Monatsh Chem DOI 10.1007/s00706-013-0945-3
ORIGINAL PAPER
Novel and selective synthesis of unsymmetrical azine derivatives via a mild reaction Javad Safari • Soheila Gandomi-Ravandi Mohammad Monemi
•
Received: 23 June 2012 / Accepted: 5 February 2013 Ó Springer-Verlag Wien 2013
Abstract A new, convenient, benign procedure for the one-step synthesis of unsymmetrical azines by reaction of aromatic aldehyde derivatives and hydrazine sulfate in triethylamine and absolute ethanol solution is presented. This methodology is more useful than the well-known twostep variant (involving preparation of a hydrazone and subsequent reaction with an aldehyde), because it enables the formation of unsymmetrical azines in a one-pot and rapid method without formation of any by-product or without requirement to separate the hydrazone intermediate. The mildness, selectivity, short reaction time, easy work-up, and applicability for large-scale reactions are the main advantages of this protocol for the synthesis of unsymmetrical azine derivatives. Structures of all new compounds were elucidated by 1H and 13C NMR, IR, MS, and CHN measurements. Keywords Unsymmetrical azine Aldehyde Triethylamine Selectivity One-pot synthesis
Introduction Azines are a class of compounds that undergo a wide variety of chemical processes and have interesting chemical properties [1]. The two imine bonds that make up the
J. Safari S. Gandomi-Ravandi (&) M. Monemi Laboratory of Organic Compound Research, Department of Organic Chemistry, College of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, I.R., Iran e-mail:
[email protected] J. Safari e-mail:
[email protected]
azine moiety can be considered as polar acceptor groups oriented in opposite directions because they are joined by an N–N bond [2]. The presence of an azine bridge between two conjugated systems prevents electron delocalization occurring between the two systems, and azines are therefore considered to be ‘‘conjugation stoppers’’ [3]. Azines are good synthons in heterocyclic synthesis [4], can be employed in certain useful synthetic transformations, and are useful in the generation of conducting materials [5], dye lasers, and image-recording materials [6]. Both hydrazones and azines have been evaluated for possible use in analytical chemistry [7]. Among other useful properties, azines have received attention owing to their potential in bond-forming reactions [8, 9]. Some unsymmetrical azines have antibacterial properties, some are used as organic luminophores, and others have been used in the synthesis of unsymmetrical diarylethylenes [7]. Azine compounds with aromatic and various heterocyclic substituents are photochromic and can be used to make dosimeters of ultraviolet radiation, screens to protect the eyes, and optical devices [10]. Although symmetrical azine systems are readily synthesized by the reaction of hydrazine with an excess of an aldehyde or ketone, the preparation of their unsymmetrical counterparts is more challenging [11]. Among the different published procedures for the synthesis of unsymmetrical azines, a two-step procedure is reported which involves (i) preparation of hydrazone by condensation of hydrazine with aldehyde, (ii) reaction of the resulting hydrazone with other aldehyde derivatives (Scheme 1) [12–14]. We report, for the first time, a simple and one-pot approach to the preparation of unsymmetrical azines via condensation of aromatic aldehyde derivatives with hydrazine sulfate in triethylamine and absolute ethanol solution (Scheme 2).
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J. Safari et al. R2
Scheme 1
R2 NNH2
O
H
N H
H
EtOH
+
O
N
N2 H4.HSO 4 R1
R1
R1
Scheme 2
R2 O R2
N
H
+
N2 H4.HSO 4
+
N
NEt 3 H
EtOH, r.t.
R1
R1
1
O
2
The method reported is operationally simple, rapid, and gives good yields. The reaction conditions are extremely mild and the method therefore has great potential as a generally useful synthetic route to a wide variety of unsymmetrical azine compounds.
Results and discussion In recent years, multicomponent, one-pot condensations have been widely studied, as they constitute an attractive synthetic strategy for the rapid and efficient generation of complex molecules. A wide range of products can be formed in a single step, with diversity achieved simply by varying the reacting components. Unsymmetrical azines have generally been synthesized in a two-step procedure, but here we report an efficient and one-step method without the separation of the hydrazone intermediate. We studied the treatment of aromatic aldehyde derivatives with hydrazine sulfate under mild conditions and found that the anticipated azines were formed smoothly at room temperature in good to excellent yields. This approach was successfully applied to the synthesis of a range of compounds, 3a–3j (Table 1). The substituents on each aromatic aldehyde were varied to yield differently substituted azines. The yields were found to be best when one aldehyde with an electron-withdrawing substituent and one aldehyde with an electron-donating substituent were employed. Next, we examined the effect of different bases on this reaction. From Table 2, it is clear that the best results are obtained when triethylamine is applied in this reaction. The effect of the drying agent, such as magnesium sulfate, was also investigated. The results suggested that the presence of MgSO4 in the reaction had a pronounced, positive effect on
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3
Table 1 Synthesis of unsymmetrical azine derivatives by the one-pot method Entry
Product
R1
R2
Yield/%a
1 2
3a 3b
4-Me 3-NO2
3-Br 4-NMe2
96 98
3
3c
3-Cl
3-Me
88
4
3d
4-Br
4-Me
81
5
3e
2-Cl
4-NO2
89
6
3f
4-Br
3-Cl
95
7
3g
2-Cl
4-Me
92
8
3h
3-Me
4-NO2
85
9
3i
4-OMe
3-NO2
90
10
3j
4-Cl
3-OMe
83
a
Yields refer to the pure isolated products
Table 2 Comparison of different bases in synthesis of unsymmetrical azines Entry
Base
Yield/%
1
NaOH
80
2
NH3
86
3
NEt3
92
4
CaCO3
78
5
NaOAc
70
the rate of reaction and promoted the generation of the unsymmetrical azine, without concurrent formation of the corresponding symmetrical azines as by-products. Therefore, MgSO4 was used as a drying agent to ensure the success of this reaction. Compounds 3a–3j were characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, and mass spectrometry. The
Selective synthesis of unsymmetrical azine derivatives
(elimination of magnesium hydroxide) produce the product azine. Magnesium sulfate is strong and quick waterabsorbing and takes large volume of water to promotion of dehydration.
NMR spectra indicated the unsymmetrical structures of the formed azines, because two imine resonances were observed in the proton spectrum. Common features in the 1 H NMR spectra for these compounds were the presence of two peaks occurring at 8–9 ppm, which correspond to the protons of two different imine groups. Analysis of the 13C NMR spectra of the unsymmetrical azines revealed that none of the products were contaminated with the corresponding aldehyde precursors, as evidenced by the absence of any peaks at approximately 199 ppm for the carbonyl, as well as the absence of all other characteristic peaks of the aldehydes. A new peak for the azine group was observed at approximately 160 ppm. The presence of two imine carbons in the 13C NMR spectra confirmed the proposed structures. Signals in the C=N stretching region of 1,520–1,600 cm-1 indicated that two C=N bonds were present in the azine molecules. Compounds 3a–3j exhibited peaks corresponding to their molecular ions in the high resolution mass spectrum. The yields of the azines obtained using this procedure are significantly higher than those obtained by other reported procedures. A plausible mechanism has been proposed for the synthesis of unsymmetrical azines in presence of magnesium sulfate in Scheme 3. This reaction scheme offers detailed information, enabling a better understanding of the mechanism of this reaction. At first, hydrazine reacts with one aldehyde derivative to form imine. The metal ion coordinates to oxygen atom for help to dehydration. The imine obtained from the reaction of an aldehyde with hydrazine in a 1:1 molar ratio is a hydrazone I. This hydrazone condenses with the secondary aldehyde forming an intermediate II which on dehydration
Conclusions We have developed a one-pot and selective synthesis of unsymmetrical azines in good yields without separation of the hydrazone intermediate and without formation of any symmetrical azine. The major advantages of this method are its selectivity, operational simplicity, mild reaction conditions, short reaction times, and excellent yields.
Experimental All reactions were carried out at room temperature. Solvents and chemicals were purchased from Merck and used without prior purification. Thin-layer chromatography (TLC) was performed to monitor reaction progress and the purity of the products. NMR spectra were obtained on a Bruker DRX-400 spectrometer in 5-mm quartz tubes and in CDCl3 solution. NMR (1H and 13C) are reported in ppm downfield from TMS and were referenced to the residual solvent signal. Infrared spectra were obtained using a Perkin-Elmer FT-IR 550 spectrophotometer. Melting points were measured using an Electrothermal MK3 apparatus. Chemical ionization mass spectral data were obtained using a Micromass UK mass spectrometer.
Scheme 3
2
O
O H
H2N
N H
H
NH2
O NH2 H
R
N H
NH2 - Mg (OH)2
R1
R1
1
Mg
O H
N
H
H
H R2
NH2
N
NH
N OH
NH H
O R1
R1
R1
R2
R2
2
Mg
II
I R2 H N 1
N +
H
Mg (OH)2
R
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J. Safari et al.
Two ethanolic solutions containing the two different precursor aldehydes (1 mmol) were added dropwise and simultaneously to a stirred solution of hydrazine sulfate (1 mmol) and triethylamine in 5 cm3 absolute ethanol at room temperature. After the addition was complete, the mixture was cooled on ice, at which point a yellowish solid separated. The solid was collected by filtration, washed with water, and dried under vacuum. Then the dried yellow solid was recrystallized from ethanol to afford the desired products as cream to orange crystals, depending on the azine.
(1E,2E)-1-(3-Chlorobenzylidene)-2-(3-methylbenzylidene)hydrazine (3c, C15H13ClN2) Cream solid; yield 88 %; m.p.: 112–114 °C; Rf = 0.87 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 2.43 (s, 3H, CH3), 7.36 (m, 1H, CH), 7.38 (s, 1H, CH), 7.40 (m, 1H, CH), 7.42 (m, 1H, CH), 7.44 (m, 1H, CH), 7.64 (m, 1H, CH), 7.70 (m, 1H, CH), 7.89 (s, 1H, CH), 8.61 (s, 1H, CH=N), 8.65 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 21.34 (CH3), 126.89 (CH), 128.04 (CH), 128.45 (CH), 130.03 (CH), 131.30 (CH), 132.09 (CH), 132.33 (C), 135.74 (C), 136.60 (C), 137.78 (CH), 138.30 (CH), 140.09 (C), 161.10 (CH), 162.24 (CH) ppm; FT-IR (KBr): m = 3,025 (C–H), 1,616 (C=N), 1,563 (C=N) cm-1; UV–Vis (CHCl3): kmax = 305 nm; MS (70 eV): m/z = 258 (M ? 2), 256 (M), 255 (M - H), 241 (M - CH3), 221 (M - Cl), 165 (M - Ph–CH3), 146 (M - Ph–Cl), 124 (M - Ph–CH3 - CH3CN), 105 (M - Ph–Cl - CH3CN).
(1E,2E)-1-(3-Bromobenzylidene)-2-(4-methylbenzylidene)hydrazine (3a, C15H13BrN2) Cream solid; yield 96 %; m.p.: 106–108 °C; Rf = 0.79 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 2.42 (s, 3H, CH3), 7.27 (dd, 2H, CH), 7.32 (dd, 2H, CH), 7.36 (dd, 1H, CH), 7.62 (dd, 1H, CH), 7.75 (dd, 1H, CH), 8.05 (s, 1H, CH), 8.59 (s, 1H, CH=N), 8.65 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 21.65 (CH3), 127.48 (C), 128.53 (C), 129.55 (CH), 130.33 (CH), 131.51 (CH), 134.21 (CH), 135.98 (CH), 141.59 (CH), 142.14 (C), 160.01 (CH), 162.87 (CH) ppm; FT-IR (KBr): m = 3,045 (=C–H), 1,621 (C=N), 1,560 (C=N) cm-1; UV– Vis (CHCl3): kmax = 312 nm; MS (70 eV): m/z = 301 (M), 300 (M - H), 286 (M - CH3), 222 (M - Br), 210 (M - Ph–CH3), 182 (M - Ph–CH3 - CH3CN), 146 (M - Ph–Br), 118 (M - Ph–Br - CH3CN).
(1E,2E)-1-(4-Bromobenzylidene)-2-(4-methylbenzylidene)hydrazine (3d, C15H13BrN2) Yellow solid; yield 81 %; m.p.: 169–170 °C; Rf = 0.91 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 2.42 (s, 3H, CH3), 7.27 (dd, 2H, CH), 7.59 (dd, 2H, CH), 7.73 (dd, 2H, CH), 8.05 (dd, 2H, CH), 8.60 (s, 1H, CH=N), 8.64 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 22.40 (CH3), 125.01 (C), 127.38 (C), 129.74 (C), 129.97 (CH), 130.77 (CH), 131.30 (CH), 133.05 (CH), 140.74 (C), 162.07 (CH), 162.84 (CH) ppm; FT-IR (KBr): m = 3,050 (=C–H), 1,621 (C=N), 3,085 (C=N) cm-1; UV– Vis (CHCl3): kmax = 312, 322 nm; MS (70 eV): m/z = 303 (M ? 2), 301 (M), 300 (M - H), 286 (M CH3), 222 (M - Br), 210 (M - Ph–CH3), 182 (M - Ph– CH3 - CH3CN), 146 (M - Ph–Br), 118 (M - Ph–Br CH3CN).
N,N-Dimethyl-4-[(E)-[(E)-(3-nitrobenzylidene)hydrazono]methyl]aniline (3b, C16H16N4O2) Orange solid; yield 98 %; m.p.: 133–135 °C; Rf = 0.93 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 3.03 (s, 3H, CH3), 3.07 (s, 3H, CH3), 6.74 (dd, 2H, CH), 7.65 (m, 2H, CH), 7.74 (dd, 1H, CH), 8.18 (m, 1H, CH), 8.35 (m, 1H, CH), 8.67 (s, 1H, CH=N), 8.73 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 40.18 (CH3), 111.65 (CH), 121.16 (C), 125.79 (CH), 129.94 (CH), 130.62 (CH), 134.19 (C), 135.52 (CH), 148.71 (C), 152.72 (C), 160.56 (CH), 164.35 (CH) ppm; FT-IR (KBr): m = 3,070 (=C–H), 1,606 (C=N), 1,573 (C=N) cm-1; UV– Vis (CHCl3): kmax = 276 nm; MS (70 eV): m/z = 296 (M), 295 (M - H), 281 (M - CH3), 266 (M - NO), 252 (M - N(CH3)2), 250 (M - NO2), 176 (M - Ph– N(CH3)2), 174 (M - Ph–NO2), 133 (M - Ph–NO2 CH3CN), 125 (M - Ph–N(CH3)2 - CH3CN).
(1E,2E)-1-(2-Chlorobenzylidene)-2-(4-nitrobenzylidene)hydrazine (3e, C14H10ClN3O2) Deep yellow solid; yield 89 %; m.p.: 262–263 °C; Rf = 0.73 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 7.39 (m, 1H, CH), 7.43 (m, 1H, CH), 7.48 (m, 1H, CH), 8.03 (m, 1H, CH), 8.20 (dd, 2H, CH), 8.34 (dd, 2H, CH), 8.72 (s, 1H, CH=N), 9.11 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 123.97 (CH), 127.26 (CH), 128.32 (C), 129.40 (CH), 129.64 (C), 130.15 (CH), 130.67 (CH), 130.74 (CH), 137.45 (C), 147.71 (C), 162.37 (CH), 163.70 (CH) ppm; FT-IR (KBr): m = 3,065 (=C–H), 1,596 (C=N), 1,519 (C=N) cm-1; UV– Vis (CHCl3): kmax = 327 nm; MS (70 eV): m/z = 289 (M ? 2), 287 (M), 286 (M - H), 257 (M - NO), 252 (M - Cl), 241 (M - NO2), 176 (M - Ph–Cl), 165 (M - Ph–NO2), 135 (M - Ph–Cl - CH3CN), 124 (M - Ph–NO2 - CH3CN).
Elemental analyses were performed using a Heraeus CHNO-RAPID elemental analyzer. UV–vis spectra of compounds were recorded on a UV–vis Varian Perkin-Elmer UV 550-S spectrophotometer. General procedure for preparation of unsymmetrical azine derivatives 3a–3j
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Selective synthesis of unsymmetrical azine derivatives
(1E,2E)-1-(4-Bromobenzylidene)-2-(3-chlorobenzylidene)hydrazine (3f, C14H10BrClN2) Cream solid; yield 95 %; m.p.: 166–167 °C; Rf = 0.74 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 7.27 (2H, m, CH), 7.44 (m, 2H, CH), 7.59 (dd, 1H, CH), 7.69 (m, 1H, CH), 7.73 (dd, 1H, CH), 7.89 (dd, 1H, CH), 8.49 (s, 1H, CH=N), 8.60 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 125.84 (C), 127.01 (C), 129.54 (CH), 129.76 (CH), 129.99 (CH), 130.13 (CH), 130.49 (C), 130.80 (CH), 132.13 (CH), 133.84 (C), 161.11 (CH), 161.18 (CH) ppm; FT-IR (KBr): m = 3,055 (=C–H), 1,625 (C=N), 1,587 (C=N) cm-1; UV–Vis (CHCl3): kmax = 317 nm; MS (70 eV): m/z = 325 (M ? 4), 323 (M ? 2), 321 (M), 320 (M - H), 286 (M - Cl), 242 (M - Br), 210 (M - Ph–Cl), 169 (M - Ph–Br - CH3CN),165 (M - Ph–Br), 124 (M Ph–Cl - CH3CN).
(1E,2E)-1-(4-Methoxybenzylidene)-2-(3-nitrobenzylidene)hydrazine (3i, C15H13N3O3) Yellow solid; yield 90 %; m.p.: 136–138 °C; Rf = 0.70 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 3.61 (s, 3H, OCH3), 7.02 (dd, 2H, CH), 8.21 (dd, 1H, CH), 8.22 (m, 1H, CH), 8.30 (m, 1H, CH), 8.44 (s, 1H, CH=N), 8.45 (s, 1H, CH), 8.58 (m, 1H, CH), 8.63 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 55.24 (OCH3), 113.59 (CH), 126.45 (CH), 126.55 (CH), 126.79 (C), 129.01 (CH), 130.60 (CH), 132.05 (C), 130.31 (CH), 146.63 (C), 160.86 (CH), 162.09 (C), 163.68 (CH) ppm; FT-IR (KBr): m = 3,062 (=C–H), 1,603 (C=N), 1,556 (C=N) cm-1; UV–Vis (CHCl3): kmax = 265 nm; MS (70 eV): m/z = 283 (M), 282 (M - H), 253 (M - NO), 252 (M - OCH3), 237 (M - NO2), 176 (M - Ph–OCH3), 161 (M - Ph–NO2), 149 (M - Ph–OCH3 - CH3CN), 134 (M - Ph–NO2 - CH3CN).
(1E,2E)-1-(2-Chlorobenzylidene)-2-(4-methylbenzylidene)hydrazine (3g, C15H13ClN2) Yellow solid; yield 92 %; m.p.: 112–113 °C; Rf = 0.87 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 2.42 (s, 3H, CH3), 7.26 (dd, 2H, CH), 7.34 (m, 2H, CH), 7.39 (m, 1H, CH), 7.40 (m, 1H, CH), 7.44 (m, 1H, CH), 8.22 (dd, 1H, CH), 8.65 (s, 1H, CH=N), 9.10 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 21.66 (CH3), 127.08 (CH), 128.18 (C), 128.56 (CH), 128.75 (C), 130.08 (CH), 131.29 (CH), 131.54 (CH), 132.20 (CH), 135.86 (C), 141.58 (C), 159.05 (CH), 161.84 (CH) ppm; FT-IR (KBr): m = 3,060 (=C–H), 1,672 (C=N), 1,624 (C=N) cm-1; UV–Vis (CHCl3): kmax = 312, 320 nm; MS (70 eV): m/z = 258 (M ? 2), 256 (M), 255 (M - H), 241 (M - CH3), 221 (M - Cl), 165 (M - Ph–CH3), 146 (M - Ph–Cl), 124 (M - Ph–CH3 - CH3CN), 105 (M - Ph–Cl - CH3CN).
(1E,2E)-1-(4-Chlorobenzylidene)-2-(3-methoxybenzylidene)hydrazine (3j, C15H13ClN2O) Cream solid; yield 83 %; m.p.: 146–148 °C; Rf = 0.76 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 3.84 (s, 3H, OCH3), 7.39 (m, 1H, CH), 7.56 (m, 1H, CH), 7.60 (m, 1H, CH), 7.68 (s, 1H, CH), 7.73 (dd, 2H, CH), 7.98 (dd, 2H, CH), 8.38 (s, 1H, CH=N), 8.45 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 55.83 (OCH3), 115.59 (CH), 118.45 (CH), 124.79 (CH), 127.56 (CH), 128.60 (CH), 129.47 (C), 130.31 (CH), 131. 63 (C), 135.86 (C), 158.42 (C), 160.68 (CH), 161.09 (CH) ppm; FT-IR (KBr): m =3,040 (=C–H), 1,569 (C=N), 1,590 (C=N) cm-1; UV–Vis (CHCl3): kmax = 310 nm; MS (70 eV): m/z = 272 (M), 271 (M - H), 241 (M - OCH3), 237 (M - Cl), 165 (M - Ph–OCH3), 162 (M - Ph–Cl), 138 (M - Ph–OCH3 - CH3CN), 134 (M - Ph–Cl CH3CN).
(1E,2E)-1-(3-Methylbenzylidene)-2-(4-nitrobenzylidene)hydrazine (3h, C15H13N3O2) Yellow solid; yield 85 %; m.p.: 264–266 °C; Rf = 0.74 (ethyl acetate/n-hexane 3:7); 1H NMR (400 MHz, CDCl3): d = 2.75 (s, 3H, CH3), 7.35 (m, 1H, CH), 7.46 (dd, 1H, CH), 7.59 (m, 1H, CH), 7.71 (m, 1H, CH), 8.01 (dd, 2H, CH), 8.31 (dd, 2H, CH), 8.67 (s, 1H, CH=N), 8.73 (s, 1H, CH=N) ppm; 13C NMR (100 MHz, CDCl3): d = 22.14 (CH3), 123.59 (CH), 127.08 (CH), 131.48 (CH), 131.85 (C), 132.00 (CH), 134.75 (C), 136.31 (CH), 137.63 (CH), 139.75 (C), 150.31 (C), 161.16 (CH), 162.68 (CH) ppm; FT-IR (KBr): m = 3,050 (=C–H), 1,596 (C=N), 1,521 (C=N) cm-1; UV–Vis (CHCl3): kmax = 325 nm; MS (70 eV): m/z = 267 (M), 266 (M - H), 252 (M - CH3), 237 (M - NO), 221 (M - NO2), 176 (M - Ph–CH3), 146 (M - Ph–NO2), 135 (M - Ph–CH3 - CH3CN), 105 (M - Ph–NO2 - CH3CN).
Acknowledgments We express our appreciation to the University of Kashan Research Council for financial support of this work (Grant No. 159198/14).
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