Electrophilic substitution, aromatic internal

The only known example of the catalytic nucleophilic aromatic substitution involves the use of Rh(III) complexes (e.g., 40). The first demonstration was made in 1980 [99]. The metal center is so electrophilic that an internal alcohol nucleophile substitutes directly without an added base. 

Although the mechanism of the Dakin reaction given in many textbooks does not involve participation of the n electron of the aromatic ring, the mandatory presence of polar substituents (e.g., OH) in the ortho or para position to the formyl group suggests that an internal electrophilic substitution of the adduct intermediate is a better representation. The uncatalyzed S Ar process is a soft-soft interaction. 

Another point worth mentioning is the selectivity of the electrogenerated electrophilic sulfur species toward aromatic substrates in the reaction with diphenylether and diphenylamine. In the first case, an internal ortho-cyclization yields in phenoxathiin, in the second, a para-substitution occurs [252] giving a linear product (8cheme 59). 

It is important to note that both aliphatic and aromatic vinyl ketones were viable substrates in which P-alkyl and P-aryl substitutions were tolerated. Diaryl alkynes, mono-aryl internal alkynes, nonaromatic internal alkynes, terminal alkynes, and free hydroxyl-bearing alkynes were all efficient participants. It should be mentioned that while ynoates were successful nucleophiles, enoate electrophiles were found to be poor participants in nickelotalyzed couplings. 

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