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Short Note

N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine

by
Georgia A. Zissimou
* and
Panayiotis A. Koutentis
Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus
*
Author to whom correspondence should be addressed.
Molbank 2017, 2017(4), M967; https://doi.org/10.3390/M967
Submission received: 13 November 2017 / Revised: 28 November 2017 / Accepted: 30 November 2017 / Published: 2 December 2017
(This article belongs to the Section Organic Synthesis)
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Abstract

:
Treating 1,5-difluoro-2,4-dinitrobenzene (1) with N1-phenyl-5-(trifluoromethyl)benzene-1,2-diamine (4) and N,N-diisopropylethylamine in EtOH at ca. 0 °C for 4 h affords a mixture of N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5) (38%) and N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) (51%) that can be separated by chromatography. Repeating the reaction in dichloromethane led to the sole formation of N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) in 96% yield.

Graphical Abstract

1. Introduction

1,5-Difluoro-2,4-dinitrobenzene (1), which can be prepared by direct nitration of 1,3-difluoro-benzene in high (77%) yield (Scheme 1) [1], is a commercially available dihalodinitroarene that is synthetically useful owing to its high reactivity towards nucleophilic aromatic substitution. The easily controlled step-wise displacement of each fluoride provides products that can be further modified via reduction of the 2,4-dinitro groups.
1,5-Difluoro-2,4-dinitrobenzene (1) has been used to prepare several biologically active fused heterocycles, such as phenazinimines [2], 1,5-benzodiazepin-2-ones [3], benzo[1,4]oxazin-3-ones [4], benzo[1,4]thiazin-3-ones [5], and indoles [6], as well as provide interesting oligo(arylamines) that show interesting H-bonding and π–π interactions in both solution and the solid state [7,8,9]. Furthermore, owing to its electron deficient arene core, difluorodinitrobenzene 1 forms 1:1 and 1:2 complexes with benzenediamine and benzidine donors [10] and solid charge transfer complexes with hydroxyl aryl Schiff bases [11]. 1,5-Difluoro-2,4-dinitrobenzene (1) can also be used to enable the chromatographic resolution of chiral secondary alcohols [12]. More recently, difluorodinitrobenzene 1 has been extensively used as a monomer unit to prepare unusual azamacrocycles via cyclooligomerisation chemistry [13,14,15,16,17,18,19].
Our interest in difluorodinitrobenzene 1 stems from its use as a starting point in the synthesis of biscyanine azaacenes such as the hexaazaanthracene 2 [20,21] and the tetraazapentacene 3 [22,23,24,25] (Figure 1) that followed a modification of Nietzki’s classical preparation of tetraazapentacene from 1,2-diaminobenzenes and the considerably less reactive 1,5-dichloro-2,4-dinitrobenzene [26].
A key initial step in preparing asymmetrically-substituted biscyanine tetraazaacene analogues [23] is the selective displacement of one fluorine by N1-aryl/alkyl 1,2-benzenediamines and this is typically achieved at 0–20 °C in solvents such as EtOH or MeOH in the presence of either triethylamine or pyridine (1 equiv) or additional N1-aryl/alkyl 1,2-benzenediamine (1 equiv). Herein, we report the synthesis of N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) (Scheme 2) using two different sets of reaction conditions: (i) the first, using EtOH as solvent that led to mixtures and only a moderate yield of the desired target and (ii) the second, using dichloromethane (DCM) as solvent which gave the target in near quantitative yield.

2. Results and Discussion

The target, N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6), was needed as a starting point for the synthesis of asymmetrically substituted biscyanine tetraazaacenes. Initially, we followed a known protocol [23] that required access to readily available 1,5-difluoro-2,4-dinitrobenzene (1) and N1-phenyl-5-(trifluoromethyl)benzene-1,2-diamine (4) [27].
As such, we treated a solution of 1,5-difluoro-2,4-dinitrobenzene (1) in EtOH at ca. 0 °C with N1-phenyl-5-(trifluoromethyl)benzene-1,2-diamine (4) and then with N,N-diisopropylethylamine (1.1 equiv) (Conditions (i), Scheme 2). Disappointingly, after 4 h, we obtained a mixture of N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5) (38%) and the desired N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) (51%) which required chromatographic separation (n-hexane/DCM) (Scheme 2).
The structural identity of both compounds was supported by mass spectrometry, UV–vis, IR, and NMR spectroscopy (see Supplementary Materials) and by elemental analysis. Presumably, the formation of the undesired ethoxy-substituted side product 5 was attributed to the presence of the para-trifluoromethyl (p-F3C) group. The powerful inductively electron withdrawing p-F3C group can deactivate the primary amine of the 1,2-benzenediamine 4 towards nucleophilic attack enabling a competitive displacement of fluoride by the ethoxide that must originate from the abundant EtOH used as the reaction solvent.
Considering this, we switched the solvent from EtOH to various non-nucleophilic aprotic solvents (1,4-dioxane, tetrahydrofuran, DCM, dimethylformamide, and dimethylacetamide). Gratifyingly, repeating the reaction in DCM at ca. 0 °C led to completion of the reaction (by thin-layer chromatography (TLC)) within 30 min to afford only the desired N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) in 96% yield, enabling its chromatography free isolation.

3. Materials and Methods

3.1. General Methods

All chemicals were commercially available except those whose synthesis is described. Reactions were protected from moisture with CaCl2 drying tubes. All volatiles were removed under reduced pressure. All reaction mixtures and column eluents were monitored by TLC using commercial glass-backed TLC plates (Merck Kieselgel 60 F254, Darmstadt, Germany). The plates were observed under UV light at 254 and 365 nm. The technique of dry flash chromatography [28] was used throughout for all non-TLC scale chromatographic separations and employed Merck Silica Gel 60 (less than 0.063 mm). Melting and decomposition points were determined using a PolyTherm-A, Wagner & Munz, Kofler hot-stage microscope apparatus (Wagner and Munz, Munich, Germany). The solvents used for recrystallization are indicated after the respective melting points. UV–vis spectra were obtained using a Perkin-Elmer Lambda-25 UV–vis spectrophotometer (Perkin-Elmer, Waltham, MA, USA) and inflections are identified by the abbreviation ‘inf’. IR spectra were recorded on a Shimadzu FTIR-NIR Prestige-21 spectrometer (Shimadzu, Kyoto, Japan) with a Pike Miracle Ge ATR accessory (Pike Miracle, Madison, WI, USA) and strong, medium and weak peaks are represented by s, m, and w, respectively. 1H- and 13C-NMR spectra were recorded on a Bruker Avance 500 machine (at 500 and 125 MHz, respectively) (Bruker, Billerica, MA, USA). Deuterated solvents were used for homonuclear lock and the signals are referenced to the deuterated solvent peaks. APT (Advance Proton Test) NMR studies identified carbon multiplicities, which are indicated by (s), (d), (t), and (q) notations. MALDI-TOF (matrix assisted laser desorption/ionization-time of flight) mass spectra (+ve mode) were conducted on a Bruker Autoflex III Smartbeam instrument (Bruker, Billerica, MA, USA). Elemental analyses were run by the London Metropolitan University Elemental Analysis Service. N1-Phenyl-5-(trifluoromethyl)-benzene-1,2-diamine (4) was synthesized according to a literature procedure [27].

3.2. Method A: N1-(5-Ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5) and N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6)

To a stirred solution of 1,5-difluoro-2,4-dinitrobenzene (1) (204 mg, 1.0 mmol) in EtOH (5 mL) at ca. 20 °C, was added in several portions over a period of 30 min N1-phenyl-5-(trifluoro-methyl)benzene-1,2-diamine (4) (252 mg, 1.0 mmol). A crystalline orange precipitate formed within 30 min, at which time and while keeping the temperature at ca. 0 °C, was added N,N-diisopropylethylamine (192 μL, 1.1 mmol). The mixture was slowly left to warm to at ca. 20 °C. Upon completion (by TLC, 4 h), the orange precipitate was filtered and chromatographed (n-hexane/DCM, 50:50) to give N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5) (178 mg, 38%) as orange needles, m.p. (hot-stage) 174.1–176.7 °C (from EtOH); Rf 0.44 (n-hexane/DCM, 50:50); found: C, 54.63; H, 3.76; N, 12.57. C21H17F3N4O5 requires C, 54.55; H, 3.71; N, 12.12%; λmax (DCM)/nm 284 (log ε 4.66), 360 inf (4.20); νmax/cm−1 3391 m and 3347 m (N–H), 1626 m, 1609 m, 1593 m, 1568 s, 1535 m, 1518 w, 1497 m, 1481 w, 1443 m, 1410 s, 1339 s, 1327 m, 1292 s, 1279 s, 1250 m, 1225 m, 1179 m, 1159 m, 1126 s, 1074 m, 1030 w, 939 m, 905 w, 878 m, 825 w, 814 m, 777 w, 766 w, 750 m, 740 m, 721 w; δH (500 MHz, CDCl3) 9.66 (1H, s, NH), 9.02 (1H, s), 7.58 (1H, d, J 1.5), 7.38 (1H, d, J 8.0), 7.35 (2H, dd, J 7.8, 7.8), 7.22 (1H, dd, J 8.5, 1.5), 7.12 (1H, dd, J 7.5, 7.5), 7.08 (2H, d, J 8.0), 6.27 (1H, s), 5.89 (1H, s), 3.97 (2H, q, J 7.0, OCH2), 1.43 (3H, t, J 7.0, CH3); δC (125 MHz, CDCl3) 158.4 (s), 147.0 (s), 140.8 (s), 140.1 (s), 131.0 (q, 2JCF 32.6), 130.6 (s), 129.8 (d), 127.7 (d), 127.4 (s), 127.2 (d), 125.5 (s), 123.9 (d), 123.6 (q, 1JCF 271.1, CF3), 120.5 (d), 117.2 (q, 3JCF 3.6), 113.1 (q, 3JCF 3.6), 97.7 (d), 66.3 (t, OCH2), 14.0 (q, CH3); m/z (MALDI-TOF) 462 (M+, 20%), 445 (100), 444 (53), 415 (47). Further elution (n-hexane/DCM, 40:60) gave N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) (222 mg, 51%) as red cubes, m.p. (hot-stage): 185.4–188.6 °C (from toluene); Rf 0.38 (n-hexane/DCM, 50:50); found: C, 52.35; H, 2.81; N, 12.86. C19H12F4N4O4 requires C, 52.30; H, 2.77; N, 12.84%; λmax(DCM)/nm 248 (log ε 4.40), 300 (4.54), 358 (4.33), 403 (4.14); νmax/cm−1 3399 m and 3321 m (N–H), 3061 w (aryl C–H), 1632 s, 1595 s, 1537 m, 1530 m, 1508 s, 1499 s, 1443 m, 1418 m, 1369 m, 1331 s, 1300 s, 1275 s, 1254 m, 1221 m, 1180 m, 1128 s, 1117 s, 1072 m, 1055 m, 1030 w, 941 m, 918 w, 883 w, 845 m, 827 m, 773 w, 760 m, 743 s, 727 m, 708 m; δH (500 MHz, CDCl3) 9.61 (1H, s, NH), 9.14 (1H, d, J 8.0), 7.53 (1H, s), 7.38 (1H, d, J 8.0), 7.34 (2H, dd, J 8.0, 8.0), 7.25 (1H, d, J 7.5, overlap with CDCl3), 7.12 (1H, dd, J 7.5, 7.5), 7.05 (2H, d, J 8.0), 6.67 (1H, d, JHF 13.0), 5.83 (1H, s); δC (125 MHz, CDCl3) one C (s) resonance missing, 159.8 (d, 1JCF 270.9, CF), 147.7 (d, 2JCF 12.5), 140.9 (s), 140.0 (s), 131.7 (q, 2JCF 32.5), 129.8 (d), 128.5 (s), 127.9 (d), 127.5 (d), 126.9 (s), 124.3 (d), 123.5 (q, 1JCF 271.3, CF3), 121.0 (d), 117.6 (q, 3JCF 3.6), 113.9 (q, 3JCF 3.6), 103.7 (d, 2JCF 27.5); m/z (MALDI-TOF) 437 (MH+, 41%), 436 (M+, 100).

3.3. Method B: N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6)

To a stirred solution of 1,5-difluoro-2,4-dinitrobenzene (1) (204 mg, 1.0 mmol) in DCM (5 mL) at ca. 0 °C, was added in one portion N1-phenyl-5-(trifluoromethyl)benzene-1,2-diamine (4) (252 mg, 1.0 mmol), followed by addition of N,N-diisopropylethylamine (192 μL, 1.1 mmol). The reaction mixture was allowed to warm to ca. 20 °C (over 30 min), after which time the reaction was complete (by TLC). The solvent was evaporated in vacuo to afford an orange-red residue that was recrystallized to afford the title compound 6 as red cubes (418 mg, 96%); identical to that described above.

Supplementary Materials

The following are available online: www.mdpi.com/1422-8599/2017/4/M967. 1H and 13C NMR spectra of N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5) and N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6).

Acknowledgments

G.A.Z. thanks the University of Cyprus “PhD Scholarships 2015-16” for financial support. The authors also thank the Cyprus Research Promotion Foundation (Grant: NEKYP/0308/02) and the following organizations and companies in Cyprus for generous donations of chemicals and glassware: The State General Laboratory, the Agricultural Research Institute, the Ministry of Agriculture, MedoChemie Ltd., Medisell Ltd., and Biotronics Ltd. Furthermore, we thank the A.G. Leventis Foundation for helping to establish the NMR facility at the University of Cyprus.

Author Contributions

P.A. Koutentis conceived the experiments; G.A. Zissimou designed and performed the experiments, analyzed the data, and wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Molbank 2017 m967 sch001 550
Scheme 1. Preparation of 1,5-difluoro-2,4-dinitrobenzene (1) from 1,3-difluorobenzene [1].
Scheme 1. Preparation of 1,5-difluoro-2,4-dinitrobenzene (1) from 1,3-difluorobenzene [1].
Molbank 2017 m967 sch001
Molbank 2017 m967 g001 550
Figure 1. General structure of biscyanines 2 and 3 prepared from difluorodinitrobenzene 1 [20,21,22,23,24,25].
Figure 1. General structure of biscyanines 2 and 3 prepared from difluorodinitrobenzene 1 [20,21,22,23,24,25].
Molbank 2017 m967 g001
Molbank 2017 m967 sch002 550
Scheme 2. Synthetic scheme for the preparation of N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) and side product N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5). Reaction conditions: (i) iPr2NEt (1.1 equiv), EtOH, 0 °C, 4 h, 38% (5), 51% (6); and (ii) iPr2NEt (1.1 equiv), DCM, 0 °C, 30 min, 96% (6).
Scheme 2. Synthetic scheme for the preparation of N1-(5-fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (6) and side product N1-(5-ethoxy-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine (5). Reaction conditions: (i) iPr2NEt (1.1 equiv), EtOH, 0 °C, 4 h, 38% (5), 51% (6); and (ii) iPr2NEt (1.1 equiv), DCM, 0 °C, 30 min, 96% (6).
Molbank 2017 m967 sch002

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Zissimou, G.A.; Koutentis, P.A. N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine. Molbank 2017, 2017, M967. https://doi.org/10.3390/M967

AMA Style

Zissimou GA, Koutentis PA. N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine. Molbank. 2017; 2017(4):M967. https://doi.org/10.3390/M967

Chicago/Turabian Style

Zissimou, Georgia A., and Panayiotis A. Koutentis. 2017. "N1-(5-Fluoro-2,4-dinitrophenyl)-N2-phenyl-4-(trifluoromethyl)benzene-1,2-diamine" Molbank 2017, no. 4: M967. https://doi.org/10.3390/M967

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