Tautomerization of 5-Alkylidene-2-Oxazolidinone to 2-Oxazolone by Use of an N-Heterocyclic Carbene Catalyst

Abstract

By employing an N-heterocyclic carbene as a catalyst, the tautomerization of 5-alkylidene-2-oxazolidinone proceeded to afford the corresponding 2-oxazolone. Among N-heterocyclic carbenes, 1,3-di-tert-butylimidazol-2-ylidene was the most effective catalyst for the tautomerization of 5-alkylidene-2-oxazolidinone.

Key words

carbenes - catalysis - 5-alkylidene-2-oxazolidinone - tautomerization - 2-oxazolone

• References and Notes

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• 8 General Procedure To an i-PrOH solution (0.9 mL) of 5-alkylidene-2-oxazolidinone 1 (0.2 mmol) in glassware was added an i-PrOH solution (0.1 mL) of an NHC catalyst (0.01 mmol) under an argon atmosphere. Tautomerization proceeded by the stirring of the resulting mixture at 40 °C for 6 h. The chemical yield of 2-oxazolone 2 was determined by integrating ¹H NMR absorptions referring to an internal standard [3-hydroxybenzyl alcohol (0.2 mmol)], a CH₂Cl₂ solution (2 mL) of which was added to the reaction mixture.
• 9 Also in the case of no catalysts, the tautomerization did not proceed.
• 10 Even when the i-PrOH solution of 2-oxazolone 2a was stirred at 80 °C for 24 h under an argon atmosphere with the use of 5 mol% of ItBu in a sealed autoclave, the tautomerization of 2a to 1a did not proceed.
• 11 5-Alkylidene-2-oxazolidinones 1 were prepared by the carboxylative cyclizations of propargylic amines with CO₂.^7a,b NMR spectra of 1a,^7b 1c,^7a 1d,^7f 1e,^7f 1g ^7c and 1h ^7e are in accordance with those reported in the literature.
• 12 Selected Data for New Compounds (Z)-5-(4′-Methoxybenzylidene)-3-methyl-1,3-oxazolidin-2-one (1b) Pale yellow powder; mp 125.5–126.1 °C. IR (KBr): 3024, 2955, 2940, 2909, 2832, 1782, 1697, 1512, 1250, 1080, 1026, 849 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.50 (ddd, J = 8.8, 3.0, 2.0 Hz, 2 H, ArH), 6.86 (ddd, J = 8.8, 3.0, 2.0 Hz, 2 H, ArH), 5.46 (t, J = 2.1 Hz, 1 H, CH), 4.29 (d, J = 2.1 Hz, 2 H, CH₂), 3.81 (s, 3 H, OCH₃), 2.98 (s, 3 H, NCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 158.4, 155.9, 134.0, 129.5, 126.3, 113.9, 102.4, 55.2, 50.8, 30.4. Anal. Calcd (%) for C₁₂H₁₃NO₃: C, 65.74; H, 5.98; N, 6.39. Found: C, 65.86; H, 5.88; N, 6.16. (Z)-3-Benzyl-5-benzylidene-1,3-oxazolidin-2-one (1f) White powder; mp 164.5–165.1 °C. IR (KBr): 3086, 3063, 3032, 2932, 2855, 1782, 1697, 1474, 1265, 1049 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.55–7.52 (m, 2 H, ArH), 7.40–7.27 (m, 7 H, ArH), 7.22–7.17 (m, 1 H, ArH), 5.44 (t, J = 2.1 Hz, 1 H, CH), 4.50 (s, 2 H, NCH₂Ar), 4.14 (d, J = 2.1 Hz, 2 H, CH₂). ¹³C NMR (100 MHz, CDCl₃): δ = 155.6, 141.7, 134.9, 133.4, 129.0, 128.5, 128.3, 128.2, 128.1, 126.8, 103.1, 48.2, 47.9. Anal. Calcd (%) for ­C₁₇H₁₅NO₂·0.5H₂O: C, 75.42; H, 5.81; N, 5.17. Found: C, 75.42; H, 5.48; N, 4.98. 5-Benzyl-3-methyl-1,3-oxazol-2-one (2a) White powder; mp 41.7–42.2 °C. IR (KBr): 3148, 3024, 3001, 2955, 2916, 1751, 1443, 1396, 1049, 741, 702 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.34–7.23 (m, 5 H, ArH), 6.00 (t, J = 1.4 Hz, 1 H, CH), 3.71 (d, J = 0.8 Hz, 2 H, CH₂), 3.15 (s, 3 H, NCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 155.9, 139.6, 135.5, 128.8, 128.7, 127.1, 112.0, 32.5, 30.4. Anal. Calcd (%) for C₁₁H₁₁NO₂: C, 69.83; H, 5.86; N, 7.40. Found: C, 69.82; H, 5.81; N, 7.32. 5-(4′-Methoxybenzyl)-3-methyl-1,3-oxazol-2-one (2b) Pale yellow powder; mp 59.3–59.7 °C. IR (KBr): 3125, 3009, 2963, 2909, 2839, 1744, 1512, 1443, 1250, 1034 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.15 (ddd, J = 8.7, 3.0, 2.1 Hz, 2 H, ArH), 6.85 (ddd, J = 8.7, 3.0, 2.1 Hz, 2 H, ArH), 6.00 (t, J = 1.4 Hz, 1 H, CH), 3.78 (s, 3 H, OCH₃), 3.63 (d, J = 1.0 Hz, 2 H, CH₂), 3.13 (s, 3 H, NCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 158.7, 155.9, 140.0, 130.0, 127.5, 114.1, 111.8, 55.3, 31.6, 30.4. Anal. Calcd (%) for C₁₂H₁₃NO₂: C, 65.74; H, 5.98; N, 6.39. Found: C, 65.82; H, 5.84; N, 6.14. 3-Methyl-5-(4′-methylphenyl)-1,3-oxazol-2-one (2c) White powder; mp 60.0–60.4 °C. IR (KBr): 3148, 3024, 2924, 1736, 1512, 1450, 1396, 1180, 1049, 949, 802 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.13 (s, 4 H, ArH), 5.97 (t, J = 1.4 Hz, 1 H, CH), 3.66 (d, J = 1.2 Hz, 2 H, CH₂Ar), 3.14 (s, 3 H, NCH₃), 2.33 (s, 3 H, ArCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 155.9, 139.9, 136.7, 132.4, 129.3, 128.7, 111.9, 32.0, 30.4, 21.0. MS (EI): m/z calcd for C₁₂H₁₃NO₂: 203.0946 [M⁺]; found: 203.0944. 4-(3′-Methyl-2′-oxo-1′,3′-oxazol-5′-methyl)benzonitrile (2d) Pale yellow powder; mp 92.5–93.1 °C. IR (KBr): 3148, 2978, 2916, 2230, 1751, 1435, 1396, 1088, 1042, 741 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.64–7.61 (m, 2 H, ArH), 7.37–7.35 (m, 2 H, ArH), 6.14 (t, J = 1.3 Hz, 1 H, CH), 3.78 (s, 2 H, CH₂), 3.19 (s, 3 H, NCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 155.7, 141.3, 137.6, 132.5, 129.6, 118.6, 112.8, 111.1, 32.5, 30.5. MS (EI): m/z calcd for C₁₂H₁₀N₂O₂: 214.0742 [M⁺]; found: 214.0741. 3-Methyl-5-[4′-(trifluoromethyl)benzyl]-1,3-oxazol-2-one (2e) White powder; mp 50.7–51.1 °C. IR (KBr): 3148, 2947, 2916, 1736, 1443, 1335, 1165, 1103, 741, 687 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.59 (d, J = 8.0 Hz, 2 H, ArH), 7.37 (d, J = 8.0 Hz, 2 H, ArH), 6.08 (t, J = 1.3 Hz, 1 H, CH), 3.77 (s, 2 H, CH₂), 3.18 (s, 3 H, NCH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 155.8, 139.8 (d, ⁴ J _C–F = 1.0 Hz), 138.4, 129.6 (q, ² J _C–F = 32.3 Hz), 129.1, 125.7 (q, ³ J _C–F = 3.7 Hz), 124.1 (q, ¹ J _C–F = 270.2 Hz), 112.5, 32.3, 30.5. ¹⁹F NMR (376 MHz, CDCl₃): δ = –62.5 (s). MS (EI): m/z calcd for ­C₁₂H₁₀F₃NO₂: 257.0664 [M⁺]; found: 257.0680. 3,5-Dibenzyl-1,3-oxazol-2-one (2f) White powder; mp 79.4–79.7 °C. IR (KBr): 3132, 3032, 2947, 1744, 1443, 1404, 1358, 1188, 1072, 957, 702 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.20 (m, 10 H, ArH), 5.96 (t, J = 1.4 Hz, 1 H, CH), 4.66 (s, 2 H, NCH₂Ar), 3.69 (s, 2 H, CH₂Ar). ¹³C NMR (100 MHz, CDCl₃): δ = 154.7, 138.8, 134.5, 134.4, 127.9, 127.8, 127.6, 127.2, 126.8, 126.0, 109.4, 46.5, 31.5. Anal. Calcd (%) for C₁₇H₁₅NO₂: C, 76.96; H, 5.70; N, 5.28. Found: C, 76.83; H, 5.56; N, 5.07.