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DOI: 10.1055/s-2008-1072503
Iron(III) Chloride Catalyzed Nazarov Cyclization of 3-Substituted Thiophene Derivatives
Publikationsverlauf
Publikationsdatum:
17. März 2008 (online)
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Abstract
The Nazarov cyclization of silicon-free 3-substituted thiophene derivatives was effectively catalyzed by 10 mol% FeCl3, which provided the desired cyclized trans product in good yield.
Key words
iron catalyst - Nazarov cyclization - thiophene
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References and Notes
Scandium-catalyzed Nazarov cyclization of 2- or 3-substituted silicon-free thiophene substrates was reported by Frontier et al., see: ref. 10g.
12Nazarov Reaction of 1a; Typical Procedure (Table 1, entry 2): To a solution of FeCl3 (3.2 mg, 0.02 mmol) in toluene (0.4 mL) was added 1a (54 mg, 0.2 mmol) at r.t., and the mixture was stirred at 60 °C for 5 h. The reaction mixture was quenched with H2O, and extracted with Et2O. The combined organic layers were dried over anhyd MgSO4 and concentrated in vacuo. The residue was purified by preparative silica gel TLC (hexane-EtOAc = 4:1) to give 2a (49 mg, 90%) as a white solid; R f 0.41 (hexane-EtOAc, 4:1); mp 96-97 °C. 1H NMR (500 MHz, CDCl3): δ = 3.83 (s, 3 H), 3.92 (d, J = 3.0 Hz, 1 H), 5.11 (d, J = 3.0 Hz, 1 H), 7.20-7.22 (m, 3 H), 7.30-7.73 (m, 3 H), 7.42 (d, J = 5.0 Hz, 1 H). 13C NMR (125 MHz, CDCl3): δ = 46.9, 52.9, 68.2, 119.6, 127.2, 128.0, 129.2, 132.6, 140.6, 144.4, 168.4, 172.7, 189.7. IR (KBr): 642, 698, 733, 1013, 1161, 1240, 1321, 1433, 1701, 1740 cm-1. EI-MS: m/z = 272. HRMS (EI): m/z calcd for C15H12O3S: 272.0507; found: 272.0509.
13Most combinations resulted in no reaction, except for FeCl3 in toluene (59%), FeCl3 in dichloroethane (32%), Fe(ClO4)2·6H2O in dichloroethane (32%) or Fe(ClO4)3·nH2O in CH2Cl2 (30%).
14Decarbomethoxylated product 3a: 1H NMR (500 MHz, CDCl3): δ = 2.93 (dd, J = 2.7, 18.5 Hz, 1 H), 3.50 (dd, J = 7.1, 18.5 Hz, 1 H), 4.69 (dd, J = 2.7, 7.1 Hz, 1 H), 7.17-7.20 (m, 3 H), 7.24-7.29 (m, 1 H), 7.32-7.35 (m, 2 H), 7.38 (d,
J = 5.1 Hz, 1 H). 13C NMR (125 MHz, CDCl3): δ = 43.0, 52.2, 119.3, 127.0, 127.5, 129.1, 132.2, 142.5, 146.5, 173.6, 197.2.
This decarbomethoxylation reaction might be caused by trace amount of H2O, because the reaction was conducted under air.