Anion-Stoichiometry-Dependent Selectivity Enhancement in Ion-Paired Chiral Ligand–Palladium Complex Catalyzed Enantioselective Allylic Alkylation

Abstract

The ratio of chiral Brønsted acid to ammonium–phosphine hybrid ligand in the in situ preparation of ion-paired chiral ligands was found to have a notable effect on the stereocontrolling ability of the corresponding palladium complex. The use of supramolecular palladium complexes generated from palladium metal, ammonium phosphine, and a chiral phosphoric acid in a ratio of 1:2:3 enabled an excellent level of enantiocontrol in the catalytic asymmetric allylation of α-nitrocarboxylates with a functionalized allylic carbonate.

Key words

ion pair - chiral ligand - supramolecular catalyst - palladium - allylic alkylation

• References and Notes

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• 11 General Procedure for Asymmetric Allylation of Nitroester with Allylic Carbonate To a Schlenk flask were added Pd₂(dba)₃·CHCl₃ (1.29 mg, 1.25 μmol), ammonium phosphine 1·HSO₄ (2.8 mg, 5 μmol), chiral phosphoric acid 2b (5.1 mg, 7.5 μmol), and K₂CO₃ (1.4 mg, 0.010 mmol). The flask was degassed by alternating vacuum evacuation/Ar backfill. After the addition of toluene (1.0 mL), the resulting catalyst mixture was evacuated and refilled with Ar three times. The reaction flask was then warmed up to 30 °C, and nitroester 4a (25.1 mg, 0.10 mmol) and allylic carbonate 5 (30.2 mg, 0.12 mmol) were successively introduced. After stirring for 24 h at the same temperature, the reaction mixture was cooled to r.t. Filtration through the short silica gel pad with the aid of EtOAc and concentration of the filtrate were performed. The reaction mixture was purified by column chromatography on silica gel (hexane–EtOAc = 20:1 as eluent) to afford 6a (38.4 mg, 0.099 mmol, 99% yield, 98% ee) as a colorless liquid. Compound 6a: [α]_D ²⁴ –3.08 (c 1.0, CH₃OH) for 99% ee. ¹H NMR (400 MHz, CDCl₃): δ = 7.34 (2 H, t, J = 2.3 Hz), 7.35–7.30 (3 H, m), 7.13 (1 H, dt, J = 15.2, 7.6 Hz), 7.16–7.11 (2 H, m), 6.64 (1 H, dt, J = 15.2, 1.5 Hz), 6.34 (2 H, t, J = 2.3 Hz), 3.65 (1 H, d, J = 14.4 Hz), 3.50 (1 H, d, J = 14.4 Hz), 3.03 (1 H, ddd, J = 15.5, 7.6, 1.5 Hz), 2.99 (1 H, ddd, J = 15.5, 7.6, 1.5 Hz), 1.48 (9 H, s). ¹³C NMR (151 MHz, CDCl₃): δ = 164.6, 161.8, 142.6, 132.8, 130.1, 129.0, 128.3, 124.7, 119.4, 113.9, 95.7, 85.5, 40.3, 36.6, 27.8. IR (film): 2980, 1744, 1699, 1553, 1468, 1352, 1296, 1283, 1152, 743 cm^–1. HRMS (ESI, +): m/z calcd for C₂₁H₂₄O₅N₂Na⁺ [M + Na]⁺: 407.1577; found: 407.1581. HPLC (OX3, hexane–2-PrOH = 10:1, flow rate = 0.5 mL/min, λ = 210 nm): t _R (major) = 12.4 min; t _R (minor) = 13.5 min.