Sigma complex intermediate

Figure 5.33 presents Friedel-Crafts acylations, taking benzoylations of toluene (top line) and para-tert-butyl toluene (Figure 5.33, bottom) as an example. The methyl group of toluene preferentially directs the benzoyl residue into the para-position. The ortho-benzoylated toluene occurs only as a by-product. In para-tert-butyl toluene both the methyl- and the tert-butyl substituent direct the electrophile towards the ortho-position, since both para-positions are occupied and could at best react with de-ferf-butylation, i.e., in a—sterically hindered — ipso-substitution (cf. Figure 5.5). Indeed, we see reaction ortho to the methyl group and not ortho to the ferf-butyl group. This selectivity can be ascribed to minimized steric interactions in the preferred sigma complex intermediate.

Because electron-withdrawing groups would destabilize an adjacent carboca-tion, addition of the electrophile to the meta position gives a more stable sigma complex intermediate. Since meta is the major product obtained, electron-withdrawing groups are known as meta directors. ... 

These ideas are nicely illustrated by the results in Table 6.6, corresponding to the four reactions given later (Reichardt et al., 1988, p. 218). The first reaction is a simple type of halide interchange process, the second is similar but involves azide ion as a very effective nucleophile. The last two are aromatic nucleophilic substitution (S Ar) reactions and are shown as proceeding by way of sigma complex intermediates (Buncel et al., 1995). Note that Reaction (6.17), which... 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

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. 

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). 

Strictly speaking there is only indirect evidence for the occurrence of sigma complexes as Side Note 5.1. short-lived intermediates of Ar-SE reactions. Formed in the rate-determining step, sigma com- Stable Cyclohexadienyl plexes completely explain both the reactivity and regioselectivity of most Ar-SE reactions (cf. Cations Section 5.1.3). In addition, the viability of short-lived sigma complexes in Ar-SE chemistry is supported by the fact that stable sigma complexes could be isolated in certain instances. 

The actual Ar-SE product in Figure 5.22, namely aminonitrobenzene sulfonic acid F (the uncharged form), which is in equilibrium with the zwitterionic form G (cf. Figure 5.18), will then be desulfonylated. The mechanism of this reaction resembles the one shown in Figure 5.7. So the sigma complex E is a key intermediate. In the end, the ortho-isomer C of... 

The sigma complex for meta substitution has its positive charge spread over three 2° carbons this intermediate is similar in energy to the intermediate for substitution of benzene. Therefore, meta substitution of toluene does not show the large rate enhancement seen with ortho and para substitution. 

Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. The following reaction illustrates the preference for ortho and para substitution in the nitration of chlorobenzene. 

Draw all the resonance forms of the sigma complex for nitration of bromobenzene at the ortho, meta, and para positions. Point out why the intermediate for meta substitution is less stable than the other two. 

An intermediate in electrophilic aromatic substitution or nucleophilic aromatic substitution with a sigma bond between the electrophile or nucleophile and the former aromatic ring. The sigma complex bears a delocalized positive charge in electrophilic aromatic substitution and a delocalized negative charge in nucleophilic aromatic substitution, (p. 756)... 

Hydrogen bonds contain linear 4e,3-center E H M bridge bonds with hypervalent hydrogen, while sigma complexes contain bent 2e,3-center E-H-M bridge bonds with electron-deficient hydrogen. In the former, the metal acts as a weak base, while in the latter it acts as a weak acid. In principle, intermediate situations are possible where both interactions compete but information is still sparse in this area. 

Sigma-complexes, observed by NMR, are analagous to the well-known Wheland intermediates in hydrocarbon chemistry, is possible upon the addition of CsF to perfluoro-s-triazine derivatives [101] (Figure 4.41) but direct observation of similar o-complexes derived from less-activated perfluoroaromatic systems has not yet been reported. 

These results indicate that proton transfer occurs in the ratedetermining step and there is little carbon—carbon bond cleavage in the transition state. Clear-cut evidence for a two-step mechanism is supplied by Lynn and Bourns results of variable carbon-13 isotope effects in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers [249] (Table 22). Slow proton transfer occurs in the first step and C—C bond cleavage takes place in the second step. At high concentrations of the buffer base, the rate of reversal of the first step becomes comparable to the (relatively fast) rate of the second step and, consequently, the second step becomes partially rate-determining which causes a weak carbon isotope effect. The most reasonable structure of the intermediate is that of the sigma complex. 

As previously discussed in section III.A.l, azide ion undergoes addition-elimination Ar reactions with bond-formation being rate-limiting or sufficiently close to it to have equivalent results. At low temperatures the intermediate addition complex from the interaction of azide ion with 2,4,6-trinitroanisole has been detected by Caveng and Zollinger . These workers observed characteristic p.m.r. resonances arising from the sigma complex (147) at —40° in acetonitrile and... 

The more stable the intermediate, the lowpr th S greater stabilization to this sigma complex... 

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