Promoting or Preventing Haloaryllithium Isomerizations: Differential Basicities and Solvent Effects as the Crucial Variables

ABSTRACT Deprotonation-triggered heavy halogen migrations should become a favorite tool in arene synthesis if their occurrence and outcome could be made predictable. Particularly attractive, though extremely rare, are stop-and-go situations where a first intermediate, generated by metalation, can be trapped at -100 °C, whereas at -75 °C halogen migration gives rise to an isomer. As shown now, one can conveniently produce the initial aryllithium species by halogen/metal interconversion in toluene at -100 °C, under conditions that preclude halogen migration, and unleash the isomerization process by adding tetrahydrofuran at -75 °C.