All reagents and solvents were procured from Merck (Darmstadt, Germany) and employed without additional purification. Proton nuclear magnetic resonance (1H-NMR) spectra were acquired using a 400 MHz Bruker spectrometer, with tetramethylsilane (TMS) as the internal standard. Chemical shifts were expressed as (δ = ppm), and CDCl3 and dimethyl sulfoxide-d6 (DMSO-d6) were utilized as solvents. Positive electrospray ionization (ESI) mass spectra were recorded on an Agilent 6410 triple quadrupole mass spectrometer. Infrared (IR) spectra were obtained using a Perkin Elmer IR spectrophotometer, with values reported in cm-1 and measured on potassium bromide discs. Melting points of the compounds were determined via the capillary method using an Electrothermal 9100 melting point apparatus, and the values provided were uncorrected. Thin layer chromatography (TLC) was conducted on silica gel (60) F254 Merck plates (Germany) and visualized under UV 254 nm light. Elemental analysis for carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) was performed using a Costech 4010 elemental analyzer (Milan, Italy). For all the compounds, the calculated values closely matched the measured values within a margin of 0.4%.
3.1.2. General Procedure for the Synthesis of Phenyl Thiosemicarbazone Analogs (2a-m)
The phenyl thiosemicarbazones 2a-m were synthesized via the procedure reported in the previous studies (
26). In summary, a mixture of 1 (1 mmol) and appropriate aromatic aldehydes in absolute ethanol was stirred for 24 hours at RT. A catalytic increment of HCl (37%) was added to accelerate the reaction progress. The obtained precipitate was collected by filtration. The final products 2a-m were recrystallized from absolute ethanol.
(E)-2-benzylidene-N-phenylhydrazinecarbothioamide (2a): White solid. Yield: (80 %); mp: 190 - 191°C; 1H-NMR (400 MHz, CDCl3): δ = 7.27 (t, J = 8.0 Hz, 1H, H4a), 7.44-7.40 (m, 5H, H3a, H5a, H3b, H5b,H4b), 7.69-7.66 (m, 4H, H2a, H6a, H2b, H6b), 7.98 (s, 1H, HC=N), 9.22 (s, 1H, NH), 10.34 (s, 1H, NH); IR (KBr): ϋ = 3296, 3150 (NH), 1582 (C=N), 1201 (C=S) cm-1; ESI-MS m/z [M+H+] = 256.0, [M+Na+] = 277.9; Anal. Calcd for C14H13N3S: C, 65.85; H, 5.13; N, 16.46; S, 12.56; found: C, 64.77; H, 5.12; N, 16.45; S, 12.57.
(E)–2-(4-methylbenzylidene)-N-phenylhydrazinecarbothioamide (2b): White solid. Yield: (86%); mp: 218 - 220°C; 1H-NMR (400 MHz, CDCl3): δ = 2.39 (s, 3H, CH3), 7.22 - 7.28 (m, 3H, HC4a, H3b, H5b), 7.42 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.57 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.67 (d, J = 8.0 Hz, 2H, H2b, H6b), 7.9 (s, 1H, HC=N), 9.20 (s, 1H, NH), 9.87 - 9.97 (m, 1H, NH); IR (KBr): ϋ = 3330, 3145 (NH), 1596 (C=N), 1199 (C=S) cm-1; ESI-MS m/z [M+H+] = 270.0, [M+Na+] = 291.8; Anal. Calcd for C15H15N3S: C, 66.88; H, 5.61; N, 15.60; S, 11.90; found: C, 65.22; H, 5.62; N, 15.62; S, 11.91.
(E)-2-(4-bromobenzylidene)-N-phenylhydrazinecarbothioamide (2c): White solid. Yield: (90 %); mp: 229-230°C; IR (KBr): ϋ= 3336, 3299 (NH), 1593 (C=N), 1265 (C=S) cm-1; ESI-MS m/z [M+H+] = 333.9, 335.9, [M+Na+] = 355.9, 357.9; Anal. Calcd for C14H12BrN3S: C, 50.31; H, 3.62; N, 12.57; S, 9.59; found: C, 51.02; H, 3.63; N, 12.56; S, 9.60.
(E)-2-(2-chlorobenzylidene)-N-phenylhydrazinecarbothioamide (2d): Cream-colored solid. Yield: (46%); mp: 188 - 189°C; 1H-NMR (400 MHz, CDCl3): δ = 7.24-7.44 (m, 6H, H3a, H4a, H5a, H3b, H4b, H5b), 7.68 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.95 (dd, J = 8.0 Hz, J = 4.0 Hz, 1H, H6b), 8.30 (s, 1H, HC=N), 9.19 (s, 1H, NH), 9.48 (s, 1H, NH); IR (KBr): ϋ = 3287, 3136 (NH), 1591 (C=N), 1191 (C=S) cm-1; ESI-MS m/z [M+H+] = 289.9, [M+Na+] = 311.8; Anal. Calcd for C14H12ClN3S: C, 58.03; H, 4.17; N, 14.50; S, 11.07; found: C, 56.25; H, 4.16; N, 14.51; S, 11.06.
(E)-2-(4-fluorobenzylidene)- N-phenylhydrazinecarbothioamide (2e): White solid. Yield: (49%); mp: 199 - 200°C; 1H-NMR (400 MHz, CDCl3): δ = 7.12 (t, J = 8.0 Hz, 1H, H4a), 7.27 (t, J = 8.0 Hz, 2H, H3b, H5b), 7.42 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.65 - 7.70 (m, 4H, H2a, H6a, H2b, H6b), 7.92 (s, 1H, HC=N), 9.16 (s, 1H, NH), 10.00 (s, 1H, NH); IR (KBr): ϋ= 3291, 3229 (NH), 1634 (C=N), 1235 (C=S) cm-1; ESI-MS m/z [M+H+] = 274; Anal. Calcd for C14H12FN3S: C, 61.52; H, 4.43; N, 15.37; S, 11.73; found: C, 60.53; H, 4.41; N, 15.36; S, 11.74.
(E)-2-(4-methoxybenzylidene)-N-phenylhydrazinecarbothioamide (2f): White solid. Yield: (52%); mp: 177 - 178°C; 1H-NMR (400 MHz, CDCl3): δ = 3.85 (s, 3H, OCH3), 7.26 (t, J = 8.0 Hz, 1H, H4a), 7.41 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.62 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.67 (d, J = 8.0 Hz, 2H, H2b, H6b), 7.92 (s, 1H, HC=N), 7.93 (d, J = 8.0 Hz, 2H, H3b, H5b), 9.19 (s, 1H, NH), 10.16 (s, 1H, NH); IR (KBr): ϋ = 3323, 3151 (NH), 1607 (C=N), 1205 (C=S) cm-1; ESI-MS m/z [M+H+] = 285.8; Anal. Calcd for C15H15N3OS: C, 63.13; H, 5.30; N, 14.73; S, 11.24; found: C, 62.98; H, 5.32; N, 14.71; S, 11.23.
(E)–2-(3-methoxybenzylidene)–N-phenylhydrazinecarbothioamide (2g): Yellow solid. Yield: (70%); mp: 154 - 155°C; 1H-NMR (400 MHz, CDCl3): δ = 3.84 (s, 3H, OCH3), 6.96 (dd, J = 8.0 Hz, J=4 Hz, 1H, H4b), 7.20-7.34 (m, 4H, H4a, H2b, H5b, H6b), 7.42 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.65 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.97 (s, 1H, HC=N), 9.20 (s, 1H, NH), 10.64 (s, 1H, NH); IR (KBr): ϋ = 3327, 3150 (NH), 1596 (C=N), 1280 (C=S) cm-1; ESI-MS m/z [M+H+] = 285.9; Anal. Calcd for C15H15N3OS: C, 63.13; H, 5.30; N, 14.73; S, 11.24; found: C, 62.98; H, 5.32; N, 14.71; S, 11.23.
(E)-2-(4-chlorobenzylidene)-N-phenylhydrazinecarbothioamide (2h): White solid. Yield: (57 %); mp: 199-200°C; IR (KBr): ϋ = 3302, 3122 (NH), 1587 (C=N), 1194 (C=S) cm-1; ESI-MS m/z [M+H+] = 289.8; Anal. Calcd for C14H12ClN3S: C, 58.03; H, 4.17; N, 14.50; S, 11.07; found: C, 57.33; H, 4.15; N, 14.52; S, 11.08.
(E)-2-(3-cyanobenzylidene)-N-phenylhydrazinecarbothioamide (2i): White solid. Yield: (63 %); mp: 197-198°C; IR (KBr): ϋ = 3285, 3177 (NH), 2237 (CN), 1542 (C=N), 1205 (C=S) cm-1; ESI-MS m/z [M+H+] = 281.0, [M+Na+] = 302.8; Anal. Calcd for C15H12N4S: C, 64.26; H, 4.31; N, 19.98; S, 11.44; found: C, 65.31; H, 4.30; N, 19.99; S, 11.45.
(E)-2-(4-nitrobenzylidene)-N-phenylhydrazinecarbothioamide (2j): White solid. Yield: (77 %); mp: 233-234°C; IR (KBr): ϋ = 3227, 3170 (NH), 1584 (C=N), 1521, 1339 (NO2), 1196 (C=S) cm-1; ESI-MS m/z [M+H+] = 300.8; Anal. Calcd for C14H12N4O2S: C, 55.99; H, 4.03; N, 18.65; S, 10.68; found: C, 55.50; H, 4.02; N, 18.63; S, 10.69.
(E)–2-(3-hydroxybenzylidene)-N-phenylhydrazinecarbothioamide (2k): White solid. Yield: (55 %); mp: 186-188°C; IR (KBr): ϋ = 3386, 3310 (NH), 1577 (C=N), 1161 (C=S) cm-1; ESI-MS m/z [M+H+] = 272.0; Anal. Calcd for C14H13N3OS: C, 61.97; H, 4.83; N, 15.49; S, 11.82; found: C, 60.71; H, 4.84; N, 15.50; S, 11.80.
(E)-N-phenyl-2-(4-(trifluoromethyl)benzylidene) hydrazinecarbothioamide (2l): White solid. Yield: (58.0 %); mp: 194-195 °C; IR (KBr): ϋ = 3200, 3060 (NH), 1609 (C=N), 1176 (C=S) cm-1; ESI-MS m/z [M+H+] = 324.2, [M+Na+] = 346.2; Anal. Calcd for C15H12F3N3S: C, 55.72; H, 3.74; N, 13.00; S, 9.92; found: 56.56; H, 3.72; N, 13.02; S, 9.93.
(E)-2-(3-bromobenzylidene)-N-phenylhydrazinecarbothioamide (2m): Yellow solid. Yield: (63 %); mp: 196-197°C; IR (KBr): ϋ = 3336, 3299 (NH), 1591 (C=N), 1214 (C=S) cm-1; ESI-MS m/z [M+H+] = 333.9, 335.9; Anal. Calcd for C14H12BrN3S: C, 50.31; H, 3.62; N, 12.57; S, 9.59; found: C, 50.36; H, 3.61; N, 12.55; S, 9.61.
3.1.3. General Procedure for the Synthesis of Derivatives 3a-m
A solution of DDQ (0.53 g, 2.3 mmol) in acetonitrile was added dropwise to a solution of intermediates 2a-m (2.3 mmol) in the same solvent. The mixture was stirred in the RT overnight. The obtained precipitates 3a-m were filtered off and recrystallized from the absolute ethanol.
N-5-diphenyl-1,3,4-thiadiazol-2-amine (3a): White solid. Yield: (75%); mp: 199 - 200°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.03 (t, 1H, J = 8.0 Hz, H4a), 7.37 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.52 (m, 3H, H3b, H4b, H5b), 7.66 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.87 (m, 2H, H2b, H6b), 10.56 (s, 1H, NH); IR (KBr): ϋ= 3252, 3198 (NH), 1610 (C=N), 695 (C-S) cm-1; ESI-MS m/z [M+H+] = 254.2; Anal. Calcd for: C14H11N3S: C, 66.38; H, 4.38; N, 16.59; S, 12.66; found: C, 65.42; H, 4.39; N, 16.58; S, 12.67.
N-phenyl-5-(p-tolyl)-1,3,4-thiadiazol-2-amine (3b): White solid. Yield: (72%); mp: 219 - 220°C; 1H-NMR (400 MHz, DMSO-d6): δ = 2.37 (s, 3H, CH3), 7.02 (t, J = 8.0 Hz, 1H, H4a), 7.34 (m, 4H, H3b, H5b, H3a, H5a), 7.65 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.75 (d, J = 8.0 Hz, 2H, H2b, H6b), 10.52 (s, 1H, NH); IR (KBr): ϋ = 3254, 3214 (NH), 1626, 1605 (C=N), 674 (C-S) cm-1; ESI-MS m/z [M+H+] = 268.2; Anal. Calcd for: C15H13N3S: C, 67.39; H, 4.90; N, 15.72; S, 11.99; found: C, 65.87; H, 4.91; N, 15.73; S, 11.98.
5-(4-bromophenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3c): White solid. Yield: (70%); mp: 230 - 231°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.03 (t, J = 7.6 Hz, 1H, H4a), 7.37 (t, 2H, J = 7.6 Hz, H3a, H5a), 7.66 (d, 2H, J = 7.6 Hz, H2a, H6a), 7.72 (d, 2H, J = 8.4 Hz, H3b, H5b), 7.82 (d, 2H, J = 8.8 Hz, H2b, H6b), 10.61 (s, 1H, NH); IR (KBr): ϋ = 3259, 3209 (NH), 1618, 1605 (C=N), 668 (C-S) cm-1; ESI-MS m/z [M+H+] = 332.1, 334.1; Anal. Calcd for: C14H10BrN3S: C, 50.62; H, 3.03; N, 12.65; S, 9.65; found: C, 51.55; H, 3.02; N, 12.67; S, 9.66.
5-(2-chlorophenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3d): White solid. Yield: (68%); mp: 225 - 226°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.04 (t, J = 8.0 Hz, 1H, H4a), 7.38 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.53 (m, 2H, H4b, H5b), 7.67 (m, 3H, H2a, H6a, H6b), 8.09 (m, 1H, H3b), 10.59 (s, 1H, NH); IR (KBr): ϋ = 3261, 3203 (NH), 1622, 1604 (C=N), 671 (C-S) cm-1; ESI-MS m/z [M+H+] = 288.1; Anal. Calcd for: C14H10ClN3S: C, 58.43; H, 3.50; N, 14.60; S, 11.14; found: C, 56.59; H, 3.49; N, 14.61; S, 11.15.
5-(4-fluorophenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3e): White solid. Yield: (60%); mp: 199 - 200°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.03 (t, 1H, J = 7.2 Hz, H4a), 7.36 (m, 4H, H2a, H3a, H5a, H6a), 7.66 (d, J = 7.6 Hz, 2H, H3b, H5b), 7.92 (m, 2H, H2b, H6b), 10.56 (s, 1H, NH); IR (KBr): ϋ = 3291, 3229 (NH), 1634, 1614 (C=N), 683 (C-S) cm-1; ESI-MS m/z [M+H+] = 272.2; Anal. Calcd for: C14H10FN3S: C, 61.98; H, 3.72; N, 15.49; S, 11.82; found: C, 60.24; H, 3.71; N, 15.50; S, 11.81.
5-(4-methoxyphenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3f): White solid. Yield: (68%); mp: 217 - 220°C; 1H-NMR (400 MHz, DMSO-d6): δ = 3.83 (s, 3H, OCH3), 7.01 (t, J = 8.0 Hz, 1H, H4a), 7.07 (d, J = 8.0 Hz, 2H, H3b, H5b), 7.36 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.65 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.8 (d, J = 8.0 Hz, 2H, H2b, H6b), 10.47 (s, 1H, NH); IR (KBr): ϋ = 3270, 3122 (NH), 1628, 1609 (C=N), 691 (C-S) cm-1; ESI-MS m/z [M+H+] = 283.8; Anal. Calcd for: C15H13N3OS: C, 63.58; H, 4.62; N, 14.83; S, 11.32; found: C, 62.78; H, 4.63; N, 14.82; S, 11.30.
5-(3-methoxyphenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3g): White solid. Yield: (71%); mp: 169 - 170°C; 1H-NMR (400 MHz, DMSO-d6): δ = 3.84 (s, 3H, OCH3), 7.05 (m, 2H, H4b, H4a), 7.41 (m, 5H, H3a, H5a, H2b, H5b, H6b), 7.66 (d, J = 8.0 Hz, 2H, H2a, H6a), 10.58 (s, 1H, NH); IR (KBr): ϋ = 3280, 3224 (NH), 1628, 1609 (C=N), 661 (C-S) cm-1; ESI-MS m/z [M-H-] = 282.2; Anal. Calcd for: C15H13N3OS: C, 63.58; H, 4.62; N, 14.83; S, 11.32; found: C, 64.20; H, 4.61; N, 14.80; S, 11.29.
5-(4-chlorophenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3h): White solid. Yield: (73%); mp: 119 - 220°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.04 (t, J = 8.0 Hz, 1H, H4a), 7.38 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.58 (d, J = 8.0 Hz, 2H, H3b, H5b), 7.66 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.89 (d, J = 8.0 Hz, 2H, H2b, H6b), 10.61 (s, 1H, NH); IR (KBr): ϋ = 3360, 3228 (NH), 1606, 1592 (C=N), 660 (C-S) cm-1; ESI-MS m/z [M-H-] = 286.1; Anal. Calcd for: C14H10ClN3S: C, 58.43; H, 3.50; N, 14.60; S, 11.14; found: C, 57.74; H, 3.51; N, 14.61; S, 11.15.
3-(5-(phenylamino)-1,3,4-thiadiazol-2-yl)benzonitrile (3i): White solid. Yield: (65%); mp: 219 - 220°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.049 (t, J = 7.6 Hz, 1H, H4a), 7.38 (t, J = 8.4 Hz, 2H, H3a, H5a), 7.67 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.72 (t, J = 8.0 Hz, 1H, H5b), 7.96 (d, J = 8.0 Hz, 1H, H4b), 8.21 (d, J = 8.0 Hz,1H, H6b), 8.30 (s, 1H, H2b), 10.69 (s, 1H, NH); IR (KBr): ϋ = 3258, 3137 (NH), 2237 (CN), 1542, 1512 (C=N), 681 (C-S) cm-1; ESI-MS m/z [M+H+] = 279.1; Anal. Calcd for: C15H10N4S: C, 64.73; H, 3.62; N, 20.13; S, 11.52; found: C, 63.69; H, 3.61; N, 20.12; S, 11.53.
5-(4-nitrophenyl)–N-phenyl-1,3,4-thiadiazol-2-amine (3j): White solid. Yield: (68%); mp: 219 - 220°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.06 (t, 1H, J = 7.2 Hz, H4a), 7.39 (t, J = 7.6 Hz, 2H, H3a, H5a), 7.67 (d, J = 8.0 Hz, 2H, H2a, H6a), 8.12 (d, J = 8.4 Hz, 2H, H3b, H5b), 8.33 (d, J = 8.8 Hz, 2H, H2b, H6b), 10.78 (s, 1H, NH); IR (KBr): ϋ = 3332, 3106 (NH), 1589, 1570 (C=N), 1530, 1334 (NO2), 622 (C-S) cm-1; ESI-MS m/z [M+H+] = 299.1; Anal. Calcd for: C14H10N4O2S: C, 56.37; H, 3.38; N, 18.78; S, 10.75; found: C, 57.22; H, 3.37; N, 18.79; S, 10.74.
3-(5-(phenylamino)-1,3,4-thiadiazol-2-yl)phenol (3k): White solid. Yield: (81%); mp: 270 - 271°C; 1H-NMR (400 MHz, DMSO-d6): δ = 6.94 (d, J = 8.0 Hz, 1H, H4b), 7.08 (t, J = 8.0 Hz, 1H, H4a), 7.34 (m, 3H, H2b, H5b, H6b), 7.42 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.71 (d, J = 8.0 Hz, 2H, H2a, H6a), 9.86 (s, 1H, NH), 10.59 (s, 1H, OH); IR (KBr): ϋ = 3233 (OH), 1594, 1569 (C=N), 681 (C-S) cm-1; ESI-MS m/z [M+H+ = 269.8; Anal. Calcd for: C14H11N3OS: C, 62.44; H, 4.12; N, 15.60; S, 11.90; found: C, 61.39; H, 4.11; N, 15.61; S, 11.92.
N-phenyl-5-(4-(trifluoromethyl)phenyl)-1,3,4-thiadiazol-2-amine (3l): White solid. Yield: (65%); mp: 228 - 229°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.05 (t, J = 7.2 Hz, 1H, H4a), 7.39 (t, J = 8.0 Hz, 2H, H3a, H5a), 7.67 (d, J = 8.0 Hz, 2H, H2a, H6a), 7.87 (d, J = 8.4 Hz, 2H, H3b, H5b), 8.08 (d, J = 8.0 Hz, 2H, H2b, H6b), 10.70 (s, 1H, NH); IR (KBr): ϋ = 3253, 3199 (NH), 1610, 1598 (C=N), 660 (C-S) cm-1; ESI-MS m/z [M+H+] = 322.2; Anal. Calcd for: C15H10F3N3S: C, 56.07; H, 3.14; N, 13.08; S, 9.98; found: C, 55.96; H, 3.13; N, 13.07; S, 9.97.
5-(3-bromophenyl)-N-phenyl-1,3,4-thiadiazol-2-amine (3m): White solid. Yield: (67%); mp: 195 - 196°C; 1H-NMR (400 MHz, DMSO-d6): δ = 7.04 (t, J = 7.2 Hz, 1H, H4a), 7.38 (t, J = 8.4 Hz, 2H, H3a, H5a), 7.478 (t, J = 8.0 Hz, 1H, H5b), 7.70 (m, 3H, H2a, H6a, H4b), 7.86 (d, J = 8.0 Hz, 1H, H6b), 8.04 (s, 1H, H2b), 10.648 (s, 1H, NH); IR (KBr): ϋ = 3236, 3185 (NH), 1608, 1589 (C=N), 654 (C-S) cm-1; ESI-MS m/z [M+H+] = 332.0, 334.0; Anal. Calcd for: C14H10BrN3S: C, 50.62; H, 3.03; N, 12.65; S, 9.65; found: C, 51.52; H, 3.02; N, 12.64; S, 9.66.