Halogen Migration on Aromatic Compounds

FIGURE 28.2 (a) Chemo- and (b) regioselectivity of the halogen/metal exchange. 

SCHEME 28.4 Regioselective bromine/lithium interconversions on equivalent positions. 

These examples underline that the outcome of monolithiation in polybrominated starting materials are under thermodynamic control. When the chemical environment of the different bromine atoms is not identical, always the most stable organolithium intermediate is formed first. The relative basicities of aryUithiums bearing methoxy, chlorine, fluorine, trifluoromethyl, and trifluo-romethoxy substituents at the ortho, meta, and para positions have been studied by Schlosser et al., and AAG increments have been calculated from the equilibrium constants [26]. These thermodynamic values allow to explain these regioselectivity issues. 

SCHEME 28.6 Selective single halogen/metal exchange with magnesates. 

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