The Role of Ni-Carboxylate During Catalytic Asymmetric Iodolactonization Using PyBidine-Ni(OAc)₂

Abstract

The combination of a PyBidine-Ni(OAc)₂ complex with a catalytic amount of iodine efficiently catalyzed asymmetric iodo­lactonization to generate chiral iodolactones with up to 89% enantiomeric excess. The formation of an intermediate Ni-carboxylate species from the alkenyl carboxylic acid is a key role in promoting the iodolactonization.

Key words

asymmetric catalysis - iodolactone - nickel - carboxylate - ligands

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• 14 Analytical data for 2a: R_f = 0.40 (hexane–EtOAc, 2:1). ¹H NMR (500 MHz, CDCl₃): δ = 7.41–7.33 (m, 5 H), 3.58 (dd, J = 12.3, 11.2 Hz, 2 H), 2.57–2.33 (m, 4 H), 1.84–1.80 (m, 1 H), 1.60–1.56 (m, 1 H); ¹³C NMR (125 MHz, CDCl₃): δ = 170.5, 140.3, 129.1, 128.5, 125.3, 84.5, 32.1, 29.1, 17.8, 16.6; HRMS: m/z [M + H]⁺ calcd for C₁₂H₁₄O₂I: 317.0033; found: 317.0031; 78% ee; [α]_D ²⁴ +27.5 (c = 1.2, CHCl₃); enantiomeric excess was determined by HPLC analysis with a Chiralpak AD-H column (hexane–i-PrOH, 95:5; 1.0 mL/min; 250 nm): R _t = 14.2 (major enantiomer), 15.8 (minor enantiomer) min.
• 15 Enantioselective Iodolactonization Catalyzed by PyBidine-Ni(OAc)₂ Complex; General Procedure: A mixture of PyBidine (7.3 mg, 0.0105 mmol) and Ni(OAc)₂·4H₂O (2.5 mg 0.01 mmol) was stirred at r.t. for 3 h in anhydrous CH₂Cl₂ (1.0 mL). After cooling the mixture to –78 °C, a carboxylic acid (0.1 mmol) in toluene (3.0 mL) was slowly added and the mixture was stirred for 0.5 h at the same temperature. I₂ (5.0 mg, 0.02 mmol) and NIS (24.6 mg, 0.11 mmol) were added and the mixture was stirred for 22–48 h. The reaction was quenched by the addition of saturated aq Na₂SO₃ and 1 M aq NaOH, and extracted with CH₂Cl₂ (3×). The collected organic layer was dried over Na₂SO₄. After removal of the solvent under reduced pressure, the residue was purified by silica-gel column chromatography (hexane–EtOAc, 6:1) to afford the iodolactone. The enantiomeric excess of the product was determined by HPLC analysis.