Vinyl radical stability

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

The incorporation of four different classes of important ultraviolet stabilizers into high polymer chains has been accomplished by synthesis of polymerizable, vinyl-substituted stabilizer derivatives followed by radical polymerization. 

Figure 13.1 The relative stability of the allyl radical compared to 1°, 2°, 3°, and vinyl radicals. (The stabilities of the radicals are relative to the hydrocarbon from which was formed, and the overall order of stability is allyl > 3° > 2° > 1° > vinyl).

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

The substituent effects predicted for vinyl radicals are rather similar to those already observed for alkyl radicals (Table 4). Attachment of alkyl groups or it systems to the radical center stabilize the radical while the introduction of a-acceptors in the a- or / -position are destabilizing. The nature of the... 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

The addition to activated acetylenes (R- -R (b)) proceeds even npon irradiation at 300 nm. In this case, the intermediate vinyl radical is stabilized by the carbonyl, vinyl or aryl gronps, and the reverse reaction to the starting materials is snppressed. 

The thiotelluration of 1-octyne requires prolonged photoirradiation, giving rise to an isomeric mixture of products in moderate yields. This result depends on the different stability of the two vinylic radicals. 

One measure of the resonance component of radical stability in polymerizations is the so-called Q value of the monomer, which quantifies the resonance stabilization of the radical (Stevens 1990). However, the experimentally determined value of Q can be influenced by other factors unrelated to resonance. To evaluate the extent to which their measured Q values were consistent with resonance stabilization of the monomer radical, the authors compared isodesmic energies from Eq. (6.14) to measured Q values for R = Me, rBu, PhO, CN, Ph, vinyl, and phenylethynyl. The largest stabilization energy was computed for the R = phenylethynyl case, about 101 kJ mol-1, although at the HF/3-21G level the expected linear correlation between log(2 and stabilization energy was only fair (R2 = 0.86 a better correlation for the non-phenylethynyl substituents had been obtained previously at a higher level of theory). 

It is more difficult to conduct the addition reactions of nucleophilic radicals to electron poor alkenes because the resulting atom transfer steps are often endothermic and are too slow to propagate chains, even with iodides. An exception is illustrated in Scheme 57 resonance-stabilized vinyl radicals (especially if they are secondary or tertiary) are reactive enough to abstract iodine from alkyl iodides.178... 

Moreover, there are two types of radicals, the a radicals and the tt radicals. An unpaired electron in the a-radical is in the a orbital, and an unpaired electron in the it radical is in the tt orbital, respectively. Therefore, the radicals (4) and (5) above are tt radicals. LButyl radical (3) is also tt radical, since this radical is stabilized by the hyperconjugation. However, the phenyl radical and the vinyl radical are typical a radicals. Normally, tt radicals are stabilized by the hyperconjugation effect or the resonance effect. However, a radicals are very reactive because there is no such stabilizing effect (Figure 1.3). 

The anti stereochemistry of the sodium-ammonia reduction appears to result from the greater stability of the vinyl radical in the trans configuration, where the alkyl groups are farther apart. An electron is added to the trans radical to give a trans vinyl anion, which is quickly protonated to the trans alkene. 

Most of the radical stabilization energy parameters utilized in Figure 2 (R = H or D, -2.65 Me or Et, - 0.5 CN, 6.6 vinyl or propen-2-yl, 13.2 (E )-propenyl, 15.4) are SE values calculated by Leroy and coworkers " those for phenyl (9.4) and cinnamyl (19.1) were set empirically. These particular parameters are serviceable and reasonable, but cannot be considered preferred . Other plausible radical stabilization energy values lead to very similar correlations which demonstrate, as Figure 2 demonstrates, that the rate constants for two-center epimerizations of cyclopropanes and vinylcyclopropanes are sensitive to the radical stabilizing capacities of substituent groups in a simple additive fashion. Whatever 1,3-disubstituted trimethylene diradical intermediates or transition structutes may be involved respond kinetically to substituents as though each locus of radical character were thermochemically independent. 

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