<i>N</i>-Halosuccinimide/BF₃−H₂O, Efficient Electrophilic Halogenating Systems for Aromatics

<i>N</i>-Halosuccinimides (NXS,<b> 1</b>) are efficiently activated in trifluoromethanesulfonic acid and BF₃−H₂O, allowing the halogenations of deactivated aromatics. Because BF₃−H₂O is more economic, easy to prepare, nonoxidizing, and offers sufficiently high acidity (−<i>H</i>₀ ≈ 12, only slightly lower than that of trifluoromethanesulfonic acid), an efficient new electrophilic reagent combination of NXS/BF₃−H₂O has been developed. DFT calculations at the B3LYP/6-311++G**//B3LYP/6-31G* level suggest that protonated <i>N</i>-halosuccinimides undergo further protosolvation at higher acidities to reactive superelectrophilic species capable either in the transfer of X⁺ from the protonated forms of NXS to the aromatic substrate or in forming a highly reactive and solvated X⁺ which would readily react with the aromatic substrates. Structural aspects of the BF₃−H₂O complex have also been investigated.