Copper-catalyzed synthesis of nitriles by aerobic oxidative reaction of alcohols and ammonium formate

Article abstract of DOI:10.1002/ejoc.201300361

An efficient methodology has been developed for the synthesis of nitriles through an aerobic oxidative reaction of alcohols and ammonium formate with copper as a homogeneous catalyst under a normal air atmosphere and solvent-free conditions. This protocol uses the air as a green oxidant and ammonium formate as the nitrogen source. A wide range of substrates were well tolerated in the reaction that gave water as a byproduct. A facile protocol, which uses copper as an effective homogeneous catalyst, has been developed for the oxidative synthesis of nitriles from alcohols and ammonium formate. This system uses air as a green oxidant and produces water as an environmentally benign side product. Copyright

Full text of DOI:10.1002/ejoc.201300361

Job/Unit: O30361
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Pages: 6
FULL PAPER
Nitrile Synthesis
A facile protocol, which uses copper as an
effective homogeneous catalyst, has been
developed for the oxidative synthesis of ni-
triles from alcohols and ammonium for-
mate. This system uses air as a green oxi-
dant and produces water as an environmen-
tally benign side product.
D. K. T. Yadav, B. M. Bhanage* ....... 1–6
Copper-Catalyzed Synthesis of Nitriles by
Aerobic Oxidative Reaction of Alcohols
and Ammonium Formate
Keywords: Synthetic methods / Homo-
geneous catalysis / Alcohols / Copper / Oxi-
dation
0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: O30361
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Date: 24-06-13 16:21:40
Pages: 6
D. K. T. Yadav, B. M. Bhanage
FULL PAPER
DOI: 10.1002/ejoc.201300361
Copper-Catalyzed Synthesis of Nitriles by Aerobic Oxidative Reaction of
Alcohols and Ammonium Formate
[a]
[a]
Dilip Kumar T. Yadav and Bhalchandra M. Bhanage*
Keywords: Synthetic methods / Homogeneous catalysis / Alcohols / Copper / Oxidation
An efficient methodology has been developed for the synthe-
sis of nitriles through an aerobic oxidative reaction of
alcohols and ammonium formate with copper as a homogen-
eous catalyst under a normal air atmosphere and solvent-free
conditions. This protocol uses the air as a green oxidant and
ammonium formate as the nitrogen source. A wide range of
substrates were well tolerated in the reaction that gave water
as a byproduct.
Introduction
methylsilyl azide) as the nitrogen source, and catalytic
amounts of Cu(ClO ) ·6H O in the presence of DCE
4
2
2
Nitriles are widely used in chemical as well as biological
applications as they serve as important building blocks in
the pharmaceutical, fine chemical, dye, and agrochemical
[14]
(
dichloroethane) as the solvent. However, the use of non-
green TMSN and DDQ requires tedious storage and hand-
3
ling problems. Both of them are air- and moisture-sensitive
reagents and produce highly toxic products when they come
in contact with water. Thus, the development of a highly
efficient and environmentally benign solvent-free protocol
for the oxidative conversion of alcohols into nitriles is still
a challenging task.
In this work, we have developed a highly efficient proto-
col for the oxidative synthesis of nitriles from the reaction
between alcohols and ammonium formate, an inexpensive
nitrogen source (see Scheme 1). This approach employs a
copper catalyst and is carried out under air, which serves as
a green oxidant.
[
1]
industries. Moreover, they are one of the most valuable
compounds employed in synthetic organic chemistry for the
preparation of amines, amides, carbonyl compounds, and
[
2]
heterocycles.
Traditionally, nitriles were synthesized
through the nucleophilic substitution of various leaving
groups, such as halogen, hydroxy, alkoxy, diazonium, and
sulfonyl functionalities, with inorganic cyanides. Alterna-
[3]
[4]
tive procedures include the dehydration of amides and al-
doximes, the elimination of oxime ethers[6] and oxime es-
[
5]
ters,[ and the oxidative conversion of aldehydes and pri-
7]
[8]
[
9]
mary amines into their corresponding nitriles. There are
also a few more reports regarding the synthesis of ni-
[
10]
triles. However, despite the potential utility of earlier re-
ported protocols, the use of toxic solvents and expensive
reagents along with the production of large amounts of in-
organic waste and tedious work-up procedures limits their
application from a green chemistry perspective.
The oxidative conversion of alcohols into nitriles is an
attractive alternative. In this regard, various protocols were
developed, but they required stoichiometric amounts of oxi- Scheme 1. Oxidative synthesis of nitriles from alcohols and ammo-
[
11]
dants and reagents. Mizuno and co-workers reported the nium formate.
[
12]
use of the catalysts Ru(OH) /Al O
and Ru(OH)x/
x
2
3
[
13]
TiO2 for the oxidative synthesis of nitriles from alcohols.
Recently, Prabhu and co-workers reported on the direct oxi-
dative synthesis of nitriles by using DDQ (2,3-dichloro-5,6-
dicyano-1,4-benzoquinone) as the oxidant, TMSN3 (tri-
Results and Discussion
A series of experiments were performed to optimize the
reaction conditions for the oxidative conversion of benzyl
a] Department of Chemistry, Institute of Chemical Technology, N. alcohol (1a) into benzonitrile (2a). Initially, we studied the
[
Parekh Marg, Matunga,
effect of various nitrogen sources with CuCl ·2H O as the
2
2
Mumbai 400019, India
E-mail: bm.bhanage@gmail.com
catalyst and K CO as the base (see Table 1, Entries 1–5).
2 3
bm.bhanage@ictmumbai.edu.in
Ammonium formate (HCOONH ) furnished a very good
4
Homepage: http://www.ictmumbai.edu.in/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300361.
yield of benzonitrile (2a) along with the maximum conver-
sion of 1a, and, thus, it was used for further studies (see
2
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Nitriles by Aerobic Oxidative Reaction
Table 1, Entry 5). The effect of the HCOONH4 loading Table 2. Effect of catalyst screening on the oxidative synthesis of
nitriles.[
a]
showed that the conversion of 1a and yield of 2a increased
as the 1a/HCOONH ratio increased from 1:1 to 1:4 (see
4
Table 1, Entries 5–7). When the reaction was carried out
under nitrogen, the desired product was not observed,
which indicated the importance of atmospheric air as the
oxidant (see Table 1, Entry 8).
Table 1. Effect of the nitrogen source on the oxidative synthesis of
nitriles.
Yield [%][b]
Entry
Catalyst
CuO
Base
[
a]
2a
3a
1
2
3
4
5
6
7
8
9
K
2
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
62
84
67
72
65
78
100
77
70
72
90
100
–
38
16
33
28
35
22
–
23
30
28
10
–
Cu(OAc)
2
·5H
2
O
CuI
CuCl
Cu
CuSO
CuCl
CuCl ·2H O
2
O
4
·5H
2
O
2
O
2
·2H
Entry
N source
Catalyst
Conv. [%]
Yield [%][b]
KOH
KOtBu
NaOtBu
2
2
2a
3a
CuCl ·2H O
2
2
2
2
2
2
2
2
1
0
CuCl
CuCl
CuCl
·2H
·2H
·2H
O
O
O
1
2
3
4
5
NH
NH NO
HPO
SO
4
Cl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
2
2
2
2
2
2
2
2
·2H
·2H
·2H
·2H
·2H
·2H
·2H
·2H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
55
70
65
68
85
91
100
0
33
43
39
43
57
79
100
–
22
27
26
25
28
12
–
[
c]
11
K
2
K
2
K
2
CO
CO
CO
3
3
3
4
)
3
[d]
12
(NH
(NH
4
2
)
4
13
none
–
4
2
4
HCOONH
HCOONH
HCOONH
HCOONH
4
[a] Reagents and conditions: 1a (0.5 mmol), HCOONH4
(2.0 mmol), catalyst (0.05 mmol, 10 mol-%), base (0.05 mmol),
135 °C, 24 h. [b] Yield based on GC analysis. [c] Temperature is
125 °C. [d] Temperature is 145 °C.
[
c]
6
7
8
4
4
4
[
d]
[
e]
–
[
(
2
1
a] Reagents and conditions: 1a (0.5 mmol), nitrogen source
0.5 mmol), CuCl ·2H O (0.05 mmol), K CO (0.05 mmol), 135 °C,
4 h. [b] Yield based on GC analysis. [c] Reaction carried out with
a/HCOONH in a ratio of 1:2. [d] Reaction carried out with 1a/
in a ratio of 1:4. [e] Reaction carried out under nitro-
2
2
2
3
efficiently with various benzylic and allylic alcohol deriva-
tives to provide a wide range of substituted nitriles in good
to excellent yields. The prominent feature of the present
protocol is that this system has a significant extent of func-
tional group tolerance, that is, chloro, bromo, and nitro
4
HCOONH
gen.
4
Thereafter, using the optimum conditions, we studied the substituents intrinsically remained intact under the reaction
effect of various copper catalysts such as CuO, Cu(OAc)2· conditions (see Table 3, Entries 2–5, 10, and 11). However,
5
H O, CuI, CuCl, Cu O, CuSO ·5H O, and CuCl ·2H O the steric effects of the substituents do have an influence on
2 2 4 2 2 2
on the nitrile synthesis in the presence of K CO as the base the reaction. For instance, when meta- and para-substituted
2
3
(see Table 2, Entries 1–7). Among the screened catalysts, benzyl alcohols were used, the reaction proceeded smoothly
CuCl ·2H O was the most effective to provide an excellent to provide good yields of the corresponding nitriles (see
2
2
yield of the desired nitrile 2a (see Table 2, Entry 7). Further- Table 3, Entries 3, 4, 7, 8, and 11). On the other hand, ortho
more, we studied the effect of the catalyst loading and substituents had a negative effect on the transformation (see
found that the yield of 2a decreased when the concentration Table 3, Entries 2, 6, and 10). Benzyl alcohol derivatives
of the catalyst was decreased from 10 to 5 mol-% (80%), with electron-withdrawing and electron-donating groups
and, hence, 10 mol-% of the catalyst was used for further were smoothly converted into the corresponding nitriles in
studies. Noting the importance of the base to the reaction excellent yields (see Table 3, Entries 7–9 and 11–13). When
outcome, various bases such as K CO , KOH, KOtBu, and the simple aromatic ring of benzyl alcohol was replaced
2
3
NaOtBu were screened (see Table 2, Entries 7–10), and, with more hindered biphenyl, naphthyl, and 1,3-benzodiox-
hence, further studies were carried out by using K CO as ole groups, the corresponding nitriles were also obtained in
2
3
it provided the maximum yield of the corresponding nitrile. good yields (see Table 3, Entries 14–16). Cinnamyl alcohol
Subsequently, the effect of temperature was also investi- was also converted into the corresponding unsaturated ni-
gated, and the yield of 2a decreased as the temperature was trile without isomerization and hydrogenation of the double
lowered (see Table 2, Entry 11). The decrease in the time bond (see Table 3, Entry 17). However, an aliphatic alcohol
from 24 to 20 h also resulted in a low conversion of 1a, did not produce the desired nitrile product under the opti-
which indicated that 24 h was the optimum time required mized reaction conditions (see Table 3, Entry 18).
for the completion of the reaction. When the reaction was
carried out in the absence of catalyst, no conversion of 1a nitrogen source (i.e., the ammonium salts), it gave an alde-
was observed (see Table 2, Entry 13). hyde as the major product, which indicates that the first
When the reaction was carried out in the absence of a
With these optimized reaction conditions, the scope of step of the mechanism is the oxidative conversion of an
protocol was extended to the synthesis of various nitrile alcohol to an aldehyde. Hence, on the basis of our investiga-
[
15a,15b]
derivatives. As illustrated in Table 3, the reaction proceeded tion and literature reports,
a plausible reaction
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D. K. T. Yadav, B. M. Bhanage
FULL PAPER
Table 3. Substrate study for oxidative synthesis of nitriles.[a]
[
1
a] Reagents and conditions: alcohol (0.5 mmol), HCOONH
0 mol-%), 135 °C, 24 h. [b] Isolated yield. ND = product not detected.
4
2 2 2 3
(2.0 mmol), CuCl ·2H O (0.05 mmol, 10 mol-%), K CO (0.05 mmol,
mechanism is proposed (see Scheme 2). The first step in- catalyst.[15c] Next, the dehydrative condensation of the alde-
volves the aerobic oxidative dehydrogenation of benzyl hyde by treatment with NH (ammonia is generated in situ
3
alcohol to benzaldehyde in the presence of an active copper from the ammonium salt) gives an imine through the for-
mation of a hemiaminal. At last, the imine undergoes an
aerobic oxidative dehydrogenation to form the correspond-
ing nitrile product.
Conclusions
We have established a new, efficient, and useful method-
ology for the direct oxidative synthesis of nitriles from benz-
ylic and allylic alcohols by employing ammonium formate
as the nitrogen source along with a copper catalyst. In com-
parison to earlier reported systems, the developed protocol
has significant advantages such as: (i) the use of the inex-
pensive and easily available CuCl ·6H O catalyst compared
2
2
to the use of noble metal catalysts, (ii) the solvent- and ad-
ditive-free reaction conditions, which makes the protocol
Scheme 2. Plausible reaction pathway for the copper-catalyzed oxi-
dative synthesis of nitriles.
4
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Nitriles by Aerobic Oxidative Reaction
greener than earlier methods, (iii) the present system cir- 4-Chlorobenzonitrile (2d):
[
16c]
White solid (65% yield). IR (KBr): ν˜
–
1
+
=
2225 cm . GC–MS (EI, 70 eV): m/z (%) = 137.0 (100) [M] , 102
cumvents the use of oxidants such as 2,3-dichloro-5,6-di-
cyano-1,4-benzoquinone (DDQ), I , H IO , and 1,3-di-
1
(
=
3
37), 75 (21), 50 (16). H NMR (500 MHz, CDCl ): δ = 7.60 (d, J
2
5
6
1
3
8.4 Hz, 2 H), 7.47 (d, J = 8.4 Hz, 2 H) ppm. C NMR
iodo-5,5-dimethylhydantoin (DIH), and (iv) the use of a
handy and inexpensive nitrogen source such as ammonium
formate. Thus, the developed protocol appears to be highly
efficient, green, and economically accepted as well as has a
wide substrate applicability, which is appealing for further
investigation.
(125 MHz, CDCl
3
): δ = 139.5, 133.3, 129.6, 117.9, 110.7 ppm.
[
16d]
4
=
(
(
(
-Bromobenzonitrile (2e):
White solid (60% yield). IR (KBr): ν˜
–
1
+
2240 cm . GC–MS (EI, 70 eV): m/z (%) = 181.0 (68) [M] , 102
1
100), 75 (38), 50 (25). H NMR (500 MHz, CDCl
d, J = 8.5 Hz, 2 H), 7.53–7.52 (d, J = 8.4 Hz, 2 H) ppm. C NMR
125 MHz, CDCl ): δ = 133.3, 132.5, 127.9, 117.9, 111.1 ppm.
3
): δ = 7.65–7.63
1
3
3
[
16e]
(
(
[
2m) 4-Isopropylbenzonitrile:
Colorless liquid (94% yield). IR
–
1
neat): ν˜ = 2226 cm . GC–MS (EI, 70 eV): m/z (%) = 145.0 (25)
M] , 130 (100), 103 (25), 77 (10). H NMR (500 MHz, CDCl ): δ
3
Experimental Section
+
1
General Methods: All chemicals were purchased from Lancaster
= 7.58–7.57 (d, J = 8.0 Hz, 2 H), 7.33–7.31 (d, J = 8.0 Hz, 2 H),
2.99–2.90 (septet, J = 7.0 Hz, 1 H), 1.27–1.25 (d, J = 7.0 Hz, 6 H)
ppm. C NMR (125 MHz, CDCl ): δ = 154.2, 132.1, 127.2, 119.0,
3
(Alfa-Aesar), Sigma Aldrich, and S. D. Fine Chemical and com-
1
3
mercial suppliers. Gas chromatography (GC) and TLC were used
to monitor the progress of the reactions. Gas chromatography 109.5, 34.3, 23.4 ppm.
analysis was carried out with a Perkin–Elmer Clarus 400 GC
Supporting Information (see footnote on the first page of this arti-
cle): Characterization data, and copies of the H and C NMR
spectra for selected compounds.
equipped with a flame ionization detector with a capillary column
Elite-1, 30 mϫ0.32 mmϫ0.25 μm). GC–MS was carried out with
a GC–MS-QP 2010 instrument [Rtx-17, 30 mϫ25 mm ID, film
1
13
(
–1
thickness (df): 0.25 μm, column flow: 2 mLmin , 80–240 °C at
1
1
0 °Cmin–1 rise]. The H NMR spectroscopic data were recorded Acknowledgments
3
with a Bruker 500 MHz spectrometer with CDCl as the solvent
and TMS as the internal standard. The 13C NMR spectroscopic
data were recorded with a Bruker 125 MHz spectrometer with
D. K. T. Y. is greatly thankful to the University Grant Commission
(UGC), India for its financial support under the UGC-SAP pro-
gram of the Institute of Chemical Technology (ICT), Matunga.
CDCl
3
as the solvent. Chemical shifts are reported in parts per
million (δ) relative to TMS as the internal standard. J (coupling
1
constant) values are reported in Hz. Splitting patterns in the
H
[
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2
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1
13
MS, H, and C NMR data with those of authentic samples.
General Experimental Procedure for Oxidative Synthesis of Aryl
Nitriles from Aryl Alcohols: A mixture of aryl alcohol (0.5 mmol),
HCOONH
4 2 2
(2 mmol), CuCl ·2H O (0.05 mmol, 10 mol-%), and
K
2
CO (0.05 mmol, 10 mol-%) was stirred at 135 °C under a nor-
3
mal air atmosphere in a sealed 20 mL Schlenk tube for 24 h. The
reaction was monitored by TLC and/or GC. After completion of
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1
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1
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Characterization of Selected Compounds
[
16a]
Benzonitrile (2a):
Colorless liquid (96% yield). IR (neat): ν˜ =
–1
+
2
5
7
1
224 cm . GC–MS (EI, 70 eV): m/z (%) = 103 (100) [M] , 76 (43),
3
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32.7, 132.0, 129.0, 118.8, 112.3 ppm.
): δ =
3
[
16b]
3
=
(
(
-Chlorobenzonitrile (2c):
White solid (75% yield). IR (KBr): ν˜
2
007, 46, 3922–3925.
–1
+
2224 cm . GC–MS (EI, 70 eV): m/z (%) = 137.0 (100) [M] , 102
[
6] K. Maeyama, M. Kobayashi, H. Kato, N. Yonezawa, Synth.
1
37), 75 (22), 50 (15). H NMR (500 MHz, CDCl
3
): δ = 7.65–7.55
m, 3 H, Ar), 7.45–7.41 (m, 1 H, Ar) ppm. C NMR (125 MHz,
): δ = 135.1, 133.1, 131.8, 130.4, 130.2, 117.3, 113.9 ppm.
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13
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4685–4690.
Eur. J. Org. Chem. 0000, 0–0
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D. K. T. Yadav, B. M. Bhanage
FULL PAPER
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Published Online: ᭿
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