Hydrogen Atom Transfer from Benzyl Alcohols to N-Oxyl Radicals. Reactivity Parameters

Abstract

A model for predicting the rate constants of hydrogen atom transfer (HAT) from the α–C–H bond of p-substituted benzyl alcohols to N-oxyl radicals was proposed. To quantify the factors governing the reactivity of both N-oxyl radicals and benzyl alcohols, multivariate regression analysis was performed using various combinations of reactivity parameters. The analysis was based on a 2D array of 35 HAT reactions, the rate constants of which span 4 orders of magnitude. The proposed polyparameter equation approximates the experimental rate constants of reactions with high accuracy using three independent parameters: Brown and Okamoto’s substituent constants σ+ in alcohol molecules and the spin population on O and N atoms in the N–O• fragment of N-oxyl radicals [calculated by DFT/B3LYP/6-31G­(d,p)]. The rate constants of HAT reactions from p-substituted benzyl alcohols to a series of aryl-substituted phthalimide-N-oxyl radicals containing either electron-withdrawing or electron-donating substituents (4-Cl, 4-HOOC, 4-CH3O), quinolinimide-N-oxyl, benzotriazole-N-oxyl, and violuric acid radicals were experimentally determined at 30 °C in acetonitrile.

Organic Synthesis Using Nitroxides

“…At the end of this section on transition-metal-free transformations, amidoxyl and imidoxyl radical-catalyzed alcohol oxidations will briefly be discussed. As mentioned in section , the bond dissociation energy of the O–H bond of N -hydroxyimide is significantly higher as compared to TEMPOH derivatives . Therefore, the corresponding N -oxyl radicals can well engage in intermolecular hydrogen atom abstractions.…”

“…As mentioned in section 3, the bond dissociation energy of the O−H bond of N-hydroxyimide is significantly higher as compared to TEMPOH derivatives. 517 Therefore, the corresponding N-oxyl radicals can well engage in intermolecular hydrogen atom abstractions. The most prominent imidoxyl radical for this process is the phthalimide N-oxyl radical (PINO), which is readily generated from Nhydroxyphthalimide (NHPI).…”

A computational study for the reaction mechanism of AIBN‐induced remote trifluoromethyl‐alkynylation of thioalkynes

“…[2,3] Therefore, recently, various novel chemical approaches for 1,6-difunctionalization of alkenes by remote 1,5-HAT have been designed and practiced. [4][5][6][7][8][9][10][11] 1,6-Difunctionalization of alkynes through 1,5-HAT, unlike alkenes, is not common. Zhu and co-workers demonstrated an efficient protocol for the 1,6-difunctionalization of alkynes through 1,5-HAT, which is 2,2-azobis (isobutyronitrile) (AIBN)-induced remote trifluoromethyl-alkynylation of thioalkynes using acetylenic triflones as both the trifluoromethyl and alkyne sources.…”

Using High-Power UV-LED to Accelerate a Decatungstate-Anion-Catalyzed Reaction: A Model Study for the Quick Oxidation of Benzyl Alcohol to Benzoic Acid Using Molecular Oxygen

“…In this reaction, hydrogen atom transfer (HAT) by photo-excited decatungstate ion triggers the oxidation. Benzyl alcohol has only one type of C(sp 3 )-H bond available for the HAT process and is frequently used for oxidation as a model compound [26][27][28]. Accumulated H + W 10 O 32 5− is oxidized by molecular oxygen to recover W 10 O 32 4− .…”

“…Pleasingly, we were able to find that the reaction was complete within 1 h when high-power irradiation by 480 W-irradiation was employed, giving benzoic acid 3 in both excellent selectivity and yield. C(sp 3 )-H bond available for the HAT process and is frequently used for oxidation as a model compound [26][27][28]. Accumulated H + W10O32 5− is oxidized by molecular oxygen to recover W10O32 4− .…”