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Synlett 2013; 24(12): 1573-1577
DOI: 10.1055/s-0033-1339195
DOI: 10.1055/s-0033-1339195
letter
Microwave-Assisted Molybdenum-Catalyzed Reductive Cyclization of o-Nitrobenzylidene Amines to 2-Aryl-2H-indazoles
Further Information
Publication History
Received:29.03.2013
Accepted after revision: 20 May 2013
Publication Date:
27 June 2013 (online)
Abstract
The reductive cyclization of o-nitrobenzylidene amines under microwave conditions employing MoO2Cl2(dmf)2 as catalyst and Ph3P as reducing agent delivers 2-aryl-2H-indazoles with yields ranging from 61–92%.
Supporting Information
- for this article is available online at http://www.thieme-connect.com/ejournals/toc/synlett.
- Supporting Information
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References and Notes
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- 14 General Procedure for the Synthesis of o-Nitrobenzylidene Amines 3 o-Nitrobenzaldehyde 1 (10 mmol) and aniline 2 (10 mmol) in i-PrOH (3 mL) were placed in a round-bottomed flask and heated under reflux with magnetic stirring for 10 min. The reaction mixture was cooled to r.t., and the crystalline o-nitrobenzylidene amine 3 was isolated by filtration. The crude product was purified by crystallization from i-PrOH–EtOH = 1:1.
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- 17 General Procedure for the MoO2Cl2(dmf)2-Catalyzed Reductive Cyclization of o-Nitrobenzylidene Amines 3 to 2-Aryl-2H-indazoles 4 A mixture of 3 (1 mmol), Ph3P (2.4 mmol), MoO2Cl2(dmf)2 (0.05 mmol), and toluene (3 mL) was sealed in a 10 mL septum reaction vial and irradiated with microwaves (DiscoverTM by CEM, 2450 MHz, 300 W, 150 °C, 20 bar) for 10 min. After filtration and removal of the solvent the reaction mixture was poured into H2O (50 mL) and extracted with EtOAc (3 × 30 mL). The combined organic extracts were washed with brine (30 mL). After drying over anhyd MgSO4 and concentration in vacuo the resulting residue was purified by flash chromatography on silica gel (cyclohexane–EtOAc = 20:1). Alternatively, the reaction mixture can also be purified by flash column chromatography without any workup procedure.
- 18 Selected Data for 2-(2,4,6-Trimethylphenyl)-2H-indazole (4d) Rf = 0.36 (cyclohexane–EtOAc = 3:1). IR (ATR): 1627, 1518, 1490, 1377, 1349, 1267, 1195, 1144, 1044, 964, 939, 851, 782, 755, 733, 671 cm–1. UV (MeCN): λmax (log ε) = 276 nm (4.08). 1H NMR (300 MHz, CDCl3): δ = 7.96 (d, 5 Ј = 1.1 Hz, 1 H, 3-H), 7.81 [dq, 4 Ј and 5 Ј ~ 1.0 Hz, 3 Ј (6-H, 7-H) = 8.7 Hz, 1 H, 7-H], 7.75 [dt, 4 Ј and 5 Ј ~ 1.0 Hz, 3 Ј (4-H, 5-H) = 8.4 Hz, 1 H, 4-H], 7.34 [ddd, 4 Ј (4-H, 6-H) = 1.2 Hz, 3 Ј (5-H, 6-H) = 6.7 Hz, 3 Ј (6-H, 7-H) = 8.8 Hz, 1 H, 6-H], 7.15 [ddd, 4 Ј (5-H, 7-H) = 0.9 Hz, 3 Ј (5-H, 6-H) = 6.6 Hz, 3 Ј (4-H, 5-H) = 8.5 Hz, 1 H, 5-H], 6.99 (br s, 2 H, 3′-H and 5′-H), 2.37 (br s, 3 H, 4′′-H), 1.95 (br s, 6 H, 2′′-H and 6′′-H). 13C NMR (75 MHz, CDCl3): δ = 149.1 (C-7a), 139.2 (C-4′), 137.3 (C-1′), 135.1 (C-2′ and C-6′), 128.7 (C-3′ and C-5′), 126.0 (C-6), 124.6 (C-3), 121.9 (C-5), 121.8 (C-3a), 120.3 (C-4), 118.0 (C-7), 21.1 (C-4′′), 17.1 (C-2′′ and C-6′′). MS (EI, 70 eV): m/z (%) = 336 (100) [M+], 221 (21), 220 (14), 28 (23). HRMS (EI): m/z calcd for C16H16N2 [M+]: 236.1314; found: 236.1334.