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Article

Synthesis of New Unsymmetrical 4,5-Dihydroxy-2-imidazolidinones. Dynamic NMR Spectroscopic Study of the Prototropic Tautomerism in 1-(2-Benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone

School of Chemistry, University College of Science, University of Tehran, Tehran, Iran
*
Authors to whom correspondence should be addressed.
Molecules 2006, 11(10), 768-775; https://doi.org/10.3390/11100768
Submission received: 28 September 2006 / Revised: 11 October 2006 / Accepted: 12 October 2006 / Published: 19 October 2006

Abstract

:
The acid-catalyzed cyclocondensation in refluxing acetonitrile of aqueous glyoxal with N-heteroaryl-N'-phenylureas 4a-f (heteroaryl = 2-thiazolyl, 2-pyrimidinyl, 2-pyrazinyl, 2-pyridinyl, 3-pyridinyl and 2-benzimidazolyl) led to the formation of the corresponding 1-heteroaryl-3-phenyl-4,5-dihydroxy-2-imidazolidinones 5a-f. All the products were characterized by elemental and spectroscopic analyses. The free-energy barrier (∆G) for prototropic tautomerism in 1-(2-benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5f) was determined by dynamic NMR studies to be 81 ± 2 KJ mol-1.

Introduction

Synthesis of imidazolidines through the cyclocondensation of diamines, bisamides and urea derivatives with aqueous glyoxal and other appropriate carbonyl compounds has been the subject of numerous investigations [1,2,3,4,5,6,7,8,9,10]. In 1962, Slezak et al. reported the acid-catalyzed reaction of urea derivatives with aqueous glyoxal leading to the formation of the corresponding glycolurils 1 (Figure 1) [11]. The formation of hydantoin derivatives 2 (Figure 1) had been found previously to take place under similar reaction conditions [12]. The additions of N,N'-dimethylurea and urea to aqueous glyoxal under both acidic and basic conditions to form 4,5-dihydroxy-2-imidazolidinones derivatives 3 (Figure 1) have been studied by Vail et al. [13]. On the basis of NMR spectroscopy, they showed that initially equimolar amounts of the cis and trans isomers were formed by a non-stereospecific addition, but the resulting equilibrium product mixture contained predominantly the trans isomer. In acidic solution, the products would be subject to protonation and subsequent formation of other products. 2-Imidazolidinones are important building blocks in Medicinal Chemistry as central nervous system depressants and enzyme inhibitors [14,15,16]. They are also used as textile finishing agents [17].
Figure 1. Structures of glycolurils 1, hydantoins 2 and 4,5-dihydroxy-2-imidazolidinones 3.
Figure 1. Structures of glycolurils 1, hydantoins 2 and 4,5-dihydroxy-2-imidazolidinones 3.
Molecules 11 00768 g001
In the present work, we wish to report a facile and convenient synthetic route to some new unsymmetrical 1,3-diarylsubstituted 4,5-dihydroxy-2-imidazolidinones 5a-f through the reaction of aqueous glyoxal with N-heteroaryl-N'-phenylureas 4a-f in the presence of formic acid as catalyst. The dynamic behavior of 1-(2-benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5f) was also studied by variable temperature NMR.

Results and Discussion

N-Heteroaryl-N'-phenylureas 4a-f were prepared through the reaction of suitable primary heteroarylamines (2-aminothiazole, 2-aminopyrimidine, 2-aminopyrazine, 2-aminopyridine, 3-amino-pyridine and 2-aminobenzimidazole) with phenyl isocyanate [18,19,20,21]. In refluxing acetonitrile and in the presence of formic acid as catalyst, treatment of compounds 4a-f with aqueous glyoxal afforded the corresponding trans-1-heteroaryl-3-phenyl-4,5-dihydroxy-2-imidazolidinones 5a-f (Scheme 1). The products 5a-f were purified by flash chromatography (FC) and characterized by spectroscopic techniques. Yields, melting points, reaction times and elemental analyses are presented in Table 1.
The mass spectra of compounds 5a-f exhibited medium intensity parent ions, while the radical cations of N-heteroaryl-N'-phenylureas 4a-f and heteroarylisocyanate cations appeared with high intensity. The 1H-NMR spectra of compounds 5a-f showed two well-resolved AB quartet spin systems corresponding to two chemically different CH-OH moieties. Based on the lack of coupling between two unequivalent methine protons, it seems likely that the hydroxyl groups are trans to each other. The other peaks of the spectra were those arising from the protons of two aromatic moieties. Upon addition of D2O to the NMR samples, the hydroxyl signals disappeared and the signals of the methine moieties quickly collapsed to two singlets.
Scheme 1. Formic acid catalized reaction of N-heteroaryl-N'-phenylureas 4a-f with aqueous glyoxal.
Scheme 1. Formic acid catalized reaction of N-heteroaryl-N'-phenylureas 4a-f with aqueous glyoxal.
Molecules 11 00768 g003
Table 1. Yields, melting points and elemental analyses of compounds 5a-f.
Table 1. Yields, melting points and elemental analyses of compounds 5a-f.
EntryArTime/hMp °CYield %Elemental Analysis (%) Calcd. (Found)
CHN
5a Molecules 11 00768 i0013160-1627051.98
(51.76)
3.97
(4.02)
15.16
(14.96)
5b Molecules 11 00768 i00210204-2069057.35
(57.17)
4.41
(4.45)
20.58
(20.43)
5c Molecules 11 00768 i0038164-1668057.35
(57.23)
4.41
(4.38)
20.58
(20.51)
5d Molecules 11 00768 i0040.5124-1268561.99
(61.78)
4.79
(4.90)
15.49
(15.38)
5e Molecules 11 00768 i0055173-1757561.99
(61.94)
4.79
(4.83)
15.49
(15.42)
5f Molecules 11 00768 i0064226-2286561.93
(61.90)
4.51
(4.60)
18.06
(17.95)
Physicochemical data show that the NH proton in benzimidazoles, as in imidazoles, migrates rapidly between the two nitrogen atoms (degenerate tautomerism) (Scheme 2). The imidazole ring with a nonsymmetrical substitution exhibits an HN1HN3 tautomerism that has been studied both experimentally and theoretically [22].
The 1H-NMR spectrum of 5f in DMSO-d6 at room temperature (25 °C) exhibited two rather sharp doublets at δ 7.48 and 7.50 ppm (J = 7.07 and 6.5 Hz), arising from CH-4' and CH-7' protons, each of them exhibiting a further doublet splitting due to the long range coupling with one of the benzimidazolyl hydrogens. Increasing the temperature results in coalescence of the two doublets (TC = 384 ± 1 K), as shown in Figure 2. Although no extensive line-shape analysis for 5f was undertaken, the variable temperature spectra allowed us to calculate the free energy barriers ∆G for the dynamic NMR process in 5f from coalescence of the CH-4' and CH-7' protons. By using the expression, k = π∆υ/, the first-order rate constant (k) for dynamic NMR effect in 5f was calculated as 31 s-1.
Scheme 2. Degenerate tautomerization of compound 5f.
Scheme 2. Degenerate tautomerization of compound 5f.
Molecules 11 00768 g004
Application of the absolute rate theory with a transmission coefficient of 1 gives the free energy of activation (∆G) as 81 ± 2 KJ mol-1 for compound 5f, where all known sources of errors are estimated and included [23]. The experimental data available are not suitable for obtaining meaningful values of ∆H and ∆S even though the errors in ∆G are not large [24].
Figure 2. Variable-temperature 500 MHz 1H-NMR spectra of the aromatic region of 5f in DMSO-d6.
Figure 2. Variable-temperature 500 MHz 1H-NMR spectra of the aromatic region of 5f in DMSO-d6.
Molecules 11 00768 g002
The 13C-NMR spectra of 5f also show 14 and 11 peaks at 298 and 384 K, respectively. This clearly indicates that the tautomers are interconverting fast at 384 K on the NMR time scale. No conversion occurred when we examined reaction of 4a-f with glyoxal in the presence of sodium hydroxide in different solvents. On the other hand, reaction of compounds 5a-f with sodium hydroxide regenerated the starting materials 4a-f. Imidazolidinones 5a-f are stable materials and can be stored at room temperature for extended periods.

Conclusions

In summary, the reaction between N-heteroaryl-N'-phenylureas 4a-f with aqueous glyoxal in acetonitrile under reflux conditions provides a simple one-pot entry into the synthesis of 1-heteroaryl-3-phenyl-4,5-dihydroxy-2-imidazilidinones 5a-f. Since scaling up of this easy method seems plausible, utilization of the procedure in industrial applications such as the preparation of textile finishing agents is conceivable. The free-energy barrier (∆G) for prototropic tautomerism in 1-(2-benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5f) was found to be 81 ± 2 KJ mol-1.

Experimental

General

All commercially available chemicals and reagents were used without further purification. Melting points were determined with an Electrothermal model 9100 apparatus and are uncorrected. IR spectra were recorded on a Shimadzu 4300 spectrophotometer. The 1H- and 13C-NMR spectra were recorded in DMSO-d6 on a DRX-500 AVANCE spectrometer at 298 K. Chemical shifts (δ) are reported in ppm and are referenced to the NMR solvent peak. Mass spectra of the products were obtained with a HP (Agilent Technologies) 5937 Mass Selective Detector. Elemental analyses were carried out with a Thermo Finnigan (FLASH 1112 SERIES EA) CHNS-O analyzer. Flash chromatography (FC) was carried out using silica gel 60 (63-200 mesh). Progress of the reactions was monitored by TLC using precoated sheets of silica gel Merck 60 F254 on aluminium.

General procedure for the synthesis of 1-heteroaryl-3-phenyl-4,5-dihydroxy-2-imidazolidinones 5a-f.

A stirring solution of N-heteroaryl-N'-phenylurea 4a-f (1 mmol), glyoxal (0.14 g of 40% aqueous solution, 1 mmol) and formic acid (0.005 g of 98% aqueous solution, 0.1 mmol) in acetonitrile (30 mL) was refluxed for the time given in Table 1. The solvent was then removed under reduced pressure and the crude material was purified by flash chromatography on silica gel, eluting with a 4:1 THF/hexane mixture to give a white crystalline product.
1-(2-Thiazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5a): Yield: 70%; m.p. 160-162 °C (from acetonitrile); IR (KBr): 3323 (OH), 3058, 2943, 1730 (C=O), 1596, 1487, 1402, 1280, 1139 cm-1; 1H‑NMR δ: 5.34 (d, 1H, J = 8.5 Hz, CH), 5.62 (d, 1H, J = 7.1 Hz, CH), 7.00 (d, 1H, J = 8.5 Hz, OH), 7.27 (d, 1H, J = 7.1 Hz, OH), 7.30 (d, 1H, J = 3.4 Hz, thiazole H-4), 7.50 (d, 1H, J = 3.4 Hz, thiazole H-5), 7.19, 7.42 and 7.68 (3m, 5H, Ar-H); 1H-NMR (DMSO-d6 + D2O) δ: 5.33 (s, 1H, CH), 5.61 (s, 1H, CH), 7.26 (d, 1H, J = 3.4 Hz, thiazole H-4), 7.48 (d, 1H, J = 3.4 Hz, thiazole H-5), 7.18, 7.41 and 7.64 (3m, 5H, Ar-H); 13C-NMR δ: 84.18, 86.35, 114.58, 121.21, 125.23, 129.77, 138.41, 138.60, 152.72, 158.13 ppm; MS (EI): m/z 277 (M+); Anal. Calcd for C12H11N3O3S: C, 51.98; H, 3.97; N, 15.16. Found: C, 51.76; H, 4.02; N, 14.96.
1-(2-Pyrimidinyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5b): Yield: 90%; m.p. 204-206 °C (from acetonitrile); IR (KBr): 3261 (OH), 3085, 2858, 1730 (C=O), 1569, 1456, 1380, 1299, 1060 cm-1; 1H‑NMR δ: 5.22 (d, 1H, J = 7.9 Hz, CH), 5.68 (d, 1H, J = 6.3 Hz, CH), 6.82 (d, 1H, J = 6.3 Hz, OH), 6.84 (d, 1H, J = 7.9 Hz, OH), 7.22 (t, 1H, J = 4.8 Hz, pyrimidine H-5), 7.16 and 7.39-7.68 (2m, 5H, Ar-H), 8.72 (d, 2H, J = 4.8 Hz, pyrimidine H-4, H-6); 1H-NMR (DMSO-d6 + D2O) δ: 5.22 (s, 1H, CH), 5.67 (s, 1H, CH), 7.21 (t, 1H, J = 4.8 Hz, pyrimidine H-5), 7.17 and 7.38-7.63 (2m, 5H, Ar-H), 8.69 (d, 2H, J = 4.8 Hz, pyrimidine H-4, H-6); 13C-NMR δ: 63.83, 84.80, 117.43, 121.63, 124.92, 129.61, 139.17, 151.46, 157.88, 159.14 ppm; MS (EI): m/z 272 (M+); Anal. Calcd for C13H12N4O3: C, 57.35; H, 4.41; N, 20.58. Found: C, 57.17; H, 4.45; N, 20.43.
1-(2-Pyrazinyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5c): Yield: 80%; m.p. 164-166 °C (from 1:1 THF-hexane); IR (KBr): 3400 (OH), 3066, 1708 (C=O), 1591, 1427, 1271, 1207, 1080 cm-1; 1H-NMR δ: 5.31 (d, 1H, J = 8.3 Hz, CH), 5.72 (d, 1H, J = 6.8 Hz, CH), 6.91 (d, 1H, J = 8.3 Hz, OH), 6.93 (d, 1H, J = 6.8 Hz, OH), 7.17-7.69 (m, 5H, Ar-H), 8.37-9.45 (m, 3H, pyrazine H-3, H-5, H-6); 1H-NMR (DMSO-d6 + D2O) δ: 5.30 (s, 1H, CH), 5.71 (s, 1H, CH), 7.18-7.65 (m, 5H, Ar-H), 8.35-9.40 (m, 3H, pyrazine H-3, H-5, H-6); 13C-NMR δ: 82.52, 85.74, 121.69, 125.23, 129.70, 136.63, 138.67, 139.70, 143.03, 148.71, 153.17 ppm; MS (EI): m/z 272 (M+); Anal. Calcd for C13H12N4O3: C, 57.35; H, 4.41; N, 20.58. Found: C, 57.23; H, 4.38; N, 20.51.
1-(2-Pyridinyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5d): Yield: 85%; m.p. 124-126 °C (from 1:1 THF-hexane); IR (KBr): 3290 (OH), 3066, 2962, 1724 (C=O), 1595, 1483, 1311, 1176, 1047 cm-1; 1H-NMR δ: 5.25 (d, 1H, J = 8.0 Hz, CH), 5.80 (d, 1H, J = 6.4 Hz, CH), 6.71 (d, 1H, J = 6.4 Hz, OH), 6.83 (d, 1H, J = 8.0 Hz, OH), 7.11-8.39 (m, 9H, Ar-H); 1H-NMR (DMSO-d6 + D2O) δ: 5.24 (s, 1H, CH), 5.79 (s, 1H, CH), 7.10-8.38 (m, 9H, Ar-H); 13C-NMR δ: 82.78, 85.14, 113.96, 119.63, 121.42, 124.83, 129.62, 138.86, 139.10, 148.55, 151.90, 153.53 ppm; MS (EI): m/z 271 (M+); Anal. Calcd for C14H13N3O3: C, 61.99; H, 4.79; N 15.49. Found: C, 61.78; H, 4.90; N, 15.38.
1-(3-Pyridinyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5e): Yield: 75%; m.p. 173-175 °C (from acetonitrile); IR (KBr): 3336 (OH), 3043, 2916, 1695 (C=O), 1581, 1490, 1413, 1271, 1070cm-1; 1H-NMR δ: 5.29 (d, 1H, J = 8.3 Hz, CH), 5.34 (d, 1H, J = 8.1 Hz, CH), 6.88 (d, 1H, J = 8.3 Hz, OH), 6.96 (d, 1H, J = 8.1 Hz, OH), 7.13-8.90 (m, 9H, Ar-H); 1H-NMR (DMSO-d6 + D2O) δ: 5.28 (s, 1H, CH), 5.33 (s, 1H, CH), 7.14-8.88 (m, 9H, Ar-H); 13C-NMR δ: 84.84, 85.61, 121.20, 124.42, 124.70, 127.69, 129.61, 136.08, 139.09, 142.15, 145.18, 153.84 ppm; MS (EI): m/z 271 (M+); Anal. Calcd. for C14H13N3O3: C, 61.99; H, 4.79; N, 15.49. Found: C, 61.94; H, 4.83; N, 15.42.
1-(2-Benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone (5f): Yield: 65%; m.p. 226-228 °C (from acetonitrile); IR (KBr): 3498, 3400, 3288, 3041, 2960, 1722 (C=O), 1627, 1568, 1498, 1460, 1271, 1147 cm-1; 1H-NMR δ: 5.35 (d, 1H, J = 8.5 Hz, CH), 5.65 (d, 1H, J = 6.9 Hz, CH), 7.02 (d, 1H, J = 8.5 Hz, OH), 7.32 (d, 1H, J = 6.9 Hz, OH), 7.20, 7.44 and 7.72 (3m, 5H, Ar-H), 7.10 and 7.50 (2m, 4H, benzimidazole-H), 12.05 (s, 1H, NH); 1H-NMR (DMSO-d6 + D2O) δ: 5.35 (s, 1H, CH), 5.65 (s, 1H, CH), 7.21, 7.43 and 7.66 (3m, 5H, Ar-H), 7.12 and 7.49 (2m, 4H, benzimidazole-H); 13C-NMR (298 K) δ: 83.61, 86.40, 112.25, 117.73, 121.41, 121.58, 122.03, 125.26, 129.78, 134.08, 138.52, 141.62, 146.36, 152.97; 13C NMR (384 K) δ: 84.16, 86.81, 112.55, 115.23, 121.80, 122.29, 125.44, 129.55, 138.74, 146.83, 153.42 ppm; MS (EI): m/z 310 (M+); Anal. Calcd for C16H14N4O3: C, 61.93; H, 4.51; N, 18.06. Found: C, 61.90; H, 4.60; N, 17.95.

Acknowledgements

The authors wish to thank the Research Council of the University of Tehran for financial support.

References and Notes

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  • Sample Availability: Samples of compounds 5a-f are available from the authors.

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MDPI and ACS Style

Ghandi, M.; Olyaei, A.; Salimi, F. Synthesis of New Unsymmetrical 4,5-Dihydroxy-2-imidazolidinones. Dynamic NMR Spectroscopic Study of the Prototropic Tautomerism in 1-(2-Benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone. Molecules 2006, 11, 768-775. https://doi.org/10.3390/11100768

AMA Style

Ghandi M, Olyaei A, Salimi F. Synthesis of New Unsymmetrical 4,5-Dihydroxy-2-imidazolidinones. Dynamic NMR Spectroscopic Study of the Prototropic Tautomerism in 1-(2-Benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone. Molecules. 2006; 11(10):768-775. https://doi.org/10.3390/11100768

Chicago/Turabian Style

Ghandi, Mehdi, Abolfazl Olyaei, and Farshid Salimi. 2006. "Synthesis of New Unsymmetrical 4,5-Dihydroxy-2-imidazolidinones. Dynamic NMR Spectroscopic Study of the Prototropic Tautomerism in 1-(2-Benzimidazolyl)-3-phenyl-4,5-dihydroxy-2-imidazolidinone" Molecules 11, no. 10: 768-775. https://doi.org/10.3390/11100768

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