Phenyliodine Bis(trifluoroacetate) Mediated Intramolecular Oxidative Coupling of Electron-Rich N-Phenyl Benzamides

Abstract

The intramolecular oxidative C–O coupling of N-(4-alkoxy-phenyl) and N-(4-acetamido-phenyl) benzamides was achieved under metal-free conditions by using phenyliodine bis(trifluoroacetate) as oxidant and TMSOTf as catalyst. The reactions afford benz­oxazole products in high yields.

Key words

hypervalent iodine - N-phenyl benzamides - cyclization - oxidative coupling - benzoxazoles

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• 16 General Procedure for the Reactions of 1 (or 5) To a stirred MeCN solution (40 mL) containing 1 (or 5, 0.5 mmol) and TMSOTf (1.0 mmol) was added dropwise PIFA (0.6 mmol) in MeCN (10 mL). The stirring was continued at r.t. for another 5 min. The mixture was then poured into a sat. NaHCO₃ solution (100 mL), and the product was extracted with EtOAc (2 × 100 mL). The combined organic phase was washed with brine, and then dried with anhyd MgSO₄. After filtration, the filtrate was concentrated under reduced pressure, and the residual was treated with flash column chromatography to give 2 (or 6)
• 17 Typical Spectroscopic Data for the Products
  6-Ethoxy-2-(4-bromophenyl)benzo[d]oxazole (2e) White solid; mp 136–138 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.46 (t, 3 H, J = 7.2 Hz, CH ₃CH₂), 4.08 (q, 2 H, J = 7.2 Hz, OCH₂), 6.95 (dd, 1 H, J = 8.8, 2.4 Hz, C₅-H), 7.06 (d, 1 H, J = 2.4 Hz, C₇-H), 7.61 (d, 1 H, J = 8.8 Hz, C₄-H), 7.62 (d, 2 H, J = 8.4 Hz, 4-BrPhH), 8.04 (d, 2 H, J = 8.4 Hz, 4-BrPhH). ¹³C NMR (100 MHz, CDCl₃): δ = 14.8, 64.2, 96.0, 113.5, 120.0, 125.6, 126.3, 128.5, 132.1, 135.6, 151.6, 157.8, 161.2. MS (EI): m/z (rel. int., %) = 317 (71) [M⁺], 262 (7), 210 (10), 183 (21), 79 (54), 51 (100). ESI-HRMS: m/z calcd for C₁₅H₁₂BrNO₂ + H: 318.0124; found: 318.0134 N-{2-Phenylbenzo[d]oxazol-6-yl}acetamide (6a) Light yellow solid; mp 176–177 °C. ¹H NMR (400 MHz, acetone-d ₆): δ = 2.13 (s, 3 H, CH₃CO), 7.38 (dd, 1 H, J = 8.8, 1.6 Hz, C₅-H), 7.59–7.61 (m, 3 H, PhH), 7.64 (d, 1 H, J = 8.8 Hz, C₄-H), 8.21–8.24 (m, 2 H, PhH), 8.39 (d, 1 H, J = 1.6 Hz, C₇-H), 9.50 (br s, 1 H, NH). ¹³C NMR (100 MHz, acetone-d ₆): δ = 24.4, 102.3, 117.2, 120.5, 128.1, 128.2, 130.0, 132.3, 138.6, 138.7, 152.0, 163.3, 169.2. MS (EI): m/z (rel. int., %) = 252 (39) [M⁺], 210 (100), 149 (6). ESI-HRMS: m/z calcd for C₁₅H₁₂N₂O₂ + H: 253.0972; found: 253.0973 N-{2-(4-Methoxyphenyl)benzo[d]oxazol-6-yl}acetamide (6b) Light yellow solid; mp 187–189 °C. ¹H NMR (400 MHz, DMSO-d ₆): δ = 2.09 (s, 3 H, CH₃CO), 3.85 (s, 3 H, OCH₃), 7.13 (d, 2 H, J = 8.4 Hz, 4-MeOPhH), 7.37 (d, 1 H, J = 8.4 Hz, C₅-H), 7.65 (d, 1 H, J = 8.8 Hz, C₄-H), 8.09 (d, 2 H, J = 8.8 Hz, 4-MeOPhH), 8.22 (s, 1 H, C₇-H), 10.22 (br s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 24.0, 55.5, 101.1, 114.7, 116.2, 118.9, 119.1, 128.8, 136.9, 137.1, 150.2, 161.9, 162.0, 168.4. MS (EI): m/z (rel. int., %) = 282 (15) [M⁺], 240 (18), 225 (10), 178 (12), 149 (100). ESI-HRMS: m/z calcd for C₁₆H₁₄N₂O₃ + H: 283.1077; found: 283.1080