A New Class of Squaramide-Containing Phase-Transfer Catalysts: Application to Asymmetric Fluorination of β-Keto Esters

Abstract

The squaramide moiety has proven to be a powerful hydrogen-bonding donor in organocatalysis, but chiral phase-transfer catalysts bearing the squaramide motif have not been reported so far. Reported herein is this kind of catalyst and their catalytic performance in the asymmetric fluorination of β-keto esters.

Key words

organocatalysis - phase-transfer catalysis - asymmetric ­catalysis - fluorination - squaramides

• References and Notes

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• 11 Catalyst 2e Light yellow solid; mp 173–174 °C; [α]_D ²⁵ –18.8 (c 0.5, MeOH). ¹H NMR (300 MHz, CD₃OD): δ = 8.84 (s, 1 H), 8.15–7.91 (m, 4 H), 7.85 (s, 1 H), 7.65–7.31 (m, 5 H), 7.23–6.92 (m, 2 H), 6.01–5.81 (m, 1 H), 5.41–4.95 (m, 4 H), 4.27–4.01 (m, 4 H), 3.76–3.41 (m, 3 H), 2.89–2.62 (m, 1 H), 2.32–2.07 (m, 3 H), 2.01–1.94 (m, 1 H), 1.30 (s, 21 H, 2 t-Bu⁺). ¹³C NMR (101 MHz, MeOD): δ = 185.95, 181.75, 168.26, 166.86, 160.82, 153.20, 148.62, 145.65, 144.71, 141.77, 138.80, 137.52, 133.56 (q, J = 33.3 Hz), 132.24, 129.85, 129.13, 128.32, 126.23, 125.66, 125.60, 123.21 (q, J = 332 Hz), 122.96, 119.25, 116.79, 102.00, 70.88, 69.93, 61.27, 56.92, 55.21, 50.82, 38.54, 35.73, 31.68, 28.07, 25.68, 21.47. ESI-HRMS: m/z calcd for C₃₉H₃₅F₆N₄O₃ [M – Br]⁺: 721.2608; found: 721.2606.
• 12 Typical Procedure for the Asymmetric Fluorination of β-Keto Esters Using Catalyst 2e To a solution of the β-keto ester (0.1 mmol) and catalyst 2e (4.6 mg, 5 mol%) in Et₂O (1 mL) was added 2 M K₂CO₃ solution (100 μL), then the solution was cooled to –10 °C. NFSI (37.84 mg, 0.12 mmol) was added in one portion and the mixture was stirred vigorously for 4 h, quenched with 10 mL sat. NH₄Cl solution and extracted with Et₂O (10 mL). The organic layer was dried with anhydrous Na₂SO₄ and concentrated. The crude product was purified by column chromatography on silica gel (PE–EtOAc).