tert-Butyl nitrite mediated azo coupling between anilines and imidazoheterocycles

Abstract

A ^tBuONO mediated practical and efficient method for the synthesis of azo imidazoheterocycles with broad functionalities has been achieved by the reaction between imidazoheterocycles and anilines at room temperature in goods yields. Azo indolizines were also prepared by employing this methodology. Mild reaction conditions, tolerance of wide functionalities, operational simplicity and easy purification of the products are the notable advantages of the current protocol.

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Introduction

Azoheterocycles are well known for their uses as organic dyestuffs, pharmaceutical agents and food additives.¹ Besides traditional utilities, these scaffolds have been successfully employed as photodissociable ligands in molecular switches and machines owing to their light driven cis/trans isomerisation² (Fig. 1). A few azoheterocycles exhibit a potential role in various fields of the electronic industry, namely, non-linear optics,³ liquid crystal technology⁴ and photodynamic therapy.⁵ Due to diverse applications of these compounds, continuous efforts have been made to device new synthetic methods for azoheterocyclic compounds.^1a,2a,6

Fig. 1 Important azoheteroarene scaffolds.

Imidazo[1,2-a]pyridine moiety displays an extensive array of biological activities such as antitumor, antiparasitic, anti-inflammatory, antiviral, hypnotic, analgesic, antipyretic, etc.⁷ Some marketed drugs like olprinone, alpidem, zolpidem, saripidem etc. contain this fused bicyclic 5–6 nitrogenous heterocycle. Imidazo[1,2-a]pyridine derivatives are also found to be useful in material sciences.⁸ Accordingly, much efforts have been made for the synthesis and functionalization of imidazopyridine scaffold.⁹ For the last few years our group is also actively engaged in developing strategies for synthesis¹⁰ and functionalization of imidazoheterocycles.¹¹

Based on our experiences we envisioned that incorporation of azo functionality in the imidazo[1,2-a]pyridine ring could impart marked biological properties as well as photoresponsive soft materials based applications. Generally azoimidazo[1,2-a]pyridines are synthesized through diazo coupling using conc. HCl and NaNO₂.^6e Herein, we report a straightforward and environmentally benign protocol for the synthesis of azoimidazo[1,2-a]pyridines by the reaction between anilines and imidazo[1,2-a]pyridines using ^tBuONO in EtOH at room temperature (Scheme 1).

Scheme 1 Synthesis of (E)-3-(aryldiazenyl)imidazo[1,2-a]pyridines.

Results and discussion

Our effort was initiated by the reaction between p-toluidine (2a, 1.1 equiv.) and 8-methyl-2-phenylimidazo[1,2-a]pyridine (1a) in presence of ^tBuONO (1.2 equiv.) at room temperature in DMSO affording the diazo compound 3aa in 68% (Table 1, entry 1). Next we checked the effect of various solvents like DMF, CH₃CN, THF, 1,4-dioxane, DCM, MeOH and EtOH (Table 1, entries 2–8). Among them, EtOH was found to be the best solvent to furnish the desired product with 88% yield after 8 h (Table 1, entry 8) probably due to fact that EtOH is a polar protic solvent which helps to stabilize the diazonium ion. Prolonged reaction time did not improve the yield. Next we increased the amount of p-toluidine (2a) and ^tBuONO, but no significant increase in yield was seen to occur (Table 1, entry 9). Only a trace amount of product was obtained when the reaction was performed at 0 °C for 24 h (Table 1, entry 10).

Table 1 Optimization of the reaction conditions^a

Entry	Solvent	2a (equiv.)	^tBuONO (equiv.)	Temp (°C)	Time (h)	Yield (%)
a Reaction conditions: all reactions were carried out on 0.20 mmol scale in solvent (1.5 mL).b No reaction.
1	DMSO	1.1	1.2	rt	24	68
2	DMF	1.1	1.2	rt	24	54
3	CH3CN	1.1	1.2	rt	24	43
4	THF	1.1	1.2	rt	24	63
5	1,4-Dioxane	1.1	1.2	rt	24	56
6	DCM	1.1	1.2	rt	24	nr^b
7	MeOH	1.1	1.2	rt	24	76
8	EtOH	1.1	1.2	rt	8	88
9	EtOH	1.5	2	rt	24	88
10	EtOH	1.1	1.2	0	24	Trace

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With the optimized reaction conditions in hand, the substrate scope of the present protocol was explored and the results are summarized in Scheme 2. A variety of anilines having both electron donating substituents like –Me, –OMe and electron withdrawing groups like –CN, –NO₂, –CO₂Et, –COMe, –COPh were studied under the present reaction conditions. It is notable that electron-deficient anilines gave better yields (3ad–ah) compared to electron-rich anilines (3aa and 3ab) as diazonium salts bearing electron withdrawing groups are stronger electrophiles than electron-rich diazonium salts. The anilines having different halogens like –Cl, –F and –I successfully reacted with imidazopyridine to give the corresponding products in significant yields (3ai–ak). Importantly, the reaction between 8-methyl-2-phenylimidazo[1,2-a]pyridine (1a) and 2-aminobenzenesulfonamide (2m) afforded ethyl benzenesulfonate substituted azo derivative 3am.

Scheme 2 Substrates scope^a, ^areaction conditions: 1 (0.20 mmol), 2 (0.22 mmol) and ^tBuONO (0.24 mmol) in EtOH (1.5 mL) at rt for 8 h. ^bReactions completed in 2 h. ^ctBuONO (0.4 mmol).

Next, we checked the scope of imidazopyridines having different substituents like –Cl, –Br and –CN on the pyridine ring as shown in Scheme 2. All of them gave the azoimidazopyridines (3cf, 3da and 3ef) with good to excellent yields. Imidazopyridines having different aryl (3fa and 3ga) and heteroaryl (3ha) substitutions at C-2 position furnished the products with good yields. To our delight, heteroaryl amines such as aminopyrazoles and aminopyridines also worked well under the present reaction conditions (3an and 3ao). However, 2,3-unsubstituted imidazo[2,1-a]pyridine (1i), 2-isobutyl and CF₃-substituted imidazo[2,1-a]pyridine (1j and 1k) did not afford the desired products.

The stereochemistry of 8-methyl-2-phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3aa) was further confirmed by X-ray single crystallographic analysis (Fig. 2).¹²

Fig. 2 X-ray crystal structure of 3aa (thermal ellipsoids are drawn at 30% probability level).

Next we extended our protocol to other imidazoheterocycles like benzo[d]imidazo[2,1-b]thiazole and imidazo[2,1-b]thiazole (Scheme 3). Pleasingly, the corresponding azo products were obtained with excellent yields (5aa–ca).

Scheme 3 Substrate scope of imidazoheterocycles^a, ^areaction conditions: 4 (0.20 mmol), 2 (0.22 mmol) and ^tBuONO (0.24 mmol) in EtOH (1.5 mL) at rt for 8 h. ^bReaction completed in 1 h.

Other than imidazoheterocycles, we were successful to install the azo functionality in indolizine moiety regioselectively (Scheme 4). Indolizines bearing –CO₂Me and –CN successfully reacted with differently substituted anilines to give the desired products in very good yields (7af and 7bg).

Scheme 4 Synthesis of azo indolizines^a, ^areaction conditions: 0.20 mmol of 6 and 0.22 mmol of 2 in EtOH (1.5 mL) in the presence of ^tBuONO (0.24 mmol) at rt for 4 h.

The present protocol is also applicable for the intramolecular azo coupling. Benzo[e]pyrrolo[2,1-c]-1,2,4-benzotriazine¹³ 9 was obtained with 78% yield (Scheme 5). It is worthy to mention that benzotriazine 9 is an analogue of indole[1,2-c]-1,2,4-benzotriazine derivatives which exhibit potent antifungal activities against five phytopathogenic fungi.¹⁴

Scheme 5 Synthesis of benzo[e]pyrrolo[2,1-c][1,2,4]triazine.

Practical applicability of the present methodology was examined by carrying out the gram-scale reaction in laboratory setup (Scheme 6). The reaction afforded the azo imidazo[1,2-a]pyridine (3aa) without significant decrease in yield (84%).

Scheme 6 Gram-scale reaction.^a

A plausible mechanism is outlined in Scheme 7. First, aniline 2a is converted to diazonium intermediate¹⁵ A in presence of ^tBuONO which subsequently reacted with imidazopyridine 1a at 3-postion. Finally the azo compound 3aa is formed via deprotonation of intermediate B. The presence of aryl group at 2-positon of imidazopyridine ring is necessary to stabilise the generated positive charge at 2-positon of the intermediate B. Therefore, 2-alkyl substituted imidazopyridine (1j) is inert under the present reaction conditions.

Scheme 7 Plausible reaction pathway.

Conclusions

In summary, we have developed a convenient method for the synthesis of azo imidazo[1,2-a]pyridines. A variety of (E)-3-(aryldiazenyl)imidazo[1,2-a]pyridines with wide range of functionalities were synthesized in good to excellent yields under neutral conditions. Intriguingly, the protocol is also applicable to other heterocycles like imidazo[2,1-b]thiazoles, benzo[d]imidazo[2,1-b]thiazoles and indolizines. The developed reaction procedure shows broad substrates scope, uses safe reagents and solvent, clean reaction, and product isolation by simple filtration making it a useful and preparative method. We believe it will gain much importance in academia as well as industry for the synthesis of functionalized azoheterocycles.

Experimental section

General information

All reagents were purchased from commercial sources and used without further purification, unless otherwise mentioned. Petroleum ether (PE) refers to the fraction of petroleum boiling between 60 and 80 °C. All reactions were carried out in oven-dried glassware. All solvents were dried and distilled before use. TLC was monitored on aluminium backed Silica Gel 60 (HF₂₅₄) 0.25 mm plates (Merck) using various organic solvent mixtures. Melting points were determined in open capillary tubes and are uncorrected. IR spectra were recorded on a FTIR instrument. ¹H and ¹³C NMR spectra were acquired on spectrometers operating at 300 or 400 or 500 MHz and 75 or 100 or 125 MHz respectively, using CDCl₃ (δ = 7.26 ppm for the ¹H NMR and δ = 77.16 ppm for the ¹³C NMR) or DMSO-d₆ (δ = 2.50 ppm for the ¹H NMR and δ = 39.50 ppm for the ¹³C NMR) solvent. High resolution mass spectra were obtained using Qtof Micro YA263 instrument. X-ray single crystal data were collected using MoKα (λ = 0.7107 Å) radiation on a SMART APEX diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement were carried out using the software package of SMART APEX. Imidazoheterocycles were prepared by our reported methods,^10c and indolizine heterocycles were prepared by Hu et al. report.¹⁶

Typical experimental procedure for the synthesis of (E)-8-methyl-2-phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3aa)

A mixture of p-toluidine 2a (0.22 mmol, 21 mg) and ^tBuONO (0.24 mmol, 25 mg, 29 μL) in EtOH (1.5 mL) was taken in a round bottom flask and was stirred for 3 min at room temperature. To this solution 8-methyl-2-phenylimidazo[1,2-a]pyridine 1a (0.20 mmol, 42 mg) was added and allowed to stir for 8 h (TLC). The orange precipitate was collected, washed with EtOH (5 mL) and 10% ethyl acetate/petroleum ether (3 mL). The solid product was dried under vacuum to afford pure (E)-8-methyl-2-phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine 3aa in 88% yield.

Synthesis of benzo[e]pyrrolo[2,1-c][1,2,4]triazine (9). To a solution of 2-(1H-pyrrol-1-yl)aniline 8 (0.20 mmol) in EtOH (1 mL) was added ^tBuONO (0.24 mmol, 25 mg, 29 μL) and the reaction mixture was stirred at room temperature for 8 h (TLC). The yellow precipitate was collected, washed with EtOH (5 mL) and 10% ethyl acetate/petroleum ether (3 mL). The solid product was dried under vacuum to furnish pure benzo[e]pyrrolo[2,1-c][1,2,4]triazine 9.

(E)-8-Methyl-2-phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3aa). Orange solid (88%, 57 mg), mp 104–105 °C. IR (KBr) ν_max: 3443, 3022, 2465, 1599, 1524, 1477, 1445, 1350, 1252 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.81 (d, J = 6.8 Hz, 1H), 8.46 (d, J = 8.4 Hz, 2H), 7.76 (d, J = 8.0 Hz, 2H), 7.54 (t, J = 7.6 Hz, 2H), 7.48–7.44 (m, 1H), 7.30–7.22 (m, 3H), 6.91 (t, J = 6.8 Hz, 1H), 2.72 (s, 3H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 152.0, 149.7, 146.0, 139.7, 133.5, 132.5, 130.1, 129.9, 129.1, 128.5, 128.4, 127.4, 127.2, 122.0, 115.1, 21.5, 17.0; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₁H₁₉N₄: 327.1610; found: 327.1602.

(E)-3-((4-Methoxyphenyl)diazenyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ab). Orange solid (83%, 57 mg), mp 106–108 °C. IR (KBr) ν_max: 3566, 3431, 3408, 3137, 2825, 1603, 1582, 1497, 1439, 1375, 1308, 1250 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.80 (d, J = 6.4 Hz, 1H), 8.44 (d, J = 7.6 Hz, 2H), 7.80 (d, J = 8.8 Hz, 2H), 7.53 (t, J = 7.6 Hz, 2H), 7.45 (t, J = 7.2 Hz, 1H), 7.28 (t, J = 6.8 Hz, 1H), 7.00–6.95 (m, 3H), 3.88 (s, 3H), 2.78 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 161.2, 148.0, 144.7, 132.3, 132.1, 130.3, 129.4, 129.2, 128.5, 127.10, 127.06, 123.8, 115.6, 114.5; 55.7, 17.3; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₁H₁₉N₄O: 343.1559; found: 343.1555.

(E)-8-Methyl-2-phenyl-3-(phenyldiazenyl)imidazo[1,2-a]pyridine (3ac). Orange solid (87%, 54 mg), mp 98–100 °C. IR (KBr) ν_max: 3059, 3028, 2395, 1618, 1591, 1579, 1525, 1483, 1471, 1444, 1379, 1350, 1290, 1250 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.86 (d, J = 6.8 Hz, 1H), 8.44 (d, J = 7.2 Hz, 2H), 7.87 (d, J = 7.6 Hz, 2H), 7.54–7.46 (m, 5H), 7.39 (t, J = 7.2 Hz, 1H), 7.29 (d, J = 6.8 Hz, 1H), 6.97 (t, J = 6.8 Hz, 1H), 2.74 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 153.8, 150.2, 146.0, 133.1, 130.2, 129.6, 129.4, 129.32, 129.26, 129.0, 128.5, 127.4, 127.2, 122.1, 115.5, 17.0; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₇N₄: 313.1453; found: 313.1447.

(E)-2-Phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3ba). Orange solid (81%, 51 mg), mp 103–104 °C. IR (KBr) ν_max: 3049, 3026, 1626, 1597, 1570, 1552, 1445, 1385, 1350, 1246 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.97 (d, J = 6.8 Hz, 1H), 8.45–8.43 (m, 2H), 7.78–7.74 (t, 3H), 7.55–7.51 (m, 2H), 7.48–7.44 (m, 2H), 7.30 (d, J = 8.0 Hz, 2H), 7.03 (t, J = 6.8 Hz, 1H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 152.0, 150.3, 146.0, 139.9, 133.4, 132.3, 130.1, 130.0, 129.4, 129.27, 129.25, 128.6, 122.1, 117.6, 115.2, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₇N₄: 313.1453; found: 313.1447.

(E)-2-((8-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)benzonitrile (3ad). Orange solid (91%, 61 mg), mp 141–142 °C. IR (KBr) ν_max: 3496, 3061, 2916, 2393, 2289, 2225, 1925, 1619, 1522, 1478, 1384, 1282, 1242, 1212 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.02 (s, 1H), 8.42 (d, J = 6.0 Hz, 2H), 7.88 (d, J = 4.4 Hz, 1H), 7.77 (d, J = 4.8 Hz, 1H), 7.64 (d, J = 7.2 Hz, 1H), 7.54–7.47 (m, 3H), 7.43–7.38 (m, 2H), 7.16 (d, J = 6.4 Hz, 1H), 2.76 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 156.0, 152.9, 147.1, 133.6, 133.2, 132.6, 131.1, 130.4, 130.0, 128.73, 128.66, 128.4, 127.6, 118.3, 117.0, 115.6, 112.0, 17.0; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₁H₁₆N₅: 338.1406; found: 338.1399.

(E)-8-Methyl-3-((3-nitrophenyl)diazenyl)-2 phenylimidazo[1,2-a]pyridine (3ae). Orange solid (92%, 66 mg), mp 152–153 °C. IR (KBr) ν_max: 3281, 3099, 1618, 1526, 1508, 1474, 1456, 1416, 1350, 1319, 1286, 1246 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.80 (d, J = 6.8 Hz, 1H), 8.60 (t, J = 6.8 Hz, 1H), 8.40 (d, J = 8.0 Hz, 2H), 8.17–8.16 (m, 1H), 8.09 (d, J = 7.2 Hz, 1H), 7.62–7.46 (m, 4H), 7.35 (d, J = 7.2 Hz, 1H), 7.02 (t, J = 7.2 Hz, 1H), 2.72 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 154.7, 152.8, 149.3, 147.1, 133.1, 132.5, 130.2, 129.98, 129.95, 129.8, 128.7, 127.9, 127.6, 126.8, 122.8, 117.4, 116.0, 16.9; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₆N₅O₂: 358.1304; found: 358.1298.

(E)-Ethyl-4-((8-methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)benzoate (3af). Red solid (97%, 75 mg), mp 126–128 °C. IR (KBr) ν_max: 3369, 2980, 2902, 2841, 2446, 1711, 1599, 1477, 1273, 1248 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.85 (d, J = 6.8 Hz, 1H), 8.44–8.42 (m, 2H), 8.15 (d, J = 8.4 Hz, 2H), 7.85 (d, J = 8.4 Hz, 2H), 7.55–7.45 (m, 3H), 7.32 (d, J = 7.2 Hz, 1H), 6.99 (t, J = 7.2 Hz, 1H), 4.41 (q, J = 6.8 Hz, 2H), 2.72 (s, 3H), 1.43 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 166.4, 156.8, 152.1, 146.9, 133.3, 132.9, 130.8, 130.2, 129.6, 128.6, 127.7, 127.5, 121.8, 115.8, 61.2, 16.9, 14.5; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₃H₂₁N₄O₂: 385.1665; found: 385.1660.

(E)-1-(4-((8-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)phenyl)ethanone (3ag). Orange solid (96%, 68 mg), mp 114–116 °C. IR (KBr) ν_max: 3854, 3838, 3821, 3749, 3736, 3674, 3651, 3616, 3061, 2991, 2918, 1680, 1651, 1595, 1560, 1541, 1524, 1512, 1491, 1477, 1460, 1443, 1377, 1350, 1292, 1263, 1248 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.85 (d, J = 6.8 Hz, 1H), 8.43 (d, J = 7.2 Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H), 7.87 (d, J = 8.4 Hz, 2H), 7.55–7.46 (m, 3H), 7.33 (d, J = 6.8 Hz, 1H), 7.00 (t, J = 6.8 Hz, 1H), 2.72 (s, 3H), 2.64 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 197.4, 156.6, 152.0, 146.7, 136.4, 133.1, 132.8, 130.1, 130.0, 129.6, 129.5, 128.5, 127.5, 127.4, 121.8, 115.7, 26.8, 16.8; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₂H₁₉N₄O: 355.1559; found: 355.1552.

(E)-(2-((8-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)phenyl) (phenyl)methanone (3ah). Orange solid (94%, 78 mg), mp 111–112 °C. IR (KBr) ν_max: 3309, 3134, 3061, 2918, 2367, 2338, 1901, 1663, 1589, 1522, 1478, 1441, 1383, 1314 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.61 (d, J = 6.8 Hz, 1H), 8.38–8.36 (m, 2H), 7.92–7.89 (m, 3H), 7.66–7.59 (m, 2H), 7.55–7.47 (m, 5H), 7.43–7.39 (m, 2H), 7.21 (d, J = 6.8 Hz, 1H), 6.57 (t, J = 7.2 Hz, 1H), 2.69 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 198.2, 151.8, 146.0, 138.4, 137.8, 133.4, 132.7, 132.6, 131.2, 130.3, 130.1, 129.8, 129.7, 129.4, 128.9, 128.7, 128.6, 127.2, 127.0, 116.3, 115.6, 17.0; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₇H₂₁N₄O: 417.1715; found: 417.1712.

(E)-3-((4-Chlorophenyl)diazenyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ai). Orange solid (88%, 61 mg), mp 100–101 °C. IR (KBr) ν_max: 3043, 2918, 2849, 2422, 1618, 1583, 1572, 1522, 1472, 1445, 1383, 1375, 1350, 1308, 1294, 1252 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.82 (d, J = 6.8 Hz, 1H), 8.41 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 6.8, 2H), 7.55–7.51 (m, 2H), 7.48–7.44 (m, 3H), 7.31 (d, J = 6.8, 1H), 6.98 (t, J = 6.8 Hz, 1H), 2.73 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 152.5, 151.1, 146.5, 134.8, 133.4, 130.2, 129.5, 129.4, 129.1, 128.6, 127.7, 127.3, 123.2, 115.5, 16.9; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₆ClN₄: 347.1063; found: 347.1066.

(E)-3-((4-Fluorophenyl)diazenyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3aj). Orange solid (90%, 59 mg), mp 102–103 °C. IR (KBr) ν_max: 3130, 3053, 3032, 2229, 2129, 1587, 1477, 1445, 1425, 1379, 1348, 1294, 1247 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.75 (d, J = 6.8 Hz, 1H), 8.42–8.39 (m, 2H), 7.83–7.79 (m, 2H), 7.55–7.51 (m, 2H), 7.48–7.43 (m, 1H), 7.26–7.24 (m, 1H), 7.18–7.13 (m, 2H), 6.92 (t, J = 7.2 Hz, 1H), 2.71 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 163.2 (d, ¹J_CF = 249 Hz), 150.5, 150.4 (d, ⁴J_CF = 2 Hz), 146.3, 133.5, 132.4, 130.1, 129.2, 128.6, 128.5, 127.5, 127.1, 123.7 (d, ²J_CF = 9 Hz), 116.1 (d, ²J_CF = 23 Hz), 115.2, 16.9; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₆FN₄: 331.1359; found: 331.1368.

(E)-3-((2-Iodophenyl)diazenyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ak). Orange solid (83%, 73 mg), mp 111–112 °C. IR (KBr) ν: 3117, 2916, 1522, 1481, 1373, 1238 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.13 (d, J = 6.4 Hz, 1H), 8.44 (d, J = 8.0 Hz, 2H), 8.00 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.55–7.52 (m, 2H), 7.49–7.41 (m, 2H), 7.36 (d, J = 7.2 Hz, 1H), 7.06 (t, J = 7.6 Hz, 2H), 2.74 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 153.1, 152.1, 146.9, 139.8, 133.4, 133.0, 130.4, 130.3, 129.7, 129.5, 129.1, 128.7, 128.6, 127.5, 117.2, 116.0, 101.5, 16.9; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₆IN₄: 439.0420; found: 439.0436.

(E)-Ethyl-2-((8-methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)benzenesulfonate (3al). Compound 3al was obtained as described for 3aa by using 2-aminobenzenesulfonamide 2l (0.22 mmol, 38 mg), ^tBuONO (0.4 mmol, 42 mg, 48 μL), 8-methyl-2-phenylimidazo[1,2-a]pyridine 1a (0.20 mmol, 42 mg). Orange solid (80%, 67 mg), mp 182–183 °C. IR (KBr) ν_max: 3089, 1586, 1510, 1382, 1346, 1243 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.27 (d, J = 6.4 Hz, 1H), 8.44 (d, J = 7.2 Hz, 2H), 8.13 (d, J = 7.6 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.72 (t, J = 7.2 Hz, 1H), 7.56–7.41 (m, 5H), 7.13 (t, J = 7.2 Hz, 1H), 4.18 (q, J = 6.8 Hz, 2H), 2.75 (s, 3H), 1.25 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, DMSO-d₆): δ 143.8, 132.9, 131.7, 131.1, 130.6, 130.1, 129.9, 129.8, 129.4, 128.8, 128.4, 127.7, 127.0, 125.1, 123.3, 117.1, 114.2, 56.1, 18.6, 16.3; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₂H₂₁N₄O₃S: 421.1334; found: 421.1327.

(E)-Ethyl-4-((6-chloro-2-phenylimidazo[1,2-a]pyridine-3-yl)diazenyl)benzoate (3cf). Orange solid (95%, 78 mg), mp 140–141 °C. IR (KBr) ν_max: 3043, 2980, 2368, 1934, 1721, 1599, 1522, 1479, 1385, 1317 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.07 (s, 1H), 8.40 (d, J = 8.0 Hz, 2H), 8.19 (d, J = 8.4 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H), 7.74 (d, J = 9.6 Hz, 1H), 7.56–7.47 (m, 4H), 4.42 (q, J = 7.2 Hz, 2H), 1.44 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 166.3, 156.4, 144.7, 132.4, 132.3, 131.4, 131.1, 130.9, 130.2, 130.1, 128.8, 127.5, 123.8, 122.0, 117.9, 61.3, 14.5; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₂H₁₈ClN₄O₂: 405.1118; found: 405.1110.

(E)-6-Bromo-2-phenyl-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3da). Orange solid (76%, 59 mg), mp 136–137 °C. IR (KBr) ν_max: 3130, 3038, 3001, 2917, 2857, 1933, 1894, 1599, 1474, 1382, 1325, 1237 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.15 (s, 1H), 8.41 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 8.4 Hz, 2H), 7.64 (d, J = 9.2 Hz, 1H), 7.54–7.46 (m, 4H), 7.32 (d, J = 8.0 Hz, 2H), 2.44 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 151.7, 150.4, 144.3, 140.6, 133.0, 132.3, 132.1, 130.1, 130.0, 129.5, 129.3, 128.7, 122.3, 118.1, 109.5, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₆BrN₄: 391.0558; found: 391.0552.

(E)-Ethyl-4-((6-cyano-2-phenylimidazo[1,2-a]pyridin-3-yl)diazenyl)benzoate (3ef). Orange solid (88%, 70 mg), mp 184–186 °C. IR (KBr) ν_max: 3401, 3120, 3046, 2984, 2234, 1934, 1713, 1599, 1524, 1482, 1387, 1334, 1308 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 10.35 (s, 1H), 8.40 (d, J = 6.4 Hz, 2H), 8.20 (d, J = 8.4 Hz, 2H), 7.91 (d, J = 8.4 Hz, 2H), 7.82 (d, J = 8.8 Hz, 1H), 7.60–7.53 (m, 4H), 4.43 (d, J = 7.2 Hz, 2H), 1.44 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 166.1, 156.0, 135.3, 132.0, 131.7, 131.0, 130.9, 130.6, 130.3, 129.8, 128.9, 122.2, 121.8, 118.6, 116.4, 101.5, 61.4, 14.5; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₈N₅O₂: 396.1460; found: 396.1452.

(E)-2-(Naphthalen-2-yl)-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3fa). Orange solid (90%, 65 mg), mp 111–113 °C. IR (KBr) ν_max: 3080, 3018, 1506, 1487, 1458, 1394, 1385, 1352, 1323, 1302, 1248, 1240 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.97 (s, 1H), 8.97 (s, 1H), 8.60 (d, J = 8.8 Hz, 1H), 8.00–7.96 (m, 2H), 7.90–7.88 (m, 1H), 7.82–7.76 (m, 3H), 7.54–7.45 (m, 3H), 7.31 (d, J = 7.6 Hz, 2H), 7.03 (s, 1H), 2.44 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 152.0, 150.1, 146.1, 140.0, 133.8, 133.5, 132.5, 130.8, 130.1, 130.0, 129.5, 129.4, 129.1, 128.0, 127.8, 127.1, 126.9, 126.3, 122.1, 117.5, 115.2, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₄H₁₉N₄: 363.1610; found: 363.1603.

(E)-2-(3-(p-Tolyldiazenyl)imidazo[1,2-a]pyridin-2-yl)phenol (3ga). Orange solid (82%, 54 mg), mp 120–122 °C. IR (KBr) ν_max: 3086, 3024, 2916, 2858, 2806, 2725, 2644, 2557, 1618, 1587, 1578, 1512, 1497, 1408, 1385, 1354, 1323, 1303, 1245, 1234 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 13.09 (brs, 1H), 10.00 (d, J = 6.4 Hz, 1H), 8.74 (d, J = 7.6 Hz, 1H), 7.88 (d, J = 6.4 Hz, 1H), 7.75 (d, J = 8.0 Hz, 2H), 7.66 (d, J = 8.8 Hz, 1H), 7.50 (t, J = 7.2 Hz, 1H), 7.38–7.28 (m, 3H), 7.10–6.99 (m, 2H), 2.45 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 159.6, 151.8, 143.6, 140.2, 131.3, 131.2, 130.0, 129.9, 129.5, 122.8, 122.6, 122.1, 119.4, 117.8, 116.5, 116.4, 115.8, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₇N₄O: 329.1402; found: 329.1396.

(E)-2-(Thiophen-2-yl)-3-(p-tolyldiazenyl)imidazo[1,2-a]pyridine (3ha). Yellow solid (85%, 54 mg), mp 120–121 °C. IR (KBr) ν_max: 3059, 3029, 2917, 2856, 2395, 1952, 1901, 1787, 1716, 1554, 1484, 1431, 1384, 1331 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.86 (d, J = 6.8 Hz, 1H), 8.13–8.12 (m, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.69 (d, J = 9.2 Hz, 1H), 7.55–7.53 (m, 1H), 7.46–7.42 (m, 1H), 7.31 (d, J = 8.4 Hz, 2H), 7.22–7.20 (m, 1H), 7.00 (t, J = 7.0 Hz, 1H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 152.0, 146.4, 146.1, 140.0, 135.7, 130.9, 130.0, 129.8, 129.6, 129.2, 128.1, 122.3, 117.3, 115.0, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₈H₁₅N₄S: 319.1017; found: 319.1015.

(E)-8-Methyl-2-phenyl-3-(pyridin-3-yldiazenyl)imidazo[1,2-a]pyridine (3am). Orange solid (79%, 49 mg), mp 145–146 °C. IR (KBr) ν_max: 3030, 2918, 2849, 1672, 1580, 1526, 1473, 1447, 1423, 1381, 1350, 1306, 1292, 1261, 1248 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.78 (d, J = 6.8 Hz, 1H), 9.08 (s, 1H), 8.56 (d, J = 3.6 Hz, 1H), 8.39 (d, J = 7.2 Hz, 2H), 8.02 (d, J = 8.0 Hz, 1H), 7.52–7.43 (m, 3H), 7.40–7.30 (m, 1H), 7.30 (d, J = 6.8 Hz, 1H), 6.97 (t, J = 6.8 Hz, 1H), 2.70 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 151.8, 149.6, 149.4, 146.7, 146.4, 133.2, 132.9, 130.1, 129.5, 128.6, 127.7, 127.4, 126.1, 124.1, 115.7, 16.8; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₆N₅: 314.1406; found: 314.1401.

(E)-8-Methyl-2-phenyl-3-((3-phenyl-1H-pyrazol-5-yl)diazenyl)imidazo[1,2-a]pyridine (3an). Brown solid (74%, 56 mg), mp 135–137 °C. IR (KBr) ν_max: 3142, 3067, 3013, 2880, 1483, 1373, 1308, 1249 cm⁻¹; ¹H NMR (500 MHz, DMSO-d₆): δ 9.69 (d, J = 6.5 Hz, 1H), 8.41 (d, J = 7.5 Hz, 2H), 7.87 (d, J = 7.5 Hz, 2H), 7.58 (t, J = 7.5 Hz, 2H), 7.50–7.47 (m, 5H), 7.39 (t, J = 7.5 Hz, 1H), 7.21 (t, J = 7.0 Hz, 1H), 6.91 (s, 1H), 2.61 (s, 3H); ¹³C NMR (125 MHz, DMSO-d₆): δ 148.5, 145.7, 132.7, 132.1, 129.7, 129.6, 129.4, 129.1, 128.9, 128.7, 128.5, 126.9, 125.4, 116.2, 90.8, 16.3; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₉N₆: 379.1671; found: 379.1668.

(E)-2-(p-Tolyl)-3-(p-tolyldiazenyl)benzo[d]imidazo[2,1-b]thiazole (5aa). Yellow solid (92%, 70 mg), mp 128–129 °C. IR (KBr) ν_max: 3095, 3058, 3020, 2708, 2272, 1485, 1474, 1356, 1335, 1236 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.14 (d, J = 8.0 Hz, 1H), 8.20 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 8.4 Hz, 2H), 7.67 (d, J = 7.6 Hz, 1H), 7.46 (t, J = 7.2 Hz, 1H), 7.38–7.30 (m, 5H), 2.45 (s, 3H), 2.44 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 153.4, 151.7, 150.9, 140.4, 139.1, 138.8, 134.4, 130.8, 130.1, 130.0, 129.7, 129.3, 126.0, 125.4, 123.5, 122.3, 119.6, 21.63, 21.57; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₉N₄S: 383.1330; found: 383.1327.

(E)-3-((2-Bromo-5-(trifluoromethyl)phenyl)diazenyl)-2-phenylbenzo[d]imidazo[2,1-b]thiazole (5bo). Yellow solid (93%, 93 mg), mp 161–162 °C. IR (KBr) ν_max: 3086, 3057, 3020, 2930, 1516, 1474, 1423, 1377, 1342, 1317, 1256, 1232 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.51 (d, J = 8.8 Hz, 1H), 8.27 (d, J = 7.6 Hz, 2H), 8.00 (s, 1H), 7.87 (d, J = 8.4 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.51–7.41 (m, 6H); ¹³C NMR (75 MHz, CDCl₃): δ 156.1, 155.5, 151.9, 137.6, 134.7, 133.9, 132.9, 130.0 (q, ²J_CF = 33.0 Hz), 130.3, 129.9, 129.8, 128.6, 126.9, 126.5, 126.1 (q, ³J_CF = 3.8 Hz), 126.0, 123.8 (q, ¹J_CF = 270.8 Hz), 123.5, 120.4, 116.3 (q, ³J_CF = 4.5 Hz); HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₂H₁₃BrF₃N₄S: 500.9996; found: 501.0000.

(E)-6-Phenyl-5-(p-tolyldiazenyl)imidazo[2,1-b]thiazole (5ca). Yellow solid (89%, 57 mg), mp 126–127 °C. IR (KBr) ν_max: 3138, 3109, 3096, 3061, 3030, 2804, 2249, 1601, 1522, 1475, 1456, 1437, 1369, 1348, 1325, 1294, 1250 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.50 (s, 1H), 8.37 (d, J = 7.6 Hz, 2H), 7.74 (d, J = 7.6 Hz, 2H), 7.50 (t, J = 7.2 Hz, 2H), 7.41 (t, J = 7.2 Hz, 1H), 7.28 (d, J = 8.0 Hz, 2H), 6.88 (d, J = 3.6 Hz, 1H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 154.2, 151.5, 149.6, 140.3, 136.1, 133.4, 129.9, 129.0, 128.9, 128.6, 123.2, 122.3, 113.1, 21.6; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₈H₁₅N₄S: 319.1017; found: 319.1015.

(E)-Methyl-3-((4-(1-ethoxycarbonyl)phenyl)diazenyl) indolizine-1-carboxylate (7af). Red solid (94%, 66 mg), mp 97–98 °C. IR (KBr) ν_max: 3404, 3370, 3125, 3096, 3083, 2979, 2956, 2900, 2866, 2835, 2639, 2327, 2260, 1956, 1911, 1707, 1700, 1599, 1521, 1418, 1388, 1343 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.32 (brs, 1H), 8.27 (d, J = 8.8 Hz, 1H), 8.08 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 7.6 Hz, 3H), 7.34 (t, J = 7.6 Hz, 1H), 6.99 (t, J = 6.8 Hz, 1H); 4.38 (q, J = 7.2 Hz, 2H), 3.89 (s, 3H), 1.41 (t, J = 6.8 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 166.3, 164.7, 156.1, 138.2, 130.6, 130.5, 127.6, 121.7, 119.9, 115.5, 108.3, 61.2, 51.5, 14.4; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₈N₃O₄: 352.1297; found: 352.1291.

(E)-3-((4-Acetylphenyl)diazenyl)indolizine-1-carbonitrile (7bg). Red solid (92%, 53 mg), mp 143–145 °C. IR (KBr) ν_max: 3135, 3107, 2369, 2217, 1934, 1677, 1490, 1422, 1350 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 9.42 (brs, 1H), 8.06 (d, J = 8.0 Hz, 2H), 7.89 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 8.0 Hz, 1H), 7.65 (s, 1H), 7.44 (t, J = 6.8 Hz, 1H), 7.13 (t, J = 7.6 Hz, 1H), 2.65 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 197.4, 156.0, 139.6, 137.6, 129.6, 127.6, 126.3, 126.21, 126.15, 122.3, 118.1, 116.1, 115.3, 87.2, 26.9; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₇H₁₃N₄O: 289.1089; found: 289.1086.

Benzo[e]pyrrolo[2,1-c][1,2,4]triazine (9). Yellow solid (78%, 26 mg), mp 176–178 °C. IR (KBr) ν_max: 3119, 3109, 3055, 3020, 1792, 1706, 1610, 1578, 1524, 1506, 1491, 1456, 1406, 1356, 1308, 1277, 1236 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.42 (d, J = 8.4 Hz, 1H), 7.85–7.83 (m, 2H), 7.72 (t, J = 7.6 Hz, 1H), 7.57 (t, J = 8.0 Hz, 1H), 7.42 (d, J = 4.0 Hz, 1H), 7.10 (t, J = 4.0 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 141.3, 136.5, 132.3, 130.8, 125.9, 124.9, 116.4, 113.5, 111.0, 109.0; HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₁₀H₈N₃: 170.0718; found: 170.0715.

Acknowledgements

A. H. acknowledges the financial support from DST, GoWB (ST/P/S&T/4G-2/2014). A. C. is thankful to UGC (Dr D. S. Kothari Post Doctoral Fellowship) and A. D. is thankful to CSIR for their fellowships.

Notes and references

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12. CCDC 1420103.†.
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Footnote

† Electronic supplementary information (ESI) available. CCDC 1420103. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra03070j

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