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.

A Comprehensive Investigation of the Chemistry and Basicity of a Parent Amidoruthenium Complex

Abstract: trans-(DMPE)(2)Ru(H)(NH(2)) (1) dehydrogenates cyclohexadiene and 9,10-dihydroanthracene to yield benzene (or anthracene), (DMPE)(2)Ru(H)(2), and ammonia. Addition of fluorene to 1 results in the formation of the ion pair [trans-(DMPE)(2)Ru(H)(NH(3))(+)][A(-)] (A(-) = fluorenide, 4a). Complex 1 also reacts with weak acids A-H (A-H = phenylacetylene, 1,2-propadiene, phenylacetonitrile, 4-(alpha,alpha,alpha-trifluoromethyl)phenylacetonitrile, cyclobutanone, phenol, p-cresol, aniline) to form ammonia and trans-(DMPE)(2)Ru(H)(A) (7, 8, 9a, 9b, 10, 11b, 11c, 12, respectively). In the cases where A-H = phenylacetylene, cyclobutanone, aniline, phenol, and p-cresol, the reaction was observed to proceed via ion pairs analogous to 4a. Compound 1 is reactive toward even weaker acids such as toluene, propylene, ammonia, cycloheptatriene, and dihydrogen, but in these cases deuterium labeling studies revealed that only H/D exchange between A-H and the ND(2) group is observed, rather than detectable formation of ion pairs or displacement products. Addition of triphenylmethane to 1 results in the formation of an equilibrium mixture of 1, triphenylmethane, and the ruthenium/triphenylmethide ion pair 4h. If the energetics of ion-pair association are ignored, this result indicates that the basicity of 1 is similar to that of triphenylmethide. All these observations support the conclusion that the NH(2) group in amido complex 1 is exceptionally basic and as a result prefers to abstract a proton rather than a hydrogen atom from a reactive C-H bond. The energetics and mechanism of these proton-transfer and -exchange reactions are analyzed with the help of DFT calculations.

Production of Formic Acid via Hydrogenation of CO2 over a Copper-Alkoxide-Functionalized MOF: A Mechanistic Study

Abstract: Conversion of greenhouse gases to more valuable chemicals is important from both the environmental and industrial points of view. Herein, the reaction mechanisms of the hydrogenation of carbon dioxide (CO2) to formic acid (HCOOH) over Cu-alkoxide-functionalized metal organic framework (MOF) have been investigated by means of calculations with the M06-L density functional. The reaction can proceed via two different pathways, namely, concerted and stepwise mechanisms. In the concerted mechanism, the hydrogenation of CO2 to formic acid occurs in a single step. It requires a high activation energy of 67.2 kcal/mol. For the stepwise mechanism, the reaction begins with the hydrogen atom abstraction by CO2 to form a formate intermediate. The intermediate then takes another hydrogen atom to form formic acid. The activation energies are calculated to be 24.2 and 18.3 kcal/mol for the first and second steps, respectively. Because of the smaller activation barriers associated with this pathway, it therefore seems to...

Hydroxyl radical oxidation of cylindrospermopsin (cyanobacterial toxin) and its role in the photochemical transformation.

Abstract: Cylindrospermopsin (CYN), an alkaloid guanidinium sulfated toxin, is produced by a number of cyanobacteria regularly found in lakes, rivers, and reservoirs. Steady-state and time-resolved radiolysis methods were used to determine reaction pathways and kinetic parameters for the reactions of hydroxyl radical with CYN. The absolute bimolecular reaction rate constant for the reaction of hydroxyl radical with CYN is (5.08 ± 0.16) × 109 M–1 s–1. Comparison of the overall reaction rate of CYN with hydroxyl radical with the individual reaction rate for addition to the uracil ring in CYN indicate the majority of the hydroxyl radicals (84%) react at the uracil functionality of CYN. Product analyses using liquid chromatography–mass spectrometry indicate the major products from the reaction of hydroxyl radical with CYN involve attack of hydroxyl radical at the uracil ring and hydrogen abstraction from the hydroxy-methine bridge linking the uracil ring to the tricyclic guanidine functionality. The role of hydroxyl ra...

Detailed Investigation of the OH Radical Quenching by Natural Antioxidant Caffeic Acid Studied by Quantum Mechanical Models.

Abstract: The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate ...