Deactivation regioselective

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

When used with A-chloro-, A-bromo-, and A-iodosuccinimide, iron(III) chloride catalyzes the introduction of halogens into arenes. The reaction works well even with deactivated aromatic rings but in some cases the regioselective course is difficult to control (Scheme 39) [49]. 

Factors that determine the regioselectivity are the deactivation of one of the two oxygen nucleophiles by stereochemically controlled dimerization of the dioxastannolane, and by equilibration of the possible isomeric products (Equation (141)).389-391... 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

The palladium catalyzed coupling of haloazines and olefins is a robust process, which is usually run at an elevated temperatures in the presence of a simple catalyst and at least an equimolar amount of base to neutralise the formed hydrogen halide. The presence of a ring heteroatom might in certain cases lead to complex formation, which deactivates the catalyst and kills the process.63 The regioselectivity of the coupling is predominantly governed by... 

For any of the nucleophilic aromatic reactions covered in this chapter, regioselectivity, when more than one activated position is available, depends in most cases on the selectivity of attack by the nucleophile. However, when the conversion of the cr-adduct to product is the rate-limiting step, as in the VNS reaction, the final product distribution may differ from that expected, based on the relative electrophilic reactivity of the possible reaction sites.15 Important roles are played by deactivating steric effects and by stabilizing specific interactions such as those, for example, between an ion-paired nucleophile and a nitro activating group, which favor attack at the ortho position. 

Nitration of strongly deactivated aromatics was carried out with nitric acid mixed with triflatoboric superacid.482 The method is characterized by high yields (penta-fluorobenzene, 99%, 1,2,3,5-tetrafluorobenzene, 89% 2,3,4-trifluoronitrobenzene, 96% methyl phenyl sulfone, 78%), usually high regioselectivity, and mild reaction conditions (room temperature), and it also tolerates many functional groups. 

It seems unlikely that deactivation of the HT transition state would be much different than that of the HH isomer. Clearly, the sharp change in regioselectivity must be due to factors other than changes in solvent deactivation. Because the sizes of the regioisomers are similar, this effect can not be attributed to the repulsive contribution to the equation of state in transition state theory. However, in this near-critical region, large solute-solute fluctuations (i.e. solute-solute clusters) must... 

A powerful electrophilic species is obtained from C120 and a strong protic acid xuch as trifluoroacetic acid. This reagent effects exclusive ring chlorination of deactivated arenes in high yield with the usual regioselectivity. 

Regioselective deactivation of the dianions of carbohydrate diols by complexing with copper salts (see Sect. 3.5) has been employed for their effective monobenzylation [165, 166]. When benzyl iodide was used, the yield of monosubstituted product was higher than 85 % in almost every instance, the rest being accounted for an unreacted starting material. In no case was any disubstituted product isolated. Benzyl bromide gave <20% of alkylation in a reasonable time, benzyl chloride showed no reaction at all. 

---