Norcaradiene radical cation

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

However, orbital factors may override thermodynamic control. For example, the regiochemistry of nucleophilic attack on the bridged norcaradiene radical cation 122 shows a significant deviation from thermodynamic control. Although attack on the cyclopropane ring should be favored by both release of ring strain and formation of delocalized free radicals (cf. Scheme 6.8), methanol attacks 122 " selectively at C2 (and C5), generating 123 and 124. There is little stereoselectivity Products derived from 123 and 124 were formed in comparable yields. ... 

Figure 6.16. Spartan representation of SOMO (b) and LUMO (a) for norcaradiene radical cation (see color insert). [Adapted from Ref. 152.]...

Significant insights into the nature of the nucleophilic capture of radial cations is provided by the regiochemistry of the attack on the bridged norcaradiene radical cation, 18 +. The products suggested regiospecific attack of methanol on 18"+ with capture at C2 and C5 generating 104" and 105". The attack occurs with limited stereoselectivity, because products derived from 104" and 105" were formed in comparable yields [131]. 

The norcaradiene radical cation has been implicated in the mass spectrum of toluene, and studies of metastable ions have been undertaken to elucidate the thermochemistry and ion structures of CjHg generated from a variety of sources. Cyclopropabenzenes have been proposed as fragment ions from tetracyanoethylene oxide adducts of benzo[b]furan, though with little justification. ... 

The structure of the corresponding radical cations should be determined by the symmetry of the two fragment FMOs at the points of union. The butadiene HOMO is antisymmetric at the positions of attachment in the norcaradiene framework, it may interact with the antisymmetric cyclopropane HOMO (as shown above). Indeed, norcaradiene derivatives provide the most promising examples of radical... 

While the nuclei of the aromatic segments show the identical signal directions, the cyclopropane protons show characteristic differences. This suggests significantly different spin-density distributions for the cyclopropane moieties of the two species and, thus, different structures. Like the norcaradiene HOMO, the styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO. In the norcaradiene system, the natural structure ( Aj) of the cyclopropane radical cation is altered by the interaction with the diolefin entity. 

Perhaps the most interesting mechanism stabilizing radical cations of type B involves homoconjugation. The interaction with the butadiene frontier molecular orbital (FMO) can lift the degeneracy of the cyclopropane in-plane e orbitals S,A) and favor the type B structure. This principle was shown to stabilize substrates such as norcaradiene and derivatives [110, 131] and, to a lesser extent. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

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