Kinetics of Self-Decomposition and Hydrogen Atom Transfer Reactions of Substituted Phthalimide N-Oxyl Radicals in Acetic Acid

Abstract

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO(.)) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO(.) which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO(.), e.g., PINO(.) + 4-Me-NHPI <=> NHPI + 4-Me-PINO(.). The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO(.) from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO(.) is more electron donating. Analysis by the Hammett equation gave rho = 1.1 and 1.8 for the reactions of PINO(.) with p-xylene and toluene, respectively.

Hydrogen abstraction from H‐donor substrates by the 6‐CF3‐benzotriazol‐N‐oxyl radical (TFNO)

“…[9,14,15,17,18,25] The contribution from tunnelling is clear, as also documented elsewhere. [14,17] c -Products study for reaction of TFNO with a probe substrate.…”

“…[14,16,17] Summing it up, the reactivity features of TFNO in the HAT process (Table 2) are akin to those of other aminoxyl radicals, [10] including the finding of an accelerating effect from electron-donor and the retarding effect from electron-withdrawing substituents on the benzylic substrate. Significant figures for this comparison of reactivity among aminoxyl radicals are selected and given in Table 3, proper allowance being made to the different solvents employed (no major effects upon the rates are however expected in these radical processes [14,15,25] ).…”

“…1), and the synthetic value of PINO-induced oxidations is recognised and has prompted studies upon the reactivity features of the aminoxyl radical intermediates. [9,10,12,13] Kinetic surveys have been already performed with PINO or else with its X-aryl-substituted cognates, [13][14][15][16][17] and have been recently complemented by a kinetic investigation with another aminoxyl radical, that is benzotriazole N-oxyl (BTNO), [18] originating from precursor 1-hydroxybenzotriazole (HBT). [19] Another kinetic study had been reported with bis(trifluoromethyl)aminoxyl radical, [20] (CF 3 ) 2 N-O .…”

A New and Efficient Heterogeneous System for the Phthalimide N‐Oxyl (PINO) Radical Generation

“…However, Espenson and co-workers, have recently reported [26] that the rate constants for hydrogen atom abstraction by PINO from p-xylene and toluene using NHPI/Co III were "slightly different" from those obtained using NHPI/Pb(OAc) 4 .…”

A spray reactor concept for catalytic oxidation of p-xylene to produce high-purity terephthalic acid

“…Chester et al [68] found that the optimum Co/Zr ratio in neat acetic acid was 6-7 for maximum TPA yield and minimum induction period. Promotors such as CoBr 2 -MnBr 2 [69,70] instead of HBr as a bromine source, N-hydroxyphthalimide [71,72], and N-hydroxy succinimide [73,74] have also been reported for pX oxidation to TPA under milder conditions (~100 °C), but the pX conversion and TPA selectivity are below the typical industrial values.…”