Enantioselective Fenton-type oxidation of glycerol using chiral Iron(II) and Copper(II) (L)-Malate complexes: Preliminary evidence for asymmetric induction

We report the enantioselective Fenton-type oxidation of glycerol (GLY, 5 g, 50 mL H ₂ O) catalysed by iron(II) (L)-malate (188 mg) and copper(II) (L)-malate (180 mg) complexes in the presence of hydrogen peroxide (15 mL, 30 wt%) and catalytic sulfuric acid (2 drops, 98 wt%), at 50 °C for 30 min. The reaction was initiated exclusively upon addition of H ₂ SO ₄ ; without acid, no measurable optical activity was detected. Optical rotation measurements (589 nm, 1 dm cell, aqueous solution) suggested the formation of optically active oxidation products consistent with preferential formation of (R)-glyceraldehyde (GLA): [α] ²⁰ _D = +5.0° for the iron system (apparent ee ≈ 57.5%) and [α] ²⁰ _D = +2.5° for the copper system (apparent ee ≈ 28.7%), referenced provisionally to [α] _pure = ±8.7° for pure glyceraldehyde enantiomers. The optical rotation was within ±10% upon halving or doubling the catalyst loading, consistent with optical activity that is relatively insensitive to catalyst loading under the conditions studied. The higher ee attained with iron is attributed to the classical Fenton mechanism (Fe ²⁺ /Fe ³⁺ cycle, k ≈ 76 M ^-1 s ^-1 ), which generates •OH radicals within a more persistent chiral coordination shell, whereas copper operates via a Haber–Weiss/Cu-Fenton pathway with a less rigid Cu ²⁺ /Cu ⁺ redox couple. These results provide preliminary evidence for acid-triggered, chiral-metal-malate-induced optical activity in the Fenton oxidation of glycerol, and a working mechanistic hypothesis for the observed iron/copper differential in optical rotation.