Ion, arenium

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... 

To involve allylic resonance in stabilizing the arenium ion formed during attack at C 2 the benzenoid character of the other ring is sacrificed... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

Koptyug, V. A. Contemporary Problems in Carbonium Ion Chemistry III Arenium Ions — Structure and Reactivity. 122, 1-245 (1984). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

In the arenium ion mechamism the attacking species may be produced in various ways, but what happens to the aromatic ring is basically the same in ail cases. For this reason most attention in the study of this mechanism centers around the identity... 

Isotope Effects. If the hydrogen ion departs before the arrival of the electrophile (SeI mechanism) or if the arrival and departure are simultaneous, there should be a substantial isotope effect (i.e., deuterated substrates should undergo substitution more slowly than nondeuterated compounds) because, in each case, the C—H bond is broken in the rate-determining step. However, in the arenium ion mechanism, the C—H bond is not broken in the rate-... 

However, in many instances, isotope effects have been found. Since the values are generally much lower than expected for either the Sgl or the simultaneous mechanisms (e.g., 1-3 for instead of 6-7), we must look elsewhere for the explanation. For the case where hydrogen is the leaving group, the arenium ion mechanism can be summarized ... 

Evidence for the arenium ion mechanism has also been obtained from other kinds of isotope-effect experiments, involving substitutions of the type... 

Isolation of Arenium Ion Intermediates. Very strong evidence for the arenium ion mechanism comes from the isolation of arenium ions in a number of instances. For example, 7 was isolated as a solid with melting point — 15°C... 

In Chapter 3, it was mentioned that positive ions can form addition complexes with 7T systems. Since the initial step of electrophilic substitution involves attack by a positive ion on an aromatic ring, it has been suggested that such a complex, called a % complex (represented as 10), is formed first and then is converted to the arenium ion 11. Stable solutions of arenium ions or 7t complexes (e.g., with Br2, l2> picric... 

TABLE 11.1 Relative Stabilities of Arenium Ions and ir Complexes and Relative Rates of Chlorination and Nitration ... 

Substituents Relative Arenium Ion stability Relative n-Complex Stability Rate of Chlorination" Rate of Nitration ... 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

For the reason given above and for other reasons, it is unlikely that the encounter complex is a n complex, but just what kind of attraction exists between Y+ and ArH is not known, other than the presumption that they are together within a solvent cage (see also p. 694). There is evidence (from isomerizations occurring in the alkyl group, as well as other observations) that n complexes are present on the pathway from substrate to arenium ion in the gas-phase protonation of alkylbenzenes. ... 

Some substituents have a pair of electrons (usually unshared) that may be contributed toward the ring. The three arenium ions would then look like this... 

We have discussed orientation in the case of monosubstituted benzenes entirely in terms of attack at the ortho, meta, and para positions, but attack at the position bearing the substituent (called the ipsoposition ) can also be important. Ipso attack has mostly been studied for nitration. When NOj attacks at the ipso position there are at least five possible fates for the resulting arenium ion (13). 

Path a. The arenium ion can lose NOj and revert to the starting compounds. This results in no net reaction and is often undetectable. 

Path b. The arenium ion can lose Z" ", in which case this is simply aromatic substitution with a leaving group other than H (see 11-36-11-44). 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

SO the Sgl mechanism and not the usual arenium ion mechanism is operating. Aromatic rings can also be deuterated by treatment with D2O and a rhodium(III) chloride or platinum catalyst or with CeDs and an alkylaluminum dichloride catalyst," though rearrangements may take place during the latter procedure. Tritium ( H, abbreviated T) can be introduced by treatment with T2O and an alkylaluminum dichloride catalyst. " Tritiation at specific sites (e.g., >90% para in... 

In a few cases, depending on the substrate and solvent, there is evidence that the arenium ion is not formed directly, but via the intermediacy of a radical pair (see p. 694) ... 

Unusual orientation has been reported for amination with halomines and with NCI3 in the presence of AICI3. For example, toluene gave predominately meta amination. It has been suggested that initial attack in this case is by Cl and that a nitrogen nucleophile (whose structure is not known but is represented here as NHJ for simplicity) adds to the resulting arenium ion, so that the initial reaction is addition to a carbon-carbon double bond followed by elimination of HCl ... 

The decarbonylation of aromatic aldehydes with sulfuric acid" is the reverse of the Gatterman-Koch reaction (11-16). It has been carried out with trialkyl- and trialkoxybenzaldehydes. The reaction takes place by the ordinary arenium ion mechanism the attacking species is H and the leaving group is HCO, which can lose a proton to give CO or combine with OH from the water solvent to give formic acid." Aromatic aldehydes have also been decarbonylated with basic catalysts." When basic catalysts are used, the mechanism is probably similar to the SeI process of 11-38. See also 14-39. 

---