Triphenylphosphine as an efficient and reusable organocatalyst for the synthesis of substituted pyrano[2,3-c]pyrazoles
Rajasekar Perumal, Panduragan Baskaran, Balaji Bathrinarayanan, Syed Sheik Mansoor*
Research Department of Chemistry, Bioactive Organic Molecule Synthetic Unit,
C. Abdul Hakeem College (Autonomous), Melvisharam – 632 509.
*Corresponding Author E-mail: mansoorcah@gmail.com
ABSTRACT:
Triphenylphosphine (PPh3) efficiently catalyzes the three-component synthesis of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles starting from 3-methyl-1-phenyl-2-pyrazolin-5-one, aromatic aldehydes and malononitrile. The method provided several advantages such as mild conditions, good to excellent yields, easily available catalyst and simple work procedure.
The synthesis of pyrano[2,3-c]pyrazole is important in organic synthesis due to their wide range of biological and therapeutic properties6-16.
During the last few years, synthesis of dihydropyrano [2,3-c]pyrazoles has received great interest17.
Several methods have been reported for the synthesis of dihydro pyrano [2,3-c] pyrazoles using various catalysts, such as Choline chloride based thiourea18, Fe3O4-supported N-pyridin-4-amine-grafted grapheme oxide19,
γ-alumina20 and mandelic acid21. Organic compounds are very good pharamacophores performing wide range of biological activities22-27.
Based on the above facts, the synthesis of pyrano[2,3-c]pyrazole derivatives is currently of great interest in organic synthesis. In continuation of our ongoing research program on the development of new catalysts and methods for organic transformations, very recently we have reported the triphenylphosphine catalysed multicomponent reactions28-30. Herein, a novel and facile way to the access of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole derivatives via a reaction between 3-methyl-1-phenyl-2-pyrazolin-5-one, aromatic aldehydes and malononitrile in aqueous ethanol at reflux in the presence of triphenylphosphine catalyst (Scheme 1).
Scheme 1 Synthesis of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles
EXPERIMENTAL:
Apparatus and analysis:
All the solvents, chemicals were taken from Merck, Sigma and Aldrich. Melting points of the products were measured using Electrothermal 9100 apparatus. Fourier transform infrared spectra (FT-IR) were taken by Shimadzu spectrometer by KBr discs. 1H and 13C NMR spectra were taken by Bruker DRX-500 and 300 Avance spectrometers.
General procedure for the preparation of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole derivatives:
To a mixture of aromatic aldehyde (1 mmol), 3-methyl-1-phenyl-2-pyrazolin-5-one (1 mmol), malononitrile (1 mmol), PPh3 (10 mol%) was added. The mixture was stirred for appropriate time in EtOH-H2O (1:1) under reflux. After completion of the reaction (TLC) and evaporation of EtOH, CH2Cl2 (20 mL) was added, and PPh3 was removed by filtration. The solid acid catalyst was dried at room temperature under vacuum and then reused. The solvent was evaporated and the crude product was purified by silica gel column chromatography (n-hexane-ethyl acetate 6:4).
Spectral data for the synthesized compounds (4a-4j)
6-Amino-5-cyano-3-methyl-1,4-diphenyl-1,4-dihydropyrano[2,3-c]pyrazole (4a):
IR (KBr, cm-1): 3466, 3311, 2196, 1664, 1588, 1255, 1127, 1019, 755. 1H NMR (500 MHz, DMSO-d6): δ 1.93 (s, 3H, CH3), 4.77 (s, 1H, CH), 6.80 (s, 2H, NH2), 7.16-7.51 (m, 10H, Ar-H ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.6, 22.4, 60.9, 118.0, 118.6, 119.6, 124.8, 126.1, 126.6, 127.8, 128.4, 129.3, 130.5, 137.9, 150.8, 177.1 ppm. MS (ESI): m/z 329 (M+H)+.
6-Amino-4-(4-fluorophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4b):
IR (KBr, cm-1): 3470, 3314, 2194, 1655, 1593, 1262, 1121, 1023, 761. 1H NMR (500 MHz, DMSO-d6): δ 1.82 (s, 3H, CH3), 4.70 (s, 1H, CH), 6.92 (s, 2H, NH2), 7.12 (d, 2H, J=8.4 Hz, Ar-H), 7.31 (d, 2H, J=8.4 Hz, Ar-H), 7.47-7.65 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 8.8, 22.6, 60.6, 117.4, 118.2, 119.7, 124.5, 125.5, 127.0, 127.7, 128.6 129.2, 130.7, 137.7, 150.5, 177.4 ppm. MS (ESI): m/z 347 (M+H)+.
6-Amino-4-(4-bromophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4c):
IR (KBr, cm-1): 3482, 3321, 2199, 1667, 1599, 1260, 1126, 1026, 762. 1H NMR (500 MHz, DMSO-d6): δ 1.96 (s, 3H, CH3), 4.82 (s, 1H, CH), 6.90 (s, 2H, NH2), 7.17 (d, 2H, J=8.4 Hz, Ar-H), 7.35 (d, 2H, J=8.4 Hz, Ar-H), 7.49-7.72 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 8.6, 21.4, 60.3, 117.7, 118.3, 119.5, 125.3, 125.7, 126.4, 128.4, 128.8, 129.7, 130.6, 138.3, 151.3, 176.8 ppm. MS (ESI): m/z 407.9 (M+H)+.
6-Amino-4-(4-methylphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4d):
IR (KBr, cm-1): 3477, 3324, 2202, 1671, 1586, 1270, 1132, 1025, 758. 1H NMR (500 MHz, DMSO-d6): δ 1.90 (s, 3H, CH3), 2.21 (s, 3H, CH3), 4.75 (s, 1H, CH), 6.84 (s, 2H, NH2), 7.11 (d, 2H, J=8.4 Hz, Ar-H), 7.24 (d, 2H, J=8.4 Hz, Ar-H), 7.41-7.59 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.4, 19.6, 22.3, 60.7, 117.9, 118.7, 120.0, 125.6, 125.8, 126.8, 128.2, 128.6, 129.6, 129.9, 137.8, 150.9, 177.0 ppm. MS (ESI): m/z 343 (M+H)+.
6-Amino-4-(4-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole(4e):
IR (KBr, cm-1): 3474, 3316, 2206, 1663, 1597, 1263, 1134, 1018, 750. 1H NMR (500 MHz, DMSO-d6): δ 1.85 (s, 3H, CH3), 3.63 (s, 3H, OCH3), 4.69 (s, 1H, CH), 6.89 (s, 2H, NH2), 7.09 (d, 2H, J=8.4 Hz, Ar-H), 7.21 (d, 2H, J=8.4 Hz, Ar-H), 7.48-7.64 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.5, 21.2, 55.8, 61.0, 118.4, 118.8, 120.3, 125.7, 125.9, 127.2, 128.0, 128.8, 129.1, 129.8, 138.5, 151.4, 176.6 ppm. MS (ESI): m/z 359 (M+H)+.
6-Amino-4-(3-nitrophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4f):
IR (KBr, cm-1): 3478, 3324, 2193, 1659, 1589, 1261, 1121, 1024, 753. 1H NMR (500 MHz, DMSO-d6): δ 1.97 (s, 3H, CH3), 4.79 (s, 1H, CH), 6.93 (s, 2H, NH2), 7.18 (d, 2H, J=8.4 Hz, Ar-H), 7.28 (d, 2H, J=8.4 Hz, Ar-H), 7.54-7.66 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 8.8, 22.0, 61.2, 117.3, 118.3, 120.1, 124.9, 126.3, 126.9, 127.8, 128.2, 129.0, 129.9, 138.0, 151.1, 176.9 ppm. MS (ESI): m/z 374 (M+H)+.
6-Amino-4-(4-chlorophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4g):
IR (KBr, cm-1): 3478, 3300, 2188, 1662, 1594, 1258, 1122, 1017, 751. 1H NMR (500 MHz, DMSO-d6): δ 1.88 (s, 3H, CH3), 4.72 (s, 1H, CH), 6.86 (s, 2H, NH2), 7.06 (d, 2H, J=8.4 Hz, Ar-H), 7.22 (d, 2H, J=8.4 Hz, Ar-H), 7.40-7.60 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.3, 21.7, 60.1, 118.4, 118.8, 119.3, 124.7, 126.3, 126.8, 127.9, 128.3, 129.6, 130.0, 138.4, 151.0, 176.7 ppm. MS (ESI): m/z 363.45 (M+H)+.
6-Amino-4-(3-chlorophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4h):
IR (KBr, cm-1): 3480, 3322, 2200, 1666, 1592, 1271, 1120, 1029, 756. 1H NMR (500 MHz, DMSO-d6): δ 1.89 (s, 3H, CH3), 4.80 (s, 1H, CH), 6.95 (s, 2H, NH2), 7.08-7.24 (m, 4H, Ar-H), 7.40-7.63 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.7, 22.5, 60.5, 117.9, 118.4, 119.9 125.8, 126.2, 127.4, 127.9, 128.3, 129.3, 130.3, 137.9, 150.6, 177.8 ppm. MS (ESI): m/z 363.45 (M+H)+.
6-Amino-4-(4-nitrophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4i):
IR (KBr, cm-1): 3461, 3317, 2205, 1672, 1600, 1266, 1131, 1030, 754. 1H NMR (500 MHz, DMSO-d6): δ 1.96 (s, 3H, CH3), 4.73 (s, 1H, CH), 6.86 (s, 2H, NH2), 7.14-7.30 (m, 4H, Ar-H), 7.46-7.67 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 9.9, 21.9, 60.8, 118.3, 118.7, 120.5, 125.5, 126.4, 127.2, 128.1, 128.6, 129.0, 129.5, 137.7, 151.0, 176.1 ppm. MS (ESI): m/z 374 (M+H)+.
6-Amino-4-(2-chlorophenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (4j):
IR (KBr, cm-1): 3466, 3310, 2204, 1675, 1588, 1276, 1120, 1043, 744. 1H NMR (500 MHz, DMSO-d6): δ 1.94 (s, 3H, CH3), 4.86 (s, 1H, CH), 6.89 (s, 2H, NH2), 7.11-7.22 (m, 4H, Ar-H), 7.440-7.59 (m, 5H, Ar-H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 10.3, 22.0, 58.9, 117.1, 118.7, 120.9 124.8, 125.2, 127.4, 127.9, 128.0, 129.7, 131.3, 138.1, 150.7, 176.9 ppm. MS (ESI): m/z 363.45 (M+H)+.
RESULTS AND DISCUSSION:
In an initial study, in order to examine the catalytic activity of catalyst and the amount of catalyst in this condensation reaction, 4-fluorobenzaldehyde (1b) was first reacted with 3-methyl-1-phenyl-2-pyrazolin-5-one (2) and malononitrile (3) in the presence of PPh3 at different solvents like CH3CN, DMF, CHCl3, EtOH, MeOH, 1,4-dioxane and EtOH-H2O (1:1) (Table 1, entries 1-7). The highest yield was obtained with EtOH-H2O (1:1) (Table 1, entry 7).
Using lower amounts of catalyst resulted in lower yields, while higher amounts of catalyst did not affect the reaction times and yields at reflux condition in EtOH-H2O (1:1) (Table 1, entries 7-9). From the results it is predicted that 10 mol% of catalyst was the best choice for completing the reaction (Table 1, entry 7).
We have conducted the model reaction at different temperatures in the presence of 10 mol % PPh3 catalyst in the presence of EtOH-H2O (1:1) (Table 1, entries 10–12). The highest yield was obtained in the shortest reaction time was at reflux condition in EtOH-H2O (1:1) (Table 1, entry 7). The reusability of the catalyst is tested for three consecutive cycles with almost the same catalytic activity (Table 1, entry 13).
After optimization of the reaction conditions various aromatic aldehydes, 3-methyl-1-phenyl-2-pyrazolin-5-one and malononitrile were subjected to condensation in the presence of 10 mol% of PPh3 as catalyst in EtOH-H2O (1:1) at reflux condition. The results are summarized in Table 2. In all the cases these three-component reaction gives the corresponding 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano [2,3-c]pyrazoles derivatives in excellent yields (Table 2, entries 1–10).
Table 1: Optimization of reaction conditions for the synthesis of 4ba
|
Entry |
Catalyst loading (mol%) |
Solvent |
Temperature (oC) |
Time (min) |
Yield (%)b |
|
1 |
10 |
CH3CN |
Reflux |
160 |
46 |
|
2 |
10 |
DMF |
Reflux |
160 |
57 |
|
3 |
10 |
CHCl3 |
Reflux |
170 |
66 |
|
4 |
10 |
EtOH |
Reflux |
110 |
75 |
|
5 |
10 |
MeOH |
Reflux |
110 |
81 |
|
6 |
10 |
1,4-dioxane |
Reflux |
130 |
70 |
|
7 |
10 |
EtOH-H2O (1:1) |
Reflux |
80 |
94 |
|
8 |
5 |
EtOH-H2O (1:1) |
Reflux |
110 |
74 |
|
9 |
15 |
EtOH-H2O (1:1) |
Reflux |
80 |
94 |
|
10 |
10 |
EtOH-H2O (1:1) |
RT |
180 |
46 |
|
11 |
10 |
EtOH-H2O (1:1) |
50 |
140 |
63 |
|
12 |
10 |
EtOH-H2O (1:1) |
60 |
120 |
73 |
|
13 |
10 |
EtOH-H2O (1:1) |
Reflux |
80 |
94,92,90,87 |
aReaction conditions: 4-fluorobenzaldehyde (1mmol), 3-methyl-1-phenyl-2-pyrazolin-5-one (1mmol) and malononitrile (1 mmol) were refluxed in the presence of PPh3 with various solvents (5ml) and also refluxed in EtOH-H2O (1:1) ; bIsolated yield.
Table 2 Synthesis of various 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles in the presence of PPh3 a
|
Entry |
Aldehyde |
Product |
Time (min) |
Yield (%)b |
M.p (oC) |
|
1 |
C6H5-CHO |
4a |
85 |
93 |
171 – 173 |
|
2 |
4-F-C6H4-CHO |
4b |
80 |
94 |
174 – 177 |
|
3 |
3-Br-C6H4-CHO |
4c |
80 |
91 |
176 – 177 |
|
4 |
4-CH3-C6H4-CHO |
4d |
90 |
88 |
176 – 178 |
|
5 |
4-OCH3-C6H4-CHO |
4e |
90 |
89 |
170 – 172 |
|
6 |
3-NO2-C6H4-CHO |
4f |
85 |
90 |
188 – 190 |
|
7 |
4-Cl-C6H4-CHO |
4g |
80 |
91 |
180 – 182 |
|
8 |
3-Cl-C6H4-CHO |
4h |
85 |
92 |
158 – 159 |
|
9 |
4-NO2-C6H4-CHO |
4i |
80 |
90 |
194 – 196 |
|
10 |
2-Cl-C6H4-CHO |
4j |
80 |
91 |
144 – 146 |
aReaction conditions: benzaldehyde (1 mmol), 3-methyl-1-phenyl-2-pyrazolin-5-one (1 mmol) and malononitrile (1 mmol) were refluxed in the presence of PPh3 in EtOH-H2O (1:1) (10 mol%); bIsolated yield
The proposed mechanism for the preparation of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles derivatives is illustrated in Scheme 2. At first the olefination of aldehyde with malononitrile is started to form 2-arylidenemalononitrile (a). Then 2-arylidenemalononitrile reacts with 3-methyl-1-phenyl-2-pyrazolin-5-one (2) to form intermediate (b). Intermediate (b) gives the desired product (4) via intermediate (c) by cyclisation of intermediate (c).
Scheme 2 Plausible mechanism of formation of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles
Scheme 2 Plausible mechanism of formation of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles
The synthesized compounds (4a-4j) were characterized by various spectral techniques like infrared, nuclear magnetic resonance and mass.
CONCLUSIONS:
In conclusion, a procedure for the preparation of 6-amino-4-aryl-5-cyano-3-methyl-1- phenyl-1,4-dihydropyrano[2,3-c]pyrazoles via a reaction between 3-methyl-1-phenyl-2-pyrazolin-5-one, aromatic aldehydes and malononitrile catalyzed by Triphenylphosphine under reflux condition in EtOH-H2O (1:1) have been developed.
CONFLICT OF INTEREST:
The authors have no conflicts of interest regarding this investigation.
ACKNOWLEDGEMENTS:
All the authors are grateful to C. Abdul Hakeem College Management for the facilities and support.
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Received on 17.05.2023 Modified on 13.08.2023
Accepted on 06.10.2023 ©AJRC All right reserved
Asian J. Research Chem. 2023; 16(6):433-437.
DOI: 10.52711/0974-4150.2023.00071