Cyclohexadienyl cation stabilities

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate In spite of being allylic a cyclohexadienyl cation IS not aromatic and possesses only a fraction of the resonance stabilization of benzene... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

Sections How substituents control rate and regioselectivity m electrophilic aro 12 10-12 14 matic substitution results from their effect on carbocation stability An electron releasing substituent stabilizes the cyclohexadienyl cation inter mediates corresponding to ortho and para attack more than meta... 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

Wheland intermediates, a complexes, or arenium ions In the case of benzenoid systems they are cyclohexadienyl cations. It is easily seen that the great stability associated with an aromatic sextet is no longer present in 1, though the ion is stabilized by resonance of its own. The arenium ion is generally a highly reactive intermediate and must stabilize itself by a further reaction, although it has been isolated (see p. 504). 

Cycloalkadienyl cations, particularly cyclohexadienyl cations (benzenium ions), the intermediate of electrophilic aromatic substitution, frequently show remarkable stability. Protonated arenes can be readily obtained from aromatic hydrocarbons244 251 in superacids and studied by 1H and 13C NMR spectroscopy.252,253 Olah et al.252 have even prepared and studied the parent benzenium ion (C6H7+) 88. Representative 1H NMR spectra of benzenium253 and naphthalenium ions25488 and 89 are shown in Figures 3.11 and 3.12, respectively. 

This ion is less stable than the cyclohexadienyl cation formed during bromination of benzene. (g) Sulfonation of furan takes place at C-2. The cationic intermediate is more stable than the cyclohexadienyl cation formed from benzene because it is stabilized by electron release from oxygen. 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadienyl cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

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