Sigma complexes, generalizations

Although several different rate laws have been drawn for different substrates, the kinetic equations are generally consistent with a mechanism involving an electrophilic attack by I2 on an anionic imidazole ring system, followed by proton abstraction from the sigma complex as the rate-determining step. 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

The magnitude of the solvent isotope effect and the absence of a carbon isotope effect confirm that proton transfer is rate-determining in the reactions referring to s. As far as the reactions referring to are concerned, the experimental values of these rate coefficients for the decarboxylation of 2- and 4-aminobenzoic acids, as well as the Arrhenius parameters, are comparable to those of the substituted salicylic acids if expected substituent effects are taken into account (Table 21) there is a correlation between log A and Ea. Therefore, it is reasonable to expect that the mechanism is the same. The observed general catalysis supplies additional evidence for rate-determining proton transfer from H30+ to S (sigma complex formation) in the decarboxylation of 4-aminobenzoic acid. 

The stability of the sigma complex is the guiding principle when determining the site of electrophilic attack on any aromatic ring. The more approximately equal energy resonance structures the carbocation has, generally the more stable it is. Resonance forms that place the positive charge next to an ewg are poor and should not be counted. 

A variety of electrophiles (E ) will react with a benzene ring, and we will explore many of them in the upcoming sections of this chapter. It will be helpful to realize that all of these reactions operate via the same general mechanism that has only two steps (1) the aromatic ring functions as a nucleophile and attacks an electrophile to form a sigma complex followed by (2) deprotonation of the sigma complex to restore aromaticity (Mechanism 19.3). 

Recent studies have established the following generalizations regarding a (sigma-bond) complexes ... 

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