Kinetics of N-oxyl Radicals’ Decay

Abstract

N-oxyl radicals of various structures were generated by oxidation of corresponding N-hydroxy compounds with iodobenzene diacetate, [bis(trifluoroacetoxy)]iodobenzene, and ammonium cerium(IV) nitrate in acetonitrile. The decay rate of N-oxyl radicals follows first-order kinetics and depends on the structure of N-oxyl radicals, reaction conditions, and the nature of the solvent and oxidant. The values of the self-decay constants change within 1.4 × 10 −4 s −1 for the 3,4,5,6-tetraphenylphthalimide-N-oxyl radical to 1.4 × 10 −2 s −1 for the 1-benzotriazole-N-oxyl radical. It was shown that the rate constants of the phthalimide-N-oxyl radicalsʼ self-decay with different electron-withdrawing or -donor substituents in the benzene ring are higher than that of the unsubstituted phthalimide-N-oxyl radical in most cases. The solvent effect on the process of phthalimide-N-oxyl radical self-decomposition was investigated. The dependence of the rate constants on the Gutmann donor numbers was shown.

Design and Synthesis of Multipurpose Derivatives for N‐Hydroxyimide and NHPI‐based Catalysis Applications**

“…Inspired by all these considerations, we embarked in the synthesis of 1 a . The most common synthetic route to N‐hydroxyimides consists in the dehydration of the two vicinal carboxylic groups, followed by reaction of the intermediate cyclic anhydride with hydroxylamine ( anhydride approach (Scheme 4a) [8b,15c,21] . Thus, 4,5‐imidazoledicarboxylic acid 3 a and its N‐benzyl‐protected analogue 4 a (Scheme 5) were selected, as potential starting materials to carry out this two‐step approach.…”

“…NHPI is cheap, non‐toxic, and commercially available, and it can promote the free‐radical C−H bond functionalization of a wide variety of organic substrates [8a,9] . The mechanism of NHPI catalysis cycle is nowadays well established [8a,b,10] . Briefly, homolytic NHPI NO−H bond dissociation leads to phthalimido‐N‐oxyl (PINO) radical which represents the effective hydrogen acceptor species in the hydrogen atom transfer (HAT) process, which is described in Scheme 1 for a generic HC R−H.…”

Breaking C–C Bonds via Electrochemically Mediated Hydrogen Atom Transfer Reactions

“…Figure S4 shows a representative chronopotentiometry trace for these experiments. A steady increase of potential throughout the electrolysis was observed, likely due to the irreversible decomposition of PINO, 27,[37][38][39] which reduces the concentration of the NHPI/PINO mediator in the solution. On average, each molecule of NHPI carries out 7.5 turnovers before undergoing decomposition based on the charge passed during the electrolysis (see the Supporting Information for calculation details).…”

“…For di-NHPI, its PINO radical is significantly less stable than PINO. 39 As a result, the faster decomposition of di-NHPI led to both lower bibenzyl conversion and product selectivity. We also tested other types of N-oxyl radicals including aminoxyls (e.g., TEMPO) and iminoxyls (e.g., oxime radicals) (Table S1).…”

A Catalyst-Free Synthesis of Fused Perfluoroalkylated 2,3-Dihydroisoxazoles via Oxa-Michael-Aldol Annulation

“…5-Bromo-2-hydroxyisoindoline-1,3-dione (1h) 12 13 C NMR (125 MHz, DMSO-d 6 ):  = 124.8, 125.8, 127.7, 127.9, 130.8, 137.1, 162.9, 163.5.…”