Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols

Abstract

The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO* is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degrees C. The reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma+ (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degrees C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E(a)(D) - E(a)(H), was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.

Remarkable Effect of PEG-1000-based Dicationic Ionic Liquid for N-hydroxyphthalimide-catalyzed Aerobic Selective Oxidation of Alkylaromatics

“…20 Then a series of substituted toluene derivatives was examined and the conversions were better than that in acetic acid. 8,21 Generally, para-or ortho-substituted toluene (Run 3-4, 9-10 and 12-17) converted into the corresponding benzoic acid in excellent yields (>85), no matter whether the substituents were electron-donating or electron-withdrawing. However, meta-substitution of the phenyl group decreased the degree of conversions dramatically.…”

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

“…[18] [24] Therefore, a higher HAT reactivity for weaker bonds can be inferred, a result consistent with analogous findings for PINO and BTNO. [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] ).…”

“…[9] For example, PINO was generated from precursor N-hydroxyphthalimide (HPI) by oxidation with Pb(OAc) 4 in AcOH solution, and showed an electronic spectrum with maximum of absorbance at 380 nm. [14] It underwent spontaneous decay with a half-life of 7900 s. Analogously, BTNO was generated from λ max at 474 nm and a spontaneous decay with half-life of 110 s. [19] Additional characterization of these aminoxyl radicals was possible by means of laser flash photolysis, cyclic voltammetry and EPR spectroscopy. [9,10] The latter technique was the one formerly employed in the kinetic investigation with BTFN ( Fig.…”

“…This is indeed a small slope, as expected for a radical process, [18] and compares well with a ρ value of -0.55 for the reaction of BTNO vs the same substrates in MeCN, [19] as well as with a ρ of -0.41 for PINO whenever generated from HPI by Pb(OAc) 4 in AcOH solution. [14] Additional ρ values (in the -0.5 to -0.7 range) are reported for X-substituted PINO's generated from X-substituted HPIs in MeCN solution. [17] In all these cases better Hammett correlations are obtained vs the σ + parameter of the X-substituents than with the simple σ parameter, in line with the electrophilicity of the aminoxyl radicals.…”

“…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 .…”