Biomimetic Oxidation of Veratryl Alcohol with H₂O₂ Catalyzed by Iron(III) Porphyrins and Horseradish Peroxidase in Ionic Liquid [1]

Abstract

3,4-Dimethoxybenzyl alcohol (veratryl alcohol) is a model compound for lignin substructures and its oxidation with H₂O₂ catalyzed by iron(III) porphyrins and horseradish peroxidase (HRP) in ionic liquids has been described. Veratraldehyde and 2-hydroxymethyl-5-methoxy-2,5-cyclohexadiene-1,4-dione were the two major products of the oxidation of veratryl alcohol at room ­temperature. The iron(III) porphyrin or HRP immobilized in ionic liquids showed increased catalytic activity and can be recycled and this is believed due to the stabilization of highvalent oxo­iron(IV) π-cation radical intermediate generated in the reaction.

ionic liquid - veratryl alcohol - oxidation - iron(III) porphyrins - horseradish peroxidase (HRP)

References and Notes

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Part of the work was presented at: Modern Trends in Inorganic Chemistry-X (MTIC-X), Bombay, India, December, 2003.

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Representative Procedure for Oxidation of Veratryl Alcohol ( 1) with H ₂ O ₂ Catalyzed by Iron(III) Porphyrins in Ionic Liquid: H₂O₂ (30%; 113 µL, 100 mmol) was added with stirring at r.t to a solution of water-soluble iron(III) porphyrin (1 mmol, 10%) and veratryl alcohol (100 mmol) in the ionic liquid [bmim][PF₆] (2.0 mL). The 20-µL aliquots were taken at 30 min interval and analyzed by HPLC analysis at 280 nm. After completion of the reaction the reaction mixture was extracted with Et₂O and the ionic liquid containing iron(III) porphyrin was recovered. The Et₂O layer was dried over anhyd Na₂SO₄ and evaporated under reduced pressure. The residue was column chromatographed over silica gel and the products were identified by ¹H NMR, IR and MS spectral data.
Representative Procedure for Oxidation of Veratryl Alcohol ( 1) with H ₂ O ₂ Catalyzed by HRP in Ionic Liquid: Veratryl alcohol (100 mmol) was dissolved in the ionic liquid [bmim][PF₆] (2.0 mL) in a 5-mL round-bottomed flask fitted with a magnetic stirrer. HRP [100 µL, 1.0 mg dissolved in 1 mL buffer (pH 4.5)] was added to the reaction mixture and H₂O₂ (30%; 113 µL, 100 mmol) was added to the reaction mixture with stirring. The stirring was continued at r.t. and 20-µL aliquots were taken at 30 min interval and analyzed by HPLC at 280 nm. After completion of the reaction the reaction mixture was extracted with Et₂O and the ionic liquid containing HRP was recovered. The Et₂O layer was dried over anhyd Na₂SO₄ and evaporated under reduced pressure. The residue was column chromatographed over silica gel and the products were identified by ¹H NMR, IR and MS spectra.

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Spectroscopic Data3,4-Dimethoxybenzaldehyde (Veratraldehyde; 2): IR (KBr): 2987, 2934, 1729, 1623, 1511, 1256, 1167, 1046, 986, 913, 789, 725 cm^-1. ¹H NMR (CDCl₃): δ = 3.89 (s, 6 H), 6.86 (s, 1 H), 6.98 (d, J = 7.35 Hz, 1 H), 7.03 (d, J = 7.35 Hz, 1 H), 10.67 (s, 1 H). MS (ESI): m/z = 166 [M⁺], 151, 138, 136, 123, 94, 73.
2-Hydroxymethyl-5-methoxy-2,5-cyclohexadiene-1,4-dione ( 3): IR (KBr): 3310, 2948, 1665, 1644, 1619, 1607, 1226, 1144 cm^-1. ¹H NMR (CDCl₃): δ = 3.85 (s, 3 H), 3.92 (s, 1 H), 4.52 (d, J = 2.3 Hz, 2 H), 6.10 (s, 1 H), 6.78 (t, J = 2.3 Hz, 1 H). MS (ESI): m/z = 168 [M⁺], 153, 150, 44.