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.

Red fluorescence from tautomers of 2′-hydroxychalcones induced by intramolecular hydrogen atom transfer

Abstract: Tautomer fluorescence in the longer wavelength region at 600 nm produced by intramolecular hydrogen atom transfer was observed in several 2′-hydroxychalcones having an electron donating group at the para position of the phenyl ring. The quantum yield of tautomer fluorescence increased by 1000 times upon decreasing the temperature from room temperature to 77 K. The introduction of dendritic substituents also increased the intensity of the tautomer fluorescence. One can control the photochemical and photophysical properties of the olefins by introduction of intramolecular hydrogen bonding and photoinduced hydrogen atom transfer.

Hydrogen Atom Abstraction by Metal−Oxo and Metal−Superoxo Complexes: Kinetics and Thermodynamics

Abstract: The superoxochromium complex CraqOO2+ abstracts a hydrogen atom from CMe3CHO in acidic aqueous solution with k = 0.16 M-1 s-1. This rate constant is only ∼102 times smaller than that for the reaction of CraqO2+ with the same aldehyde, k = 23 M-1 s-1, in contrast to the much greater reactivity difference between alkoxyl and alkylperoxyl radicals, kt-BuO/kt-BuOO ≈ 106. The absolute rate constants for hydrogen atom abstraction from a common reagent by metal-oxo and -superoxo species and the corresponding organic oxygen-centered radicals, RO• and ROO•, can now be compared for the first time: kBuO (9 × 107 M-1 s-1) > kCrO(23) ≥ kBuOO(8) > kCrOO (0.16). The reactivity of individual species is explained by the energetics of the O−H bonds in ROH, ROOH, CraqOH2+, and CraqOOH2+.

Parallel and competitive pathways for substrate desaturation, hydroxylation, and radical rearrangement by the non-heme diiron hydroxylase AlkB.

Abstract: A purified and highly active form of the non-heme diiron hydroxylase AlkB was investigated using the diagnostic probe substrate norcarane. The reaction afforded C2 (26%) and C3 (43%) hydroxylation and desaturation products (31%). Initial C-H cleavage at C2 led to 7% C2 hydroxylation and 19% 3-hydroxymethylcyclohexene, a rearrangement product characteristic of a radical rearrangement pathway. A deuterated substrate analogue, 3,3,4,4-norcarane-d(4), afforded drastically reduced amounts of C3 alcohol (8%) and desaturation products (5%), while the radical rearranged alcohol was now the major product (65%). This change in product ratios indicates a large kinetic hydrogen isotope effect of ∼20 for both the C-H hydroxylation at C3 and the desaturation pathway, with all of the desaturation originating via hydrogen abstraction at C3 and not C2. The data indicate that AlkB reacts with norcarane via initial C-H hydrogen abstraction from C2 or C3 and that the three pathways, C3 hydroxylation, C3 desaturation, and C2 hydroxylation/radical rearrangement, are parallel and competitive. Thus, the incipient radical at C3 either reacts with the iron-oxo center to form an alcohol or proceeds along the desaturation pathway via a second H-abstraction to afford both 2-norcarene and 3-norcarene. Subsequent reactions of these norcarenes lead to detectable amounts of hydroxylation products and toluene. By contrast, the 2-norcaranyl radical intermediate leads to C2 hydroxylation and the diagnostic radical rearrangement, but this radical apparently does not afford desaturation products. The results indicate that C-H hydroxylation and desaturation follow analogous stepwise reaction channels via carbon radicals that diverge at the product-forming step.

Radical Addition to Isonitriles: A Route to Polyfunctionalized Alkenes through a Novel Three-Component Radical Cascade Reaction

Abstract: The reaction of aromatic disulfides, alkynes, and isonitriles under photolytic conditions affords polyfunctionalized alkenesβ-arylthio-substituted acrylamides or acrylonitrilesin fair yields through a novel three-component radical cascade reaction. The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical to the isonitrile. A fast reaction, e.g., scavenging by a nitro derivative or β-fragmentation, is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The stereochemistry of the reaction is discussed, particularly with respect to the stereochemical outcome of related hydrogen abstraction reactions by the same vinyl radicals. The lower or even inverted preference for either geometrical isomer observed in our cases with respect to that encountered in hydrogen abstraction reactions is explained in terms of transition-state interactions and/or isomerization of the fina...