A Novel and Efficient Synthesis of 2-Aryl-2H-indazoles via SnCl₂-Mediated Cyclization of 2-Nitrobenzylamines

 

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Abstract

A mild, efficient, and novel synthesis of 2-aryl-2H-indazoles via cyclization of 2-nitrobenzylamines promoted by SnCl₂·2H₂O has been described. This method applies to a wide scope of substrates containing electron-donating and electron-withdrawing substituents.

References and Notes

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General Procedure for the Synthesis of 2-Aryl-2 H -indazoles
A solution of 2-nitrobenzylamine (3 mmol) and SnCl₂·2H₂O (6 mmol) in 95% EtOH (20 mL) was allowed to stir at 40 °C for 2 h. The reaction mixture was quenched with 3% HCl (100 mL), filtrated, the crude product was purified by crystallization from 95% EtOH to give the pure products 2.

Representative Spectral Data for 2b
¹H NMR (400 MHz, DMSO-d ₆): δ = 9.12 (s, 1 H, H-3), 8.10 (d, 2 H, J = 8.0 Hz, ArH), 7.78 (d, 2 H, J = 8.4 Hz, ArH), 7.72 (d, 2 H, J = 8.4 Hz, ArH), 7.46 (t, 1 H, J = 7.2 Hz, ArH), 7.32 (t, 1 H, J = 8.0 Hz, ArH), 7.11 (t, 1 H, J = 7.6 Hz, ArH) ppm. ¹³C NMR (100 MHz, DMSO-d ₆): δ = 149.12, 140.12, 129.86, 128.02, 126.96, 122.64, 122.28, 121.74, 121.11, 120.42, 117.65 ppm. IR (KBr): ν = 3131, 3066, 1626, 1593, 1518, 1494, 1464, 1407, 1378, 1349, 1314, 1250, 1231, 1201, 1143, 1129, 1073, 1044, 950, 907, 820, 780, 754, 685 cm^-1. MS: m/z (%) = 194 (100) [M⁺]. Anal. Calcd for C₁₃H₁₀N₂: C, 80.39; H, 5.19; N, 14.42. Found: C, 80.55; H, 5.29; N, 14.26.

Crystallographic data for the structure of 2i has been deposited at the Cambridge Crystallographic Data Centre under the deposit number CCDC-655981. Copies of available material can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 (1223)336033; e-mail: deposit@ccdc.cam.ac.uk]. Crystal data for 2i: C₁₄H₁₀Cl₂N₂; M = 277.14, colorless block crystals, 0.43 × 0.40 × 0.32 mm, monoclinic, space group P2₁/c, a = 14.234(3) Å, b = 14.438(3) Å, c = 6.0511(15) Å, α = 90°, β = 92.338(7)°, γ = 90°, V = 1242.5(5) ų, Z = 4, D _c = 1.481 g cm^-1, F(000) = 568, µ (Mo Kα) = 0.503 mm^-1. Intensity data were collected on a diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) using ω scan mode with 1.43° < θ < 25.00°; 2183 unique reflections were measured and 1532 reflections with I > 2σ(I) were used in the refinement. The structures were solved by direct methods and expanded using Fourier techniques. The final cycle of full-matrix least squares technique to R = 0.0460 and wR = 0.1283.