Palladium-Catalyzed Allylation of Polyfluoroarenes with Allylic Pivalates

Abstract

An efficient 1,5-cyclooctadiene–PdCl₂/dicyclohexyl(2′,4′,6′-triisopropylbiphenyl-2-yl)phosphine (XPhos) catalytic system was developed for C–H allylation of polyfluoroarenes with allylic pivalates. The reactions showed excellent functional-group tolerance, good yields, and high regioselectivities. Mechanistic investigations supported a (π-allyl)palladium complex pathway through a directed oxidative addition of the allylic pivalate to palladium, followed by sequential nucleophilic attack by the polyfluorobenzene and reductive elimination. In a gram-scale reaction, a palladium loading of 0.5 mol% was enough to afford the required product in good yield.

Key words

polyfluoroarenes - allylation - palladium catalysis - C–H functionalization - fluoro compounds

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• 14 1,2,3,4,5-Pentafluoro-6-[(2E)-3-phenylprop-2-en-1-yl]benzene (4a); Typical Procedure COD–PdCl₂ (5 mol%), XPhos (10 mol%), Cs₂CO₃ (1.2 mmol, 1.2 equiv), and toluene (3 mL) were added sequentially to a Schlenk tube containing allylic pivalate (E)-3a (1.0 mmol, 1.0 equiv) and pentafluorobenzene (1a; 2.0 mmol, 2.0 equiv) under argon, and the mixture was stirred at 120 ℃ (oil bath) for 8 h until the reaction was complete (TLC). The mixture was then diluted with EtOAc (40 mL), washed with brine (3 × 20 mL), and dried (Na₂SO₄). After filtration and evaporation of the solvent, the residue was purified by chromatography (silica gel, hexane) to give a white solid; yield: 279 mg (98%); mp 62–64 °C. ¹H NMR (600 MHz, CDCl₃): δ = 7.33–7.25 (m, 4 H), 7.24–7.20 (m, 1 H), 6.47 (d, J = 15.7 Hz, 1 H), 6.21 (dt, J = 15.7, 6.7 Hz, 1 H), 3.59 (dd, J = 6.7, 1.3 Hz, 2 H). ¹³C NMR (151 MHz, CDCl₃): δ = 145.00 (dm, J = 246.4 Hz), 139.87 (dm, J = 252.4 Hz), 137.52 (dm, J = 250.4 Hz), 136.57, 132.44, 128.55, 127.64, 126.19, 124.24, 113.23 (t, J = 19.1 Hz), 25.60. ¹⁹F NMR (565 MHz, CDCl₃): δ = –143.90 (dd, J = 22.3, 8.4 Hz, 2 F), –157.23 (t, J = 22.3 Hz, 1 F), –162.48 (td, J = 22.3, 8.4 Hz, 2 F). MS (EI): m/z = 284 [M]⁺.