Novel, Mild and Chemoselective Dehydrogenation of 2-Imidazolines with Trichloroisocyanuric Acid

Abstract

A rapid, mild and high-yielding method for dehydrogenation of a variety of structurally diverse 2-imidazolines to imidazoles using trichloroisocyanuric acid (TCCA) in the presence of DBU is reported. Chemoselective oxidation of 2-imidazolines can be achieved in the presence of sulfide and alcohol. The mild conditions of this procedure and the absence of any transition metal make this reaction suitable for safe laboratory use.

Key words

2-imidazolines - imidazoles - trichloroisocyanuric acid - dehydrogenation

References

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12

Typical Experimental Procedure: 2-(4-Methyl-phenyl)imidazoline (1b, 2.0 mmol) was dissolved in MeCN (40 mL) and DBU (0.54 g, 3.6 mmol) was added. The solution was cooled to -15 °C and a solution of TCCA (0.185 g, 0.8 mmol) in MeCN (10 mL) was added dropwise. The reaction mixture was stirred for 3 min. After completion of the reaction (as monitored by TLC), the mixture was allowed to warm up to r.t. and passed through a short pad of alumina. The solvent was evaporated and the resulting crude material was purified by column chromatography on alumina with appropriate eluent to afford the pure 2-(4-methylphenyl)imidazole (2b, 94%), mp 218-220 °C (lit. [9] mp 223 ºC). IR (KBr): 3435, 3150, 2985, 1658, 1610, 1575, 1514, 1435, 1100, 815, 725 cm^-1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 2.31 (s, 3 H), 6.99 (br s, 1 H), 7.18 (br s, 1 H), 7.23 (d, J = 7.5 Hz, 2 H), 7.81 (d, J = 8 Hz, 2 H), 12.39 (s, 1 H).