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Synlett 2009(1): 63-66  
DOI: 10.1055/s-0028-1087383
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of 1-Methylbenzimidazoles from Carbonitriles

Jonas Sluiter, Jens Christoffers*
Institut für Reine und Angewandte Chemie, Carl von Ossietzky-Universität Oldenburg, 26111 Oldenburg, Germany
Fax: +49(441)798 3873; e-Mail: jens.christoffers@uni-oldenburg.de;
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Publication History

Received 24 September 2008
Publication Date:
12 December 2008 (online)

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Abstract

The NaH-mediated N-methylbenzimidazole formation starting from carbonitriles and N-methyl-1,2-phenylenediamine is reported to be a procedure compatible with acid-labile acetal protective groups within the starting materials. Products were further converted in Suzuki, Sonogashira, Heck and Buchwald-Hartwig reactions.

Key words

benzimidazoles - heterocycles - carbonitriles - cross-coupling

11

2-Cyclohexyl-1-methyl-1 H -benzimidazole (3a)
Diamine 1 (244 mg, 2.00 mmol), NaH (80.0 mg, 60% dispersion in mineral oil, 2.00 mmol), and abs. toluene (1.0 mL) were successively added to nitrile 2a (218 mg, 2.00 mmol). The mixture was stirred for 3 h at 120 ˚C in a tightly closed reaction vial, then completely transferred on top of a column (SiO2), and chromatographed (hexane-EtOAc, 1:1; R f  = 0.32) to give the title compound 3a (384 mg, 1.79 mmol, 90%) as a light brown solid, mp 102 ˚C (lit.¹³a 104 ˚C). ¹H NMR (500 MHz, CDCl3): δ = 1.30-1.48 (m,
3 H), 1.74-2.03 (m, 7 H), 2.84 (tt, J = 3.5, 11.6 Hz, 1 H), 3.74 (s, 3 H), 7.20-7.31 (m, 3 H), 7.72-7.77 (m, 1 H) ppm. ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 25.82 (CH2), 26.32 (2 CH2), 29.52 (CH3), 31.46 (2 CH2), 36.33 (CH), 108.80 (CH), 119.27 (CH), 121.66 (CH), 121.87 (CH), 135.64 (C), 142.55 (C), 159.00 (C) ppm. IR (ATR): ν =  3173 (w), 3052 (w), 2929 (s), 2851 (m), 1730 (m), 1615 (m), 1506 (m), 1441 (s), 1278 (m), 1239 (m), 842 (m), 736 (s) cm-¹. MS (EI, 70 eV): m/z (%) = 214 (14) [M+], 159 (100). HRMS (CI, isobutane): m/z calcd for C14H19N2: 215.1548; found: 215.1548 [M + H+]. Anal. Calcd for C14H18N2 (214.31): C, 78.46; H, 8.47; N, 13.07. Found: C, 78.40; H, 8.55; N, 13.05.

23

1-Methyl-2-phenyl-1 H -benzimidazole (3h)
Diamine 1 (6.1 g, 50 mmol), NaH (2.0 g, 60% dispersion in mineral oil, 50 mmol) and abs. mesitylene (10 mL) were successively added to nitrile 2h (5.2 g, 50 mmol). The mixture was heated to reflux for 16 h (inner temp ca. 170 ˚C, oil bath 210 ˚C). After cooling to ambient temperature, the mixture was filtered through SiO2 (1 cm), the residue washed with CH2Cl2 (5 mL), and the combined filtrates evaporated. The residue was chromatographed (SiO2, hexane-EtOAc, 1:1; R f  = 0.34) to give the title compound 3a (6.2 g, 30 mmol, 60%) as a light yellow solid, mp 91 ˚C (lit.¹8b 92-93 ˚C). ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 31.57 (CH3), 109.54 (CH), 119.76 (CH), 122.33 (CH), 122.67 (CH), 128.58 (2 CH), 129.35 (2 CH), 129.63 (CH), 130.16 (C), 136.51 (C), 142.91 (C), 153.70 (C) ppm. All other data were in accordance with the literature.¹8b

25

2-(Biphenyl-4-yl)-1-methyl-1 H -benzimidazole (4)
A mixture of bromophenyl compound 3i (91 mg, 0.32 mmol, 1.0 equiv), PhB(OH)2 (42 mg, 0.35 mmol, 1.1 equiv), Na2PdCl4 (1.2 mg, 4.2 µmol, 1.3 mol%), cataCXium Fsulf (3.3 mg, 4.5 µmol, 1.5 mol%), and K2CO3 (131 mg, 945 µmol, 3 equiv) was placed in a Schlenk tube and twice evacuated and flushed with N2. Degassed H2O (0.5 mL) and BuOH (0.5 mL) were added, and the resulting mixture was stirred for 16 h at 100 ˚C. Subsequently, the organic layer was separated and the aqueous layer extracted with CH2Cl2 (5 mL). The combined organic layers were dried (MgSO4), and after filtration, the solvent was evaporated and the residue chromatographed (SiO2, hexane-EtOAc = 1:1, R f  = 0.48) to give the title compound 4 (82 mg, 0.29 µmol, 92%) as a yellow solid, mp 164 ˚C. ¹H NMR (500 MHz, CDCl3): δ = 3.87 (s, 3 H), 7.29-7.34 (m, 2 H), 7.35-7.40 (m, 2 H), 7.46 (t, J = 7.6 Hz, 2 H), 7.62-7.67 (m, 2 H), 7.71-7.76 (m, 2 H), 7.81-7.87 (m, 3 H) ppm. ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 31.70 (CH3), 109.57 (CH), 119.75 (CH), 122.42 (CH), 122.74 (CH), 127.08 (2 CH), 127.26 (2 CH), 127.78 (CH), 128.86 (2 CH), 128.94 (C), 129.77 (2 CH), 136.61 (C), 140.09 (C), 142.40 (C), 142.93 (C), 153.40 (C) ppm. IR (ATR): ν = 3031 (w), 2925 (w), 1463 (m), 1375 (m), 1325 (m), 1005 (m), 850 (m), 765 (m), 737 (s), 690 (s) cm-¹. MS (EI, 70 eV): m/z (%) = 284 (98) [M+], 283 (100), 179 (10), 152 (10), 78 (12), 77 (29). HRMS (EI, 70 eV): m/z calcd for C20H16N2: 284.1313; found: 284.1313 [M+].

26

cataCXium FBu = (9-butyl-9-fluorenyl)dicyclohexylphos-phonium tetrafluoroborate; for use in Sonogashira reactions, see ref. 24a

27

1-Methyl-2-[4-(phenylethynyl)phenyl]-1 H -benzimi-dazole (5) ²8
A suspension of bromophenyl compound 3i (91 mg, 0.32 mmol, 1 equiv), phenylacetylene (38 mg, 0.38 mmol, 1.2 equiv), Na2PdCl4 (1.5 mg, 5.1 µmol, 1.6 mol%), cataCXium FBu (3.6 mg, 7.2 µmol, 2.3 mol%), and CuI (1.5 mg, 7.8 µmol, 2.5 mol%) in i-PrNH2 (1.0 mL) was stirred under an inert atmosphere for 16 h at 100 ˚C. Subsequently, all volatile materials were removed in vacuo, the residue dissolved in CH2Cl2 (5 mL), and washed with H2O (2 ×
3 mL). The organic layer was dried (Na2SO4), and after filtration, the solvent was evaporated. Chromatography of the residue (SiO2; hexane-EtOAc, 2:1; R f  = 0.29) gave the title compound 5 (81 mg, 0.26 mmol, 84%) as a light yellow solid, mp 152 ˚C (lit.²8 153-155 ˚C). ¹H NMR (500 MHz, CDCl3): δ = 3.80 (s, 3 H), 7.26-7.37 (m, 6 H), 7.50-7.57 (m, 2 H), 7.63-7.68 (m, 2 H), 7.71-7.77 (m, 2 H), 7.79-7.86 (m, 1 H) ppm. ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 31.63 (CH3), 88.71 (C), 91.27 (C), 109.58 (CH), 119.75 (CH), 122.47 (CH), 122.77 (C), 122.87 (CH), 124.62 (C), 128.31 (2 CH), 128.48 (CH), 129.20 (2 CH), 129.68 (C), 131.57
(2 CH), 131.70 (2 CH), 136.56 (C), 142.86 (C), 152.83 (C) ppm. IR (ATR): ν = 3058 (w), 2950 (w), 2216 (w), 1457 (m), 1437 (m), 1381 (m), 1329 (m), 1249 (m), 1103 (m), 1007 (m), 919 (m), 840 (s), 740 (s), 730 (s), 690 (s) cm-¹.
MS (EI, 70 eV): m/z (%) = 308 (100) [M+], 307 (80), 203 (23), 176 (12), 154 (12). HRMS (EI, 70 eV): m/z calcd for C22H16N2: 308.1313; found: 308.1313 [M+].

30

( E )-1-Methyl-2-[4-(2-phenylethenyl)phenyl]-1 H -benz-imidazole (6)
A mixture of bromophenyl compound 3i (60 mg, 0.21 mmol, 1 equiv), freshly distilled styrene (44 mg, 0.42 mmol, 2.0 equiv), Pd(OAc)2 (5.0 mg, 21 µmol, 0.1 equiv), Ph3P (14 mg, 51 µmol, 0.24 equiv), Et3N (85 mg, 0.84 mmol, 4.0 equiv), and abs. DMF (3.0 mL) was stirred for 16 h at 100 ˚C under an inert atmosphere. Subsequently, all volatile materials were removed in vacuo, the residue dissolved in CH2Cl2 (5 mL), and washed with H2O (2 × 3 mL). The organic layer was dried (Na2SO4), and after filtration, the solvent was evaporated. Chromatography of the residue (SiO2; hexane-EtOAc, 2:1; R f  = 0.27) gave the title compound 6 (31 mg, 0.10 mmol, 48%) as a light yellow solid, mp 150 ˚C. ¹H NMR (500 MHz, CDCl3): δ = 3.86 (s, 3 H), 7.15 (d, J = 16.3 Hz, 1 H), 7.21 (d, J = 16.3 Hz, 1 H), 7.23-7.34 (m, 3 H), 7.33-7.41 (m, 3 H), 7.50-7.56 (m, 2 H), 7.60-7.68 (m, 2 H), 7.72-7.79 (m, 2 H), 7.80-7.87 (m, 1 H) ppm. ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 31.74 (CH3), 109.56 (CH), 119.74 (CH), 122.45 (CH), 122.76 (CH), 126.63 (4 CH), 127.67 (CH), 127.97 (CH), 128.71 (2 CH), 129.01 (C), 129.68
(2 CH), 130.17 (CH), 136.62 (C), 136.90 (C), 138.68 (C), 142.91 (C), 153.40 (C) ppm. IR (ATR): ν = 3025 (w), 2953 (w), 1461 (m), 1379 (m), 1326 (m), 960 (m), 821 (s), 741 (s), 688 (s) cm-¹. MS (EI, 70 eV): m/z (%) = 310 (100) [M+], 309 (75). HRMS (EI, 70 eV): m/z calcd for C22H18N2: 310.1470; found: 310.1470 [M+].

32

2-[4-(Cyclohexylamino)phenyl]-1-methyl-1 H -benzimi-dazole (7)
A suspension of Pd(OAc)2 (3.0 mg, 11 µmol, 5.0 mol%) and rac-BINAP (7.0 mg, 11 µmol, 7.5 mol%) in abs. toluene (1.5 mL) was stirred for 5 min at 23 ˚C under an inert atmo-sphere. Bromophenyl compound 3i (60 mg, 0.21 mmol, 1 equiv), CyNH2 (27 mg, 0.27 mmol, 1.3 equiv), and NaOt-Bu (34 mg, 0.36 mmol, 1.7 equiv) were added, and the resulting mixture was stirred for 16 h at 85 ˚C. Subsequently, it was diluted with CH2Cl2 (5 mL) and washed with sat. NaHCO3 solution (5 mL). The organic layer was dried (MgSO4), and after filtration, the solvent was evaporated. Chromatography of the residue (SiO2; hexane-EtOAc, 1:2; R f  = 0.44) gave the title compound 7 (42 mg, 0.14 mmol, 65%) as a light yellow solid, mp 145 ˚C. ¹H NMR (500 MHz, CDCl3): δ = 1.13-1.29 (m, 3 H), 1.32-1.46 (m, 2 H), 1.62-1.70 (m, 1 H), 1.73-1.83 (m, 2 H), 2.01-2.13 (m, 2 H), 3.25-3.39 (m, 1 H), 3.77-3.93 (m, 4 H), 6.63-6.69 (m, 2 H), 7.23-7.28 (m, 2 H), 7.29-7.34 (m, 1 H), 7.54-7.60 (m, 2 H), 7.74-7.80 (m, 1 H) ppm. ¹³C{¹H} NMR (125 MHz, CDCl3): δ = 24.89 (2 CH2), 25.78 (CH2), 31.74 (CH3), 33.24 (2 CH2), 51.40 (CH), 109.23 (CH), 112.51 (2 CH), 117.80 (C), 119.20 (CH), 121.97 (CH), 122.00 (CH), 130.65 (2 CH), 136.61 (C), 142.97 (C), 148.49 (C), 154.62 (C) ppm. IR (ATR): ν = 3335 (w), 3055 (w), 2928 (m), 2853 (m), 1608 (s), 1542 (m), 1456 (s), 1379 (m), 1323 (m), 1180 (m), 1006 (m), 888 (s), 828 (m), 814 (m), 743 (s), 731 (s) cm-¹. MS (EI, 70 eV): m/z (%) = 305 (100) [M+], 304 (16), 263 (12), 262 (62), 222 (14), 131 (10). HRMS (EI, 70 eV): m/z calcd for C20H23N3: 305.1892; found: 305.1892 [M+].