Synthesis of Some New Anils: Part 1. Reaction of 2-Hydroxy-benzaldehyde and 2-Hydroxynaphthaldehyde with 2-Aminopyridene and 2-Aminopyrazine

Abstract

New Schiff bases derived from 2-aminopyridene and 2-aminopyrazine have been synthesized. The UV-Visible spectra of the compounds have been investigated in acetonitrile and toluene. The compounds were in tautomeric equilibrium (enol-imine O–H···N, keto-amine O···H–N forms) in polar and nonpolar solvents. For some derivatives the keto-amine form was observed in both toluene and acetonitrile. ¹H-NMR and IR results showed that all Schiff bases studied favor the enol-imine form over the keto form in a weakly polar solvent such as deuterochloroform.

Introduction

2-Hydroxy Schiff base ligands and their complexes, derived from the reaction of salicylaldehyde and 2-hydroxy-1-naphthaldehyde with amines have been extensively studied [1,2,3,4,5,6]. 2-Hydroxy Schiff base ligands are of interest mainly due to the existence of (O–H···N and N–H···O) type hydrogen bonds and tautomerism between the enol-imine and keto-enamine forms (Scheme 1). Tautomerism in 2-hydroxy Schiff bases both in solution and in the solid state was investigated using different spectroscopic techniques [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. In the spectra of solutions of these compounds different Schiff bases have been studied in both polar and non-polar solvents [5,6,8,9,11,13,16,20]. A new band at greater than 400 nm has been observed in polar solvents, and in acidic media in some solvents, but this band has not been observed in hydrocarbon solvents. The results indicate that the absorption band at greater than 400 nm belongs to the keto-enamine form of the Schiff base; the enol form has no appreciable absorbance in this region. Different explanations for the source of this band have been proposed [5,8,15,17,20]. It was suggested that this new band is due to the keto form in salicylidene anilines (Scheme 1). It was also observed that ortho-hydroxy naphthalidene anilines show two bands in the visible region located above 400 nm [8]. These two bands were assigned to the keto form. In the solid state, salicylideneanilines exist mostly in the enol-imino tautomeric form. In naphthaldimines both forms are possible and O–H···N or N–H···O intramolecular hydrogen bonds can occur [2,3,14,19,22]. The hydrogen bonding and tautomerism of Schiff bases and the tautomeric forms at 50% abundance have been reported in the crystalline state for the Schiff base formed by 3-haloanilines and 2-hydroxy-1-naphthaldehyde [3]. The Schiff base compounds can be classified according to their photo- and thermochromic characteristics [24,25]. Thermochromism is due to a change in the -electron configuration induced by proton transfer, which can occur in the ground state and requires a planar molecular system. Non-planar molecules exhibit photochromism. Studies on the photochromic compounds have been increasing ever since the potential applications of photochromic materials were realized in various areas such as the control and measurement of radiation intensity, optical computers and display systems.

Scheme 1. Tautomerism in 2-hydroxy Schiff bases 1.

Scheme 1. Tautomerism in 2-hydroxy Schiff bases 1.

Results and Discussion

The Schiff bases studied were prepared as shown in Scheme 2 and Scheme 3. FT-IR spectra for compounds 5a-b and 8a-b showed weak and broad absorption bands in the 3400–3200 cm^{- 1} region. This is interpreted to be a sign of the presence of the enol forms 6a-b and 8a-b. The peaks with wave numbers between 1710–1590 cm^{- 1} arise due to stretching vibrations of C═N and C═O bands. In this region one band was observed for compounds 5a-b and 8b, indicating a fully enol form. The FT-IR data indicated that compound 7a crystallized as a mixture of both enol and keto forms 8a and 9a, respectively. The enol form was inferred from the presence of strong absorption in the region of 3400–3200 cm^{- 1}. On the other hand, the keto form was suggested by the strong absorption in the 1660-1700 cm^-1 region (Scheme 3).

Scheme 2. Synthesis of Schiff bases 5a-b and 6a-b.

Scheme 2. Synthesis of Schiff bases 5a-b and 6a-b.

Scheme 3. Synthesis of Schiff bases 5a-b and 8a-b.

Scheme 3. Synthesis of Schiff bases 5a-b and 8a-b.

From the ¹H-NMR spectra of compounds 5a-b and 8a-b it was possible to assign the value of the chemical shifts of the labile hydroxyl group protons. This proton exists in the spectra of all four compounds, which indicated the presence of the enol form of these compounds. On the other hand compound 8a exists as a mixture of both enol and keto forms.

Electronic absorption spectral properties

The UV-Visible spectra of the compounds were studied in polar acetonitrile and non-polar toluene solvents. The spectra are presented in Figure 1, Figure 2, Figure 3 and Figure 4 and summarized in Table 1. Some of the compounds studied show one absorption band in the range of less than 400 nm in toluene. The new band (greater than 400 nm) belongs to the keto form of the 2-hydroxy Schiff bases in polar and non-polar solvents [1,2]. The band was observed in some cases at greater than 400 nm in both polar acetonitrile and non-polar toluene solutions. The enol-imine tautomer is dominant only in the polar solutions of acetonitrile, in contrast to the keto-enamine tautomer 6.

Solvent effects can be explained in term of the capability of solvents to form hydrogen bonding both as proton donors and as proton acceptors, thereby permitting proton transfers that result in formation of the keto form (Scheme 3). As a proton donor, the solvent interacts with the non-bonding electron pair of the azomethine nitrogen. Thus, it may be assumed that, in those Schiff base derivatives where a non-bonding electron pair is no longer available, a solvent effect will not be observed.

Table 1. Electronic Spectral Data of Compounds 5a-b and 8a-b.

Compound	Toluene	Acetonitrile
	λ max (nm)	λ max (nm)	Molar coefficient
5a	315	315	5956.4
	435	430	67867.7
	455	455	65277.6
5b	315 360	220 270 310 350	415219.1 289652.8 340425.5 310710.8
8a	285 385	235 270 340	47667.5 48247.6 43482.6
8b	345 410	230 400	441340.0 125780.3

Table 1. Electronic Spectral Data of Compounds 5a-b and 8a-b.

Compound	Toluene	Acetonitrile
	λ max (nm)	λ max (nm)	Molar coefficient
5a	315	315	5956.4
	435	430	67867.7
	455	455	65277.6
5b	315 360	220 270 310 350	415219.1 289652.8 340425.5 310710.8
8a	285 385	235 270 340	47667.5 48247.6 43482.6
8b	345 410	230 400	441340.0 125780.3

The spectroscopic data suggest that these compounds exist mainly in the enol-imine form in solid state for compounds 5a-b, and a mixture of enol and the keto form for compounds 8b. The new compounds prepared in this work showed no photochromic nor thermochromic properties in both solution and solid rigid polymer matrices.

Figure 1. UV-Visible spectra of compound 5a in toluene.

Figure 1. UV-Visible spectra of compound 5a in toluene.

Figure 2. UV-Visible spectra of compound 5b in toluene.

Figure 2. UV-Visible spectra of compound 5b in toluene.

Figure 3. UV-Visible spectra of compound 8a in toluene.

Figure 3. UV-Visible spectra of compound 8a in toluene.

Figure 4. UV-Visible spectra of compound 8b in toluene.

Figure 4. UV-Visible spectra of compound 8b in toluene.

Conclusions

In summary, structural analysis using IR, ¹H-NMR and UV-visible spectroscopy show that the anils prepared from 2-amino substituted pyridine and pyrazine exist manly in enol forms and mixture of both enol and keto tautomeric form in some instances.

Experimental

General

Melting points were recorded on a Thomas-Hoover capillary melting apparatus without correction. IR spectra were taken as KBr disk on a Nicolet Magna 520 FTIR spectrometer, ¹H-NMR were recorded in CDCl₃ on a Bruker DPX 400 spectrometer using TMS as internal standard. Mass spectra were obtained on a Varian MAT CH5 Spectrometer using EI technique. UV-visible spectra were recorded on a Shimadzu 260 spectrometer for solutions. All reagents and chemicals were obtained from Aldrich Chemical Company (USA) and were used as received unless otherwise noted.

General procedure for preparation of anil derivatives:

A solution of amine (1 mol) in pure ethanol was heated to its boiling temperature, and then aldehyde (1 mol) dissolved in hot ethanol was added to the amine solution and the resulting mixture was then refluxed for 3 hrs. Cooling the mixture, filtering the precipitates and recrystalization from ethanol gave the pure products.

2-(2-Hydroxybenzylidene)aminopyridine: Yield: 64.6 %; mp: 172.9 ºC; colour: yellow; IR ν (cm^-1); 1612.5 (C=N), 1474.3 (C=C), 1322.9 (C-O) and 1141 (C-N); ¹H-NMR (CDCl₃) δ: 12.92 (s, OH), 8.64 (s, 1H, CH_olef) and 8.07-6.92 (m, 8H, CH_arom).

2-(2-Hydroxynahthlidene)aminopyridine: Yield: quantitative; mp: 273.4 ºC; colour: yellow; IR ν (cm^-1); 1685.5 (C=N), 1495.2 (C=C), 1291.3 (C-O) and 1180.3 (C-N); ¹H-NMR (CDCl₃) δ: 15.44 (s, OH), 9.97 (s, 1H, CH_olef), and 852-6.93 (m, 10H, CH_arom).

2-(2-Hydroxybenzylidene)aminopyrazine: Yield: 33.9 %; mp: 96.6 ºC; colour: yellow; IR ν (cm^-1); 1594.9 (C=N), 1470 (C=C) and 1191.9 (C-N); ¹H-NMR (CDCl₃) δ: 12.67 (s, OH), 8.80 (s, 1H, CH_olef), 8.67 (d, CH_arom), 8.51 (s, CH_arom), 8.50 (d, CH_arom), 7.69 (d, CH_arom), 7.27 (dd, CH_arom), 6.92 (d, CH_arom) and 6.84 (dd, CH_arom); Analysis for C₁₁H₉N₃O (199.21) % Calcd: C: 66.32, H: 4.55, N: 21.09; % Found: C: 66.05, H: 4.43 N: 21.14.

2-(2-Hydroxynahthlidene)aminopyrazine: Yield: 49.7 %; mp: 177.2ºC; colour: orange; IR ν (cm^-1); 1607 (C=N), 1507 (C=C), 1298.7 (C-O) and 1141.4 (C-N); ¹H-NMR (CDCl₃) δ: 14.81 (s, OH), 9.31 (s, 1H, CH_olef), 8.67 (d, CH_arom), 8.55 (d, CH_arom), 8.50 (m, 2CH_arom), 8.17 (d, CH_arom), 7.76 (dd, CH_arom), 7.67 (dd, CH_arom), 7.59 (d, CH_arom) and 6.93 (d, CH_arom). Analysis for C₁₅H₁₁N₃O (249.27) Calcd: C: 72.28; H: 4.45; N: 16.86; % Found: C: 72.28; H: 4.53; N: 16.79.