Ortho substituents arylation

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

If the aryl halide contains two ortho substituents, the reaction should not be able to occur. This is indeed the case. ... 

Fig. 3.9 Catalysts 27 and 28 lacking ortho-substituents on the iV-aryl side chains...

Several studies confirm an ortho effect leading to a dominating aryl sulphur bond cleavage. For example, the introduction of a bulky ortho substituent will provide the formation of the aliphatic sulphinic acid. A series of cyclic sulphones was studied at the mercury cathode and the results (see Table 2) appear to be fully in agreement with those expected when considering the preliminary works presented above. 

Of course, we do not mean to imply any kind of moral imperative here. For example, if you were checking out some dubious looking starting material that supposedly contained a given functionality (e.g., Ar-CH2-Br with no ortho substituent on the aryl ring and no other obvious steric clash between this aryl system and any other), then it would be quite acceptable - and even desirable - to exploit some obvious handle in the form of a cutdown interpretation . In this case for example, if there is no two-proton singlet (or AB system if the molecule is chiral) at around 4.6-4.3 ppm, then you can reasonably conclude that the stuff is not what you wanted. Job done. Move on. 

Aryl nitroxides frequently produce quite complex spectra arising from interaction of the spin with the aryl hydrogen nuclei. This can be associated with delocalization of spin into the aromatic rings (see structure [8]), an effect which is usually accompanied by an attenuation of the principal aN-value. When, however, the aryl group contains bulky ortho-substituents, it is twisted out of conjugation with the nitroxide aN then reverts almost to its value in an alkyl nitroxide, and coupling with the aromatic hydrogens is dramatically... 

The effect on 15N and 13C chemical shifts in anilines75-77 and nitrobenzenes74 of a reduced resonance interaction caused by ortho substituents which sterically twist the amino and nitro groups from coplanarity with the aryl ring has been extensively investigated and is fairly well documented. 

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