Stoichiometry and Kinetics of p-Methoxytoluene Oxidation by Electron Transfer. Mechanistic Dichotomy between Side Chain and Nuclear Substitution.

Article abstract of DOI:10.1021/jo00191a019

The oxidation of p-methoxytoluene by tris(phenanthroline)iron(III) or Fe(phen)3³⁺ in acetonitrile containing pyridine bases affords both side chain and nuclear substitution products in the form of the isomeric N-benzylpyridinium ion I and N-anisylpyridinium ion II, respectively.The relative rates of formation of I and II show an unusual and pronounced dependence on the structure of the base-pyridine yielding mainly II, and 2,6-lutidine producing exclusively I.The mechanistic dichotomy between these side chain and nuclear substitution products is resolved by the complete analysis of the complex oxidation kinetics.Thus the mechanism in Scheme II involves initial electron transfer from p-methoxytoluene to Fe(phen)3³⁺ to afford the p-methoxytoluene cation radical +.> as a reactive intermediate which can be directly observed by transient ESR spectroscopy.Products I and II arise from +.> and the pyridine base in a follow-up step with a second-order rate constant k₂.The base depedence of k² together with the deuterium kinetic isotope effect for deprotonation help to delineate the separate transition states leading to side chain and nuclear substitution.The kinetics also allow the evaluation of the rate constant k¹ for electron transfer from p-methoxytoluene to Fe(phen)3³⁺.The correlation of log k₁ with analogous values for electron transfer from a series of polymethylated benzenes according to the linear free energy relationship from the Marcus rate theory shows the essential outer sphere character of the transition state for electron transfer.