Enantioselective Sulfide Oxidation with H₂O₂: A Solid Phase and Array Approach for the Optimisation of Chiral Schiff Base-Vanadium Catalysts

Abstract

Two libraries of chiral Schiff base ligands were synthesised and screened in the vanadium-catalysed oxidation of alkyl aryl sulfides with hydrogen peroxide as terminal oxidant. The vanadium-chiral Schiff base complex 10, readily prepared from 3,5-diiodo-salicylaldehyde and (S)-tert-leucinol, was found to be highly enantioselective. Optically active sulfoxides could thus be obtained in good yields with up to 97% ee.

Key words

asymmetric catalysis - Schiff bases - hydrogen peroxide - enantioselective sulfoxidation - supported catalysts

References

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13

Compound 5: ¹H NMR (CDCl₃, 250 MHz) δ = 10.30 (s, 1 H), 7.15 (m, 2 H), 6.08 (ddt, 1 H, J = 17.0, 10.5, 5.0 Hz), 5.52 (dd, 1 H, J = 17.0, 1.5 Hz), 5.33 (dd, 1 H, J = 10.5, 1.5 Hz), 4.46 (br. d, 2 H, J = 5.0 Hz), 3.98 (br. t, 2 H, J = 5.0 Hz), 2.46 (br. t, 2 H, J = 7.0 Hz), 1.85 (m, 4 H), 1.41 (s, 9 H); ¹³C NMR (CDCl₃, 62 MHz) δ = 190.4, 179.2, 156.5, 154.8, 145.6, 132.7, 130.3, 122.5, 117.5, 108.3, 79.9, 67.7, 35.2, 33.5, 30.7 (3 C), 28.5, 21.4; MS (ES) m/z = 335 (M + H⁺), 279, 251, 233.

15

Chiral amino acids failed to react with supported salicylaldehyde 6.

16

Typical procedure: Solid supported Schiff base 7 (6 µmol, 0.012 equiv) was weighed in an Alltech tube and the resin was swollen in CH₂Cl₂ for 1 h. A 0.04 M solution of VO(acac)₂ in CH₂Cl₂ (1 mL, 40 µmol) was added and the mixture was shaken for 1 h. The solution was filtered and the resin washed with CH₂Cl₂ (5 × 2 mL) and transferred into a reaction test tube. A 0.5 M solution of thioanisole (1 mL, 0.5 mmol, 1 equiv) and 1,2,3-trimethoxybenzene (0.1 mmol, 0.2 equiv, internal standard) in CH₂Cl₂ was added, followed by 7% H₂O₂ in H₂O (240 µL, 1.1 equiv). The reaction mixture was stirred for 16 h and analysed by chiral HPLC (Chiralcel OD-H, 5% EtOH in heptane, 1 mL/min, 227 nm). Retention times: 4.2 min(thioanisole), 7.1 min (internal standard), 11.7 min (R-methyl-phenylsulfoxide), 13.1 min (S-methyl-phenylsulfoxide), 14.3 min (methyl-phenylsulfone).

18

4-Bromo-1-hydroxy-2-naphthaldehyde was prepared by bromination of 1-hydroxy-2-naphthaldehyde with N-bromosuccinimide according to a literature procedure [19] and isolated in 60% yield. Spectroscopic data: ¹H NMR (CDCl₃, 250 MHz) δ = 12.60 (s, 1 H), 9.92 (s, 1 H), 8.49 (d, 1 H, J = 8.5 Hz), 8.19 (d, 1 H, J = 8.5 Hz), 7.80 (t, 1 H, J = 8.5 Hz), 7.80 (s, 1 H), 7.63 (t, 1 H, J = 8.5 Hz); ¹³C NMR (CDCl₃, 62 MHz) δ = 195.2, 161.3, 135.5, 131.8, 129.4, 127.2, 126.9, 125.8, 124.8, 114.8, 112.1.

20

Compound 10: ¹H NMR (CDCl₃, 250 MHz) δ = 14.85 (br. s, 1 H), 8.10 (s, 1 H), 8.00 (d, 1 H, J = 2.0 Hz), 7.51 (d, 1 H, J = 2.0 Hz), 3.99 (dd, 1 H, J = 11.5, 2.5 Hz), 3.69 (dd, 1 H, J = 11.5, 9.5 Hz), 3.07 (dd, 1 H, J = 9.5, 2.5 Hz), 1.00 (s, 9 H); ¹³C NMR (CDCl₃, 62 MHz) δ = 166.5, 164.6, 149.9, 141.0, 117.0, 92.6, 78.2, 75.9, 61.8, 32.9, 26.8 (3 C); MS (ES) m/z = 474 (M + H⁺).

21

Compound 11: ¹H NMR (CDCl₃, 250 MHz) δ = 13.57 (br. s, 1 H), 8.39 (d, 1 H, J = 8.0 Hz), 7.98 (d, 1 H, J = 8.0 Hz), 7.70 (m, 1 H), 7.67 (t, 1 H, J = 8.0 Hz), 7.49 (t, 1 H, J = 8.0 Hz), 7.01 (s, 1 H), 4.06 (dd, 1 H, J = 11.5, 3.0 Hz), 3.76 (br. t, 1 H, J = 10.5 Hz), 3.16 (m, 1 H), 1.07 (s, 9 H); ¹³C NMR (CDCl₃, 62 MHz) δ = 177.1, 162.2, 135.9, 131.5, 130.7, 127.7, 126.1, 125.5, 109.4, 107.0, 75.2, 62.3, 33.5, 27.2 (3 C); MS (ES) m/z = 350 and 352 (M + H⁺).

22

Typical experimental procedure: To a 0.03 M solution of ligand in CH₂Cl₂ (0.25 mL, 7.5 µmol, 0.015 equiv) was added a 0.02 M solution of VO(acac)₂ in CH₂Cl₂ (0.25 mL, 5 µmol, 0.01 equiv) and the resulting mixture was stirred at r.t. for 30 min. A 1 M solution of sulfide in CH₂Cl₂ (0.5 mL, 0.5 mmol, 1 equiv) was added and after 30 min stirring at r.t., the reaction mixture was cooled down to 0 °C. After 15 min at 0 °C, 27% H₂O₂ in H₂O (65 µL, 1.2 mmol, 1.2 equiv) was added dropwise. The mixture was stirred at 0 °C for 16 h and the solvent evaporated. The crude residue was purified by column chromatography (silica gel, EtOAc-cyclohexane).