Catalytic Strecker reaction: g-C3N4-anchored sulfonic acid organocatalyst for the synthesis of α-aminonitriles

Article abstract of DOI:10.1007/s11164-020-04370-x

Abstract: Efficient organocatalyst for enantioselective Strecker reaction was synthesized using g-C₃N₄ sheets (CN). CN-anchored sulfonic acid (CN-Bu-SO₃H) was found to be a highly efficient and recoverable organocatalyst f

Full text of DOI:10.1007/s11164-020-04370-x

Research on Chemical Intermediates (2021) 47:1489–1502
https://doi.org/10.1007/s11164-020-04370-x
Catalytic Strecker reaction: g‑C N ‑anchored sulfonic acid
3
4
organocatalyst for the synthesis of α‑aminonitriles
Monavar Rahmati · Hossein Ghafuri1
1
Received: 14 August 2020 / Accepted: 11 December 2020 / Published online: 10 February 2021
©
The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature 2021
Abstract
Efcient organocatalyst ꢀor enantioselective Strecker reaction was synthesized using
g-C N sheets (CN). CN-anchored sulꢀonic acid (CN-Bu-SO H) was ꢀound to be a
3
4
3
highly efcient and recoverable organocatalyst ꢀor the synthesis oꢀ α-aminonitriles
with good to high yields. The synthesized organocatalyst (CN-Bu-SO H) was char-
3
acterized by FTIR, EDS, XRD, FESEM, and TGA analyses. The simple experimen-
tal method, using nontoxic solvents, easy recovery, and the reusability oꢀ the catalyst
have led to the development oꢀ an environmentally ꢀriendly approach to the synthe-
sis oꢀ α-aminonitriles.
Graphic abstract
Keywords Organocatalyst · g-C N sheets · Strecker reaction · α-Aminonitriles ·
3
4
Reusability catalyst
*
Hossein Ghaꢀuri
ghaꢀuri@iust.ac.ir
1
Catalysts and Organic Synthesis Research Laboratory, Department oꢀ Chemistry, Iran University
oꢀ Science and Technology, Tehran 16846-13114, Iran
Vol.:(012
1
3456789)
3

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M. Rahmati, H. Ghafuri
Introduction
Organocatalysis has appeared in the past decade as one oꢀ the most important
subjects in the development oꢀ the catalysts. Through utilization oꢀ organocata-
lysts, a huge number oꢀ products are expected to be prepared using the least num-
ber oꢀ organic compounds. By working with novel organocatalysis, compounds
would be gained in a similar way with the classical methods. Thereꢀore, plenty oꢀ
important reactions have been made by using organic catalysts [1] including aldol
condensation [2, 3], multicomponent reactions [4], Mannich reactions [5–7],
Strecker reaction, and α-heteroꢀunctionalization oꢀ carbonyl compounds [8–10].
Graphical carbon nitride sheet (g-C N ) is classiꢁed as a layered material and
3
4
also as a signiꢁcant category oꢀ nonmetallic compounds which have unique prop-
erties such as special electronic band structure, high thermal stability, large sur-
ꢀ
ace area, and environmentally ꢀriendly ꢀeatures [11, 12]. The 2D g-C N sheets
3 4
have attracted much scientiꢁc ꢀondness due to their potential applications in solar
energy conversion, photosynthesis, electrocatalysis, bioimaging application, and
photocatalytic perꢀormance [13, 14]. Recently, covalent ꢀunctionalization oꢀ CN
sheets has been used widely in catalysis science [15–19]. Thereꢀore, using CN as
a suitable support to make new heterogeneous catalysts is a suitable method ꢀor
new catalytic systems.
Today, it is important to make the connection between chemical reactions and
environmental problems. Several methods have been used ꢀor green chemistry,
such as ultrasounds or microwaves. Green chemistry addresses many classical
challenges by inventing novel methods and designing new synthetic routes to
maximize desired products and minimize by-products. Fundamental innovations
in chemical sciences will lead science to a new generation oꢀ chemical syntheses.
The Strecker reaction, even 160 years aꢀter its initial presentation, is one oꢀ
the main direct methods ꢀor the synthesis oꢀ α-amino acids in the laboratory and
industrial scale. α-aminonitriles have received much attention due to the presence
oꢀ the cyanide group in their structure. These compounds can lead to the produc-
tion oꢀ α-amino acids [20, 21], diamines, thiadiazoles and imidazoles [22]. To
solve the problems caused by toxic cyanide sources ꢀor Strecker reaction [23],
diꢂerent sources oꢀ cyanide such as Bu SnCN [24], K [Fe(CN) ] [25], Zn(CN)
3
2
6
2
[
[
26], Et AlCN [27], (EtO) P(O)CN [28], CH (OH)(CN)CH [29], and TMSCN
2 2 3 3
30] have been used. Among these sources, trimethylsilyl cyanide (TMSCN) is
the most suitable one.
Till now, the Strecker reaction has been promoted by diꢂerent Lewis or Bron-
sted acid catalysts like Fe(Cp) PF [31], InI [32], RhI [33], Cu(OTꢀ) [34],
2
6
3
3
2
BiCl [22], La(NO ) .H O, GdCl .H O [35], NiCl [36], Ga(OTꢀ) [37], GaCl
3
3 3
2
3
2
2
3
3
[
38], CeCl [39], Pr(OTꢀ) [40], RuCl [41], FeCl [42], and p-toluenesulꢀonic
3 3 3 3
acid [43]. However, most oꢀ these methods suꢂer ꢀrom complications such as
high reaction temperature, long reaction time, use oꢀ toxic solvents, low ef-
ciency, and severe reaction conditions. Thus, much attention has been paid to the
development oꢀ more environmentally ꢀriendly catalytic methods ꢀor the synthesis
oꢀ α-amine nitrile derivatives.
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1491
3
4
Scheme 1 Schematic preparation oꢀ CN-Bu-SO H
3
O
NH2
CN-Bu-SO3H
EtOH / rt
CH3
H C Si C N
R
3
NH
CN
4)
R
CH3
H
(
1)
(2)
(3)
(
Scheme 2 Three-component condensation oꢀ aldehydes (1), aniline (2), TMSCN (3) catalyzed by CN-
Bu-SO H
3
Based on the ꢀeatures which were mentioned ꢀor CN sheets, the aim oꢀ this
research was set ꢀor designing and synthesizing oꢀ green and eꢂective CN-based
organocatalyst ꢀor the multicomponent reactions. In this study, the CN-Bu-SO H
3
was synthesized as an eꢂective organocatalyst and the efciency oꢀ it was inves-
tigated ꢀor the preparation oꢀ α-aminonitriles via condensation oꢀ aldehydes with
aniline and trimethylsilyl cyanide (TMSCN) under the mild reaction condition
(
Scheme 1).
Results and discussion
Characterization of CN‑Bu‑SO3H
The prepared CN-Bu-SO H was characterized by FTIR, XRD, FESEM, EDX, and
3
TGA analyses (Scheme 2).
The FTIR spectra oꢀ CN (a) and CN-Bu-SO H (b) are compared to each other in
3
−
1
Fig. 1. In FTIR oꢀ pure CN sheet, a strong peak appeared at 3147 and 3336 cm ,
which belongs to the NH group, peaks related to C=N stretching vibration were
−1
observed in 1627 and 1473 cm , the C–N groups oꢀ the heterocyclic rings appeared
−1
−1
at around 1415, 1323, and 1207 cm , and a sharp peak appeared at 794 cm which
is due to the presence oꢀ the tri-s-triazine units (Fig. 1a). The signals at 2781 and
−
1
2
758 cm were attributed to the C-H groups, and the broad peak in the range oꢀ
1
176 cm− (see the CN-Bu-SO H spectra) is attributed to the S-OH group (Fig. 1b).
1
3
1
3

1492
M. Rahmati, H. Ghafuri
Fig. 1 FTIR spectra oꢀ CN (a),
CN-Bu-SO H (b)
3
Correspondingly, two symmetrical and asymmetrical vibration bands around 1220
−
1
and 1348 cm belong to the SO groups [44].
2
In Fig. 2, the XRD pattern oꢀ the CN and CN-Bu-SO H is shown. Diꢂraction
3
peaks at 2θ=27.35° (002) and 13.04° (100) are related to the in-plane structure
packing motiꢀ oꢀ tri-s-triazine units (Fig. 2a). Also, diꢂraction peaks at 2θ=27.4°
(
002), 17.9°, and 14.8° (100) are related to the CN-Bu-SO H (Fig. 2b).
3
In addition, FESEM images oꢀ the CN-Bu-SO H catalyst are shown in Fig. 3. The
3
morphology study oꢀ CN-Bu-SO H was perꢀormed using ꢁeld emission scanning
3
electron microscopy (FESEM). The CN-Bu-SO H images clearly show that the CN
3
surꢀace is diꢂerent to pure CN sheets and was ꢀunctionalized successꢀully (Fig. 3).
As shown in Fig. 4, the EDX analysis oꢀ the CN-Bu-SO H conꢁrms the presence
3
oꢀ C (28.27%), O (23.89%), N (43.88%), and S (3.96%), respectively. These results
indicate the presence oꢀ acidic groups (SO H) in the synthesized catalyst.
3
The thermogravimetric analysis (TGA) oꢀ the CN-Bu-SO H is shown in Fig. 5.
3
The TGA curve oꢀ CN-Bu-SO H (Fig. 1b) shows three steps oꢀ weight loss. In the
3
ꢁ
rst stage, a weight loss oꢀ about 5% is observed at temperatures oꢀ 100–200 °C,
which is related to the evaporation oꢀ water/solvent which was absorbed into the
pores oꢀ the structure. The TGA plot oꢀ CN-Bu-SO H shows a weight reduction
3
oꢀ~17% in 350 to 500 °C, which was attributed to the removal oꢀ the sulꢀonic group.
Also, owing to the disintegration oꢀ the CN sheet, a sharp weight reduction was
observed at>600 °C. So, the CN sheet was shown to be stable up to 600 °C.
Fig. 2 The XRD pattern oꢀ CN (a), CN-Bu-SO H (b)
3
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1493
3
4
Fig. 3 EDX analysis oꢀ CN-Bu-SO H
3
Fig. 4 EDX analysis oꢀ CN-Bu-SO H
3
To evaluate the eꢂect oꢀ the CN-Bu-SO H as an introduced organocatalyst under
3
diꢂerent reaction conditions, the reaction oꢀ 1 mmol oꢀ aldehyde (1a), 1 mmol oꢀ
aniline (2), and 1.2 mmol oꢀ TMSCN (3) was investigated as the model reaction to
aꢂord α-aminonitriles. Various reaction parameters such as the temperature, solvent,
and amount oꢀ catalyst were optimized. The results are summarized in Table 1. The
results showed that the reaction by using 20 mg oꢀ the catalyst in EtOH and room
temperature proceeded in 10 min and 96% yield (Table 1, entry 6). When the reac-
tion was studied in the presence oꢀ 20 mg oꢀ the catalyst in EtOH and 80 °C, the
1
3

1494
M. Rahmati, H. Ghafuri
Fig. 5 TGA curve oꢀ CN (a), CN-Bu-SO H (b)
3
Table 1 Optimization
oꢀ
aniline
conditions
in
the
(3)
reaction
under
oꢀ
4-chlo-
condi-
robenzaldehyde
tio ns
(1),
(2)
TMSCN
diꢂerent
a
Entry
Amount oꢀ
Solvent
Temperature (°C)
Time (min)
Yield (%) 4a
catalyst (mg)
1
2
3
4
5
6
7
8
9
20
20
20
20
20
20
20
15
10
5
MeOH
CH Cl
Reꢃux
Reꢃux
60
Reꢃux
60
r.t
80
80
80
25
90
120
100
55
10
11
25
40
78
45
56
52
50
96
93
69
60
57
15
2
2
CH CN
3
DMF
Solvent-ꢀree
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1
1
0
1
80
80
55
180
0
Reaction conditions: 4-chlorobenzaldehyde (1, 1 mmol), aniline (2, 1 mmol) TMSCN (3, 1.2 mmol),
CN-Bu-SO H under diꢂerent conditions
3
a
The yields relate to the isolated product
desired product was gained in lower yield (Entry 7). Also, by reducing the amount
oꢀ catalyst, the desired product was obtained under similar conditions over a longer
period oꢀ time and lower efciency.
Additionally, other reaction conditions were carried out to study the solvent eꢂect
in the presence oꢀ an optimized amount oꢀ catalyst. MeOH, CH Cl , CH CN, and
2
2
3
DMF were used ꢀor this study. According to the gained results, EtOH was selected
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1495
3
4
as the optimal solvent ꢀor the reaction. Compared to other solvents, EtOH led to the
production oꢀ the desired product in a shorter time and higher efciency (entry 1–4)
(
Table 2).
Observably, the ꢀavorable products were gained in good to high yields. These
observations strongly approve the appropriate catalytic activity oꢀ the CN-Bu-SO H
3
organocatalyst to promote the Strecker reaction with a wide range oꢀ aldehydes, ani-
line and TMSCN. It should be mentioned that good yields are achieved even with
aldehydes with electron-donating groups which would be expected to slow the ꢀor-
mation oꢀ the putative imine intermediate.
In view oꢀ the above considerations, the ꢀollowing mechanism can be suggested
ꢀ
or the synthesis oꢀ α-aminonitriles (Scheme 3). First, using aldehydes and ani-
line, the iminium is ꢀormed as an intermediate [56]. In the ꢀollowing, TMSCN with
iminium intermediate by the nucleophilic attack oꢀ CN led to the production oꢀ the
product.
Aꢀter the reaction ꢁnishes, the introduced catalyst was separated by ꢁltration and
washed several times with water and n-hexane, dried, and reused in the reaction
without loss oꢀ activity aꢀter six cycles (Fig. 6).
To demonstrate the catalytic activity oꢀ the CN-Bu-SO3H as a heterogeneous
organocatalyst, its efciency has been compared with several reported catalysts ꢀor
the synthesis oꢀ 4a (Table 3). The results show that the present study is better than
other reports in terms oꢀ product efciency, reaction time, and efciency oꢀ the recy-
cled catalyst.
Conclusions
In this study, a new heterogeneous organocatalyst ꢀor the synthesis oꢀ α-aminonitriles
is presented. Efcient and selective synthesis oꢀ α-aminonitriles was carried out
using simple and available raw materials like benzaldehyde, aniline, and TMSCN
in the presence oꢀ CN-Bu-SO3H as an organocatalyst. High efciency, reusability
oꢀ the catalyst, easy separation method, simple experimental procedure, and using
nontoxic and noncorrosive solid organic organocatalyst are some oꢀ the important
ꢀeatures oꢀ this method ꢀor the preparation oꢀ α-aminonitriles.
Experimental section
Reagents and apparatus
All materials were bought ꢀrom Merck or Aldrich with high purity, except ben-
zaldehyde, which was utilized in the ꢀorm oꢀ newly distilled. Melting points were
determined using an electrothermal 9100 device and are unmodiꢁed. Characteri-
zation oꢀ new organocatalyst was perꢀormed by FESEM TESCAN-MIRA3, EDX
Numerix DXP-X10P, Shimadzu FTIR-8400S and TGA Bahr company STA 504.
The XRD pattern oꢀ the catalyst was obtained using TW 1800 diꢂractometer
with Cu Ka radiation (λ = 1.54050 Å). The analytical thin-layer chromatography
1
3

1496
M. Rahmati, H. Ghafuri
Table 2 Optimization oꢀ conditions in the reaction oꢀ 4-chlorobenzaldehyde (1), aniline (2) TMSCN (3)
under mild conditions
a
o
o
Entry
Aldehyde
Amine
Product
Time Yield Mp ( C)
Mp ( C)
(
min)
(%)
Reference
O
1
NH2
Cl
1
2
1
2
0
6
5
2
96
111–112
108–111
113–116
131–133
[45]
NH
CN
H
Cl
(
4a)
O
2
3
4
NH2
NC
90
92
91
[46]
[23]
[47]
NH
CN
H
CN
(
4b)
O
NH2
Cl
Cl
NH
CN
Cl
H
Cl
(
4c)
O
NO2
NH2
NH2
NH2
NO2
NH
CN
H
(
4d)
O
5
NH
CN
2
3
0
0
94
79–81
85–87
[48]
[49]
H
(
4e)
O
NO2
6
93
NH
CN
NO2
H
(4f)
(
TLC) experiments were perꢀormed using Merck 0.2 mm silica gel 60F-254Al-
1
plates. All compounds well characterized by IR and H NMR (500 MHz, Bruker
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1497
3
4
Table 2 (continued)
O
NH2
Cl
Cl
7
18
87
93
91
67–70
87–88
[50]
[45]
[45]
NH
CN
H
(
4g)
O
8
NH2
Br
1
2
5
0
NH
CN
H
Br
(
4h)
O
9
NH2
O N
2
115–117
NH
CN
H
NO2
CH3
OMe
OH
(
4i)
O
1
0
NH2
H C
3
3
0
85
79–81
[51]
NH
H
CN
(4j)
O
1
1
NH2
MeO
3
0
93
90–91
[52]
NH
CN
H
(
4k)
O
1
1
2
3
NH2
HO
2
3
6
5
91
89
119–121
[53]
NH
CN
H
(4l)
O
NH2
Oil
[54]
NH
CN
HO
OH
H
(4m)
1
3

1498
M. Rahmati, H. Ghafuri
Table 2 (continued)
O
NH2
Cl
1
1
1
4
5
6
Cl
Cl
2
3
3
7
0
8
90
87
89
81
88
77–78
95–97
[52]
[51]
[47]
[45]
[55]
NH
CN
Cl
H
(
4n)
NH2
NH2
NH2
NH2
H
S
S
NH
CN
O
H
(4o)
H
68–70
O
O
NH
CN
O
H
(4p)
O
1
1
7
8
O
4
3
0
5
156–158
109–111
NH
CN
H
O
(4q)
O
CH3
N
H C
3
NH
CN
H
N
H C CH3
3
(4r)
Reaction conditions: benzaldehyde (1, 1 mmol), aniline (2, 1 mmol) TMSCN (3, 1.2 mmol), CN-Bu–
SO H under mild conditions
3
a
The yields relate to the isolated product
DRX-500 Avance spectrometers in DMSO-d6) spectral data in comparison with
those obtained ꢀrom reliable samples or reported. All yields related to isolated
products.
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1499
3
4
O S
O
O
H
O S O
H
O
H N
2
OH
R
Si OH
HN
H O
2
Si
O
O
S
R
O
Si CN
R
NH
CN
H
=
O S O
OH
Scheme 3 proposed mechanism ꢀor the synthesis oꢀ α-aminonitriles catalyzed by CN-Bu-SO H
3
1
3

1500
M. Rahmati, H. Ghafuri
Fig. 6 Reusability oꢀ the hetero-
geneous acidic organocatalyst
CN-Bu-SO H ꢀor the synthesis
3
oꢀ 4a
Table 3 Comparison oꢀ the catalytic activity oꢀ the CN-Bu-SO H with other catalysts
3
Entry Catalyst
Amount oꢀ catalyst Solvent
Time (min) Yield (%) Reꢀerences
1
2
3
4
5
Cellulose sulꢀuric acid 50 mg
CH3CN
Solvent-ꢀree 20
CH2Cl2
EtOH
EtOH
80
90
91
91
98
96
[63]
[37]
[43]
[29]
Fe(Cp) PF
5 mol%
52 mg
50 mg
20 mg
2
6
Ga(OTꢀ)3
120
90
10
B-MCM-41
CN-Bu-SO H
This work
3
General procedure for preparation of the g‑C N sheets
3
4
g-C N sheets were prepared by the known procedure [57]. One gram oꢀ bulk
3
4
g-C3N4 was added to a mixture oꢀ concentrated H SO (20 ml) and HNO
2
4
3
(
20 ml) at room temperature and stirred ꢀor 2 h. The mixture was diluted with
distilled water (1 l), and the gained white solid was ꢁltered and washed with dis-
tilled water (100 ml) and dried in the oven at 60 °C. Next, 0.1 g oꢀ bulk g-C N in
3
4
1
00 ml oꢀ water/isopropanol (1:1) was mixed in an ultrasonic bath ꢀor 6 h. Even-
tually, it was centriꢀuged to remove the remaining g-C N nanoparticles.
3
4
General procedure for preparation of the CN‑Bu‑SO3H
One gram oꢀ g-C N sheets was dispersed in toluene (25 ml), and then, 3 g oꢀ
3
4
1
,4-butane sultone was added, and the reaction mixture was reꢃuxed ꢀor 6 h under
an inert atmosphere. Then, the mixture was cooled at room temperature, and was
muster with centriꢀugation, washed with chloroꢀorm and diethyl ether, and drying
at 60 °C ꢀor 12 h (Scheme 1).
1
3

Catalytic Strecker reaction: g‑C N ‑anchored sulfonic…
1501
3
4
General procedure for the synthesis of α‑aminonitriles
In a round-bottom ꢃask, 1 mmol aldehyde, 1 mmol aniline, 1.2 mmol TMSCN, and
mg oꢀ CN-Bu-SO H in 3 ml oꢀ EtOH were stirred at room temperature. Aꢀter the
2
3
end oꢀ the reaction, as illustrated by TLC, 5 ml oꢀ ethanol was added to the reaction
container. The gained mixture was ꢁltered to separate the catalyst ꢀrom the reaction
mixture that could be reused. EtOH was vaporized, and precipitate oꢀ α-aminonitriles
was obtained. The precipitate was washed with n-hexane or petroleum ether to
remove any remaining primary material and dried. More puriꢁcation oꢀ the products
was carried out by silica gel column chromatography (EtOAc–hexane, 1:10).
Selected spectral data
2
‑(4‑Chlorophenyl)‑2‑(phenylamino)acetonitrile, 4a White solid, mp 111-112 °C; IR
−1 1
(
KBr): ʋ 3299, 2925, 2361, 1600, 1490, 755 cm ; H NMR (500 MHz, DMSO-d6):
δ 6.03 (d, J=9.5 Hz, 1H), 6.7 (d, J=9.4 Hz, NH, 1H), 6.73 (d, J=7.35 Hz, 1H),
.8 (d, J=7.8 Hz, 2H), 7.17 (t, J=7.92 Hz, 1H), 7.55 (d, J=8.6 Hz, 2H), 7.60 (d,
J=8.4 Hz, 2H) ppm.
6
2
‑(3‑Nitrophenyl)‑2‑(phenylamino)acetonitrile, 4f pale yellow solid; mp 85-87 °C;
−1
1
IR (KBr): ʋ 3365, 2927, 2361, 1603, 1532, 1496, 1347, 749.68 cm ; H NMR
500 MHz, DMSO-d6): δ=6.25 (d, J=9.85 Hz 1H), 6.76 (t, J=7.35 Hz 1H), 6.83 (d,
J=8.45 Hz, 2H), 6.88 (d, J=9.85 Hz, 1H), 7.2 (t, J=8.35 Hz, 2H), 7.8 (t, J=8.0 Hz,
H), 8.05 (d, J=7.75 Hz, 1H), 8.29 (d, J=10.25 Hz, 1H), 8.44(s, 1H, NH).
(
1
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-020-04370-x.
Acknowledgements We are grateꢀul ꢀor the ꢁnancial support ꢀrom the Research Council oꢀ Iran Univer-
sity oꢀ Science and Technology (IUST), Tehran, Iran.
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