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.

The reaction of acetylene with hydroxyl radicals.

Abstract: The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.

Mechanism of site-selective DNA nicking by the hydrodioxyl (perhydroxyl) radical.

Abstract: In previous studies, the ability of the hydrodioxyl (perhydroxyl) radical [HOO•, the conjugate acid of superoxide (O2•-)] to “nick” DNA under biomimetic conditions was demonstrated, and a sequence selectivity was observed. A background level of nonspecific nicking also was noted. This paper provides support for 5‘-hydrogen atom abstraction from the deoxyribose ring as the initial event in the sequence-selective nicking by O2•-/HOO•. Two experiments support the proposed mechanism. First, using a defined sequence 5‘-32P-labeled restriction fragment as the DNA substrate, only free (unalkylated) 3‘-phosphate is produced at the site of nicking. Second, using poly (dA)·poly (T) as the substrate, furfural is formed in the reaction from deoxyribose ring breakdown. Both results are consistent with 5‘-hydrogen atom abstraction for initiation of the site-selective nicking. Hydrogen atom abstraction at other sites of the deoxyribose ring and/or base oxidation and loss followed by strand scission likely are responsibl...

Thiyl Radicals Abstract Hydrogen Atoms from the αC−H Bonds in Model Peptides: Absolute Rate Constants and Effect of Amino Acid Structure

Abstract: Thiyl radicals are important intermediates in biological oxidative stress and enzymatic reactions, for example, the ribonucleotide reductases. On the basis of the homolytic bond dissociation energies (BDEs) only, the αC−H bonds of peptides and proteins would present suitable targets for hydrogen abstraction by thiyl radicals. However, additional parameters such as polar and conformational effects may control such hydrogen-transfer processes. To evaluate the potential of thiyl radicals for hydrogen abstraction from αC−H bonds, we provide the first absolute rate constants for these reactions with model peptides. Thiyl radicals react with αC−H bonds with rate constants between 1.7 × 103 M-1 s-1 (N-acetylproline amide) and 4 × 105 M-1 s-1 (sarcosine anhydride). However, the correlation of rate constants with BDEs is poor. Rather, these reactions may be controlled by conformation and dynamic flexibility around the αC−H bonds.

Transition metal salts catalysis in the aerobic oxidation of organic compounds: Thermochemical and kinetic aspects and new synthetic developments in the presence of N-hydroxy-derivative catalysts

Abstract: The catalytic system formed by Mn(II) and Co(II) or Cu(II) nitrates has shown to be particularly effective for the oxidation of ketones and aldehydes by molecular oxygen under mild conditions. The process has a general character, but it is particularly selective for the oxidation of alkyl–aryl ketones to aromatic carboxylic acids, alkyl-cyclopropyl ketones to cyclopropane carboxylic acids and cycloalkanones to dicarboxylic acids. Mn salt plays a key role in this catalysis, which catalyses the enolisation of the carbonyl compound and initiates a free-radical redox chain with oxygen by an electron-transfer process. Co and Cu salts alone are inert, but they exalt the catalytic activity of the Mn salt, being more effective in the decomposition of the hydroperoxides. The same metal salt complexes, associated with TEMPO revealed particularly effective for the aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones under mild conditions (with air or oxygen at room temperature and atmospheric pressure); the mechanism of the catalysis is discussed. Thermochemical and kinetic investigations by EPR spectroscopy, concerning N-hydroxy compounds, have allowed to evaluate the Bond Dissociation Enthalpies (BDE) of several OH bonds and the absolute rate constants for the formation of the phthalimide-N-oxyl (PINO) radical from N-hydroxyphthalimide (NHPI) and for the hydrogen abstraction from several CH bonds by the PINO radical. The thermochemical and kinetic results have allowed us to explain the opposite catalytic behaviour of the two nitroxyl radicals, TEMPO and PINO, the former being an inhibitor of free radical processes, the latter a promoter of free-radical chain, and to develop new selective processes concerning the aerobic oxidation of alcohols, amines, amides, silanes and the substitution of heteroaromatic bases.