Sigma-complexes, formation

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. 

Equation (3) has good quantitative predictive power and is a successful extra-thermodynamic relationship like the Hammett sigma function. No other approach to modeling complex formation equilibria, including HSAB itself, can predict log values for unidentate ligands so accurately. 

Once formed, the electrophile behaves like any other electrophile, so the mechanism of the attack is the same as that for the previous situation where a nucleophile attacked the electrophile (described in the earlier section Basics of Electrophilic Substitution Reactions ). The attack leads to the formation of the resonance-stabilized sigma complex, followed by the loss of a hydrogen ion to a base. 

The authors speculated that Pd(ii) was reduced by reaction with the IL, followed by formation of sigma complex between the olefin and copper triflate. This polarized complex then reacts with the Pd(0)-7r-complex with the substrate to form the final product as shown by the scheme below. Scheme 7. 

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. 

Because of the high electron density in the aromatic ring, toluene behaves as a base both in the formation of charge transfer r complexes and in the formation of sigma complexes. When only n-electrons are involved, toluene behaves much like benzene and xylene. When o-bonds and complexes arc involved, toluene reacts much faster than benzene and much slower than xylenes. 

Fig. 5.3. Formation of a sigma complex (with delocalized positive charge) instead of an ammonium cation (with localized positive charge) upon protonation of l,3,5-tris(pyrro-lidinyl)benzene.

The regioselectivity and reactivity of Ar-SE reactions of naphthalene are explained correctly by comparing the free activation enthalpies for the formation of the sigma complexes 1- E— C10H10+ and 2-E— E— C10H10+ from the electrophile and naphthalene and for the formation of the sigma complex E— C6H6+ from the electrophile and benzene, respectively. 

With respect to the reactivities, the decisive effect is the following in the formation of either of the two isomeric sigma complexes from naphthalene, the difference between the naphthalene resonance energy (66 kcal/mol)—which is lost—and the benzene resonance energy (36 kcal/mol)—which is maintained—is about 30 kcal/mol. By contrast, the formation of a sigma complex from benzene costs the full 36 kcal/mol of the benzene resonance energy. This explains why fcmphthalene > enzene... 

Step 2 Electrophilic attack and formation of the sigma complex. 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

Formation of the sigma complex from ArCOO and H30 + may become reversible at high concentrations of a sufficiently strong base.)... 

Gillian, A. L., and Nibert, M. L. (1998). Amino terminus of reovirus nonstructural protein sigma NS is important for ssRNA binding and nucleoprotein complex formation. Virology 240, 1-11. 

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... 

An alkene is an average electron source, and an aromatic compound is usually worse therefore to get electrophilic addition to alkenes and aromatic compounds to occur one needs a good electron sink. Often a loose association of an electrophile with the pi electron cloud (called a pi-complex) occurs before the actual sigma bond formation step. The best electrophiles, carbocations, add easily. For an overview of electrophilic additions to alkenes, see Section 4.4. 

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