chlorobenzaldehyde and 2,4-dichlorobenzaldehyde used in the preparation of benzoins22 were commercially available and were used after purification.
General procedures for the synthesis of 2,3-diphenylquinoxaline(4a). Thermal method
A mixture of benzoin (0.42 g, 2 mmol), o-phenylenediamine (0.22 g, 2 mmol) and 10 mL of glacial acetic acid was heated to reflux. After 5 minutes a brown solution was observed; the progress of the reaction was monitored by TLC, and reflux was continued for 2 h. After cooling to room temperature, the reaction mixture was poured into cold water, extracted with dichloromethane (3×10 mL), dried with anhydrous calcium chloride, and then the solvent was evaporated under reduced pressure to afford the crude product. 2,3-Diphenylquinoxaline 4a was obtained as white needles after recrystalisation from ethanol. (0.55 g, m.p. 127-128 °C [reported m.p. 128-12 9°C]5b,16e,i,k, yield 98%), IR (KBr) (. max, cm-1): 3055, 1542, 1495, 1448, 1355. 1H NMR (CDC13, 500 MHz) d 7.33-7.37 (m, 3H), 7.56 (dd, 2H, J = 7.8 Hz, 1.2 Hz), 7.76 (dd, 1H, J = 6.3 Hz, 3.4 Hz), 8.2 (dd, 1H, J = 6.3 Hz, 3.4 Hz) ppm. 13C NMR (125 MHz, CDCl3) d 128.27 (CH), 128.81 (CH), 129.25 (CH), 129.91 (CH), 129.92 (CH), 139.18 (C), 141.28 (C), 153.46 (C) ppm.
Microwave irradiation method
A mixture of benzoin (0.42 g, 2 mmol), o-phenylenediamine (0.22 g, 2 mmol), and 2 mL glacial acetic acid was irradiated in a microwave oven at a power output of 450 W. After 10 seconds, a brown solution was observed; the progress of the reaction was monitored by TLC. After completion of the reaction (upon three minutes), the reaction mixture was poured into cold water, extracted with dichloromethane (3×10 mL), the extract dried with calcium chloride, and then the solvent was evaporated under reduced pressure to afford the crude product. 2,3-Diphenylquinoxaline 4a was obtained as white needles after recrystalisation from ethanol. (0.54 g, m.p. 127-128°C [reported m.p. 128-129°C]5b,16e,i,k, yield 96%).
2,3-Di(4-methoxyphenyl)quinoxaline (4b). (0.60 g, m.p. 151-152 °C [reported m.p. 151- 152.5 °C],5b,16e,i,k,17 yield 88%), IR (KBr) (. max, cm-1): 3056, 2957, 1617, 1517, 1448, 1355. 1H NMR (500 MHz, CDCl3) d 3.83 (s, 3H, OCH3), 6.89 (d, 2H, J = 8.3 Hz), 7.52 (d, 2H, J = 8.3 Hz), 7.72 (dd, 1H, J = 6.1 Hz, 3.2 Hz), 8.14 (dd, 1H, J = 6.05 Hz, 3.2 Hz) ppm. 13C NMR (125 MHz, CDCl3) d 55.30 (OCH3), 113.79 (CH), 129.03 (CH), 129.49 (CH), 131.29 (CH), 131.79 (C), 141.09 (C), 153.00 (C), 160.21(C) ppm.
2,3-Di-p-tolylquinoxaline (4c). (0.57 g, m.p. 149-150 °C [reported m.p. 145-146 °C],14,16j,16m yield 91%), IR (KBr) (. max, cm-1): 3031, 2958, 1617, 1517, 1448, 1355. 1H NMR (500 MHz, CDCl3) d 2.40 (s, 3H, CH3), 7.18 (d, 2H, J = 7.9 Hz), 7.48 (d, 2H, J = 7.9 Hz), 7.76 (dd, 1H, J = 6.3 Hz, 3.4 Hz), 8.19 (dd, 1H, J = 6.3 Hz, 3.4 Hz) ppm. 13C NMR (125 MHz, CDCl3) d 21.36 (CH3), 128.99 (CH), 129.15 (CH), 129.64 (CH), 129.78 (CH), 136.47 (C), 138.74 (C), 141.20 (C), 153.49(C) ppm.
4-(2-(4-Chlorophenyl)quinoxalin-3-yl)-N,N-dimethylbenzenamine (4d). (0.56 g, m.p. 168-170 °C, yield 83%), IR (KBr) (. max, cm-1): 3056, 2932, 1617, 1542, 1448, 1355. 1H NMR (500 MHz, CDCl3) d 3.01 (s, 3H, 2CH3), 6.67 (dd, 1H, J = 10.0 Hz, 2.5 Hz), 7.37 (dd, 1H, J = 5.0 Hz, 2.5 Hz), 7.46 (dd, 1H, J = 5.0 Hz, 2.5 Hz), 7.57 (dd, 1H, J = 10.0 Hz, 2.5 Hz), 7.71-7.75 (m, 1H), 8.11-8.14 (m, CH) ppm. 13C NMR (125 MHz, CDCl3) d 40.00 (CH3),
