2,6-Bis(phenylamino)-4-(iminophenyl)benzoquinone

Abstract

The reaction of 2,4-difluoronitrobenzene with an excess of aniline at 175 °C led to the isolation of an unexpected brown quinone substituted with two phenylamino groups and one phenylimino group. This product was easily distinguished from other expected derivatives because it is brown rather than yellow. The UV/Vis has a weak long wavelength absorption at 480–600 nm accounting for the colour.

1. Introduction

The condensation of amines or carbanions with nitro-substituted aromatic compounds where hydrogen is substituted is comparatively rare. Figure 1 shows an early example from the 19th century in which the reaction of aniline with nitrobenzene is catalysed by acid [1]. A total of 56 pounds of aniline hydrochloride 2 was reacted with 44 pounds of nitrobenzene 2 to produce red dye 3.

Figure 1. Acid catalysed reaction of nitrobenzene 1 with aniline hydrochloride 2 forming red dye 3 which is violet in acid.

Figure 2 shows the products which form—4-nitrodiphenylamine 4 and 4-nitrosodiphenylamine 5—from the KOH base-catalysed reaction of nitrobenzene 1 with aniline 2 along with azobenzene 6, phenazine 7 and (phenylamino)azobenzene 3 [2]. The catalytic hydrogenation of products 4 and 5 produces anti-oxidant 8. This has been used for making pseudo-mauveine which is mauveine lacking any methyl groups [3].

Figure 2. Anti-oxidant 8 formed from the KOH catalysed reaction of aniline with nitrobenzene followed by catalytic hydrogenation of products 4 and 5.

Figure 3 shows a synthesis of a photoacid generator called Irgacure PAG 103 [4,5]. The reaction of o-xylyl cyanide 9 and 2-nitrothiophene 10 with KOH gives adduct 11 which can be sulfonated to produce photoacid generator 12. The photochemical decomposition, which involves an unusual 6π photocyclisation of the C=N of an oxime leading to an acridine derivative and acid, has been explored [6,7]. Heterocyclic examples with a similar synthesis to compound 11 are also known [8]. When the carbanion of compound 9 reacts with the 2-nitrothiophene 10, the elimination of the ring hydrogen of the tetrahedral carbon leads to the reduction of the nitro group to a nitroso group.

Figure 3. KOH catalysed condensation of o-xylyl cyanide 9 with 2-nitrothiophene 10 on route to photoacid generator PAG 103 12.

2. Discussion

Treatment of 2,4-difluoronitrobenzene 13 with aniline at 175 °C followed by purification on a silica column produced compound 14 as a brown solid in 14% yield. Nitrobenzenes substituted with amino groups are usually yellow coloured [9,10], so this compound was a different chromophore. The UV/Vis spectrum showed a strong absorption at 383 nm and a weaker broad absorption at 480–600 nm which accounts for the stronger colour. It had 18 carbon resonances in the ¹³C NMR spectrum with one carbonyl group at 181 ppm and the ¹H NMR spectrum had 17 aromatic protons. Two broader resonances were observed further downfield at 7.68 and 8.69 ppm and were assigned to the two NH hydrogens. There were two sharp resonances at 6.21 and 6.22 ppm which were assigned to the two quinone hydrogen atoms. The accurate mass gave a formulae of C₂₄H₁₉N₃O. The supplementary materials (Figures S1–S5) have the proton and carbon spectra and the UV/Vis spectrum (Figure S6).

The structure of compound 14 (Figure 4) was deduced from the spectroscopic data and the anticipated chemistry which is shown in Figure 5. Initially, aniline was expected to displace the two fluorine atoms to give 2,4-bis(phenylamino)nitrobenzene 15. Forceful conditions are required because aniline is not a strong nucleophile for displacing the second fluorine of 2,4-difluoronitrobenzene 13. Only one site ortho to the nitro group remained unsubstituted. In line with the previous literature reactions of nitrobenzene 1 and aniline 2 [1,2], aniline may attack the unsubstituted site, thereby producing intermediate 16, followed by the elimination of hydrogen from the tetrahedral carbon which reduces the nitro group to a nitroso group forming intermediate 17 and water. The tautomerism of compound 17 to compound 18 followed by the hydrolysis of the oxime will produce a final product of 14. 4-Nitrosodiphenylamine 19 undergoes a similar type of base-catalysed isomerism to oxime 20 (Figure 6) [11].

Figure 4. Synthesis of compound 14.

Figure 5. Proposed mechanistic scheme for the formation of compound 14.

Figure 6. The base-catalysed tautomerism of 4-nitrosodiphenylamine to an oxime.

3. Materials and Methods

IR spectra were recorded on a Diamond Anvil Fourier transform infrared (FTIR) spectrometer (ThermoScientific, Hemel Hempstead, UK). Ultraviolet (UV) spectra were recorded using a PerkinElmer Lambda 25 UV-Vis spectrometer (PerkinElmer, Coventry, UK) with EtOH as the solvent. The term sh means shoulder. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded at 400 and 100.5 MHz, respectively, using a Varian 400 spectrometer. Chemical shifts, δ, are given in ppm and were measured by performing a comparison with the residual solvent. Coupling constants, J, are given in Hz. High-resolution mass spectra were obtained at the University of Wales, Swansea, using an Atmospheric Solids Analysis Probe (ASAP) (Positive mode) Instrument: Xevo G2-S ASAP (SpectraLab, Markham, ON, Canada). Melting points were determined on a Kofler hot-stage microscope.

• Synthesis of 2,6-Bis(phenylamino)-4-(iminophenyl)benzoquinone (14)

2,4-Difluoronitrobenzene 13 (500 mg, 3.15 mmol) in aniline 2 (20 mL) was heated at 170 °C for 24 h. The reaction mixture was dark and complex. It was scraped up with a spatula into DCM and applied to a long 12 inch silica column. Elution with DCM gave the title compound (165 mg, 14%) as a brown powder, mp 197–198 °C (from dichloromethane:light petroleum ether). It runs closely with another yellow product. λ_max (EtOH)/nm 537sh (log ε 3.9), 383 (4.0) and 276 (4.0); λ_max (Diamond)(cm⁻¹) 3259(s), 1632(s), 1573(vs), 1499(vs), 1442(vs), 1292(vs), 1174(s), 847(w), 739(s), 693(vs) and 531(s); δ_H (400 MHz; CDCl₃) 6.21 (1H, s), 6.22 (1H, s), 6.99 (2H, d, J = 4.0), 7.06 (1H, t, J = 8.0 and 8.0), 7.09 (2H, d, J = 8.0), 7.18 (1H, t, J = 8.0 and 8.0), 7.24 (1H, t, J = 8.0 and 8.0), 7.29 (2H, m), 7.36 (2H, d, J = 8.0), 7.42 (2H, t, J = 8.0 and 8.0), 7.45 (2H, J = 8.0 and 8.0), 7.68 (1H, s, NH) and 8.69 (1H, s, NH); δ_C (100.1 MHz; CDCl₃) 90.5, 97.1, 120.8, 120.9, 123.1, 124.0, 124.8, 125.4, 129.0, 129.4, 129.6, 137.9, 138.6, 140.7, 149.4, 149.6, 153.8 and 181.1 (C = 0); m/z (Orbitrap ASAP) 366.1607 (M + H⁺, 100%) C₂₄H₁₉N₃O + H⁺ requires 366.1606.

4. Conclusions

We previously demonstrated that a low 4% yield of 2,4-bis(phenylamino)nitrobenzene 15 can be obtained from the reaction of an excess of aniline with 2,4-difluoronitrobenzene 13 [12]. Efforts to improve this reaction have not improved the yield but instead led to the isolation of an unexpected brown solid characterised by spectroscopic methods as a quinone substituted with three phenylamino groups. There is only one similar compound reported in the literature which is an isomer of compound 14 and can be formed by an enzyme-catalysed condensation of aniline with substituted catechols or by other methods [13].