Arenium ions isolation

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... 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

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). 

Product formation was interpreted in terms of transalkylation of substituted triphenylmethanes. Protonation at the ipso position of the substituted phenyl ring to form arenium ion 64 followed by the C—C bond breaking yields the diphenylmethyl cation, which alkylates benzene or is stabilized by hydride transfer (Scheme 5.30). The protonated intermediate 64 is highly unstable when the ring has an electron-withdrawing substituent. Consequently, its transformation is extremely slow and the primary product triphenylmethane can be isolated. 

Note that both the bromination and the acylation of naphthalene result in the substitution of the electrophile at the 1 position. None of the isomeric product with the electrophile bonded to the 2 position is isolated in either case. The higher reactivity of the 1 position can be understood by examination of the resonance structures for the arenium ion. When the electrophile adds to the 1 position, the arenium ion has a total of seven resonance structures, whereas only six exist for the arenium ion resulting from addition of the electrophile to the 2 position. 

It corresponds to addition followed by elimination, and is symbolised by AE + De. The departing X+ is often a proton, while Z is a general substituent. The key step in this scheme is the formation of an intermediate arenium ion (Wheland intermediate, a complex), and the relative stability of this species is crucial to the outcome of the reaction. Isolable arenium ions are known, and the benzenonium ion itself C6Hy has been inferred from NMR of strongly acidic solutions [255],... 

The remaining two steps proceed along very similar lines in most electrophilic aromatic substitution reactions. The attack by the electrophile is usually the rate limiting step. The cationic intermediate is called a Wheland intermediate, or o-complex or an arenium ion, and can sometimes be isolated. 

A simple example illustrates the reaction. When benzene reacts with benzyl chloride in the presence of 0.4 equivalents of AICI3, diphenylmethane (45) is isolated in 59% yield. If this reaction proceeds by electrophilic aromatic substitution, then the sp carbon of benzyl chloride is a precursor to a carbocation. To form a carbocation from benzyl chloride, the chlorine atom must react as a Lewis base with AICI3 to form PhCH2 AlCL. Benzene reacts with this carbocation via electrophilic aromatic substitution in the same manner as the reaction with Br in the previous section to form an arenium ion intermediate (see 40) to give 45. 

When 1-bromobutane reacts with aluminum chloride (AICI3) in the presence of benzene, the isolated product is 48, not the expected n.-butylbenzene (1-phe-nylbutane). If 1-bromobutane reacts with aluminum chloride, the expected product is the primary carbocation shown however, to explain formation of 48, benzene must react with a secondary carbocation. Therefore, the initially formed primary cation must rearrange to the more stable secondary cation, 46, via a 1,2-hydride shift before it reacts with benzene. This type of 1,2-hydride shift was introduced in Chapter 10 (Section 10.2.2). Carbocation 46 reacts with benzene by the expected mechanism to give arenium ion 47 once formed, loss of a proton by the El reaction gives 48. 

Another type of Friedel-Crafts reaction also involves a carbocation, but not the simple alkyl carbocations discussed in the previous section. When benzene reacts with butanoyl chloride (49) in the presence of AICI3, the isolated product is a ketone butyrophenone (1-phenyl-l-butanone, 50) in 51% yield. It is known that benzene does not react with 49 without the presence of aluminum chloride. Clearly, the acyl unit has reacted with benzene to form the new carbon-carbon bond, with loss of the chlorine atom of 49, but there must be a prior reaction between AICI3 and the acid chloride. This is an aromatic substitution and, based on knowledge of benzene, there must be an arenium ion intermediate. To form an arenium ion, benzene must react with a cationic species, which must arise from 49. What is the nature of this cationic intermediate ... 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

The evidence for the first step of Figure 6.34 being rate determining and the intermediacy of the resonance-stabifized (Scheme 6.82) a-complex or arenium ion following that transition state comes from (a) the isolation of some suitably substituted arenium ion intermediates that then go on to product, (b) low-temperature studies of the NMR spectra of arenium ions, and (c) the presence or lack of a primary isotope effect when the carbon-hydrogen bond is broken. If the bond-breaking step is rate determining then an isotope effect on the rate will be seen. 

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