Arene rings, direct metallation

Bimetallic complexes where the a-bonded metal center is bonded to an arene ring carbon atom, and is thus in direct contact with the v system of the arene ring via metal d orbitals, are distinguished from those where the o-metal fragment forms part of a nonconjugated side chain of the ring. 

Also the lithium salts of macrocyclic compounds can be used as C-nucleophiles. Indeed, a new approach to modify the meso-position of tetramethoxy-substituted calix[4]arenes through the direct metal-free C-C cross-coupling of their lithium salts with 1,2,4-triazines has recently been suggested. It has been shown in our laboratory that the carbanions generated from tetraalkoxycalix[4]arenes are able to react easily with 3,6-diphenyl-1,2,4-triazine in THF at —78°C to give the o -adduct at C-6 of the triazine ring. Oxidation of this C-adduct with DDQ at ambient temperature affords meso-substituted calixarene derivatives in good yields (Scheme 40). 

Attachment of the arene ring to the electron-withdrawing metal preferentially directs the nucleophilic attack on the ring to the meta position. 

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