Carbocations arenonium ions

Cyclohexadienol was prepared by Rickborn in 1970 from reaction of the epoxide of 1,4-cyclohexadiene with methyl lithium.100 A hydrate of naphthalene, 1-hydroxy-1,2-dihydro-naphthalene was prepared by Bamberger in 1895 by allylic bromination of O-acylated tetralol (1-hydroxy-l,2,3,4-tetrahydronaphthalene) followed by reaction with base.101 Hydrates of naphthalene and other polycylic aromatics are also available from oxidative fermentation of dihydroaromatic molecules, which occurs particularly efficiently with a mutant strain (UV4) of Pseudomonas putida.102,103 The hydrates are alcohols and they undergo acid-catalyzed dehydration to form the aromatic molecule by the same mechanism as other alcohols, except that the thermodynamic driving force provided by the aromatic product makes deprotonation of the carbocation (arenonium ion) a fast reaction, so that in contrast to simple alcohols, formation of the carbocation is rate-determining (Scheme 6).104,105... 

These species are also referred to as arenium or arenonium ions, as well as by the more general term of carbocation intermediate. 

Step (1) is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation (a benzenonium ion) loses a cation (most often to give the substitution product, rather than adding a nucleophile to give the addition product. The benzenonium ion is a specific example of an arenonium ion, formed by electrophilic attack on an arene (Section 11.4). It is also called a sigma complex, because it arises by formation of a o-bond between E and the ring. See Fig. 11-1 for a typical enthalpy-reaction curve for the nitration of an arene. 

This correlation is shown for a range of carbocation structures in Fig. 1, with the arenonium ions shown as filled circles. The correlation includes protonated mesitylene, studied by Marziano,107 and earlier measurements of the protonation of methoxy-substituted benzenes108 and azulenes.109 It also... 

Stabilization conferred by aromatic hyperconjugation resolves a puzzle concerning the relative stabilities of arenonium ions. As judged by rates of solvolysis reactions, normally a phenyl group is more effective than vinyl in stabilizing a carbocation center.166 This difference is moderated for cycloalkyl substrates, so that benzoannelation has little effect, for example, on the rate of hydrolysis of 3-chlorocyclohexene (Cagney H, Kudavalli JS, More O Ferrall... 

Fig. 3 Plot of log ku2o against pklR. The main correlation line based on arenonium ions and secondary benzylic carbocations the dashed line and filled circles are for tertiary cations.

The alternative explanation assuming the transition state of the 1,2-shift to be similar to the initial one (cf Ref >) is hardly acceptable for the case of rearrangements of long-lived carbonium ions. The 1,2-methyl-shift-retardation effect of the phenyl group and of the substituted phenyl groups in arenonium ions may be, to a certain degree, due to the predominance of conformation with the aryl group in the quasi-axial position this is favourable for the intramolecular donor-acceptor interaction with the carbocation centre (cf. Ref. 

For a consistent quantitative description of rearrangements it seems reasonable to divide the whole set of degenerate carbocations into series of structurally related ones within which the changes in rearrangement rates can be treated as caused mainly by one structural effect. There is every reason to believe (cf. Ref. ) that in many cases the extent of electron deficiency in the carbonium centre may serve as a valuable parameter for such a purpose. For the arenonium ions with an aromatic transition state the decisive factor may prove to be the value of jt-electron deficiency which for a series of related structures may be characterized by the chemical shift of the carbonium carbon (5C ) in the C NMR spectrum 259,270)... 

An example of degenerate 1,2-methyl shifts are arenonium ions of the type A it has been found that the electronic influence of the MCR of the carbocation on the rate of rearrangement is characterized not only by 8C but also by such structural parameters as the <7 -constants of substituents and the infrared frequencies of C = 0, V( of ketones related to the rearranging carbocations . ... 

The most appropriate parameter seems to be the chemical shift of the carbon atom — the carbocation centre (SC ). Its main advantage is its close relationship with the value of -electron deficiency in the carbocation centre which can be estimated by the MO method. For the series of structurally related arenonium ions - a linear correlation is found between the free activation energy of the... 

G. Olah proposed to name the classical carbocations carbenium ions (cf. and those with penta-coordinated carbon atoms carbonium ions . In keeping with his recommendations the authors d ing with the above ion groups in their English-language publications now use the term arenium ions more often than the one used earlier ( arenonium ions ). 

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