Palladium-Catalyzed Wacker-Type Oxidation of N-Boc Indoles under Mild Conditions

Abstract

Palladium-catalyzed Wacker-type oxidation of N-Boc indoles for the synthesis of 3-oxyindolines has been developed. The palladium-catalyzed Wacker-type oxidation of N-Boc indoles can readily occur in MeCN using H₂O₂ as oxidant. And 3-oxyindolines were obtained in medium to high yield. Furtherly, the mechanism of palladium-catalyzed Wacker-type oxidation of N-Boc indoles was proposed.

Key words

palladium - catalysis - Wacker-type oxidation - indoles - 3-oxyindolines

• References and Notes

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Sigman’s work of Pd-catalyzed Wacker oxidation:
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• 15 Procedure for Wacker-Type Oxidation of N-Boc Indoles A mixture of N-Boc indole 1 (0.25 mmol) and Pd(acac)₂ (4.6 mg, 0.015 mmol, 6.0 mol%) in MeCN (3 mL) was added into a Schlenk flask (25 mL) and stirred at room temperature, followed by addition of NaIO₄ (80.2 mg, 0.375 mmol, 1.5 equiv), and the mixture was stirred at 70 °C until the reaction was finished. Then the solvent was evaporated under reduced pressure, and the residue was purified by column chromatography. tert-Butyl 2-Methyl-3-oxoindoline-1-carboxylate (2a) Yield 94%, 56.9 mg, colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.54–1.61 (m, 12 H), 4.22–4.24 (m, 1 H), 7.13 (dd, J = 7.6, 8.5 Hz, 1 H), 7.61–7.72 (m, 1 H), 7.71 (d, J = 7.6 Hz, 1 H), 8.17 (br, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 17.0, 28.3, 61.7, 116.9, 123.0, 124.0, 137.1, 150.9, 199.6. IR (neat): 2977, 2932, 1712, 1606, 1468, 1377, 1296, 1275, 1159, 1095, 1054, 978, 850, 757, 737 cm^–1. HRMS (EI): m/z calcd for C₁₄H₁₇NO₃: 247.1208 [M]⁺; found: 247.1198.