Bridged norcaradiene

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. ... 

The interaction of 1 with dipolar reagents has led to bridged norcaradienes (Section V.A.3) of varying stability , but with electrophilic C,V-diphenylnitrone ring expanded 123 is formed (Scheme 23) Similarly , reaction with sulphonyl isocyanates provides isoindolinones 124. Simple 1,3-dipoles such as methyl azidoformate and ethyl diazoacetate are without effect on 1, indicating too large an energy gap between the... 

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]. 

Electrocyclic reactions have been performed with three of the bridged [ 11 Jan-nulenones. Both 11 and 13, which both contain a cycloheptatriene unit, undergo Diels-Alder additions with dienophiles via their norcaradiene valence tautomers 41 and 43 and yield adducts of the type 42 and 44. Annulenone 13 was found to react only with 4-phenyl-l,2,4-triazoline-3,5-dione whilst 11 underwent reaction with a variety of dienophiles. 3,8-Methano[ 11 Jannulenone 12 contains a tetraene system and undergoes addition reactions, apparently of the (8 + 2)-type, at the termini of the tetraene system. Thus with maleic anhydride the adduct 46, the valence tautomer of the initial adduct 45, was isolated. 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

In contrast, stabilization of the cycloheptatriene form is achieved by extending it to the 10-Tt-electron aromatic, l,6-methano-[10]annulene 8. Again disubstitution of the methano-bridge (as in 9 10) shifts the equilibrium, so that both forms coexist in measurable amounts/ Aperi-C i2-CU.2 bridge does not alter the enthalpy difference between 8 and its norcaradiene valence tautomer, whereas a / m-CH = CH bridge provides considerable de-stabilization of 8. Consequently 11, that exists in the norcaradiene form, on selective hydrogenation of the peri-double bond gives 12, that exists in the [10]-annulene form. ... 

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