Halobenzenes substitution

Polarographic studies on haloisoxazoles in anhydrous DMF containing R4N" were performed and the magnitude of the half-wave potentials were recorded. Cleavage of the C—X bond was faster in phenylhaloisoxazoles than in halobenzenes. Substitution patterns affected the reduction (79ZOB1322). 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

Approximate Relative Rates of Halogen Displacement FROM 4-Substituted Halobenzenes and 4-Substituted 2-Nitrohalobenzenes... 

Halobenzenes undergo nucleophilic aromatic substitution through either of two mechanisms. If the halobenzene has a strongly electron-withdrawing substituent in the ortho or para position, substitution occurs by addition of a nucleophile to the ring, followed by elimination of halide from the intermediate anion. If the halobenzene is not activated by an electron-withdrawing substituent, substitution can occur by elimination of HX to give a benzyne, followed by addition of a nucleophile. 

When arenesulfonyl radicals are generated in benzene the only reported products are those of disproportionation50,95 (vide supra). However, Camaggi and coworkers95 have found that arenesulfonyl radicals in halobenzene replace the halogen atom at 150-190 °C, the relative reactivities being for Cl Br I, 1 5.9 18.6. These authors95 proposed that the reaction proceeds via a reversible ip.vo-substitution,... 

Cu-exchanged zeolites have been examined in the nucleophilic substitution of halobenzenes towards aminated and oxygenated systems. Selectivities are dependent on the zeolite s pore sizes. 

Electrophilic addition to 1-haloalkenes (e.g. 27), presents a number of parallels to the electrophilic substitution of halobenzenes (p. 155). Thus it is the involvement of the electron pairs on Br that controls the orientation of addition (cf. o-/p-direction in C6H5Br) ... 

The a-, f3- and y-halogeno substituents in pyridines and their benzo analogues are each more susceptible to nucleophilic substitution than is the case for halobenzenes because of the overall electron deficiency of the heteroaromatic ring. Furthermore, halogen substituents a and y to nitrogen are usually more reactive than /3-halogens and this is particularly so in pyridinium type systems. 

A limited number of reports of indoles arising from o-bromoaniline and enolates have appeared (equation 107) (80JOC1546,8lT(S9)393). The final cyclization step is of the condensation type already recognized in several other procedures. The inertness of unactivated halobenzenes, such as o-bromoaniline, requires an alternative to direct aromatic nucleophilic substitution and those cases where success has been reported depend upon photoinitiated substitution by an electron transfer process. The scope of this method remains to be explored but it appears that alkyl, alkoxy and carboxy groups can be tolerated on the aromatic ring. When the enolates are derived from an unsymmetrical ketone in which one group is methyl, there appears to be a preference for exclusive involvement of the less substituted enolate, leading to 2-alkylindoles. 

THE GAS PHASE ISOMERISATION OF SUBSTITUTED HALOBENZENES ON ZEOLITES B.COQ, J.PARDILLOS and F.FIGUERAS... 

In recent years, a large body of work emphasized the use of zeolites for production of fine chemicals (refs.1-4). The interests stand in replacement of liquid acids to lower corrosion of equipment and pollution, and to reach specific selectivities. However, the hopes raised up in a rapid development of processes seems restrained nowadays. Many patents claimed zeolites as catalysts but very few have received industrial applications. Actually, basic research on the stability, the origin of deactivation, the regenerability of the catalysts have to be developed. Moreover, fundamental aspects of the mechanism of this new kind of reactions are lacking, in particular, the possibility of radical mechanisms, which are rather scarce with hydrocarbons, but can likely occur when heteroatoms are involved in the reactant. Those were our objectives in the study of the isomerisation of substituted halobenzenes on zeolites (refs.5-7). Indeed this reaction was claimed to occur readily on zeolites (refs.8-9), but it is supposed that no industrial development has followed. 

The gas phase isomerisation of substituted halobenzenes occurs readily on zeolites. Similarities appear with the same reaction catalyzed by AlClj in the homogeneous phase, in particular the applicability of the Hammett equation. However, several differences stand in the mechanism and the reaction scheme. Part of the transformation of halobenzenes occurs by means of a radical dechlorination/chlorination mechanism. For the same reasons the formal reaction follows a triangular scheme which differs from the consecutive scheme occuring in the liquid phase. Moreover, selectivity can be strongly affected by the restriction to diffusion in the porous volume of the solid. 

The reactivities of aryl halides, such as the halobenzenes, are exceedingly low toward nucleophilic reagents that normally effect displacements with alkyl halides and activated aryl halides. Substitutions do occur under forcing conditions of either high temperatures or very strong bases. For example, chlorobenzene reacts with sodium hydroxide solution at temperatures around 340° and this reaction was once an important commercial process for the production of benzenol (phenol) ... 

The isolation of the first halobenzene complex, (q6-chlorobenzene)tricarbonylchromium(0), allowed a test for a direct analog of classical SNAr reactivity.15 The activating effect of the Cr(CO>3 unit was found to be comparable to a single p-nitro substituent in reaction with methoxide in methanol and the substituted arene ligand was detached with mild oxidation (equation 2). 

Very few examples of vinyl substitution with polynuclear aromatic halides have been reported but indications are that they generally react like the halobenzenes. 

---