Complex or Wheland Intermediates

As described earlier, the S Ar reaction mechanism generally involves the formation of a a-complex intermediate. This species is formally a cyclohexadienyl cation, and it has also been called the Wheland intermediate, Pfeiffer-Wizinger complex, arenium ion, benzonium ion, and benzeniumion. Since it was first proposed as an intermediate in Friedel-Crafts reactions, there has been a considerable amount of evidence for its involvement in the reactions. As described previously, the involvement of the a-complex provides a good basis for the understanding of some activating and dbecting effects in S Ar reactions. 

The final step in the S Ar reaction mechanism involves deprotonation of the a-complex intermediate to regenerate the aromatic x-system, and this is expected to be a very fast step. Since the C—H bond is not being broken in a rate-determining step, there is usually little or no detectable kinetic isotope effect (KIE) for S Ar reactions [61]. Thus, studies of KlEs are also consistent with the involvement of the a-complex. Larger KIEs have been observed in conversions involving weak electrophiles, such as nitrosations and diazonium coupling reactions [61c]. 

Several examples have been reported of S Ar reactions providing different products under thermodynamic and kinetic control. For example, sulfonation of naphthalene at 80°C gives predominantly the a-isomer (the kinetic product), while reaction at an elevated temperature provides the P-isomer (the 

Several a-complex structures have also been studied using X-ray crystallography. The crystalline adducts 73, 75, 76, 81, and 83 from hexamethylbenzene have all been characterized by X-ray diffraction [65f-j]. In complex 75, the C—Cl bond distance is measured to be 1.81 A, a value typical 

FIGURE 1.7 Preparation of (T-complex 84. Adapted with permission from Rathore et al. [47]. (1994) 

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However, the intermediate carbocation is stabilized by resonance, with the positive charge shared formally by three carbon atoms of the benzene ring (Scheme 2.2). The resonance hybrid structure 1 indicates the delocalization of the charge. The carbocation is also referred to as a a-complex or Wheland intermediate-... 

Electrophilic substitution of aromatic (and heteroaromatic) molecules proceeds via a two-step sequence, initial addition (of EF) giving a positively charged intermediate (a o-complex, or Wheland intermediate), then elimination (normally of H ), of which the former is usually the slower (rate-determining) step. Under most circumstances such substitutions are irreversible and the product ratio is determined by kinetic control. 

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

Nitration of the toluene appears to be a three-step process, as demonstrated with DFT calculations of benzene nitration in the gas phase by Olah and coworkers." The acetyl nitrate readily transfers a nitronium -like moiety to the Jt-system of the toluene close to the para or ortho sites. This is a jt-complex with no real bonding interaction. An sp to sp hybridization of the para or ortho carbon must then occur to create a o-complex (aka Wheland intermediate or arenium cation). Finally, this 0-complex must transfer the proton from the para or ortho site to a Brpnsted acid site in the zeolite wall. Upon optimizing the H-beta structure with the acetyl nitrate site and toluene, it became clear that the most likely acid transfer site after creation of the o-complex is the All-02 site in our model, which, as we already... 

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). 

Electrophilic aromatic substitution normally proceeds via a positively charged intermediate 71 (known as a Wheland intermediate or cr-complex) (equation 27)84. 

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

The mechanism of mercuration is shown in Scheme 2. In the first step, the mercury salt forms a Jt-complex with the aromatic substrate [15, 16]. In 1982, Lau, Huffman, and Kochi [17] reported the first isolation and full characterization (including X-ray molecular and crystal structure) of such an intermediate, a complex of hexamethylbenzene with Hg(TFA)2. The X-ray structure revealed a Hg2( -TFA)i framework with a molecule of C6Me6 -coordinated to each of the Hg atoms. Analogous Jt-complexes have also been observed and studied by Dean and co-workers [18] and more recently by Barron s [19] and Gabbai s [20] groups. The Jt-complex intermediate can rearrange to the ej-complcx (a Wheland intermediate) directly, or sometimes via electron transfer, to produce a radical ion pair which then collapses (Scheme 2) [21,22]. 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

Electrophilic aromatic substitution represents one of the most important applications of the transformation of an arene via a CT (precursor) complex. The process is considered to proceed via an (encounter) -complex between the electrophile (E+) and the aromatic substrate (ArH), which collapses in a single rate-limiting step to the Wheland intermediate or a-... 

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