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A study of manganese (III) oxidation of hindered phenols Poh, Bo Long

Abstract

The complex of manganese(ill) with trans-1,2-diaminocyclohexane- tetraacetic acid was prepared and its formula shown to be KMn[sup III] CyDTA(H₂O), where CyDTA is an abbreviation for trans-1,2-diaminocyclohexanetetraacetic acid anion. A mechanistic study of the oxidation of 2,4,6-tri-t-butylphenol with this complex was carried out in methanol under three conditions: (1) in the presence of acid; (2) in the presence of base; (3) in the absence of acid and base. Under acidic conditions, the oxidant HMn[sup III] CyDTA(H₂O) and the rate-determining step of the reaction involves a hydrogen atom abstraction from the O-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic isotope effect of 4.8 was obtained at 25°. The activation parameters are ΔH =15.7 Kcal.mole⁻¹ and ΔS* = -10.4 e.u. The small effect observed for substitution in the para position is consistent with the radical character of the reaction. A ℓ value of 0.39 was obtained from a Hammett plot of log k₃/k[sub 3H] versus σ[sub P]. Under basic conditions, the oxidant is Mn[sup III]CyDTA(OH)²⁻ and there are two competitive reactionst one that involves electron-transfer from 2,4,6-tri-t-butylphenoxy anion to Mn[sup III]CyDTA(OH)²⁻ and another that involves hydrogen atom abstraction from 2,4,6-tri-t-butylphenol by Mn[sup III]CyDTA(OH)²⁻. The activation parameters are ΔH* = 10.2 Kcal.mole⁻¹, ΔS* = -23 e.u. for the former reaction; ΔH* =10.2 Kcal.mole⁻¹, ΔS* = -29 e.u. for the latter reaction. A kinetic isotope effect of 1.9 was obtained for the latter reaction at 25°. The rate of electron-transfer from 2,4,6-tri-t-butylphenoxy anion to Mn[sup III]CyDTA(OH)²⁻ is 104 times faster than the rate of hydrogen atom abstraction from 2,4,6-tri-t-butyl- phenol by Mn[sup III]CyDTA(OH)²⁻. The effect on reaction rate for substitution in the para position is large, a ℓ value of 2.3 being obtained from a Hammett plot of log k₉/k[sub 9H] versus σ[sub p]⁻ (k₉ is the rate constant for the reaction between 2,4,6-tri-t-butylphenol and Mn[sup III]CyDTA(OH)²⁻). Under neutral conditions, the oxidant is Mn[sup III]CyDTA(H₂O) and the rate-determining step of the reaction involves hydrogen atom abstraction from the O-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic isotope effect of 5-9 was obtained at 25°. The activation parameters are ΔH* = 10.9 Kcal.mole⁻¹ and ΔS* = -39 e.u. The small effect observed for substitution in the para position is consistent with the radical character of the reaction, a ℓ value of -0.07 being obtained from a Hammett plot of log k₃/k[sub 3aH] versus σ[sub p]. The compound 3,5-di-t-butyl-4-hydroxybenzaldehyde, however, has an anomalously high rate. This anomaly is also observed under acidic conditions. The oxidising power of Mn[sup III]CyDTA(OH)²⁻ is 360 times greater than that of Mn[sup III]CyDTA(H₂O) with respect to hydrogen atom abstraction from 2,4,6-tri-t-butylphenol. This difference in oxidising strength suggests that an 'inner-sphere'-type mechanism is involved in the reactions. The difference in oxidising strength between Mn[sup III]CyDTA(OH)²⁻ and Mn[sup III]CyDTA(H₂O)⁻ is even greater in 95 % dimethyl sulfoxide solvent -5.4 x 10⁷. Oxidation of 2,4,6-tri-t-butylphenol with potassium ferrlcyanide in methanol in the presence of hydrochloric acid is shown to be a reversible reaction. The lack of data on the ionization constants of H₄Fe(CN)₆ and H₃Fe(CN)₆ prevented a detailed study of the equilibria involved.

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