Here, we highlight an efficient and novel approach for the one-pot multicomponent preparations of 2-amino-4,6-disubstituted-3-cyanopyridine derivatives through the cyclo-condensation of substituted aromatic aldehydes, ammonium acetate, malononitrile, and aryl ketones under neat conditions. This study presents a novel approach for the preparations of 2-amino-4,6-disubstituted-3-cyanopyridine derivatives with numerous advantages, such as rapid reaction, environmentally friendly conditions, a broad range of compatible substrates, catalyst recyclability, and a simple workup process. The reported method harnesses the catalytic potential of guanidine hydrochloride, serving as a highly effective organo-catalyst, which exhibits exceptional mildness and catalytic activity, facilitating the reaction with high yield.
Research Article
Guanidine hydrochloride assisted one-pot synthesis of 2-amino-4,6- disubstituted-3-cyanopyridine derivatives: a sustainable and greener strategy with catalyst recyclability
https://doi.org/10.21203/rs.3.rs-4872464/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 28 Sep, 2024
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Guanidine Hydrochloride
Multicomponent
2-Amino-3-cyanopyridine
Solvent-free
Catalyst Recyclability
Heterocycles have significant roles in organic chemistry because of their diverse structures and chemical functionalities [1, 2]. The pyridine nucleus is one of the prominent heterocycles since it is found in numerous natural substances and holds significant importance in medicinal and pharmaceutical chemistry [3, 4]. Pyridine derivatives are a broad class of chemical compounds that have attracted significant attention because of their diverse medicinal properties such as antimicrobial [5, 6], anticoagulant [7], anticancer [8] antiviral [9], and anti-inflammatory [10].
In particular, 2-amino-3-cyanopyridine compounds have emerged as focal points of interest due to their reported biological activities such as carbonic anhydrase inhibitors [11], SIRT1 inhibitors [12], novel IKK-b inhibitors [13], dual VEGFR-2/HER-2 inhibitors [14], A2A adenosine receptor antagonists [15], antioxidant [16] and their use as essential intermediates in the synthesis of intricate heterocyclic molecules [17]. These compounds provide intriguing possibilities for drug discovery and development, prompting a continued pursuit of innovative synthetic methodologies to access and explore their potential applications [18–20].
Multicomponent reactions (MCRs) represent a predominant pathway for the preparation of 2-Amino-4,6-disubstituted-3-cyanopyridine derivatives by coupling reaction of four-components viz. aldehydes, malononitrile, ammonium acetate and aryl ketones using different catalysts reported in the literature such as diester cationic gemini surfactant (DCGS) [21], Cu(OAc)2 [22], (Fe3O4/SiO2/PTS-GO‐ZnO) [23], ytterbium perfluorooctanoate [Yb(PFO)3] [24], tetrabutyl ammonium bromide[25], 8,8′,8″-([1,3,5-triazine-2,4,6-triyl]tris[oxy])tris(1-sulfoquinolin-1-ium)chloride (TTS) [26], nano-Fe (4-bromo phenyl-salicylaldimine pyranopyrimidinedione) Cl2 {Nano-[Fe-4BSP] Cl2}[27], β-Cyclodextrin [28], Urease [29]. However, conventional methods often entail disadvantages such as toxic solvents, harsh reaction conditions, utilization of costly metal catalysts, and intricate work-up procedures, thus necessitating the exploration of safer, more efficient alternatives.
Guanidine hydrochloride (GuHCl), a versatile salt composed of the guanidinium cation and chloride anion, plays a catalytic role in organic synthesis, particularly in multicomponent reactions (MCR) like the Mannich reaction [30] Ugi reaction [31], multicomponent Hantzsch reaction [32], and Strecker Reaction [33]. With dual capability as a nucleophile activator, attributable to its primary amino nucleophilic groups and chloride ion, GuHCl emerges as a cost-effective and non-toxic catalyst for driving chemical conversions. The preference for neat conditions stems from its safety, high atom economy, and redox stability, aligning with our commitment to green chemistry principles [33, 34].
By continuing our pursuit of developing efficient methodologies for the synthesis of bioactive intermediates [35–39], we present a green and efficient protocol for the preparation of 2-amino-3-cyanopyridines through a four-component condensation reaction involving aryl ketones (3), malononitrile (2), aldehydes (1) and ammonium acetate (4) by utilizing GuHCl as an organo-catalyst in solvent-free condition (Scheme 1).
All chemicals and solvents utilized in this work were procured from reliable vendors, such as Spectrochem, Avra, Eurisotop, Sigma Aldrich, and SD Fine Chemicals, and used as received, without any further purification. The purity of the starting materials and progress of the reaction was determined by using Merk silica gel G254 0.22mm plates, and visualized under UV light at a wavelength of 254nm. H1 and C 13 NMR spectra were recorded on a 400 MHz NMR spectrometer from Bruker NMR instrument Technologies, utilizing tetramethylsilane (TMS) as an internal standard and either CDCl3 or DMSO-d6 as a deuterated solvent. The melting point (MP) of compounds were analyzed using the Analab ThermoCal melting point apparatus, employing one-end open capillary tubes.
General procedure for the synthesis of 2-amino-3,4-diphenyl-3-cyanopyridines derivatives
A mixture of aromatic aldehyde (3 mmol), aryl ketone (3 mmol), malononitrile (3 mmol), ammonium acetate (4.5 mmol), and guanidine hydrochloride (10 mol %) was added in a round bottom flask (RBF) and stirred by using a magnetic stirrer at 800C for 30 -45 min. The progress of the reaction has been observed every 15 minutes by TLC (n-Hexane: Ethyl acetate 8:2). After successful completion of the reaction, the reaction mixture is cooled at room temperature followed by a water wash and filtered. The residue underwent vacuum drying and was subsequently recrystallized utilizing ethanol to produce the desired product. Also, the filtrate was dried under a vacuum till the complete evacuation of water, and crystals of GuHCl were recovered and reused.
Characterization
2-Amino-4,6-diphenyl-nicotinonitrile (5a)
White Solid; M.P. 184-186 0C (Lit [40] 183-185 0C) 1H NMR (400 MHz, CDCl3) δ 7.94 (d, J = 2.8 Hz, 2H), 7.57 (d, J = 5.6 Hz, 2H), 7.42 (d, J = 19.2 Hz, 6H), 7.14 (s, 1H), 5.29 (s, 2H) ; 13C NMR (101 MHz, CDCl3) δ 160.40, 159.97, 155.27, 138.08, 137.06, 130.33, 129.95, 129.06, 128.93, 128.30, 127.46, 117.26, 111.40, 88.43.
2-Amino-4-(4-Chlorophenyl)-6-phenylnicotinonitrile (5b)
White solid; M.P. 190-192 0C (Lit [27] 188-190 0C) 1H NMR (400 MHz, CDCl3) δ 7.96 – 7.88 (m, 2H), 7.50 (d, J = 2.1 Hz, 1H), 7.44 (s, 1H), 7.43 – 7.36 (m, 4H), 7.19 (s, 1H), 7.10 (s, 1H), 5.32 (s, 2H).
2-amino-4-(3-chlorophenyl)-6-phenylnicotinonitrile (5c)
Yellow Solid; M.P. 170-172 0C (Lit[41] 168-170 0C) 1H NMR (400 MHz, CDCl3) δ 8.04 – 7.97 (m, 2H), 7.59 (s, 1H), 7.54 (d, J = 6.8 Hz, 1H), 7.50 – 7.45 (m, 4H), 7.26 (s, 1H), 7.18 (s, 1H), 5.38 (s, 2H).
2-amino-4-(2-chlorophenyl)-6-phenylnicotinonitrile (5d)
White Solid; M.P. 194-196 0C (Lit[41] 193-196 0C) 1H NMR (400 MHz, CDCl3) δ 7.83 (d, J = 48.8 Hz, 2H), 7.61 – 7.17 (m, 7H), 7.07 (s, 1H), 5.48 (s, 2H).
2-amino-6-(4-methoxyphenyl)-4-phenylnicotinonitrile (5e)
White Solid; M.P. 170-172 0C (Lit[42] 170 0C) 1H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 8.9 Hz, 2H), 7.56 (d, J = 9.6 Hz, 2H), 7.49 – 7.40 (m, 3H), 7.08 (s, 1H), 6.92 (d, J = 8.9 Hz, 2H), 5.26 (s, 2H), 3.80 (s, 3H).
2-amino-4-(3-methoxyphenyl)-6-phenylnicotinonitrile (5f)
Yellow Solid; M.P. 146-148 0C (Lit[43] 145-147 0C) 1HNMR (400 MHz, CDCl3) δ 8.00 (d, J = 7.9 Hz, 2H), 7.49 – 7.41 (m, 3H), 7.29 – 7.14 (m, 4H), 7.04 (dd, J = 8.3, 2.5 Hz, 1H), 5.36 (s, 2H), 3.89 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 160.37, 159.96, 155.12, 138.34, 138.05, 130.34, 130.18, 128.94, 127.46, 120.63, 117.22, 115.69, 113.78, 111.34, 88.44, 55.59.
2-amino-4-(3,4-dimethoxyphenyl)-6-phenylnicotinonitrile (5g)
Brown Solid; M.P. 240-242 0C (Lit [44] 249-2510C) 1H NMR (400 MHz, CDCl3) δ 8.00 (d, J = 9.6 Hz, 2H), 7.54 – 7.42 (m, 3H), 7.28 – 7.22 (m, 1H), 7.21 – 7.16 (m, 2H), 7.00 (d, J = 8.3 Hz, 1H), 5.37 (s, 2H), 3.96 (d, J = 5.6 Hz, 6H).
2-amino-4-(4-methoxyphenyl)-6-phenylnicotinonitrile (5h)
White Solid; M.P. 180-182 0C (Lit[43] 178-180 0C) 1H NMR (400 MHz, CDCl3) δ 7.92 (d, J = 8.0 Hz, 2H), 7.54 (d, J = 8.9 Hz, 2H), 7.40 (d, J = 7.3 Hz, 3H), 7.12 (s, 1H), 6.97 (d, J = 8.9 Hz, 2H), 5.28 (s, 2H), 3.81 (s, 3H).
2-amino-6-phenyl-4-(thiophen-3-yl)-nicotinonitrile (5i)
White Solid; M.P. 211-213 0C (Lit[43] 212-215 0C) 1H NMR (400 MHz, CDCl3) δ 8.06 – 7.80 (m, 3H), 7.57 – 7.39 (m, 4H), 7.28 (d, J = 17.2 Hz, 1H), 7.24 – 7.16 (m, 1H), 5.37 (s, 2H).
2-amino-6-phenyl-4-(p-tolyl)nicotinonitrile (5j)
White Solid; M.P. 220-222 0C (Lit[43] 216-219 0C) 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 7.9 Hz, 2H), 7.54 (d, J = 8.1 Hz, 2H), 7.47 (dd, J = 5.1, 2.2 Hz, 3H), 7.33 (d, J = 7.9 Hz, 2H), 7.20 (s, 1H), 5.35 (s, 2H), 2.44 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 160.43, 159.86, 155.28, 140.20, 138.14, 134.15, 130.25, 129.75, 128.90, 128.18, 127.44, 117.43, 111.28, 88.33, 21.48.
2-amino-6-(4-chlorophenyl)-4-phenylnicotinonitrile (5k)
Yellow Solid; M.P. 240-242 0C (Lit [19] 248-250 0C) 1H NMR (400 MHz, CDCl3) δ 7.89 (d, J = 8.7 Hz, 2H), 7.57 – 7.54 (m, 2H), 7.45 (dd, J = 5.1, 1.9 Hz, 3H), 7.38 (d, J = 8.7 Hz, 2H), 7.11 (s, 1H), 5.30 (s, 2H).
2-amino-6-(4-hydroxyphenyl)-4-phenylnicotinonitrile (5l)
Yellow Solid; M.P. 230-232 0C (Lit [19] 231-233 0C) 1H NMR (400 MHz, DMSO-d6) δ 9.97 (s, 1H), 8.01 (d, J = 8.8 Hz, 2H), 7.66 (d, J = 9.5 Hz, 2H), 7.56 – 7.52 (m, 4H), 6.90 – 6.84 (m, 4H).
2-amino-6-(4-bromophenyl)-4-(p-tolyl)nicotinonitrile (5m)
Yellow Solid; M.P. 220-222°C (Lit: [19] 219-2210C) 1H NMR (400 MHz, CDCl3): δ 8.1 (m,2H), 7.86-7.59 (m,4H), 7.36-7.28 (m,2H), 7.16 (s, 1H), 5.97 (s,2H), 2.49 (s,3H).
2-amino-4-(4-nitrophenyl)-6-phenylnicotinonitrile (5n)
White Solid; M.P. 220-222°C (Lit:[20] 219-2210C) 1H NMR (400 MHz, CDCl3): δ 8.24-8.18 (d, 2 H), 7.96-7.93(s, 2 H), 7.72-7.78 (d, J = 8.6 Hz 3 H), 7.56 (m, 2 H), 7.04 (s 1H), 5.6 (s, 2H)
2-Amino-4-(4-Fluorophenyl)-6-phenylnicotinonitrile (5o)
Yellow Solid; M.P. 218-220°C (Lit:[20] 219-2210C) 1H NMR (400 MHz, DMSO-d6) δ 8.10-8.14 (m, 2 H), 7.88 (m, 2 H), 7.72-7.76 (d, J = 6.2 Hz, 3 H), 7.54-7.65 (d, J = 8.4 Hz, 2 H), δ7.34(s 1H), 7.02 (s, 2H)
2-amino-6-(p-tolyl)-4-phenyl-nicotinonitrile (5p)
White Solid;M.P. 178-180°C (Lit: [29] 182-1830C) 1H NMR (400 MHz, CDCl3): δ 7.99 (s, 2H), 7.58 – 7.50 (m, 2H), 7.45 (s, 3H), 7.31 (s, 2H), 7.18 (s, 1H), 5.41 (s, 2H), 2.43 (s, 3H).
Building upon our previous research endeavours in catalysis, we embarked on a systematic exploration to assess the efficacy of GuHCl as a novel catalyst for synthesizing 2-amino-4,6-(disubstituted)-3-cyanopyridines. Initially, we selected a model reaction involving benzaldehyde (3 mmol), acetophenone (3 mmol), malononitrile (3 mmol), and ammonium acetate (4.5 mmol) as reactants and a catalytic amount of GuHCl under neat conditions in the formation of product 5a.
The impact of catalyst loading on the reaction time and product yields was determined (Table 1, entries 3-8). Notably, in the absence of any catalyst at normal room temperature and 100°C, the reaction proceeded sluggishly, yielding only traces of the desired product even after prolonged reaction times (Table 1, entries 2-3). However, with the utilization of GuHCl at optimized concentrations, significant changes in both the reaction time and yields were observed. For instance, at a catalyst loading of 10 mol% and a temperature of 80°C, the reaction exhibited an impressive yield of 96% within 35 minutes (Table 1, entry 6).
Table 1. Effect of catalyst loading for the preparations of 2-amino-4,6-diphenyl-3-cyanopyridinesa
|
Entry |
Catalyst (mol %) |
Temperature (oC) |
Time (min.) |
Yield (%) |
|
1 |
0 |
rt |
240 |
Traces |
|
2 |
0 |
100 |
150 |
Traces |
|
3 |
2.5 |
80 |
45 |
50 |
|
4 |
5 |
80 |
35 |
68 |
|
5 |
8 |
80 |
35 |
84 |
|
6 |
10 |
80 |
35 |
96 |
|
7 |
12.5 |
80 |
35 |
94 |
|
8 |
15 |
80 |
30 |
90 |
aReaction conditions: 3 mmol of benzaldehyde, 3 mmol of acetophenone, 3 mmol of malononitrile, and 4.5 mmol of ammonium acetate. bIsolated yield.
The influence of various solvents on the duration and efficiency of the reaction was evaluated, as depicted in Table 2. Surprisingly, none of the solvents exhibited a significant benefit in terms of reaction duration and product yield compared to the solvent-free settings. A significant yield was observed when GuHCl (10 mol%) was employed under neat conditions at 80 0C, resulting in a yield of 96% within 36 minutes (Table 2, entry 2). Consequently, solvent-free settings emerged as the preferred choice due to their cost-effectiveness and environmental compatibility.
Table 2. Effect of various solvents for the preparations of 2-amino-4,6-diphenyl-3-cyanopyridinea.
|
Sr. No |
Catalyst (mol %) |
Solvent |
Temperature (oC) |
Time (mins) |
Yieldb (%) |
|
1 |
10 |
Solvent-free |
Rt |
120 |
Traces |
|
2 |
10 |
Solvent-free |
80 |
36 |
96 |
|
3 |
10 |
Water |
80 |
60 |
72 |
|
4 |
10 |
Ethanol |
reflux |
120 |
90 |
|
5 |
10 |
Acetonitrile |
80 |
200 |
91 |
|
6 |
10 |
MeCN |
reflux |
140 |
65 |
|
7 |
10 |
THF |
reflux |
120 |
60 |
aReaction conditions: 3 mmol of benzaldehyde, 3 mmol of acetophenone, 3 mmol of malononitrile, and 4.5 mmol of ammonium acetate, and Gu.HCl (10 mol %). bIsolated yield.
A comparative analysis of catalysts documented in the literature for the preparations of the 2-amino-4,6-disubstituted-3-cyanopyridine compounds is presented (Table 3, entries 1-9). These observations underscore the efficacy of our approach, particularly in light of its notable advantages over existing protocols offering favorable reaction conditions, streamlined workup procedures, minimal reaction duration, and enhanced product yields.
Table 3. Comparison with various reported catalysts for the preparations of 2-Amino-3-cyanopyridines
|
Entry |
Catalyst |
Reaction Condition |
Time |
Yield % |
Ref. |
|
1. |
graphene oxide |
H2O / 800 C |
5 hr |
96 |
[45] |
|
2. |
β-cyclodextrin |
H2O / 900 C |
2 hr |
85 |
[28] |
|
3. |
Bu4N+Br -- (TBAB) |
H2O /reflux |
2 hr |
92 |
[25] |
|
4. |
iron(III) phosphate |
ethanol /reflux |
4 hr |
93 |
[44] |
|
5. |
Cu(OAc)2 |
ethanol/ reflux |
2 hr |
95 |
[22] |
|
6. |
diester cationic gemini surfactant (DCGS) |
H2O /reflux |
1 hr |
96 |
[21] |
|
7. |
Yb(PFO) |
ethanol/800 C |
4 hr |
90 |
[24] |
|
8. |
cellulose–SO3H |
H2O / 600 C |
2.5 hr |
95 |
[46] |
|
9. |
guanidine hydrochloride |
neat / 800 C |
0.55 hr |
96 |
This work |
bIsolated yield.
A plausible reaction mechanism for the synthesis of 2-amino-4,6-disubstituted-3-cyanopyridine derivatives is proposed, as depicted in Scheme 2. Initially, GuHCl activates the carbonyl group aromatic aldehyde, thereby enhancing its susceptibility to nucleophilic attack by malononitrile, leading to the formation of the arylidenemalononitrile derivative (I) via Knoevenagel condensation. Subsequently, the formation of imino derivative (II) takes place by the reaction of aryl ketone with ammonium acetate. This imino derivative (II) reacts with arylidene malononitrile (I), to form a reaction intermediate (III). The resulting intermediate (II) then undergoes cyclization to form a dihydropyridine derivative, which is followed by tautomerization and aerial oxidation to afford the final product, 2-amino--4,6-disubstituted-3-cyanopyridine derivative (5).
Further, we optimized the reaction condition to explore the range of substrates for the synthesis of 2-amino-4,6-disubstituted- 3-cyanopyridine derivatives (Table 4). We noticed that aldehydes and aryl ketones bearing both electron-donating (EDG) and electron-withdrawing groups (EWG) underwent the reaction with good results. Heterocyclic aldehydes were also well-tolerated under our protocol. The broader application of our procedure enabled the synthesis of numerous 2-amino-3-cyanopyridines in very good to excellent yields (84-96%) within a short reaction duration.
Table 4. Substrate scope for 3-cyanopyridine derivatives[a,b]
Eco‑scale and E‑Factor calculation
It is difficult to retrieve any publication on the Eco scale value and E-factor calculations for the current reaction scheme. Consequently, we employed the standard formula for the calculation of the E-Factor reported by Roger A. Sheldon [47] and compared it with the literature [44]. In our current study, the E Factor was determined to be 0.29. Notably, when compared, a higher E Factor of 3.46 has been identified in existing literature. In the context of synthesizing active pharmaceutical ingredients (APIs) and fine chemicals, it is imperative that this value be significantly lower than Sheldon's E-Factor [47,48]. Furthermore, we have evaluated Eco Scale values in our investigation, and in our current study it was found to be 95, while for literature work [49] score is 74.5 (See ESI) and this shows thar our study is more greener and environmentally benign.
In our study, we assessed the recyclability of GuHCl in the model reaction at 80°C. We found that the recovered catalyst could be recycled for up to four cycles without experiencing a significant loss in catalytic performance, with the fourth run yielding an impressive yield of 88%.
In conclusion, our research demonstrates a novel protocol designed for the efficient preparations of 2-amino-4,6-disubstituted-3-cyanopyridine derivatives by employing GuHCl as a catalyst. This study offers several advantages, including a high reaction rate, excellent yields, easy and simple workup procedures, catalyst recyclability, and solvent-free conditions, all of which contribute to environmentally friendly processes. Overall, our research shows the versatility as well as the efficacy of various synthetic approaches, offering up the possibilities for the development of more sustainable and greener protocols in organic synthesis.
Supplementary Information
Experimental details, NMR values and Spectral data, and Eco-scale value calculations are enclosed in the supplementary material as a separate electronic file.
Funding - Not applicable
Conflict of interest - The authors have no conflicts of interest to declare.
Ethical approval - Not applicable.
Acknowledgements
The authors are thankful to the Institute of Chemical Technology Indian Oil Campus Bhubaneswar for providing resources, facilities and support.
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Schemes 1-3 are available in the Supplementary Files section.
No competing interests reported.
