Rotation barrier allyl

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

The rotational barriers increase from sodium to cesium to yield an estimate of the free allyl anion barrier to rotation. The calculated barrier is higher than that determined experimentally. Hommes and colleagues proposed that the decrease could be due to solvation or dimerization. Considering both dimerization and solvation, the calculated barrier decreases by 5.5 and 0.5 kcalmol-1, respectively. 

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. 

Free energies AG have been reported for the rotational barriers in allyl alkali metal compounds (set CPI, Table 10) . As the only variable is the alkali metal ion, the AG values were correlated with equation 45 ... 

However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of O2 with alkenes. As shown latef, it is the non-bonded interactions in the isomeric transition states that control product formation and barriers to rotation are rather irrelevant. The calculated rotational barrier values, with the HF-STO-3G method, for the allylic methyls in a series of trisubstituted alkenes, as well as the experimentally observed ene regioselectivity of a series of selective substrates, are shown in Table 9. ... 

Risk labels, lATA/ICAO, 751-3 Risk phrases, 621, 748, 749 River water, peroxide determination, 642 RNA, ozone disinfection, 616 ROS see Reactive oxygen species Rose Bengal sensitized photooxidation, 890 Rotational barriers, regioselective allylic hydroperoxide formation, 836, 847-9 Rotational isomers, peroxynitrous add, 8-9 Rotational spectra, ozonides, 721, 722-3 RP-HPLC, hydrogen peroxide determination, 627... 

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 ... 

Allyl cations 80 and 81 have been generated and studied by NMR spectroscopy.237 Although sterically crowded, cation 80 proved to be surprisingly stable up to 80°C. The rotational barrier estimated on the basis of the coalescence temperature of the 13C NMR signals is 16.8 kcal mol-1, in good agreement with the calculated value (MNDO, 16.5 kcal mol-1). In contrast, the rotational barrier of cation 81 was found to be less... 

In the meantime, other Tt-delocalized systems were discovered among open-chain systems, notably the three allylic systems, Scheme 4. All of these systems showed a behavior akin to the aromatic systems they are delocalized, have uniform geometries, and are more stable than saturated analogues.90-93 Furthermore, they all possess rotational barriers with heights related to the resonance stabilization of the species. 

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. 

The Rotational Barrier of Allyl Is Not in Conflict with the jr-Distortivity... 

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).

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