Cationic structures strained ring cations

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. 

Pronounced differences between radical cation structures and their parents must be expected for strained ring compounds. The HOMO or LUMO of these systems may be localized mainly in one bond, which may be weakened or actually break upon ionization. The oxidation potentials of strained ring compounds are lower than those of unstrained substrates because strain energy is released, resulting in noticeable changes in structure. 

A plausible explanation for this fact involves the reduction of strain energy upon radical ion formation and corresponding noticeable changes in their structures. Accordingly, the radical cations of strained ring compounds are of maj or interest, particularly the distribution of spin and charge in response to the minimization of strain. 

Whereas strained ring systems are usually reactive and often unstable, molecules which satisfy the criteria for aromaticity exhibit enhanced stability. As is evident from the structural formula of 1, the cycloproparenes set these features in juxtaposition as they are strained molecules in which a single carbon atom is fused across adjacent centres of an aromatic system. The interest of the experimentalist in strained molecules has been matched by the theoretician in the search for suitable models for developing the concepts of chemical bonding and aromaticity. The cycloproparenes have been particularly important in this regard as they meet the criterion for partial aromatic bond localization and consequent bond length alternation in the aromatic ring as proposed by Mills and Nixon in 1930, viz. la vs lb. The cation 5, anion 6 and radical 7 derived from 1, and also the ketone 8 and exocyclic methylene derivative 9, are of interest in this respect. 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

Major commercial synthetic specialty polymers are made by chain-growth polymerization of functionalized vinyl monomers, carbonyl monomers, or strained ring compounds. Depending on monomer structure, the polymerization may be initiated free radically, anionically, or cationically. Copolymers or terpolymers with random, alternating, block, or graft sequences can be prepared under appropriate reaction conditions. There are numerous mediods used to prepare specialty polymers in the research laboratory. However, only a few are of commercial interest. Of particular commercial interest is synthesis of specialty polymers in solutions, dispersions, suspensions, or emulsions. 

An alternative approach to the antitumor active enediyne systems (cf. eq 47) using Nicholas cations has been extensively studied and applied to the dynemycin core structure (eq 53). Another example of the use of such cations in forming strained rings is the preparation of a cobalt complex of a trithia-crown ether (eq 54), which has been shown to complex Cu and Ag ... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

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