Abstraction, hydrogen atom, from

Step 2 Hydrogen atom abstraction from methane by a chlorine atom... 

One of the most common reactions of photoexcited carbonyl groups is hydrogen-atom abstraction from solvent or some other hydrogen donor. A second common reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group ... 

Spin density surface for the most stable radical formed by hydrogen atom abstraction from a model of a-tocopherol shows delocalization of the unpaired electron. 

Draw resonance structures for the possible radicals resulting from hydrogen atom abstraction from toluene. Which would you anticipate to be the most stable Why Compare energies for the different radicals (radical A, radical B,. ..). Is the lowest-energy radical that which you anticipated Are any of the alternatives significantly better than any of the others Explain your reasoning. 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

The most direct evidence that stereoelectronic effects are also important in these reactions follows from the specificity observed in hydrogen atom abstraction from conformationally constrained compounds,18 60 C-H bonds adjacent to oxygen113"118 or nitrogen110 and which subtend a small dihedral angle with a lone pair orbital (<30°) are considerably activated in relation to those where the dihedral angle is or approaches 90°. Thus, the equatorial H in 20 is reported to be 12 times more reactive towards /-butoxy radicals than the axial 11 in 21.115... 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

Hydrogen Atom Abstraction from the C5-Methyl Group of Thymine 356... 

General Considerations Regarding Hydrogen Atom Abstraction from the 2 -Deoxyribose Sugar... 

In this reaction, the trialkylstannane serves to initiate the chain sequence but it is present in low concentration to minimize the rate of hydrogen atom abstraction from the stannane. Under these conditions, the chain is propagated by iodine atom abstraction. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

With AT. trichosporium 0B3b, epimerization with exo, exo, exo, exo-2,3,5,6-d4-norbornane upon hydroxylation occurs (83), which parallels results for cytochrome P-450 hydroxylation with this substrate (91). The extent of epimerization with MMO, however, was significantly lower, being 2% following hydrogen atom abstraction from the endo position compared to 18% with cytochrome P-450, and 5% after abstraction at the exo position as compared to 14% with cytochrome P-450... 

Cp2TiCl . Hydrogen atom abstraction from THF seems possible, also. Most remarkably, the reaction can be employed in the efficient synthesis of cyclopropanols and cyclobutanols as shown in Scheme 28. 

O—H bonds, hydrogen atom abstraction from, 9,127 Organic materials for second-order non-linear optics, 32,121 Organic reactivity, electron-transfer paradigm for, 35, 193 Organic reactivity, structure determination of, 35,67... 

Droege, A.T., Tully, F.P. (1987) Hydrogen-atom abstraction from alkanes by OH. 6. Cyclopentane and cyclohexane. J. Phys. Chem. 91, 1222-1225. 

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

The reaction of FL with methyl alcohol gives the ether (92%). This process plays a pivotal role in the analysis of the properties of this carbene. The results are analysed within the spin-specific reaction framework where the ether is taken to be the product of the singlet carbene and this reaction rate is approximately diffusion limited (as it is for JXA). It is further assumed that 3FL will react with methyl alcohol as does 3BA, i.e., by hydrogen-atom abstraction from carbon, a relatively slow process in comparison with reaction of the singlet carbene (see Table 7). 

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