Synthesis of New Azo Compounds Based on N-(4-Hydroxypheneyl)maleimide and N-(4-Methylpheneyl)maleimide

Maleic anhydride was reacted with p-aminophenol and p-toluidine in the presence of di-phosphorus pentoxide (P2O5) as a catalyst to produce two compounds: N-(4-hydroxy-phenyl)maleimide (I) and N-(4-methylphenyl)maleimide (II). The new azo compounds I(a-c) and II(a-c) were prepared by the reaction of I and II with three different aromatic amines, namely aniline, p-aminophenol and p-toluidine. The structures of these compounds were confirmed by CHN, FT-IR, 1H-NMR, 13C-NMR, mass spectrum and UV/Vis spectroscopy.


Introduction
Small molecules and macromolecules containing imide groups exhibit great electrical properties, good solubility in polar media, resistance to hydrolysis and high thermal stability [1][2][3][4][5][6][7][8]. Due to their excellent properties many efforts have been made to produce different compounds containing imide groups consisting of two carbonyl groups bound to nitrogen. The most common unsubstituted cyclic imides were prepared by heating dicarboxylic acids or their anhydrides with reactants including ammonia, urea, formamide lithium nitride or primary amines [9][10][11][12], but the reaction needs to be carried out at high temperatures for efficient ring closure. Recently, attempts at preparing imide compounds either by the OPEN ACCESS conventional technique or via the microwave irradiation using various catalysts such as Lewis acids, hexamethyldisilazane, carbonyldiimidazole, 4-N,N-dimethylaminopyridine, ammonium chloride, hydroxylamine hydrochloride and sodium acetate to minimize the temperature and time of the reaction have been published [13][14][15][16][17][18]. In this study, the conventional technique was used to synthesize two imides by the reaction of maleic anhydride with p-aminophenol and p-toluidine, respectively, in the presence of diphosphorus pentoxide (P 2 O 5 ) as a catalyst, which decreased the temperature needed for ring closure from 150-300 °C to 20-70 °C.

Synthesis and characterization
The preparation of compounds I, II, I(a-c) and II(a-c) is shown in Scheme 1. The structure of these compounds was confirmed by elemental analysis (CHN), FT-IR, 1 H-NMR, 13 C-NMR, mass spectrum and UV/Vis spectroscopy. Scheme 1. Synthesis of N-(4-hydroxypheneyl)maleimide (I), N-(4-methylpheneyl)maleimide (II), I(a-c) and II(a-c).

Scheme 1. Cont.
The FT-IR spectra of compounds I and II showed the presence of C=O absorbances at 1,702 cm −1 , alkene group (HC=CH) ones at 3,119 cm −1 and the presence of aromatic rings indicated by bands at 1,589, 1,600 and 1,512 cm −1 . In addition a hydroxyl group at 3,481 cm −1 and a methyl group at 1,316 cm −1 were seen for compounds I and II, respectively. All these peaks clearly proved that compounds I and II were produced. The FT-IR data for compounds I(a-c) and II(a-c) showed the same characteristic bands of the coupling agents I and II, namely imide, methyl, hydroxyl group, alkene, and p-substituted band while the presence of the azo (N=N) group band in 1,630-1,575 cm −1 range confirmed the success of the synthesis. Besides, the o-substituted benzene ring absorbtion at 750-775 cm −1 proved that the azo group was attached to the ortho position of the benzene rings. The FT-IR spectrum of compound Ia as a typical example is shown in Figure 1.
The 1 H-NMR and 13 C-NMR for azo compounds I and Ia have been chosen as typical examples and the corresponding spectra are shown in Figures 2 and 3, respectively. In the 1 H-NMR spectrum, the protons of the alkene group (HC=CH) and the protons of aromatic ring appeared at 6.62-6.52 ppm and 6.75-7.39 ppm, respectively. The new peak appeared at 6.93 ppm was assigned to the ortho position and that proved the reaction between compound I and aniline has occurred. The broad peak at 9.45-9.75 ppm was assigned to the free O-H proton.
In the 13       In the UV/Visible spectra, the azo group (N=N) usually gives an absorption in the 350-370 nm range [19]. We expected compounds I(a-c) would exhibited higher λ max than compounds II(a-c) due to the presence of an auxochrome group such as hydroxyl group in the compounds [20]. However, the results showed the opposite, whereby they gave a lower absorption wavelength, which is in the range of 320-330 nm. This might be attributed to the tautomerism due to the polar or proton donor solvents, which can stabilize the carbonyl group by dipolar association or hydrogen bonding and thus decrease the magnitude of the enolization. As the result, the keto group will give a shorter wavelength [21]. The appearance of a new peak at 164 cm −1 in the 13 C-NMR spectrum (Figure 3) suggests that this phenomenon could have indeed occurred. The UV/Vis data for the azo compounds I(a-c) and II(a-c) are given in the Experimental section.

Instrumentation
FT-IR spectra were measured at room temperature using a Perkin-Elmer 2000 FT-IR equipped with a high-purity dried potassium bromide (KBr) beam splitter. The 1 H-NMR and 13 C-NMR spectra were obtained using a Bruker 400 MHz NMR spectrophotometer with tetramethylsilane (TMS) as the internal reference. CHN microanalyses were performed using Perkin Elmer 2400 Series II Combustion Analyzer. The MS were recorded on a Perkin Elmer Clarus 500 Gas Chromatography-Mass Spectrometry system (GC-MS). UV/Vis Spectroscopy were determined using a Shimadzu UV-1601 PC instrument. The entire sample was weighed and dissolved in methanol.

Synthesis of N-(4-methylphenyl)maleimide (II)
Compound II was prepared by following the procedure of the preparation of I except that p-toluidine was substituted for p-aminophenol.

General procedure for preparation of the heterocyclic azo compounds I(a-c) and II(a-c)
Solution A was prepared by mixing pure aniline (a, 0.93 g, 0.01 mol) with concentrated HCl (3 mL) and water (3 mL) and cooling at 5 °C in an ice bath. NaNO 2 (0.69 g, 0.01 mol) was dissolved in water (10 mL) at 5 °C to obtain solution B. Then solution A was added dropwise to solution B at 5 °C with stirring. The mixture was then slowly added into the solution of compound I (1.89 g, 0.01 mol), which was dissolved in 10% NaOH (20 mL) at 5 °C. The mixture was keep chilled in the ice bath and stirred continuously for 10 min. The precipitate formed was filtered and recrystallized from glacial acetic acid, and washed with methanol and finally dried in a vacuum oven at 65 °C for 24 hours. The procedure was repeated by substituted I with II, where was substituted by p-aminophenol (b) and p-toluidine (c).