Methyltrioxorhenium-Catalyzed Oxidation of Aromatic Aldoximes

Abstract

The first catalytic oxidation of aryl oximes to nitro compounds by means of methyltrioxorhenium and urea hydroperoxide is reported. The formation of carbamates, probably occurring through formation of nitrile oxide intermediates, has been observed from 2,6-disubstituted aryl oximes.

Key words

catalysis - urea hydrogen peroxide - nitro compounds - carbamates - nitrile oxides

References

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11

Typical Procedure for the Oxidation of Oximes: A 10 mL reaction flask was charged sequentially with MTO (0.02-0.04 mmol), MeOH (2 mL), and UHP (3 mmol). The stirred solution became yellow due to the formation of peroxy species and after 5 min the oxime (1 mmol) was added. The reaction mixture was stirred at r.t. for the time reported in Table [1] . The solvent was evaporated, the crude reaction mixture was added with CH₂Cl₂ and the undissolved urea was filtered off. The pure products were collected by chromatography on silica gel using the appropriate eluent.

12

All new compounds gave satisfactory analytical and spectroscopic data.

13

Selected data for carbamates:
8: R_f = 0.17 (petroleum ether-Et₂O 6:1). Pale yellow solid; mp 118-120 °C. IR (CDCl₃): 3421, 2984, 2934, 1730, 1497, 1227 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 7.39-7.34 (m, 2 H, ArH), 7.20-7.11 (m, 1 H, ArH), 6.26 (br s, 1 H, NH), 4.24 (q, 2 H, J = 7.0 Hz, OCH₂), 1.30 (t, 3 H, J = 7.0 Hz, CH₂CH₃). ¹³C NMR (50 MHz, CDCl₃): δ = 153.8, 133.8, 132.0, 128.4, 128.2, 62.0, 14.4. MS: m/z (%) = 233 (4) [M⁺ - 1], 200(13), 198 (47), 176 (32), 173 (40), 163 (54), 161 (100). Anal. Calcd for C₉H₉Cl₂NO₂: C, 46.18; H, 3.88; N, 5.98. Found: C, 46.48; H, 3.84; N, 5.76.
11: R_f = 0.20 (petroleum ether-Et₂O 4:1). White solid; mp 107-109 °C. IR (CDCl₃): 3427, 3314, 2956, 2922, 2861, 1726, 1500, 1450, 1355, 1225 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 6.90 (m, 2 H, ArH), 6.11 (br s, 1 H, NH), 3.76 (br s, 3 H, OCH₃), 2.28 (s, 3 H), 2.22 (s, 6 H). ¹³C NMR (50 MHz, CDCl₃): δ = 155.1, 136.8, 135.6, 130.9, 128.8, 52.4, 20.8, 18.1. MS: m/z (%) = 193 (91) [M⁺], 162 (34), 160 (45), 146 (27), 135 (80), 132 (100), 119 (23), 91 (51). Anal. Calcd for C₁₁H₁₅NO₂: C, 68.37; H, 7.82; N, 7.25. Found: C, 68.76; H, 7.85; N, 7.72.

15

Selected data for cycloadducts:
16: White solid; mp 207-209 °C (CH₃OH). IR (CDCl₃): 3072, 2975, 1732, 1562, 1499, 1433, 1377, 1192 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 7.48-7.26 (m, 8 H, ArH), 5.73 (d, 1 H, J = 9.3 Hz, H-6a), 4.91 (d, 1 H, J = 9.3 Hz, H-3a). ¹³C NMR (50 MHz, CDCl₃): δ = 171.1, 168.9, 150.0, 135.2, 132.1, 130.8, 129.4, 129.2, 128.4, 126.0, 125.3, 80.0, 56.7. MS: m/z (%) = 361 (10) [M⁺], 360 (27) [M⁺ - 1], 212 (51), 173 (50), 119 (100), 91 (7). Anal. Calcd for C₁₇H₁₀Cl₂N₂O₃: C, 56.53; H, 2.79; N, 7.76. Found: C, 56.37; H, 2.64; N, 8.00.
19: R_f = 0.19 (petroleum ether-Et₂O 5:1). White solid; mp 129-130 °C. IR (CDCl₃): 2956, 1743, 1437, 1310, 1269, 1225 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 6.87 (m, 2 H, ArH), 5.58 (d, 1 H, J = 6.6 Hz, H-5), 4.70 (d, 1 H, J = 6.6 Hz, H-4), 3.85 (s, 3 H, OCH₃), 3.60 (s, 3 H, OCH₃), 2.27 (s, 3 H), 2.18 (s, 6 H). ¹³C NMR (50 MHz, CDCl₃): δ = 169.3, 167.2, 153.5, 139.2, 136.9, 128.5, 123.3, 80.7, 59.7, 53.1, 53.0, 21.1, 19.6. MS: m/z (%) = 306 (1) [M⁺], 305 (5) [M⁺ - 1], 246 (32), 219 (25), 214 (10), 186 (61), 158 (29), 57 (100). Anal. Calcd for C₁₆H₁₉NO₅: C, 62.94; H, 6.27; N, 4.59. Found: C, 62.58; H, 6.02; N, 4.40.

16

Typical Procedure for the Synthesis of Cycloadducts: To a stirred solution of MTO (0.04 mmol) in MeOH (2 mL) was added UHP (3 mmol), the dipolarophile (1.5 mmol) and the oxime (1 mmol). The reaction mixture was stirred at r.t. until no more oxime was detected by TLC. The solvent was evaporated, the crude mixture was added with CH₂Cl₂ and the undissolved urea was filtered off. The residue was purified by chromatography on silica gel to afford the pure products.