 °C (lit. 146–149 °C); H NMR (500 MHz, CDCl ): δ 7.90 (m,
3
DPX-400 MHz spectrometer in DMSO-d or CDCl using TMS as an
32
6
3
2H), 8.37 (m, 2H). Data are in good agreement with the literature.
internal standard. Chemical shifts (δ) are given in ppm and coupling
constants (J) are given in Hz. Melting points were determined on a
Thomas Hoover capillary apparatus and are uncorrected. Analytical
thin-layer chromatography (TLC) was performed on pre-coated silica
gel 60 F254 plates. Yields refer to the isolated yields of the products
after purification by silica-gel column chromatography (300 mesh).
4
-Chlorobenzonitrile (Table 2, entry 6): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:15)
to give 4-chlorobenzonitrile as: White solid; yield 228 mg (83%); m.p.
2–94 °C (lit. 91–93 °C); H NMR (500 MHz, CDCl ): δ 7.48 (d,
J = 8.4 Hz, 2H), 7.62 (d, J = 8.4 Hz, 2H). Data are in good agreement
20
1
9
3
33
with the literature.
2
,4-Dichlorobenzonitrile (Table 2, entry 7): The residue was purified
by column chromatography on silica gel (ethyl acetate/n-hexane =
:15) to give 2,4-dichlorobenzonitrile as: White solid; yield 299 mg
Synthesis of nitriles; general procedure
An aldehyde (2 mmol), CuO (0.1 mmol), NH OH·HCl (3 mmol),
2
1
NaOAc (3 mmol) and acetonitrile (6 mL) were added to a 25 mL round-
bottom flask equipped with magnetic stirrer. The mixture was heated
to reflux for 3–10 h. After cooling to r.t., acetonitrile was distilled
off under reduced pressure. Water (15 mL) and ethyl acetate (30 mL)
were added to the reaction mixture, and the organic layer was washed
with saturated aqueous sodium bicarbonate (1 × 15 mL) and water
8
1
(
87%); m.p. 58–60 °C (lit. 59 °C); H NMR (500 MHz, CDCl ): δ 7.39
3
(dd, J = 8.4, 2.0 Hz, 1H), 7.55 (d, J = 1.9 Hz, 1H), 7.62 (d, J = 8.4 Hz,
2
8
1
H). Data are in good agreement with the literature.
2
-Chlorobenzonitrile (Table 2, entry 8): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:10)
to give 2-chlorobenzonitrile as: White solid; yield 242 mg (88%);
(
1 × 15 mL). The organic layer was dried with Na SO , evaporated and
2 4
8
1
m.p. 43–44 °C (lit. 42 °C); H NMR (500 MHz, CDCl ): δ 7.39 (m,
the residue was purified by column chromatography on silica gel (ethyl
3
1
H), 7.56 (m, 2H), 7.69 (m, 1H). Data are in good agreement with the
acetate/n-hexane) to give the corresponding nitriles. All the nitriles are
33
1
literature.
commercially available and were characterised by H NMR spectra,
Dodecanenitrile (Table 2, entry 9): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:15)
to give dodecanenitrile as: Colourless liquid; yield 308 mg (85%);
which are available in the ESI.
Identification of acetamide as a by-product
1
4
-(Dimethylamino)benzonitrile was synthesised using the method
H NMR (500 MHz, CDCl ): δ 0.89 (t, J = 7.0 Hz, 3H), 1.27–1.33 (m,
3
described above. After cooling to r.t., the reaction mixture was filtered
and the filtrate was concentrated under reduced pressure to provide the
crude product. The crude product was analysed by LC-MS. From the
LC-MS spectra, we found that acetamide was generated. In addition
to the product 4-(dimethylamino)benzonitrile, 4-(dimethylamino)
benzamide, 4-(dimethylamino)benzaldehyde and 4-(dimethylamino)
benzoic acid were also detected.
14H), 1.45 (m, 2H), 1.66 (m, 2H), 2.34 (t, J = 7.2 Hz, 2H). Data are in
good agreement with the literature.
3
4
Isonicotinonitrile (Table 2, entry 10): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:1)
to give isonicotinonitrile as: White solid; yield 156 mg (75%); m.p.
2
0
1
76–78 °C (lit. 76 °C); H NMR (500 MHz, CDCl ): δ 7.54 (m, 2H),
3
31
8.82 (m, 2H). Data are in good agreement with the literature.
Acetamide: MW: 59.07; found: 60.1 (M + 1).
Furan-2-carbonitrile (Table 2, entry 11): The residue was purified by
column chromatography on silica gel (ethyl acetate/n-hexane = 1:15 and
2:1) to give furan-2-carbonitrile as: Colourless liquid; yield 115 mg (62%);
4
4
4
4
4
-(Dimethylamino)benzonitrile: MW: 146.19; found: 147.1 (M + 1).
-(Dimethylamino)benzamide: MW: 164.21; found: 165.1 (M + 1).
-(Dimethylamino)benzaldehyde: MW: 149.19; found: 150.2 (M + 1).
-(Dimethylamino)benzoic acid: MW: 165.19; found: 166.2 (M + 1).
-(Dimethylamino)benzonitrile (Table 1, entry 3): The residue was
1
H NMR (500 MHz, CDCl ): δ 6.55 (m, 1H), 7.12 (m, 1H), 7.60 (m, 1H).
3
35
Data are in good agreement with the literature. Furan-2-carboxamide
was obtained as a by-product: White solid; yield 75 mg (34%; m.p.
3
6
1
purified by column chromatography on silica gel (ethyl acetate/n-
139–141 °C (lit. 140–142 °C); H NMR (400 MHz, DMSO-d ): δ 7.81 (d,
6
hexane = 1:10) to give 4-(dimethylamino)benzonitrile as: White solid;
1H, J = 0.8 Hz), 7.79 (br. s, 1H), 7.40 (br. s, 1H), 7.10 (d, 1H, J = 3.2 Hz),
6.61 (m, 1H). Data are in good agreement with the literature.
2
0
1
36
yield 266 mg (91%): m.p. 70–71 °C (lit. 69–71 °C); H NMR (500
MHz, CDCl ): δ 3.04 (s, 6H), 6.63 (m, 2H), 7.47 (m, 2H). Data are in
Thiophene-2-carbonitrile (Table 2, entry 12): The residue was
purified by column chromatography on silica gel (ethyl acetate/n-
hexane = 1:15 and 2:1) to give thiophene-2-carbonitrile as: Colourless
3
2
8
good agreement with the literature.
4
-Methoxybenzonitrile (Table 2, entry 1): The residue was purified
1
by column chromatography on silica gel (ethyl acetate/n-hexane = 1:4)
liquid; yield 150 mg (69%); H NMR (500 MHz, CDCl ): δ 7.14 (m,
3
to give 4-methoxybenzonitrile as: White solid; yield 239 mg (90%);
1H), 7.62 (m, 1H), 7.65 (m, 1H). Data are in good agreement with
2
0
1
32
m.p. 56–58 °C (lit. 57–59 °C); H NMR (500 MHz, CDCl ): δ 3.86 (s,
the literature. Thiophene-2-carboxamide was obtained as a by-
3
3
7
1
H), 6.97 (d, J = 8.8 Hz, 2H), 7.60 (d, J = 8.8 Hz, 2H). Data are in good
product: White solid; yield 66 mg (26%); m.p. 178–179 °C (lit.
2
9
1
agreement with the literature.
179.1–180.2 °C); H NMR (400 MHz, DMSO-d ): δ 7.97 (br. s, 1H),
7.74 (d, 2H, J = 4.0 Hz), 7.39 (br. s, 1H), 7.13 (t, 1H, J = 4.0 Hz). Data
6
4
-Hydroxybenzonitrile (Table 2, entry 2): The residue was purified
37
by column chromatography on silica gel (ethyl acetate/n-hexane = 1:3)
are in good agreement with the literature.

4
68 JOURNAL OF CHEMICAL RESEARCH 2017
Acknowledgements
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3
Financial support from the Doctor Project of Guizhou
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