A Method for Highly Selective Halogenation of o-Carboranes and m-Carboranes.

A facile halogenation method for highly selective synthesis of 9-X-o-carboranes, 9,12-X2-o-carboranes, 9-X-12-X'-o-carboranes, 9-X-m-carboranes, 9,10-X2-m-carboranes, and 9-X-10-X'-m-carboranes (X, X' = Cl, Br, I) has been developed on the basis of our previous work. The success of this transformation relies on the usage of trifluoromethanesulfonic acid (HOTf), the easily available strong Brønsted acid. The addition of HOTf greatly increases the electrophilicity of N-haloamides through hydrogen bonding interaction, resulting in the low loading of N-haloamides, short reaction time, and mild reaction conditions. Additionally, the solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is also essential to further increase the acidity of HOTf.