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.

Bond and mode selectivity in the reaction of atomic chlorine with vibrationally excited CH2D2.

Abstract: The title reaction is investigated by co-expanding a mixture of Cl2 and CH2D2 into a vacuum chamber and initiating the reaction by photolyzing Cl2 with linearly polarized 355 nm light. Excitation of the first C–H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C–D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D2 prepared in a local mode containing two quanta in one C–H oscillator |2000〉− or in a local mode containing one quantum each in two C–H oscillators |1100〉 lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. The Cl+CH2D2|2000〉− reaction yields methyl radical products primarily in their ground state, whereas the Cl+CH2D2|1100〉 reaction yields methyl radical products that are C–H stretch excited. The HCl(v=1) rotational distribution from the Cl+C...

Selective C–H halogenation over hydroxylation by non-heme iron(IV)-oxo

Abstract: Non-heme iron based halogenase enzymes promote selective halogenation of the sp3-C–H bond through iron(IV)-oxo-halide active species. During halogenation, competitive hydroxylation can be prevented completely in enzymatic systems. However, synthetic iron(IV)-oxo-halide intermediates often result in a mixture of halogenation and hydroxylation products. In this report, we have developed a new synthetic strategy by employing non-heme iron based complexes for selective sp3-C–H halogenation by overriding hydroxylation. A room temperature stable, iron(IV)-oxo complex, [Fe(2PyN2Q)(O)]2+ was directed for hydrogen atom abstraction (HAA) from aliphatic substrates and the iron(II)-halide [FeII(2PyN2Q)(X)]+ (X, halogen) was exploited in conjunction to deliver the halogen atom to the ensuing carbon centered radical. Despite iron(IV)-oxo being an effective promoter of hydroxylation of aliphatic substrates, the perfect interplay of HAA and halogen atom transfer in this work leads to the halogenation product selectively by diverting the hydroxylation pathway. Experimental studies outline the mechanistic details of the iron(IV)-oxo mediated halogenation reactions. A kinetic isotope study between PhCH3 and C6D5CD3 showed a value of 13.5 that supports the initial HAA step as the RDS during halogenation. Successful implementation of this new strategy led to the establishment of a functional mimic of non-heme halogenase enzymes with an excellent selectivity for halogenation over hydroxylation. Detailed theoretical studies based on density functional methods reveal how the small difference in the ligand design leads to a large difference in the electronic structure of the [Fe(2PyN2Q)(O)]2+ species. Both experimental and computational studies suggest that the halide rebound process of the cage escaped radical with iron(III)-halide is energetically favorable compared to iron(III)-hydroxide and it brings in selective formation of halogenation products over hydroxylation.

Non-classical selectivities in the reduction of alkenes by cobalt-mediated hydrogen atom transfer.

Abstract: Classical methods for alkene hydrogenation typically reduce less-substituted or more-strained alkenes, or those in proximity to a directing group, most rapidly. Here we describe a cobalt-mediated hydrogenation protocol that provides complementary selectivities in the reduction of several classes of olefins and alkynes. The selectivity of this reduction derives from a hydrogen atom transfer mechanism, which favors the generation of the more stable alkylradical intermediate. We also report the first alkene hydrobromination, hydroiodination, and hydroselenylation by a hydrogen atom transfer process.

An assessment of theoretical procedures for predicting the thermochemistry and kinetics of hydrogen abstraction by methyl radical from benzene.

Abstract: The reaction enthalpy (298 K), barrier (0 K), and activation energy and preexponential factor (600-800 K) have been examined computationally for the abstraction of hydrogen from benzene by the methyl radical, to assess their sensitivity to the applied level of theory. The computational methods considered include high-level composite procedures, including W1, G3-RAD, G3(MP2)-RAD, and CBS-QB3, as well as conventional ab initio and density functional theory (DFT) methods, with the latter two classes employing the 6-31G(d), 6-31+G(d,p) and/or 6-311+G(3df,2p) basis sets, and including ZPVE/thermal corrections obtained from 6-31G(d) or 6-31+G(d,p) calculations. Virtually all the theoretical procedures except UMP2 are found to give geometries that are suitable for subsequent calculation of the reaction enthalpy and barrier. For the reaction enthalpy, W1, G3-RAD, and URCCSD(T) give best agreement with experiment, while the large-basis-set DFT procedures slightly underestimate the endothermicity. The reaction barrier is slightly more sensitive to the choice of basis set and/or correlation level, with URCCSD(T) and the low-cost BMK method providing values in close agreement with the benchmark G3-RAD value. Inspection of the theoretically calculated rate parameters reveals a minor dependence on the level of theory for the preexponential factor. There is more sensitivity for the activation energy, with a reasonable agreement with experiment being obtained for the G3 methods and the hybrid functionals BMK, BB1K, and MPW1K, especially in combination with the 6-311+G(3df,2p) basis set. Overall, the high-level G3-RAD composite procedure, URCCSD(T), and the cost-effective DFT methods BMK, BB1K, and MPW1K give the best results among the methods assessed for calculating the thermochemistry and kinetics of hydrogen abstraction by the methyl radical from benzene.