Hydrogen atom abstraction polarization

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

The bimolecular reaction with hydrogen atom abstraction by dioxygen from the weakest C—H bond. Ester as a polar medium accelerates this reaction due to solvation of the polar TS. 

Figure 22.7. Schematic depiction of the TS for hydrogen atom abstraction from methylcuhane (6) hy an alkoxyl radical. The polarity of the TS, depicted in the bottom resonance stmcture, was confirmed by the results of population analyses. " ...

Values of k+ are known from gas phase studies for a variety of unsaturated compounds91 and with these non-polar solvents it may be assumed that the values will be the same in the liquid phase. Hence it has been possible to determine values of the rate coefficients for a wide series of hydrogen atom abstraction reactions in solution. On the basis of these, it has been possible to determine values of kA for other scavengers92. Some of these rate coefficients are shown in Table 11. It has however, been suggested that alternative physical mechanisms may be responsible for the decrease in hydrogen yield observed in the presence of scavengers93. 

The rates of hydrogen atom abstractions by radicals are subject to the same factors that control rates of alkene additions [130]. Both enthalpic and polar... 

The /1,/f-difluorinated radicals are more interesting in that RCH2CF2CH2- is ca. five times as reactive as RCH2CH2CH2- in both hydrogen atom abstractions from n-Bu3SnH and in additions to styrenes. Since the RCH2CF2CH2- radicals are effectively planar, their enhanced reactivities must derive from either polar or enthalpic factors. The latter is probably the more important. That is, the C-H... 

The polar properties of the alpha-C-H bonds and their reactivity towards hydrogen atom abstraction in acetals are enhanced by the presence of two ether groups. These hydrogens are susceptible to abstraction by excited carbonyl compounds (77). Cleavage of C—0 bonds in acetals occurs readily when an acetal radical is formed (38) even at room temperature. It has been shown that irradiation of cyclic acetals of aldehydes at room temperature in the presence of acetone leads to the appropriate carboxylic esters (22),... 

The efficiency of hydrogen atom abstraction is often influenced by several factors involving thermodynamic, polar, stereoelectronic and steric effects. For instance, C-H bonds a to oxygen are not always susceptible to abstraction when nucleophilic radical initiators are used. P. Kaushal, P. L. H. Mok,... 

For these systems, direct hydrogen abstraction by benzophenone triplets was observed in benzene whereas in a polar solvent electron transfer and hydrogen-atom abstraction were observed. Electron transfer followed by an intramolecular proton transfer was observed in these systems although such proton-transfer reactions are not observed in unlinked systems of primary and secondary amines. The observed differences between the linked and unlinked systems have been attributed to the dependence of electron transfer, proton transfer, and hydrogen transfer on mutual distance and orientation. In the unlinked systems, rotational and translational motion of two reacting molecules are usually much faster than those in linked systems. 

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