Metalation permutational interconversions

C. Permutational Metal/Metal Salts Interconversions (Transmetallations). . . 445... 

Halogen/metal permutation and hydrogen/metal permutation (usually apostrophed as "metalation") dominate the interconversion methods. They employ organoalkali reagents such as phenyllithium, methyllithium, -butyllithium, iec-butyllithium, or the superbasic LIC-KOR mixture. However, even if commercial, these reagents have to be made beforehand. The reaction of a chloro- or bromo-substituted hydrocarbon with lithium, sodium, or magnesium offers a standard entry to them. Thus, ultimately one always has to revert to the metal. 

What makes the permutation approach so attractive is the ease of execution and the mild reaction conditions. A possible drawback is its moderate exothermicity which may prevent the interconversion to go to completion but rather to stop as soon as an equilibrium composition is attained. Moreover, to rely on an organometallic exchange reagent merely defers the problem. Ultimately, the genesis of any organometallic compound can be traced back to the elemental metal. 

Bromo- and iodoarenes are the oldest and most typical substrates for permutational halogen/metal interconversions. Butyllithium is routinely used for this purpose (Table 16). Hetero-substituents such as dialkylamino or bis(trialkylsilyl)amino, cyano or nitro (Table 17), alkoxy and 2-tetrahydropyranyloxy (Table 18), lithiooxy or lithiooxycarbonyl (Table 19) and fluoro or trifluoromethyl (Table 20) and chloro, bromo or iodo (Table 21) are well tolerated. 

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