Stability aromatization

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

There is another usefiil viewpoint of concerted reactions that is based on the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground-state molecules. A stabilized aromatic transition state will lead to a low activation energy, i.e., an allowed reaction. An antiaromatic transition state will result in a high energy barrier and correspond to a forbidden process. The analysis of concerted reactions by this process consists of examining the array of orbitals that would be present in the transition state and classifying the system as aromatic or antiaromatic. 

We consider the relatively high pKA values of 6-8 to be typical value for a cation-quinone methide equilibrium. The formation of a resonance-stabilized aromatic carbocation is one reason for these high pKA values. Another reason is the high energy of the quinone methide. The thermodynamic cycle shown in... 

The parent [6]radialene has no stabilizing aromatic feature and is predicted to have a relatively low-lying HOMO as compared to the lower members of the series, 1-399. This may explain why it is so highly reactive that it is difficult to handle. As far as the chemical behavior of the [6]radialenes is concerned, it is the hexamethyl derivative 150... 

Cycloadditions of this type do not occur with isolated non-activated C-C double bonds, the 2-azaallyl system (Scheme 3) is only capable of existence with stabilizing aromatic groups and the allyl-lithium stem (Scheme 3) has no tendency at all to undergo cycloaddition, if a group is lacking which can stabilize the negative charge in the 2-position ... 

STABILITY, AROMATICITY AND CONFORMATION 2.04.5.1 Conformation and Stability in Piperidine and Derivatives... 

Conjugated ring systems offer an alternative mode for the stabilization of a carbanion center. The most common situation is where deprotonation completes a cyclic n system leading to a highly stabilized, aromatic anion. The best known example is cyclopentadiene, which leads to a six-electron, aromatic ring after... 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

The same cleavage reaction occurs in rotenoids containing the chromanochromanone system (105).63,57 From the primary fission product of the retro-Diels-Alder decomposition [107], containing rings A and B, a highly stabilized aromatic system is formed by loss of a hydrogen atom, (105)->[107]->[108]. 

Figure 23.1. Molecular structure and position of equilibrium. Resonance-stabilized aromatic amine is weaker base than ammonia. (Plots aligned with each other for easy comparison.)...

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