Relative Basicities of ortho-, meta-, and para-Substituted Aryllithiums

The relative basicities of aryllithiums bearing methoxy, chlorine, fluorine, trifluoromethyl and trifluoromethoxy substituents at the ortho, meta, and para positions have been assessed. To this end, two aryllithiums of comparable basicity were equilibrated together with the corresponding bromo- or iodoarenes in a 1: 2 mixture of pentanes with tetrahydrofuran at -50, -75, or -100 degrees C. The "basicity" (protodelithiation) increments Delta Delta G derived from the equilibrium constants are linearly correlated with the relative protonation enthalpies of the corresponding aryl anions in the gas phase. However, the correlation factor proves to be position-dependent. Compared with "naked" aryl anions, the basicity of aryllithiums mirrors the effects of ortho, meta, and para substituents to the extent of 36%, 30%, and 25%, respectively.