Hydrogen atom abstraction

About: Hydrogen atom abstraction is a research topic. Over the lifetime, 7059 publications have been published within this topic receiving 151781 citations.

Alkynylation of Csp2 (O)–H Bonds Enabled by Photoredox‐Mediated Hydrogen‐Atom Transfer

Abstract: The development of new hydrogen-atom transfer (HAT) strategies within the framework of photoredox catalysis is highly appealing for its power to activate a desired C−H bond in the substrate leading to its selective functionalization. Reported here is the first photoredox-mediated hydrogen-atom transfer method for the efficient synthesis of ynones, ynamides, and ynoates with high regio- and chemoselectivity by direct functionalization of Csp2 (O)−H bonds. The broad synthetic application of this method has been demonstrated by the selective functionalization of C(O)−H bonds within complex molecular scaffolds.

Single-Site Vanadyl Activation, Functionalization, and Reoxidation Reaction Mechanism for Propane Oxidative Dehydrogenation on the Cubic V4O10 Cluster

Abstract: Vanadyl oxide (V═O) sites are thought to play a role in a number of industrially important catalysts for activating saturated alkanes, but in no system is the mechanism for the activation, product formation, and reoxidation steps established. In this paper, we use quantum mechanical methods (B3LYP flavor of density functional theory) to examine the detailed mechanism for propane reacting with a V_4O_(10) cluster to model the catalytic oxidative dehydrogenation (ODH) of propane on the V_2O_5(001) surface. We here report the mechanism of the complete catalytic cycle, including the regeneration of the reduced catalyst using gaseous O_2. The rate-determining step is hydrogen abstraction by the vanadyl (V═O) group (in agreement with experiment) to form an iso-propyl radical that binds to an adjacent V−O−V site. Subsequently, this bound iso-propyl forms propene product by β-hydride elimination to form bound H_2O. We find that this H_2O (bound to a V^(III) site) is too stable to desorb unimolecularly. Instead, the desorption is induced by binding of gaseous O_2 to the V^(III) site, which dramatically decreases the coordination energy of H_2O from 37.8 to 12.9 kcal/mol. Further rearrangement of the O_2 molecule leads to formation of a cyclic VO_2 peroxide, which activates the C−H bond of a second propane to form a second propene (with a lower reaction barrier). Desorption of this propene regenerates the original V_4O_(10) cluster. We find that all reactions involve the single vanadyl oxygen (V═O), with the bridging oxygens (V−O−V) serving to stabilize the iso-propyl radical intermediate. We refer to this mechanism as the single-site vanadyl activation, functionalization, and reoxidation mechanism (SS-VAFR). This SS-VAFR mechanism should be applicable to propane ODH on the supported vanadium oxide catalysts where only monovanadate (VO_4) species are present.

Absolute rate constants for hydrocarbon autoxidation

Abstract: The effect of deuterium substitution on the absolute rate constants for the bi~nolecular chain termination process in the oxidation of styrene indicates that the a-hydrogen is abstracted in this reaction. The first order chain terlnination process is suppressed both by deuteration of styrene at the or-position and by the addition of heavy water. A possible mechanism for this termination is proposed. There appear to be small secondary deuterium isotope effects in the propagation reaction. The overall oxidation rates and the propagation rate constants are increased by the addition to the aromatic ring of both electron-attracting and electron-releasing substituents. This is attributed in the former case to the increased stability of the resulting styryl radicals and in the latter case to the increased stability of a dipolar transition state. In hydrogen atom abstraction from 2,6-di-t-butyl-4-methylphenol, the peroxy radical from 3-chlorostyrene is more reactive than that from styrene which, in turn, is more reactive than the peroxy radical from 4-methosystyrene.