Manganese-Mediated Aerobic Oxidative Denitroalkylation of β-Nitrostyrenes with Alkylboronic Acids

Abstract

An aerobic oxidative stereoselective denitroalkylation of β-nitrostyrenes under mild conditions by using an alkylboronic acid as the alkyl source was achieved. This protocol accepts a broad scope of β-nitrostyrenes and alkylboronic acids, leading to the formation of C(sp³)–C(sp²) bonds. Air was employed to reoxidize the resultant Mn(II) species back to Mn(III) during the catalytic cycle.

Key words

manganese catalysis - denitroalkylation - alkylboronic acids - nitrostyrenes - radical reaction - aerobic oxidation

• References and Notes

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• 11 [(E)-2-Cyclohexylvinyl]benzene (3a); Typical Procedure A 10 mL Schlenk tube equipped with a magnetic stirrer bar was charged with β-nitrostyrene (1a; 0.2 mmol), cyclohexylboronic acid (2a; 0.6 mmol, 3.0 equiv), Mn(OAc)₃·2 H₂O (0.2 mmol, 1.0 equiv), ligand L6 (0.2 mmol, 1.0 equiv), TFA (0.4 mmol, 2.0 equiv), and DCE (2 mL). The mixture was then stirred at 60 °C under air for 12 h until the reaction was complete (TLC). 10% aq NaHCO₃ (20 mL) was added, and the mixture was extracted with CH₂Cl₂ (3 × 10 mL). The combined extracts were washed with brine, dried (Na₂SO₄), and concentrated under reduced pressure. The crude residue was purified by column chromatography (silica gel, EtOAc–hexane) to give a colorless oil; yield: 87%. ¹H NMR (300 MHz, CDCl₃): δ = 7.42–7.30 (m, 4 H), 7.28–7.19 (m, 1 H), 6.40 (d, J = 16.0 Hz, 1 H), 6.23 (dd, J = 16.0, 6.8 Hz, 1 H), 2.21–2.15 (m, 1 H), 1.89–1.71 (m, 5 H), 1.45–1.34 (m, 2 H), 1.29–1.16 (m, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 138.11, 136.93, 128.53, 127.26, 126.80, 126.00, 41.25, 33.02, 26.25, 26.13. HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₁₄H₁₉: 187.1482; found: 187.1479.