One-pot synthesis of 2 , 4 , 5-tri-substituted-1 H-imidazoles promoted by trichloromelamine

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N-halo reagents with high active N-X bond have the potential to promote some important reactions.Trichloromelamine (TCM) as a common N-halo reagent is used as bleaching agents, disinfectants and bactericides and promoted some organic reactions such as N-nitrosation of secondary amines 22 , oxidation of alcohols 23,24 , and trimethylsilylation of hydroxyl group 25 .

Results and disscusion
In continuation of our investigations on solid acids in organic synthesis [26][27][28][29][30] , we report here on applying trichloromelamine as an efficient Cl + catalyst source for synthesis of 2,4,5-trisubstituted imidazoles via reaction of benzil, aldehydes and ammonium acetate.For investigation the efficiency of trichloromelamine in preparation of 2,4,5-trisubstituted imidazoles, we have examined the reaction of benzaldehyde (1 mmol), benzil (1 mmol) and ammonium acetate (2 mmol) as model reaction.The reaction in different conditions in the presence of trichloromelamine revealed that the best condition was 0.07 g of trichloromelamine under solvent-free condition at 110 °C (Entry 2, Table 1).Benzil, ammonium acetate and various aldehydes were then used as substrates for the synthesis of 2,4,5-tri substituted imidazoles (Table 2, scheme 1).Most of the products were known and identified by spectroscopic data and comparison with literature.The presence of one broad singlet signal in δ=7.22 ppm, proves the formation of 4,5-diphenyl imidazole.According to our investigation about the mechanism of 2,4,5-trisubstituted imidazole synthesis in the presence of trichloromelamine, we have found that one or two NHCl group in this reagent converts to NH 2 group.The reusability of trichloromelamine in this reaction is 2 or 3 times.Thus, we have proposed a mechanism for preparation of 2,4,5-trisubstituted imidazole in the presence of trichloromelamine as positive chlorine catalyst in scheme 3.

Conclusion
In conclusion, we have demonstrated a simple method for the synthesis of 2,4,5-trisubstituted imidazoles using trichloromelamine as an eco-friendly, inexpensive, and efficient reagent.Short reaction times, high yield, simplicity of operation and easy work-up are some advantages of this method.

Experimental
The chemicals were used without any additional purification.The products were characterized by FT-IR (ATR), 1 H-NMR, and a comparison of their physical properties with those reported in the literature.FT-IR (ATR) spectra were run on a Bruker, Eqinox 55 spectrometer.A Bruker (DRX-400 Avance) NMR was used to record the 1 H-NMR spectra.Elemental analyses were done by Costech ECS 4010 CHNS-O analyser.

General procedures for the synthesis of 2,4,5-trisubstituted imidazoles in the presence of trichloromelamine
A mixture of benzil (1 mmol), aldehyde (1 mmol), ammonium acetate (2 mmol) and trichloromelamine (0.07 g) was heated with stirring at 110 °C.After completion of the reaction (monitored by TLC), acetone was added to the cold reaction mixture and the catalyst was recovered by filtration and washed with acetone (2 × 5 ml).By adding water to the filtrate, a milky to yellow solid was obtained.The dried solid was washed with hot n-hexane to obtain highly pure product.Most of the products were known and identified by comparison of their physical and spectral data with those of authentic samples.(m, 10 H, Ar-C-H), 2.94 (sep, 3 J = 6.9 Hz, 1 H, C-H), 1.25 (d, 3 J = 6.9 Hz, 6 H, 2CH 3 ) ppm.Elemental analysis.Found, %: С 85.03; Н 6.61; N 8.36.С 24 H 22 N 2 .Calculated, %: С 85.17; Н 6.55; N 8.28,

Scheme 3 .
Scheme 3. A proposed mechanism for the synthesis of 2,4,5-trisubstituted imidazoles in the presence of TCM.