Substitution, electrophilic group ability

The substitution of an electrophile for another group on an aromatic ring is electrophilic aromatic substitution (Equation 7.55). Although the leaving group is most often H+, it may also be another Lewis acid. Perrin has found that the order of leaving group abilities is H+ I+ > Br+ > NOa+ > CU, which is... 

The leaving group ability of Me Sn group in electrophilic substitutions allowed the synthesis of diaryl ketones in good yields (40-78%) through the reaction of Me SnAr (synthesized by S l reactions) with ArCOCl [48]. These reactions were regioselective, allowing the synthesis of diaiyl ketones not usually available under Friedel-Crafts reactions. 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

The remarkable ability of the amino group to promote electrophilic substitution is illustrated by the nitration of 3,5-dinitroaniline (30) to 2,3,4,5,6-pentanitroaniline (31) in 52 % yield when treated with anhydrous mixed acid. The pre-existing nitro groups are advantageous by making the aromatic ring less prone to oxidation. 

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