Iron-Promoted Decarboxylation of Arylacetic Acids for the Synthesis of Aromatic Nitriles with Sodium Nitrite as the Nitrogen Source

Article abstract of DOI:10.1055/s-0040-1707300

A new and effective method was developed for the synthesis of aromatic nitriles from arylacetic acids by using NaNO ₂as the nitrogen source and Fe(OTf) ₃as the promoter at 50 °C. A series of arylacetic acids underwent this transformation to give the targeted products in yields of 51-90%. Because of the mild conditions, the reaction is compatible with a broad range of functional groups, including ester, carboxy, hydroxy, acetamido, halo, nitro, cyano, methoxy, and even highly reactive formyl groups.

Full text of DOI:10.1055/s-0040-1707300

SYNLETT
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
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020, 31, A–D
letter
A
en
Synlett
Z. Shen et al.
Letter
Iron-Promoted Decarboxylation of Arylacetic Acids for the Syn-
thesis of Aromatic Nitriles with Sodium Nitrite as the Nitrogen
Source
Zhenpeng Shen◊a,b
Wenbo Liu◊a
Xinzhe Tian*a
Zhe Zhaoa,b
Yun-Lai Ren*a
CN
CO
2
H
Fe(OTf)
3
R
+
NaNO2
R
5
0 °C
2
4 examples
51–90%
R = ester, carboxy, hydroxy, acetamido, halo, nitro, etc.
a
College of Science, Henan Agricultural University, Zhengzhou,
Henan 450002, P. R. of China
zzutxz@126.com
renyunlai@126.com
School of Chemical Engineering & Pharmacy, Henan University
b
of Science and Technology, Luoyang, Henan 471003, P. R. of
China
These authors contributed equally
◊
Corresponding Authors
Received: 02.07.2020
Accepted after revision: 31.08.2020
Published online: 28.09.2020
overcome these disadvantages, we attempted to develop a
new and simple method for the synthesis of aromatic ni-
DOI: 10.1055/s-0040-1707300; Art ID: st-2020-k0378-l
triles from arylacetic acids by using NaNO as the nitrogen
2
source (Scheme 1c). To the best of our knowledge, there is
no previous example of the use of NaNO2 as a nitrogen
source in the conversion of arylacetic acids into aromatic
nitriles.
Abstract A new and effective method was developed for the synthe-
sis of aromatic nitriles from arylacetic acids by using NaNO as the nitro-
2
gen source and Fe(OTf) as the promoter at 50 °C. A series of arylacetic
3
acids underwent this transformation to give the targeted products in
yields of 51–90%. Because of the mild conditions, the reaction is com-
patible with a broad range of functional groups, including ester, car-
boxy, hydroxy, acetamido, halo, nitro, cyano, methoxy, and even highly
reactive formyl groups.
Previous work:
O
CN
CO
2
H
Cu(TFA)
2
a)
b)
X
X
X
+
+
H2
N
NH2
O
2
, 95–120 °C
Key words aryl nitriles, nitriles, decarboxylation, sodium nitrite, iron
catalysis, arylacetic acids
2
CO H
CN
DIPEA
NaN
3
X
O
O
N
(
Deoxo-Fluor)
SF
3
Aromatic nitriles are a group of chemicals with a wide
range of synthetic uses due ease with which the cyano
group can be converted into many other functional groups,
This work:
c)
CN
2
CO H
Fe(OTf)
3
1
X
X
such as carboxy, amino, aldehyde, or heterocyclic groups.
+
NaNO
2
DMSO, 50 °C
This has inspired many organic chemists to strive to devel-
op effective methods for the preparation of aromatic ni-
Scheme 1 Methods for the conversion of arylacetic acids to aromatic
2
nitriles
triles. Aromatic nitriles are usually synthesized from such
3
4
substrates as arenediazonium salts, aryl halides, alde-
hydes, amines, oximes,7 arenes,8 amides,
ketones, among others.
Arylacetic acids are an attractive group of substrates be-
5
6
6b,9
azides,10 or
Our initial study was aimed at the conversion of biphe-
nyl-4-ylacetic acid (1a) into biphenyl-4-carbonitrile (2a) as
a model reaction to establish the feasibility of using NaNO2
as a reagent. The reaction did not occur in the absence of a
1
1
12
13
cause of their ready availability. Song and co-workers re-
ported a copper-catalyzed method in which the arylacetic
acid undergoes decarboxylation to give an aromatic alde-
hyde that undergoes oxidative condensation with ammonia
formed by decomposition of urea (Scheme 1a).14 Kangani et
al. used sodium azide/Deoxo-Fluor as a reagent system to
synthesize aromatic nitriles from arylacetic acids (Scheme
promoter (Table 1, entry 1). When AlCl was used as the
3
promoter, only 5% of biphenyl-4-carbonitrile (2a) was ob-
tained (entry 2). Nitrile 2a was also obtained in a low yield
with NiCl , BiCl , or HCl as the promoter (entries 3–5).
However, the substrate was smoothly converted into biphe-
nyl-4-carbonitrile (2a) in 76% yield when FeCl was used as
2
3
3
1
5
1
b). However, the use of extremely poisonous sodium
the promoter (entry 6), which prompted us to screen vari-
azide and expensive Deoxo-Fluor restricts the application
of this latter method. Moreover, the former copper-cata-
lyzed method requires high temperatures of 95–120 °C. To
ous Fe salts. Among the screened promoters, Fe(OTf) was
the most effective, giving nitrile 2a in 90% yield (entry 11),
accompanied by a small amount of biphenyl-4-carbalde-
3
©
2020. Thieme. All rights reserved. Synlett 2020, 31, A–D
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Synlett
Z. Shen et al.
Letter
hyde as a byproduct. The reaction was highly dependent on
the solvent. Reactions in dimethyl sulfoxide (DMSO), N,N-
dimethylformamide (DMF), or N-methyl-2-pyrrolidone
Table 1 Optimization of the Conversion of Biphenyl-4-ylacetic Acid
1a) into Biphenyl-4-carbonitrile (2a)a
(
CN
(
NMP) gave nitrile 2a in yields of 90, 59, and 76%, respec-
CO
2
H
NaNO , promoter
2
tively (entries 11–13), whereas 1,4-dioxane, toluene, aceto-
nitrile, and tetrahydrofuran (THF) were less effective as sol-
vents (entries 14–17). The reaction temperature also had a
significant effect on the reaction, and a 90% yield was ob-
tained when the reaction was performed at 50 °C (entry
50 °C, 10 h
Ph
Ph
1
a
2a
Entry
Promoter
Solvent
Temp
°C)
Conversionb Yieldb
(%) (%)
(
1
2
3
4
5
6
7
8
9
–
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
30
70
90
50
50
50
50
2
7
0
1
1), whereas at 30 °C, the nitrile 2a was obtained in a lower
AlCl
3
5
22
13
7
yield of 52% (entry 18). Increasing the reaction temperature
to 90 °C (entry 20) resulted in complete conversion, but the
yield of nitrile 2a decreased to 62% due to the formation of
large amounts of various byproducts, including biphenyl-4-
carbaldehyde, biphenyl-4-carboxylic acid, and biphenyl-4-
carboxamide. These results indicated that 50 °C is the opti-
mal reaction temperature in terms of the yield. We tried
gradually reducing the promoter loading to 0.8 or 0.5 equiv-
alents, but found that this was not possible without sacri-
ficing product yield, even when the reaction time was pro-
longed to 40 hours. Although it was found to be unneces-
sary to use a dried solvent (entries 11 and 21), the addition
of a larger amount of water had a markedly negative effect
on the reaction (entry 22). In addition, we found that it was
necessary to perform the reaction under an inert atmo-
sphere (entries 23 and 24).
NiCl
2
28
15
12
79
7
BiCl3
HClc
FeCl
3
76
6
Fe(acac)
3
FeSO
ferrocene
-Fe
Fe(OTf)
4
60
28
27
97
63
78
33
15
7
52
25
22
90
59
76
31
11
6
1
0
1
2
3
4
2
O
3
1
1
1
1
3
Fe(OTf)
Fe(OTf)
Fe(OTf)
3
3
NMP
3
1,4-dioxane
toluene
MeCN
THF
15
Fe(OTf)3
Having determined the optimal conditions for the reac-
tion, we evaluated the substrate scope of the protocol for
the conversion of arylacetic acids into aromatic nitriles
16
17
18
19
Fe(OTf)
Fe(OTf)
Fe(OTf)
Fe(OTf)
Fe(OTf)
3
3
3
3
3
3
2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
53
98
100
95
81
100
100
52
71
62
87
36
81
39
(
Scheme 2). A series of phenylacetic acids were smoothly
converted into the corresponding nitriles in moderate to
1
6
high yields. The conditions were compatible with a broad
range of functional groups, including ester, carboxy, hy-
droxy, acetamido, halo, nitro, cyano, and methoxy groups
20
21d
Fe(OTf)3
Fe(OTf)3
2e
2
2
(
(
2b–n); even the highly reactive formyl group was tolerated
2i). However, the reaction was susceptible to steric hin-
3f
Fe(OTf)
Fe(OTf)
3
3
2
4g
drance; for example, (4-chlorophenyl)acetic acid (1c) gave
the corresponding product 2c in 85% yield whereas (2-chlo-
rophenyl)acetic acid (1d) gave product 2d in only 46% yield.
The electronic effect of substituents on the reaction was in-
conspicuous; for example, (4-bromophenyl)acetic acid (1e)
gave product 2e in 76% yield, whereas product 2m was ob-
tained in a similar yield (73%) from (4-methoxyphenyl)ace-
tic acid (1m). Naphthylacetic acids were also good sub-
strates; 2- and 1-naphthylacetic acids were smoothly con-
verted in the corresponding naphthonitriles 2o and 2p in
moderate yields of 75 and 69%, respectively. The heterocy-
clic acids 2-thienylacetic acid (1q) and 2-furylacetic acid
a
2
Reaction conditions: 1a (0.5 mmol), NaNO (3 mmol), promoter (1 mmol),
solvent (2 mL), 10 h, under argon. The solvents were undried.
b
Determined by GC with biphenyl as an internal standard.
c
1
2 M aq HCl.
d
e
f
Anhyd DMSO.
O (0.2 mL) was added.
Under air.
H
2
g
2
Under O .
1b (Scheme 3a), and it could be converted into benzonitrile
(2b) under our reaction conditions (Scheme 3b). In addi-
tion, the experimental results shown in Schemes 3b and 3c
suggest that the presence of Fe is necessary for the effec-
1
7b
3+
(
2r) were also smoothly converted into the corresponding
tive conversion of intermediate 2c into benzonitrile (2b).
products 2q and 2r in yields of 77 and 68% respectively.
We therefore initially proposed the mechanism shown
17
17
Initially, on the basis of reports in the literature, we
surmised that (2Z)-(hydroxyimino)(phenyl)acetic acid (3)
might be an intermediate in the present reaction of acid 1b.
Indeed, this intermediate was observed in the reaction of
in Scheme 4. First, NO , formed by the reaction of the ni-
2
trite ion with the acid, abstracts a benzylic hydrogen atom
from phenylacetic acid (2a) to give the benzylic radical 5,
which reacts with NO to form intermediate 6. Intermediate
6
then undergoes the tautomerization to the oxime inter-
©
2020. Thieme. All rights reserved. Synlett 2020, 31, A–D

C
Synlett
Z. Shen et al.
Letter
NaNO
2
, Fe(OTf)
3
OH
CO
OH
H
Ar CN
N
N
Ar
2
CO H
CN
DMSO, 50 °C, 10 h
CO
2
H
Ph
Ph
1b–r
2b–r
Ph
2
H
Ph
+
1
b
2b
3
4
Cl
3 2
Fe(OTf) , NaNO
CN
b, 73%
Cl
CN
CN
a)
1b
2b
3
+
4
DMSO, 25 °C, 3 h
Fe(OTf)
3
6%
1.6%
7%
2
2
2c, 85%
2d, 46%
3
O
b)
c)
3
3
2b
DMSO, 50 °C, 10 h
CN
Br
CN
NC
O
CN
86%
HO
HOTf
e, 76%
h, 90%
2f, 61%
2g, 82%
2j, 77%
2b
DMSO, 50 °C, 10 h
8
%
O
NO
CN
CN
d)
e)
+
+
3
O
2
N
CN
1b
1b
2b
H
MeO
DMSO, 50 °C, 10 h
1
%
5%
2
2i, 65%
NO
2
2b
3
MeO
HO
2l, 62%
DMSO, 50 °C, 10 h
O
3
%
8%
MeCNH
CN
CN
CN
MeO
CN
HOTf, NaNO
2
f)
1b
4
2b
+
3
2
k, 51%
2m, 73%
DMSO, 50 °C, 10 h
1
6%
3%
CN
MeO
Fe(OTf)
3
g)
2b
3%
DMSO, 50 °C, 10 h
MeO
CN
7
2
n, 83%
2o, 75%
2p, 69%
Scheme 3 Control experiments for mechanistic investigation
CN
q, 77%
CN
S
O
.
Fe3+ + NO –
Fe2+
.
2
H+ + 2 NO –
2
.
NO + NO
2
+ H
2
O
2
+
NO
H
2
2
2r, 68%
O
Scheme 2 Conversion of various arylacetic acids into nitriles. Reagents
and conditions: 1 (0.5 mmol), NaNO (3 mmol), Fe(OTf) (1 mmol),
DMSO (2 mL), 50 °C, 10 h, under argon. The DMSO was not dried, and
the air in the tube was removed. The yields of 2b, 2q, and 2r are GC
yields, whereas the other yields are isolated yields.
.
N
NO
2
HNO
2
.
2
3
CO
2
H
CO
O
2
H
.
NO
CO
2
2a
5
6
OH
Fe
N
N
Fe3+
O
C
O
CN
mediate 3. This intermediate is converted into benzonitrile
CO
2
H
– 2 H+
3
+
[FeO]+ + CO2
(
2b) with the assistance of Fe . This pathway suggests that
NMR
7
2b
3
a large amount of (Z)-2-(hydroxyimino)-2-phenylacetic
Scheme 4 Minor mechanistic pathway for the present reaction
acid (3) should be formed by the reaction between NO and
2
phenylacetic acid in the absence of a Fe salt. However, this
intermediate was obtained in low yields in the presence of
tautomerizes to the oxime 4. This reacts with Fe3+ to pro-
duce intermediate 11, which eventually rearranges the in-
termediate 13, stabilized by an intramolecular hydrogen
bond. Finally, 13 decomposes to give the target product 2b.
+
–
NO , NO, or H /NO (Schemes 3d–f), suggesting that the
2
2
radical pathway shown in Scheme 4 is not the major path-
way. This is consistent with the experimental finding that
neither of the radical inhibitors TEMPO and BHT completely
inhibited the formation of benzonitrile (see Scheme S2 in
the Supporting Information).
As shown in Scheme 3a, the present reaction gave the
oxime 4 in 7% yield. Moreover, this intermediate underwent
dehydration to give benzonitrile (2b) in a high yield in the
CO
2
H
Fe3+
– H+
2
CO Fe
Fe HONO
[Fe(OH)]2+
– CO2
2
a
8
9
H
H
O
OH
O
Fe3+
– H+
N
N
N
Fe
presence of Fe(OTf) (Scheme 3g). These results show that
3
10
4
11
GC-MS
the mechanistic pathway involves the formation and dehy-
dration of oxime 4. Based on the observations discussed
above and reports in the literature,18 we tentatively propose
the mechanistic pathway shown in Scheme 5. Phenylacetic
acid (2a) undergoes decarboxylation with the assistance of
H
O
H
O
Fe
Fe
CN
N
N
1
2
13
[Fe(OH)]2+
2b
3
+
Fe to give intermediate 9. The reaction of intermediate 9
with nitrous acid gives (nitrosomethyl)benzene (10), which
Scheme 5 Major mechanistic pathway for the present reaction
©
2020. Thieme. All rights reserved. Synlett 2020, 31, A–D

D
Synlett
Z. Shen et al.
Letter
In conclusion, we have developed a new and effective
method for the synthesis of aromatic nitriles from aryl-
acetic acids by using NaNO2 as a nitrogen source and
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Li, J.; Tian, H.; Sun, B. J. Org. Chem. 2018, 83, 12939.
Fe(OTf) as a promoter at 50 °C. To our knowledge, there is
3
(
8) (a) Liu, M.; You, E.; Cao, W.; Shi, J. Asian J. Org. Chem. 2019, 8,
850. (b) Chen, H.; Mondal, A.; Wedi, P.; van Gemmeren, M. ACS
no previous example of the use of NaNO2 as a nitrogen
source in the conversion of arylacetic acids into aromatic
nitriles. The mild conditions permit the reaction to be com-
patible with a broad range of functional groups, such as es-
ter, carboxy, hydroxy, acetamido, halo, nitro, cyano, me-
thoxy, and even the highly reactive formyl group. Under the
present condition, a series of phenylacetic acids and naph-
thylacetic acids, as well as 2-furylacetic acid and 2-thienyl-
acetic acid, were smoothly converted into the correspond-
ing nitriles in low to high yields.
1
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Funding Information
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Vadagaonkar, K. S.; Mohanasrinivasan, V.; Chaskar, A. C. Tetrahe-
dron Lett. 2019, 60, 891. (b) Hussain, F. H.; Suria, M.; Namdeo,
A.; Borah, G.; Dutta, D.; Goswami, T.; Paharia, P. Catal. Commun.
The authors would like to thank the National Natural Science Founda-
tion of China (Grant No. 21603060) for their financial support.
N
ati
o
nal Natural
S
cience
F
o
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datio
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h
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Supporting Information
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e) Wang, D.; Fang, J.; Deng, G.-J.; Gong, H. ACS Sustainable
Supporting information for this article is available online at
Chem. Eng. 2017, 5, 6398.
https://doi.org/10.1055/s-0040-1707300.
S
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p
p
orting Informatio
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orti
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References and Notes
(
16) Nitriles 2a–r: General Procedure
A tube of approximate volume 45 mL was charged with the
appropriate arylacetic acid (0.5 mmol), NaNO2 (3 mmol),
(
1) (a) Zhao, L.; Dong, Y.; Xia, Q.; Bai, J.; Li, Y. J. Org. Chem. 2020, 85,
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Fe(OTf) (1 mmol), and undried DMSO (2 mL), and the air in the
2
019, 21, 9919. (c) Wang, Z.; Wang, X.; Ura, Y.; Nishihara, Y. Org.
3
tube was replaced by argon gas. The tube was sealed and the
mixture was heated with magnetic stirring at 50 °C for 10 h,
then cooled to r.t. The solvent was evaporated, and the residue
was purified by column chromatography (silica gel).
Lett. 2019, 21, 6779. (d) Liu, L.-Y.; Yeung, K.-S.; Yu, J.-Q. Chem.
Eur. J. 2019, 25, 2199. (e) Bhagat, S. B.; Telvekar, V. N. Synlett
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018, 29, 874. (f) Gao, G.; Sun, P.; Li, Y.; Wang, F.; Zhao, Z.; Qin,
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G.; Wu, J.; Li, F. J. Catal. 2017, 355, 53.
11b
Biphenyl-4-carbonitrile (2a)
1
White solid; yield: 77.1 mg (86%); m.p. 84–86°C. H NMR (400
(
2) (a) Hosseinian, A.; Ahmadi, S.; Monfared, A.; Nezhad, P. D. K.;
Vessally, E. Curr. Org. Chem. 2018, 22, 1862. (b) Chaitanya, M.;
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Hribernik, L. Green Chem. 2017, 19, 2286. (d) Ghodse, S. M.;
Takale, B. S.; Hatvate, N. T.; Telvekar, V. N. ChemistrySelect 2018, 3,
MHz, CDCl ):  = 7.76 (d, J = 8.4 Hz, 2 H), 7.71 (d, J = 8.4 Hz, 2 H),
3
1
3
7
.61–7.63 (m, 2 H), 7.50–7.53 (m, 2 H), 7.44–7.48 (m, 1 H).
C
NMR (100 MHz, CDCl ):  = 145.7, 139.2, 132.6, 129.1, 128.7,
3
1
27.8, 127.3, 119.0, 110.9.
5
b
Methyl 4-Cyanobenzoate (2j)
4168. (e) Yabe, O.; Mizufune, H.; Ikemoto, T. Synlett 2009, 1291.
White solid; yield: 62 mg (77%); m.p. 63–65°C. 1H NMR (400
(
(
3) Li, J.; Xu, W.; Ding, J.; Lee, K.-H. Tetrahedron Lett. 2016, 57, 1205.
4) (a) Shee, M.; Shah, S. S.; Singh, N. D. P. Chem. Commun. 2020, 56,
MHz, CDCl ):  = 8.15 (d, J = 8.1 Hz, 2 H), 7.76 (d, J = 8.1 Hz, 2 H),
3
3.97 (s, 3 H).
4240. (b) Niknam, E.; Panahi, F.; Khalafi-Nezhad, A. Eur. J. Org.
C NMR (100 MHz, CDCl ):  = 165.4, 133.9, 132.2, 130.1, 118.0,
Chem. 2020, 2020, 2699. (c) Chen, H.; Sun, S.; Liu, Y. A.; Liao, X.
ACS Catal. 2020, 10, 1397. (d) Mills, L. R.; Graham, J. M.; Patel, P.;
Rousseaux, S. A. L. J. Am. Chem. Soc. 2019, 141, 19257.
3
1
16.4, 52.7.
1
-Naphthonitrile (2p)5b
White solid; yield: 52.8 mg (69%); m.p. 37–39°C. 1H NMR (400
(5) (a) Nandi, J.; Leadbeater, N. E. Org. Biomol. Chem. 2019, 17, 9182.
MHz, CDCl ):  = 8.25 (d, J = 8.3 Hz, 1 H), 8.09 (d, J = 8.3 Hz, 1 H),
(
(
b) Fang, W.-Y.; Qin, H.-L. J. Org. Chem. 2019, 84, 5803.
c) Gurjar, J.; Bater, J.; Fokin, V. V. Chem. Eur. J. 2019, 25, 1906.
3
7
.93 (t, J = 7.6 Hz, 2 H), 7.71 (t, J = 7.0 Hz, 1 H), 7.64 (t, J = 7.2 Hz, 1
13
H), 7.54 (t, J = 8.0 Hz, 1 H). C NMR (100 MHz, CDCl ):  = 134.7,
(
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981. (b) Das, H. S.; Das, S.; Dey, K.; Singh, B.; Haridasan, R. K.;
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3
1
34.2, 132.3, 129.2, 129.1, 128.4, 128.1, 127.7, 126.4, 119.3, 109.4.
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1
(
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1
2
019, 21, 5386. (d) Achard, T.; Egly, J.; Sigrist, M.; Maisse-
(
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(
1
Russ. J. Gen. Chem. 2018, 88, 1590. (c) Li, Y.-T.; Liao, B.-S.; Chen,
H.-P.; Liu, S.-T. Synthesis 2011, 2639.
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2020. Thieme. All rights reserved. Synlett 2020, 31, A–D