Benzylic radicals, rotational barriers

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

The conformational barriers in acyclic radicals are smaller than those in closed-shell acycles, with the barrier to rotation in the ethyl radical on the order of tenths of a kilocalorie per mole. The barriers increase for heteroatom-substituted radicals, such as the hydroxymethyl radical, which has a rotational barrier of 5 kcal/mol. Radicals that are conjugated with a n system, such as allyl, benzyl, and radicals adjacent to a carbonyl group, have barriers to rotation on the order of 10 kcal/mol. Such barriers can lead to rotational rate constants that are smaller than the rate constants of competing radical reactions, as was demonstrated with a-amide radicals, and this type of effect permits acyclic stereocontrol in some cases. "... 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C, Ph2CH- and PhCH2 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of diphenylcarbene are typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals.Ketyl radicals containing the enthrone moiety and the 4-(methyl sulphonate) benzophenone ketyl radical anion are related studies in this field. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

Table 1 Rotational Barriers (kcal/mol) of Allyl and Benzyl Radicals Calculated by Various Levels of Theory ...

Use HMO theory to estimate the electronic energy barrier for rotation of an allyl radical about C2-C3 as shown in Figure 4.79. How does your result compare with an experimental activation energy How does the calculated value compare with the electronic energy barriers calculated for the allyl cation and the allyl anion How do the calculated values for the allyl systems compare with those of the corresponding benzyl systems ... 

In preceding sections we discussed in some detail the performance of various methods in predicting barriers to rotation in allyl and benzyl and singlet-triplet gaps in CB and TMM. In this section we provide some more examples, mostly from the recent literature, to illustrate how different methods fare in predicting various properties of open-shell molecules. However it is neither the purpose of the authors nor within the scope of this chapter to present a comprehensive overview. Also, the examples are not necessarily representative, but are drawn from work that is close to the research of the authors, i.e., mainly on radical ions and diradicals. 

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