N-Hydroxyphthalimides and Metal Cocatalysts for the Autoxidation of p-Xylene to Terephthalic Acid

N-Hydroxyphthalimide (NHPI) and its derivatives, such as 3-F-NHPI, 4-Me-NHPI, N-acetoxyphthalimide, and N,N-dihydroxypyromelitimide, were used as promoters with Co(OAc)2 catalyst for the autoxidation of p-xylene (pX) and other methyl arenes. All the promoters gave acceptable rates and yields of terephthalic acid. The initial reaction rates, measured by the rate of oxygen uptake, were analyzed by a rate equation in terms of [pX], [Co(II)], and [NHPI]. The metal cocatalysts Mn(II) and Ce(III) accelerated the reaction significantly at millimolar concentrations. The reaction occurs by a chain mechanism that involves formation of the phthalimide N-oxyl radical, PINO• (that is, R2NO•), which abstracts a hydrogen atom from the methyl group of p-xylene to form the carbon-centered radical ArCH2•. In a stepwise fashion, the sequence progresses through alcohol, aldehyde, and carboxylic acid; at each stage, C−H abstraction by PINO• is involved. A significant kinetic isotope effect on the overall oxidation of p-xylene was found, vi(H)/vi(D) = 3.4. The activity of the substituted NHPI promoters follows the order NHPI > 3-F-NHPI > 4-Me-NHPI, which can be interpreted in terms of kinetic stability of the corresponding PINO radical.