Rearomatization energy

The photo-NOCAS reaction is restricted to mononuclear, electron-deficient aromatics as the electron acceptors. Using polynuclear systems such as 1,4-dicyanonaphthalene, for example, leads to the ole-fin-nucleophile adduct radical adding, rather than substituting, at the ipso-position of the arene (Scheme 18). This is most Hkely due to the reduced rearomatization energy gained in binuclear, as opposed to mononuclear, systems. The photo-NOCAS reaction is Hmited to aliphatic olefins as electron... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

The steric and dipole-dipole effects of the CF3 group on valence isomerization in the Dewar pyridine-azaprismane-pyridine system have been studied. These reveal themselves in the high stability, compared to the pyridine, of the valence isomer arising out of the large activation energy for rearomatization. The transformation of a 1-Dewar to 2-Dewar pyridine was observed (89T3115). 

Formation of the ethano-dimer of a-tocopherol (12) by reduction of spiro dimer (9) proceeds readily almost independently of the reductant used. This reduction step can also be performed by tocopheroxyl radicals as occurring upon treatment of tocopherol with high concentrations of radical initiators (see Fig. 6.10). The ready reduction can be explained by the energy gain upon rearomatization of the cyclohexadienone system. Since the reverse process, oxidation from 12 to 9 by various oxidants, proceeds also quantitatively, spiro dimer 9 and ethano-dimer 12 can be regarded as a reversible redox system (Fig. 6.22). 

In the first step, the carbon centered radical is generated. The second step involves the addition of this radical to the protonated ring. The third step consists of the rearomatization of the radical adduct by oxidation. The rates of addition of alkyl and acyl radicals to protonated heteroaromatic bases are much higher than those of possible competitive reactions, particularly those with solvents. Polar effects influence the rates of the radical additions to the heteroaromatic ring by decreasing the activation energy as the electron deficiency of the heterocyclic ring increases. 

These results can be explained by the reactions in Scheme I and the potential energy profile in Figure 1. Attack by C6H50- via oxygen is kinet-ically preferred the energy barrier for this process should be lower than that for attack via carbon, because in formation of the C adduct 6 aromaticity would be disrupted. However, whereas the O-bonded adduct 8 can revert back to the reactants, the C-bonded adduct 6 initially formed will rapidly rearomatize by proton loss to give the final product 7 this pathway is effectively irreversible. The C-bonded adduct is therefore obtained as the product of thermodynamic control. 

Fort EH, Scott LT (2011) Carbon nanotubes from short hydrocarbon templates. Energy analysis of the Diels-Alder cycloaddition/rearomatization growth strategy. J Mater Chem 21 1373. doi 10.1039/c0jm02517h... 

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