3-Cyano-4,6-diphenyl-2-(phenethylamino)pyridine

Abstract

Novel synthesis of a 2-amino-3-cyano-2,6-diphenyl pyridine by a one-pot multi-component reaction of 1,3-diphenylpropane-1,3-dione, malononitrile and phenethylamine in the presence of N-hydroxybenzamide and zinc chloride has been reported. The structure of the synthesized compound was assigned on the basis of its elemental analysis, ¹H-NMR, ¹³C-NMR, IR and mass spectral data. X-ray structure analysis confirmed unambiguously the proposed structure. The photophysical properties (λ_Abs., λ_Flu.) in CH₃OH, CH₃CN, CH₂Cl₂ and the emission spectrum of the new compound in solution and in the solid state are reported.

The optoelectronic devices such as optical fibers, switches, tunable lasers and amplifiers, modulators with various applications need compounds emitting in the blue spectral region [1]. So developing a procedure for the synthesis of thermally stable, highly fluorescent materials can be urgently interesting for technology upgrading. There are some reports on using the fluorescent compounds in biochemical and medical research [2]. 2-Aminopyridine derivatives are of great importance due to their biological activities such as cardioprotective [3], antibacterial [4], antioxidant [5], anti-inflammatory [6] and anti-HIV [7]. Sulfapyridine, an example which contains 2-aminopyridine moiety, is an old marked antibacterial drug. Therefore designing new procedures to synthesize 2-aminopyridine derivatives has attracted a lot of interest. Literature shows some reports on the synthesis of 2-amino-3-cyano-4,6-dialkylpyridines but there are few reports on diaryl ones [8,9,10,11]. Recently we have reported the synthesis of 2-amino-3-cyano-4,6-dialkyl pyridines by the one-pot multi-component reaction of 2,4-pentanedione, malononitrile and primary or secondary amines in the presence of N-hydroxy benzamide or p-toluenesulfonic acid as the acidic catalyst [12]. In continuation of our research on the synthesis of fluorescent compounds [12,13,14], we herein report the synthesis of 3-cyano-4,6-diphenyl-2-(phenethylamino) pyridine using N-hydroxybenzamide and ZnCl₂ as an organic acid and Lewis acid catalysts respectively (Scheme 1).

Structure 3 was assigned on the basis of its elemental analysis, ¹H-NMR, ¹³C-NMR, IR and mass spectral data. The light yellow crystals of 3 were obtained by crystallization from ethyl acetate/n-hexane: 1/3 and its X-ray structure was determined to confirm unambiguously its structure. [15] (Figure 1).

Compound 3 has shown fluorescence activity in the blue region in solution and solid phase. The photophysical data for this compound including λ_Abs. (nm), λ_Flu. (nm) have been measured for 0.00002 M solutions in CH₃OH, CH₃CN and CH₂Cl₂ (

Table 1. Photophysical data: electronic absorption (Abs.) and fluorescence (Flu.) of 3.

	CH3OH		CH3CN		CH2Cl2
Comp	λAbs	λFlu	λAbs	λFlu	λAbs	λFlu
3	300	404	300	403	320	401

). The fluorescence emission spectrum of compound 3 solutions in CH₂Cl₂, CH₃CN and CH₃OH is shown in Figure 2.

Figure 3 shows photographs of compound 3 solutions in CH₂Cl₂, CH₃CN and CH₃OH (a): under visible light and (b): under a UV lamp with λ = 366 nm (Philips TL8W/08F8T5/BLC).

The emission spectrum of compound 3 has been determined. The sample is prepared as the following: a solution of compound 3 (0.5 mL) in CHCl₃ (10 ⁻⁸ mol L⁻¹) was first coated on a quartz glass sheet and dried at room temperature, then placed in a 1 cm length quartz cell. The excitation and emission slits were adjusted on 3 nm. The fluorescence emission intensity of compound 3 after excitation in 300 nm is shown in Figure 4. The photo in the top right of Figure 4 has been taken under a UV lamp with λ = 366 nm (Philips TL8W/08F8T5/BLC), only to show the fluorescence property of compound 3 in solid state It seems the title compound can achieve a good chance among the optoelectronic devices.

Experimental

Elemental analysis for C, H and N was performed using a Thermo Finnigan Flash EA1112 instrument. ¹H-NMR and ¹³C-NMR spectra were determined on a Bruker 250 spectrometer. IR spectra were measured on a Bruker EQUINOX 55 spectrophotometer with the ATR method. Mass spectra were recorded on a Finnigan-MAT 8430 spectrometer. Photophysical data measurements were made by a luminescence PERKIN ELMER LS 50B spectrometer.

3-Cyano-(2-phenethylamino)-4,6-diphenyl pyridine (3). To a magnetically stirred solution of 1,3-diphenylpropane-1,3-dione (0.224 g, 1 mmol) and ZnCl₂ (10 mol %) in 10 mL CH₂Cl₂, malononitrile (0.066 g, 1 mmol), phenethyl amine (0.12 mL, 1 mmol) and N-hydroxy benzamide (50 mol %, 0.68 g) were added in a one-pot manner. The solution was stirred at room temperature for 10 h. The reaction progress was monitored by IR. When one of the CN absorptions (2228 cm⁻¹) in IR spectrum of the reaction mixture had disappeared, the solvent was removed under reduced pressure and the product was purified using column chromatography (silica gel, ethyl acetate/n-hexane: 1/5). The crystals of product were obtained (0.27 g, 72% yield) as yellow crystals. Melting point: 148 °C.

Structural Characterization

IR, ν_max: 3353, 2211, 1550 cm⁻¹; δ_H (250 MHz, CDCl₃): 3.08 (2H, t, J = 7. 2 Hz, CH₂), 3.96 (2H, q, J = 6.7 Hz, CH₂-N), 5.51-5.55 (1H, br s, NH), 7.19 (1H, s, CH of pyridine ring), 7.26–7.42, 7.50–7.59, 7.64–7.68, 8.12–8.15 (15H, 4m, Ph protons) ppm; δ_C (CDCl₃): 35.93 (CH₂), 43.25 (CH₂-N), 109.61 (CN), 117.41 (CH of pyridine), 126.60, 127.39, 128.76, 128.78, 128.94, 129.73, 130.17, 137.34, 138.34, 139.17, 155.08, 159.06 (aromatic carbons) ppm; MS: m/z = 375 (M⁺), 298, 272, 255, 221, 195, 120, 91, 77, 75, 43; Anal. Calcd for C₂₆H₂₁N₃: C, 83.17; H, 5.64; N, 11.19; Found: C, 83.20; H, 5.69; N, 11.23.