Allyl radicals, rotational barriers

A comparison of the rotational barriers in allylic radicals A-D provides evidence for the stabilizing effect of the capto-dative combination ... 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

Table 11 Rotational barriers (kcal mol" ) in allylic radicals."...

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

An error-propagation analysis allows the conclusion that the captodative substituent effect on the rotational barrier in this allyl radical is at least additive and perhaps slightly greater. 

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. 

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

Benzene and other aromatics alike are stable molecules, while cyclobutadiene and other antiaromatic molecules are unstable molecules.27-76 Similarly, allylic species are stable intermediates and possess significant rotational barriers. It may appear as a contradiction that, for example, the tr-component of benzene can be distortive but it still endows the molecule with special stability or that the distortive jr-component of allyl radical can lead to a rotational barrier. We would like to show in this section that these stability patterns derive from the vertical resonance energy which is expressed as a special stability because for most experimental probes (in eluding substitution reactions) the o-frame restricts the molecule to small distortion167 in which the vertical resonance energy is still appreciable, as shown schematically in Figure 5. 

Shaik and Bar102 demonstrated that allyl anion has a distortive jr-component but at the same time exhibits a rotational barrier. This analysis was reaffirmed later for allyl radical.5 Subsequently, Gobbi and Frenking93 pointed out that the total distortion energy of allylic species is very small because it reflects the balance of jr-distortivity opposed by the a-symmetrizing propensity. They further argued that along with this jr-distortivity, the allylic species enjoys resonance stabilization which is the source of the rotational barrier. A detailed VB analysis by Mo et al.149 established the same tendency. 

Figure 9. Hess cycle used to relate the rotational barrier of allyl radical to its resonance energy. All energies are given in kcal/mol (data for the cycle is taken from ref 93 B values from ref 111).

In an ESR study of 1,1,3,3-difluoroallyl radicals, Krusic and coworkers were able to demonstrate that the barrier to rotation of such apparently planar radicals is substantially reduced [18]. Although allyl itself has a rotational barrier of 15 kcal/mol [19, 20], 1,1,3,3-tetrafluoroallyl, 1, had a barrier of but 7.2 kcal/mol. The observed 19F hfs constants (42.6 and 39.7 G) were consistent with 1 being a planar system. It is likely that the lowering of the rotational barrier of 1 derives from a destabilizing interaction between the fluorine lone pairs and the doubly-occupied allyl tt-MO which diminishes the net allylic resonance energy, as well as from stabilization of the transition state due to pyramidalization. 

Nagashima and colleagues showed subsequently that 5-10 mol% of Pd(PPh3)4 catalyzed atom transfer radical cyclizations (ATRC) of /V-allyl-difluoroiodo-acetamides 158b in fluorescent lab light at ambient temperature [194], Under these conditions, 34—98% yield of cyclized compounds 159b were obtained. No reaction occurred in the dark, while it was considerably slower in the absence of the catalyst. The low yield of 34% in one example (R=Bn) is due to the unfavorable rotational barrier in the substrate, which cannot be influenced by the presence of the catalyst. 

I McLachlan calculation of spin densities, discussion of twisting at C(2)-C(8) and C(2 )-C(8) bonds radical with all positions of the benzoring deuterated also prepared. " ) INDO calculation of coupling constants for optimized structure. - Mixtures of exo- and enatom abstraction from allyl cyanide at various temperatures in CjHCI). - Determination of rotational barrier. ... 

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). ... 

The rotational barrier in allyl radicals also illustrates the effect of the number of orbitals in the conjugated array on the conjugation. Removing one p-orbital from conjugation via the 90° twist in allyl radical imposes the energy penalty of only 15kcal/mol - considerably less costly than the penalty for the analogous rotation in ethylene (-65 kcal/mol). 

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 ... 

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. 

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