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

Abstract

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.

9,9′-Bis-o-carboranes: synthesis and exploration of properties

“…8 Palladium-catalyzed cross-coupling of B-halo-carboranes is also an efficient way to construct functional carboranes. 9 With HFIP as a solvent, our group achieved the selective B(9) electrophilic halogenation, 10 electrophilic amination, 11 and oxidation 12 of o -carboranes with >20 : 1 selectivity. With Pd(OAc) 2 as a catalyst, selective B(9) amination was also established, but with a relatively lower selectivity compared with electrophilic substitution reactions.…”

“…Based on our previous work, 10–13 a plausible reaction mechanism is proposed in Scheme 3. Firstly, the reaction of Pd(OAc) 2 and HOTf formed highly active electron-deficient Pd( ii ) species Pd(OTf) 2 .…”

“…Firstly, the electrophilic halogenation of 2b was examined based on our previous reports (Scheme 4B). 10 A mixture of 12-chloro-9,9′-bis- o -carborane 4 and 12,12′-dichloro-bis- o -carborane 5 was obtained with trichloroisocyanuric acid (TCCA) as the chlorinated agent. Meanwhile, the dihalogenated bis- o -carboranes were the main products for the reactions of electrophilic bromination and iodination, even if only one equivalent of each halogenating reagent, N -bromosuccinimide (NBS) and N -iodosuccinimide (NIS), was used.…”

Facile Friedel–Crafts alkylation of arenes under solvent-free conditions

“…Recently, we have focused our attention on the electrophilic aromatic substitution reactions of carboranes, 15 and found that 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) 16 and trifluoromethanesulfonic acid (HOTf) 17 can greatly increase the reactivity due to their excellent hydrogen bond donating ability and weak nucleophilicity. The controllable electrophilic nitration of arenes was also explored, which showed an excellent substrate scope.…”