Efficient Eco-Friendly Solvent-Free Click Synthesis and Antimicrobial Evaluation of New Fluorinated 1,2,3-Triazoles and their Conversion into Schiff Bases

A facile and convenient green click synthesis has been developed for the preparation of new fluorinated 1,2,3-triazoles under solvent-free conditions via a Huisgen 1,3-dipolar cycloaddition reaction between dimethylacetylene dicarboxylate (DMADC) and fluorophenyl azides in excellent yields within 2 min. Treatment of the resulting diesters with hydrazine hydrate furnished the corresponding dihydrazides, which, upon condensation with benzaldehyde derivatives, afforded a new series of bis-hydrazones. All of the synthesized compounds were fully characterized using infrared (IR) spectroscopy, 1 H, 13 C and 19 F nuclear magnetic resonance (NMR), mass spectrometry (MS) and elemental analysis. A preliminary bioassay indicated that some of the tested compounds exhibited significant antimicrobial activity.


Introduction
During the last few years, the development of fast, safe and high-yielding click chemistry methodologies have rapidly emerged as one of the most challenging and attractive topics in modern heterocyclic chemistry. [1][2][3][4][5][6] The most popular example of click chemistry is the synthesis of 1,2,3-triazoles via 1,3-dipolar cycloaddition reactions. 7 The 1,2,3-triazole based derivatives have been employed in various medicinal chemistry applications, such as anti-HIV, 8 antiviral, 9 anticonvulsant, 10 antiallergic, 11 anticancer, 12 and antifungal 13 agents. In addition, several fluorinated drug structures have incorporated a 1,2,3-scaffold. 14 Therefore, considerable attention has been devoted to the synthesis and applications of fluorinated compounds in medicinal chemistry and agrochemistry. [15][16][17][18][19][20] The incorporation of a fluorine atom into organic molecules results in high thermal stability and enhanced lipophilicity, which can enhance the rate of cell penetration and transport of a drug to an active site. [21][22][23][24] 1,3-Dipolar Huisgen cycloaddition of organic azides with alkynes has been recognized as the most widely reported method for the construction of 1,2,3-triazole scaffold. In addition, the reaction of dimethyl/ethylacetylene dicarboxylate with different organoazides via a 1,3-dipolar cycloaddition has been extensively adopted for the synthesis of 4,5-disubstituted 1,2,3-triazoles. Savin et al. 25 was the first to report the efficient microwave solvent-free reaction of dimethylacetylene dicarboxylate (DMADC) with benzyl azide within 1 min. Recently, Shanmugavelan et al. 26 reproduce the same synthesis without microwave irradiation via conventional heating of the reactants at 90-120 ºC for 1 min. However, few examples for the synthesis of 1-aryl-1,2,3-triazoles have been previously reported. 27 Kamalraj et al. 28 developed a one-pot synthesis of 1-aryl-4-acetyl-5-methyl-1,2,3-triazole from organoazide and acetylacetone in the presence of a base under warm conditions in ethanol. Recently, an efficient one-pot copper-catalysed synthesis of 1,4-diaryl-substituted 1,2,3-triazoles was developed using aryldiazonium silica sulfates, sodium azide, terminal alkynes, and sodium ascorbate. 29 Another method is the copper-free synthesis of 1-aryl-1,2,3-triazoles from sodium acetylide and aryl azides at room temperature. 30 Efficient Eco-Friendly Solvent-Free Click Synthesis and Antimicrobial Evaluation of New Fluorinated 1,2,3-Triazoles To the best of our knowledge, the synthesis of 1-aryl-1,2,3-triazoles under solvent-free conditions has not been previously reported. Inspired by Shanmugavelan et al.'s work, 26 we investigated the viability of utilizing the solvent-free conditions as an efficient eco-friendly protocol for the synthesis of 1-aryl-1,2,3-triazoles. Herein, we describe our modified methodology for the quick synthesis of new 1-fluoroaryl-1,2,3-triazole-4,5-diesters via 1,3-dipolar cycloaddition of DMADC with different fluorophenyl azides under solvent-free conditions. Moreover, the main purpose of the present study was to design and to synthesize some novel 1-fluoroaryl-1,2,3-triazole-hydrazone hybrids in order to improve the physiochemical properties of 1,2,3-triazole and/or synergistic effect by combining the 1,2,3-triazole nucleus with hydrazone linkage in one scaffold. The synthesized bis-hydrazones were evaluated for their antimicrobial activity against different Gram-negative and Gram-positive bacteria. In addition, the effect of introducing the fluorine substituent on the antimicrobial activity has also been investigated.

Chemistry
Initially, fluorophenyl azides were synthesized via diazotization of their corresponding fluoroanilines in the presence of a sodium nitrite solution in acidic media followed by the addition of sodium azide according to the method described by Boyer and Canter. 31 1,3-Dipolar cycloaddition of 2-fluorophenyl azide (2a) to dimethylacetylene dicarboxylate (1) in the presence of dichloromethane has been previously reported by Bouasla et al. 32 under both conventional and microwave heating methods. The resulting dimethyl 1-(2-fluorophenyl)-1,2,3-triazolo-4,5-dicarboxylate (3a) was obtained in 68% yield after regular stirring for 24 h and in 88% yield under microwave irradiation (MWI) for 8 min. In the current study, the same reaction was conducted under solvent-free conditions in a sealed tube. The reaction required heating in a water bath for 2 min to afford 1,2,3-triazole 3a in 96% yield. The promising result obtained from the cycloaddition of 2-fluorophenylazide 2a with 1 inspired us to extend the reaction to other fluorophenylazides. Accordingly, new 1-(fluorophenyl)-1,2,3-triazoles 3b-d were synthesized in 93-96% yield within 2-3 min through the same eco-friendly solvent-free protocol previously described (Scheme 1).
The synthesized 1,2,3-triazole diesters 3a-d were fully characterized based on their infrared (IR), 1 H and 13 C nuclear magnetic resonance (NMR) and mass spectrometry (MS) spectra. Their IR spectra contained two absorption bands at 1727-1737 and 2923-2988 cm -1 , which are characteristic of the ester C=O and CH 3 groups, respectively. The 1 H NMR spectra of compounds 3a-d recorded in CDCl 3 revealed the presence of two characteristic singlets at d 3.83-3.94 ppm, which confirmed the nonequivalence of the two methyl ester protons. The phenyl protons appeared in the aromatic region at a δ of approximately 7.39-8.09 ppm. In the 13 C NMR spectra, the signal corresponding to the two nonequivalent carbonyl ester groups appeared at 157.56-161.28 ppm, and the two methyl carbons appeared at 52.69-54.83 ppm.
Diesters 3a-d were refluxed with hydrazine hydrate in ethanol for 4 h to afford corresponding dihydrazides 4a-d in 88-92% yields. The formation of dihydrazides 4a-d was confirmed by IR, 1 H NMR, 13 C NMR and MS analyses. Their IR spectra revealed the appearance of characteristic hydrazide NH and NH 2 groups at 3227-3371 cm -1 . In the 1 H NMR spectra of compounds 4a-d, the disappearance of the methyl ester signals and appearance of two singlets at d 4.50-4.79 and 10.34-11.55 ppm due to the NH 2 and NH protons, respectively, confirmed the success of the hydrazinolysis reaction. In addition, carbonyl signals of the amide functionalities in the 13 C NMR spectra appeared at 157.16-163.67 ppm, which supported the proposed structures for dihydrazides 4a-d.
However, the heating of dihydrazides 4a-d with benzaldehyde derivatives in the presence of a catalytic amount of hydrochloric acid for 2 h yielded the corresponding bis-hydrazones 5a-p in 83-91% yields (Scheme 2). The structures of the synthesized bis-hydrazones have been elucidated by IR, 1 H NMR, 13 C NMR and MS analyses.
Hydrazones were reported to exist as E/Z geometrical isomers about the imine bond (HC=N) and cis/trans conformers to the carbonyl amide linkage. 33 Aouad 2107 Vol. 26, No. 10, 2015 steric rearrangements of the hydrazone functionality in the geometric syn and anti isomers. In addition, the E-geometrical isomer and the cis/trans conformers were the more predominant forms in highly polar solvent, such as deuterated dimethylsulfoxide (DMSO-d 6 ), and the Z-isomer was only observed in less polar solvents. 36,37 Scheme 3 summarizes the general E/Z geometrical isomers and cis/ trans conformers (Scheme 3).
The 1 H NMR and 13 C NMR spectra of all of the synthesized Schiff bases 5a-p recorded in DMSO-d 6 confirm the existence of a mixture of diastereomers (i.e., E/cis and E/trans) for each imino-amide moiety.
The 1 H NMR spectrum of bis-hydrazone 5f derived from p-methoxybenzaldehyde contained four different characteristic singlet peaks at d 7.95, 8.21, 8.48 and 8.59 ppm with a ratio of 1:1:1:1, which have a total integration of two protons that are related to the two nonequivalent imine protons (HC=N). The spectrum also contained four singlets at d 12.25, 12.39, 12.47 and 12.79 ppm that have a total integration of two NH protons with the same ratio. In addition, the two methoxy groups split into four singlets at d 3.71, 3.79, 3.80 and 3.81 ppm and have a total integration of six protons. The aromatic protons appeared at their expected chemical shifts and have a total integration of 12 protons. The 13 C NMR spectrum also confirmed the presence of the E/cis and E/trans isomers due the appearance of three signals at d 55.18, 55.25 and 55.29 ppm, which are characteristic of the two methoxy groups. In addition, the nonequivalent carbonyl (C=O) and imine (C=N) signals resonated in the downfield region between d 152.92-161.40 ppm.
The absence of this pairing of signals in the 1 H NMR spectrum of bis-hydrazides 4a-d compared to bis-hydrazones 5a-p confirmed the formation these compounds as a mixture of E/cis and E/trans diastereomers.
To confirm the solvent effect for the isomerism of hydrazones, the 1 H NMR spectrum of compound 5f was measured in a less polar solvent (chloroform-d). Two singlet signals were observed at 12.01 and 14.68 ppm for the NH proton, 8.61 and 8.79 ppm for the HC=N proton and 5.10 and 5.58 ppm for the OCH 3 protons corresponding to the cis or trans conformers of the E isomer.

Antimicrobial screening
The synthesized 1,2,3-triazoles were evaluated for their in vitro antimicrobial activity against three Grampositive bacteria, three Gram-negative and two fungal strains. Both microbial screenings were assessed by the minimum inhibitory concentration (MIC) using the broth dilution method. 38,39 The MIC is defined as the minimum concentration of compounds required to completely inhibit bacterial growth. The antibacterial and antifungal screening data expressed as MIC are listed in Table 1. The 1,2,3-triazole diesters 3a-d exhibit good to moderate antibacterial activity against all of the bacterial strains with a MIC range of 16 to 31.25 µg mL -1 . However, a lack of activity was observed against all of the fungal strains. The antimicrobial activity of dihydrazides 4a-d indicated that compounds 4a and 4b possessing 2-F and/or 4-F substitution on the phenyl ring exhibited excellent antibacterial activity against all of the Gram-positive bacteria with a MIC range of 4 to 8 µg mL -1 and good activity against all of the Gram-negative bacterial strains at a MIC of 16 µg mL -1 . Compound 4d, which contains both fluoro and iodo substitutions, exhibited excellent antifungal activity at a MIC of 8 µg mL -1 . The antimicrobial screening revealed that of the tested bis-hydrazones 5a-p, compounds 5a, 5c, 5e, 5g, 5m and 5o exhibited excellent activity against all of the tested bacterial strains with a MIC range of 4 to 8 µg mL -1 . In addition, the highest antifungal  activity with a MIC range of 4 to 8 µg mL -1 was exhibited by compounds 5c, 5g, 5m and 5o, possessing NO 2 and/or iodo substitution. The remaining compounds were found to be more active at higher concentrations (16-31.25 µg mL -1 ). Therefore, the antimicrobial activity and structure activity relationship indicated that the higher inhibition exhibited by the bis-hydrazones against all of bacterial and fungal strains is most likely due to the presence of imine linkage in addition to the 1,2,3-triazole moiety. In addition, the incorporation of fluorine, nitro and/or iodo groups increased the antimicrobial activity.

Conclusions
In conclusion, an eco-friendly, safe, high-yielding and experimentally simple method has been developed for the synthesis of 1-(fluorinated phenyl)-1,2,3-triazole diesters in shorter reaction times. These reactions were carried out via interaction of fluorophenyl azides with dimethylacetylene dicarboxylate under solvent-free conditions in a water bath. The dihydrazides, which were synthesized through hydrazinolysis of the diesters, were condensed with different benzaldehydes to afford the corresponding bis-hydrazones. Some of the tested 1,2,3-triazoles exhibited significant antibacterial and antifungal activities with a MIC range of 4-16 µg mL -1 .

General procedures
The melting points were determined on a Melt-temp apparatus and are uncorrected. Thin-layer chromatography (TLC) was performed on Merck silica gel 60 F254, and  General procedure for the synthesis of dimethyl 1-(substituted phenyl)-1H-1,2,3-triazole-4,5-dicarboxylate 3a-d Dimethyl acetylenedicarboxylate (0.015 mmol) and the appropriate fluorophenyl azide (0.02 mmol) were heated in a water bath for 2-3 min. The reaction mixture was cooled, and then, ether was added to precipitate the product. The solid was filtered and washed with ether to obtain the desired product.  13 13  General procedure for the synthesis of Schiff bases 5a-q A mixture of 4a-d (0.01 mmol) and the appropriate benzaldehyde derivative (0.02 mmol) was refluxed in ethanol (30 mL) containing HCl (0.5 mL) for 2 h. After cooling, the obtained precipitate was filtered and recrystallized from ethanol to afford the desired product.