Oxidation of isoeugenol to vanillin by the “H2O2–vanadate–pyrazine-2-carboxylic acid” reagent

Isoeugenol [2-methoxy-4-(prop-1-en-1-yl)phenol; compound 1] can be oxidized to vanillin (4-hydroxy-3-methoxybenzaldehyde, compound 2) by H2O2 in the presence of the n-Bu4NVO3/pyrazine-2-carboxylic acid catalytic combination. A 50% yield of 2 was attained in a 2-h reaction at 80 °C, with a turnover number being higher than 1200. Kinetics of the transformation of 1 into 2 have been studied. The results obtained indicate that the first step of the 1 activation is its interaction with a hydroxyl radical. Further transformations of the primary product formed in this step proceed via several routes, one of which leads to the formation of 2. It has been proposed that the interaction of vanadate with the first molecule of H2O2 gives a V(IV)-containing species and hydroperoxyl radical: “VV–OOH” → “VIV” + HOO. In the next step, the V(IV) complex reacts with the second molecule of H2O2 affording a hydroxyl radical and recovering a starting V(V)-containing species: “VIV” + H2O2 → “VV” + HO− + HO. Then, the hydroxyl radical attacks a double bond in the substrate: HO + CHR′CHR″ → CHR′–CH(OH)R″. The carbon-centered radical formed in the latter step can rapidly capture a molecule of O2 from the atmosphere: CHR′–CH(OH)R″ + O2 → C(OO)HR′–CH(OH)R″. Finally, a rupture of the CC bond gives rise to the formation of aldehyde 2.

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► Combination n-Bu4NVO3/pyrazinecarboxylic acid catalyzes the oxidation of isoeugenol.
► A 50% yield of vanillin was attained with a turnover number being higher than 1200.
► A kinetic study indicates the participation of hydroxyl radicals.