Arenonium ion

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

Antiperiplanar conformation, 118, 253 Arenium ions, 131 Arenonium ions, 131 Arndt-Eistert reaction, 119 Aromaticity, 14... 

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. 

PtI MeC6H3(CH2NMe2)2-o,o ]BF4 (49) is considered to be a metal-substituted arenonium ion it should be susceptible to nucleophilic attack. This concept has been used to effect a fully reversible, aryl to cyclohexadiene conversion in a metal coordination sphere.524... 

Evaluation of pATR from measurements of rate and equilibrium constants for the protonation of carbon-carbon double bonds of alkenes suggests the possibility of a similar approach for aromatic double bonds. Protonated aromatic molecules are the parent structures of the arenonium ion intermediates of electrophilic aromatic substitution. For these cations the equilibrium constant Kk refers to equilibria with the corresponding aromatic hydrates, as is illustrated in Scheme 5 for the benzenonium ion (cyclohexadienyl cation) 9 for which the hydrate is cyclohexadienol 10. 

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... 

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... 

It is also possible to examine the effect of oxygen substituents on the stability of arenonium ions. Wirz has studied keto-enol equilibria for phenol,151 naphthol (Wirz J, Personal communication), and anthrol.152,153 The tautomeric constants may be combined with p/y,s for protonation of the keto tautomer and ionization of the phenol to provide pifas f°r protonation of the aromatic ring of phenol and the phenoxide ion. As illustrated in Scheme 18 the unstable keto tautomer of phenol 22 was produced by photolysis of the bicyclooctene dione 21. Except in the case of the anthrone a pA a for protonation of the keto tautomer has not been measured directly. However, values can be estimated from the pfor protonation of the 4,4-dimethylated analog136 with a correction for the substituent effect of the methyl groups. 

Table 3 Acid dissociation constants of arenonium ions and their a-HO and a-O derivatives3...

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.

Thus if we axe interested only in the relative values of equilibrium or rate constants for closely similar reactions, we need only concern ourselves with changes in delocalization energy. This of course is a useful simplification. A good illustration is provided by the protonation of aromatic hydrocarbons. Aromatic hydrocarbons react with strong acids to form salts. These are now known4 to be arenonium ions thus benzene is converted to benzenonium ion (I) by strong acids ... 

Now the bond lengths of the CC bonds in aromatic compounds vary over only a small range it is reasonable to suppose that the bond energies of their a components will all be much the same. This should also be true of the bonds in arenonium ions such as I. 

The arguments given above may be best illustrated by an example the rates of substitution reactions in simple aromatic systems. We shall assume first that the transition states are arenonium ions or radicals, i.e. 

A new member of the series of arenonium ions undergoing degenerate rearrangements, the 9,9,10-trimethylpyrenium ion [321], has been studied by Borodkin et al. (1974a) using H-nmr spectroscopy. The rate of rearrangement via 1,2-methide shifts was very close to that of [307] and this was attributed... 

Sangalov et al., prepared arenonium ions in the form of Gustavson complexes starting from various substituted benzenes, hydrogen chloride and aluminium chloride. These complexes were drown to be active initiators for the polymerisation of styrene and isobutene at —30 and —78°C in methylene chloride. No kinetic investigation was carried out on these systems. The authors claimed that initiation took place by protonation of the olefin and that incorporation of aromatic groups from the cataylst in the products was due to transfer reactions. 

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. 

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