untitled European Journal of Chemistry 8 (1) (2017) 13‐14 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2017 Atlanta Publishing House LLC ‐ All rights reserved ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.8.1.13-14.1525 European Journal of Chemistry Journal webpage: www.eurjchem.com Microwave assistant synthesis of trans‐4‐nitrostilbene derivatives in solvent free condition Ming Jun Cen, Yu Wang and Yong Wu He * Department of Chemistry, Hebei University of Engineering, Handan, 056038, China * Corresponding author at: Department of Chemistry, Hebei University of Engineering, Handan, 056038, China. Tel.: +86.310.7977081. Fax: +86.310.8578796. E‐mail address: heyongwu@hebeu.edu.cn (Y.W. He). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.8.1.13-14.1525 Received: 23 December 2016 Received in revised form: 13 January 2017 Accepted: 14 January 2017 Published online: 31 March 2017 Printed: 31 March 2017   A general method for the synthesis of trans‐4‐nitrostilbenes has been developed. The trans‐ 4‐nitrostilbene could be synthesized in good yields under microwave irradiation within 10 min through Perkin reaction by using 4‐nitrophenylacetic acid, benzaldehydes and pyrrolidine. KEYWORDS Stilbene Perkin reaction Trans‐4‐nitrostilbene Microwave chemistry Solvent free condition Microwave irradiation Cite this: Eur. J. Chem. 2017, 8(1), 13‐14 1. Introduction The microwave‐assisted synthesis in organic chemistry has been receiving great attention as a promising heating technique in recent decades [1]. Compared with conventional heating methods, higher yields, milder reaction conditions and shorter reaction times can be achieved [2,3]. Some reviews have been published on the application of microwaves in chemical synthesis [1,4,5]. Stilbene, which is one kind of very useful material in chemistry, life sciences and materials science, has received great attention in organic synthesis [6]. The conventional synt‐ heses of such derivatives through transition‐metal‐catalyzed cross‐coupling and Mizoroki‐Heck reactions or Wittig‐type reactions usually need complicated multi‐steps and suffer from byproducts [7‐13]. Several papers have been published for the synthesis of stilbene derivatives under microwave. Srinivasan group reported the synthesis of dinitrostilbene under microwave, which only a few compounds have been studied [14]; and Sinha et al. reported the synthesis of hydroxylated stilbene under microwave [15], which needed to add PEG into the system. It is still necessary to study the general method for the synthesis of stilbene under microwave. In present study, a fast microwave‐assisted synthesis of trans‐ 4‐nitrostilbene derivatives in solvent free condition has been reported. 2. Experimental 2.1. Instrumentation All the commercial reagents and solvents were used without further purification. Microwave irradiation was performed in XH‐100A, Beijing microwave reactor (800 W). The 1H NMR spectra were measured on a Bruker AVANCE 400 (400 MHz) spectrometer using TMS as internal standard and CDCl3 as solvent. 2.2. General procedure for the synthesis of trans‐4‐nitrostil benes A mixture of 4‐nitrophenylacetic acid (1) (0.1 mol) and aldehyde (2) (0.1 mol) was irradiated under microwave of 800 W in a microwave reactor in presence of pyrrolidine (0.6 mL) for 10 min. The resultant mixture was recrystallized from ethanol (Scheme 1). The solid product was filtered and dried. The structure of trans‐4‐nitrostilbenes was consistent with their spectral data in references [7,16]. 14 Cen et al. / European Journal of Chemistry 8 (1) (2017) 13‐14 Table 1. The reaction of aldehydes and 4‐nitorphenylacetic acid in presence of pyrrolidine under microwave irradiation. Entry Time (min) Pyrrolidine (mL) Yield (%) 1 5 2.00 62.1 2 10 2.00 90.3 3 20 2.00 90.1 4 10 0.60 90.0 5 10 0.20 78.2 6 120a 2.00 85.7 a Under refluxed condition. Table 2. Synthesis of trans‐4‐nitrostilbenes under microwave irradiation. Entry Substrates Isolated yields (%) Conventional methods yields (%) a 1 Benzaldehyde 90.3 85.7 2 4‐Bromobenzaldehyde 92.6 87.2 3 2‐Bromobenzaldehyde 92.1 88.6 4 4‐Chlorobenzaldehyde 91.2 86.3 5 4‐Fluorobenzaldehyde 89.6 83.8 6 4‐Cyanobenzaldehyde 90.1 89.2 7 4‐Nitrobenzaldehyde 92.3 80.2 8 4‐Methoxybenzaldehyde 86.2 60.5 9 2‐Methoxybenzaldehyde 85.3 65.3 10 4‐Methylbenzaldehyde 87.3 70.2 11 4‐Dimethylaminobenzaldehyde 85.5 85.3 a The yields were obtained under refluxed conditions for 2 h. COOH NO2 + CHO R2 R1 MW, 10 min Pyrrolidine O2N R1 R2 1 2 3 R1 = -H, -Br, -Cl, -F, -CN, -NO2, -OCH3, -CH3, -N(CH3)2 R2 = -H, -Br, or -OCH3 Scheme 1 3. Results and discussion The results were summarized in Tables 1 and 2. As shown in Table 1, the reaction of aldehydes and 4‐nitrophenyl acetic acid could be carried out in presence of pyrrolidine to give trans‐4‐nitrostilbene through a Perkin type reaction under microwave irradiation. The trans‐4‐nitrostilbene was pre‐ viously prepared in 85% yield under refluxed condition in an oil bath [16]; while 4‐nitrostilbene was obtained in 90.3% yield under microwave irradiation. We also did experiment in the absence of microwave, the mixture of 4‐nitrophenylacetic acid, benzaldehyde and pyrrolidine was refluxed for 2 h to produce trans‐4‐nitrostilbene in 85.7% yield. Obviously, the microwave could accelerate the Perkin type reaction. The effect of irradiation time was studied. The product was obtained in 62.1% under microwave irradiation for 5 min, and if prolonged the irradiation time to 10 or 20 min, higher yields (90.3 or 90.1%, respectively) could be achieved. In addition, the amount of pyrrolidine was also evaluated. Similar yields of product were obtained in presence of 2.0 mL or 0.6 mL pyrrolidine, however, if the amount of pyrrolidine reduced to 0.2 mL, only much lower yield was obtained. Therefore, the synthesis of trans‐4‐nitrostilbene derivatives was carried out in presence 0.6 mL pyrrolidine for 10 min under microwave irradiation. The reaction of aldehydes and 4‐nitrophenylacetic acid under microwave irradiation was summarized in Table 2. According to Table 2, the trans‐4‐nitrostilbene derivatives could be synthesized in very good yield under microwave irradiation within 10 min. Moreover, the aldehydes with electron withdrawing groups could give higher yields for product than the aldehydes with electron donating groups, and the position of substituent had little effect on the yield, which indicated that the yield of this Perkin reaction could be improved with the electron withdrawing group. 4. Conclusion In summary, a facile synthesis of trans‐4‐nitrostilbene in excellent yields under microwave irradiation has been developed. This method provided a general and very convenient procedure for the synthesis of trans‐stilbenes. 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