Metal-Free Addition of Boronic Acids to Silylnitronates

Abstract

We report for the first time a metal-free addition of boronic acids to silylnitronates to afford oxime derivatives through aryl transfer on the carbon nitrogen double bond. A reaction mechanism has been proposed in relation with a DFT study on the key aryl transfer. This arylation process is effective for cycloalkenyl nitro derivatives leading to oximes that may be oxidatively converted into 3-arylisoxazole derivatives.

Key words

nitro - silyl nitronate - boronic acid - oxime - isoxazole

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• 11 Typical Procedure: Preparation of 4a from 1a Et₃N (1.1 mmol, 0.15 ml) was added to a stirred solution of allylic nitro compound 1a (1 mmol, 141 mg) and tert-butyldimethylsilyl chloride (1.1 mmol, 166 mg) in acetonitrile (2 mL). The reaction mixture was stirred for 30 min at room temperature and quickly filtered through a short column of silica gel, eluting with dichloromethane (100 mL) to give, after evaporation of the solvent, nitronate 2a as a yellow oil formed in quantitative yield (256 mg, 1 mmol). 4-Ethylphenyl boronic acid (3a; 2 equiv, 1 mmol, 150 mg) was added to a solution of 2a (1 equiv, 0.5 mmol, 128 mg) in acetonitrile (1 mL). After degassing with argon, the reaction mixture was heated at 70 °C under argon for 30 min. Evaporation of the solvent under reduced pressure followed by purification by flash chromatography on silica gel (PE/Et₂O, 100:0 to 80:20) afforded oxime 4a as an orange solid (78 mg, 0.34 mmol, 68%); mp 161 °C; R_f = 0.35 (PE/Et₂O, 80:20). ¹H NMR (400 MHz, CDCl₃): δ = 8.97 (br, 1 H), 7.27 (d, J = 8.1 Hz, 2 H), 7.15 (d, J = 8.1 Hz, 2 H), 5.71–5.61 (m, 1 H), 2.70 (q, J = 7.6 Hz, 2 H), 2.39–2.29 (m, 2 H), 2.10–2.04 (m, 2 H), 1.71–1.54 (m, 4 H), 1.28 (t, J = 7.6 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 160.3, 144.4, 135.3, 135.0, 130.1, 128.7, 127.7, 28.8, 26.2, 24.7, 22.5, 22.2, 15.4. HRMS: m/z calcd for C₁₅H₁₉NO: 229.1467; found: 229.1464. IR (thin film): ν = 3214, 3038, 2932, 2866, 1551, 1447, 1262, 997, 968, 823, 737, 722 cm^–1.
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• 13 For more classical cycloaddition-based conversions of nitronate derivatives into isoxazoles, see: Namboothiri IN. N, Rastogi N. Isoxazolines from Nitro Compounds: Synthesis and Applications, In Synthesis of Heterocycles via Cycloadditions, Vol. 12. Gupta RR, Hassner A. Springer; Berlin/Heidelberg: 2008: 1-44