Application of Triphenylammonium Tricyanomethanide as an Efficient and Recyclable Nanostructured Molten-Salt Catalyst for the Synthesis of N-Benzylidene-2-arylimidazo[1,2-a]pyridin-3-amines

Abstract

Triphenylammonium tricyanomethanide (Ph₃NH)[C(CN)₃] was synthesized and used as an efficient and recyclable nanostructured molten-salt (NMS) catalyst for the synthesis of N-benzylidene-2-arylimidazo[1,2-a]pyridin-3-amines by the reaction of various (het)aryl aldehydes with trimethylsilyl cyanide and pyridin-2-amine under solvent-free conditions at 50 °C. The NMS catalyst could be simply recycled and reused several times without significant loss of its catalytic activity.

Key words

triphenylammonium tricyanomethanide - imidazopyridines - nanostructured molten salt - aldehydes - pyridinamine - organocatalysis

• References

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• 32 Triphenylammonium Tricyanomethanide (Ph₃NH)[C(CN)₃] Ph₃N (3 mmol, 0.736 g) was added to a solution of HC(CN)₃ (3 mmol, 0.273 g) in H₂O (5 mL), and the mixture was stirred for 3 h at 50 °C. The solvent was then evaporated under reduced pressure and the resulting white powder was dried under reduced pressure at 100 °C for 3 h. The white solid was washed with Et₂O to remove any unreacted starting materials and then dried under vacuum to give a white solid; yield: 0.979 g (97%); mp >350 °C. IR (KBr): 3377, 3037, 2987, 2228, 2110, 1639, 1474, 1389, 1062, 887 cm^–1. ¹H NMR (400 MHz, DMSO-d ₆): δ = 6.99 (d, J = 8.4 Hz, 6 H, ArH), 7.04 (t, J = 7.4 Hz, 3 H, ArH), 7.31 (t, J = 8.0 Hz, 6 H, ArH), 8.55 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 41.9, 123.3, 124.1, 129.9, 166.4, 166.6. MS: m/z = 336.3 [M⁺]. For full analytical data, see the Supporting Information. N-benzylidene-2-arylimidazo[1,2-a]pyridin-3-amines 4; General Procedure (Ph₃NH)[C(CN)₃] (1 mol%; 0.0034 g) was added to and mixed with aldehyde 1 (2 mmol), TMSCN (2; 2 mmol; 0.099 g), and pyridin-2-amine (3; 1 mmol; 0.094 g), and the mixture was heated at 50 °C for the appropriate time (Table 3). When the reaction was complete [TLC, hexane–EtOAc (5:2)], the mixture was washed with H₂O (10 mL) and filtered to separate the catalyst from the other materials. (The reaction mixture was insoluble in H₂O, and the catalyst was soluble in H₂O.) The crude product was purified by crystallization from EtOH–H₂O (10:1). All products were identified by comparison of their physical data with those reported in the literature; for spectroscopic data, see the Supporting Information.