Reactivity of Peroxomonophosphoric Acid in the Oxidation of Benzaldehydes and Dimethyl Sulfoxide in Aqueous Ethanol

Abstract

Abstract Oxidation of benzaldehydes and dimethyl sulfoxide with peroxomonophosphoric acid (PMPA) has been studied in 40% aqueous EtOH in order to examine the reactivity of undissociated and dissociated PMPA in comparison with that of peroxycarboxylic acid. The reactivity for DMSO was in the order RCO3−>H3PO5>H2PO5−>RCO3H>HPO52−. As in the peroxycarboxylic acid oxidation, PMPA oxidation of substituted benzaldehydes afforded phenols by aryl migration and benzoic acids by hydride migration. The product ratio changed with substituents and pH; salicylaldehyde (o-HO) gave only catechol by aryl migration, while p-chloro and unsubstituted benzaldehydes afforded benzoic acids predominantly. The oxidation of aldehydes bearing p-MeO and p-Me was a borderline case and the migratory ratio of Ar∼/H∼ was higher at lower pH, i.e., the order being H3PO5>H2PO5−>HPO52−. The substituent effect in the apparent kobsd values was very small (i.e., ρ≈0), but the effect for aryl migration (kAr∼) estimated from product selectivity gave ρ=−2.88(σ). The rate for hydride migration (kH∼) resulted in ρ=1.7 and 2.0 (σ). Reactivity of PMPA in each dissociated form was compared with that of RCO3H.