Palladium-Catalyzed Selective Fluorination of o‑Carboranes

ABSTRACT: A Pd(II)-catalyzed direct selective fluorina-tion reaction of carboranes using a F+ reagent has been developed, leading to a series of polyfluorocarboranes in high isolated yields. The mechanism involving electrophilic B−H activation, oxidation of Pd(II) by F+ species, and reductive elimination is proposed. Functionalization of carboranes has received growinginterest owing to their applications in medicine as boron neutron capture therapy (BNCT) agents,1 in supramolecular design as building blocks,2 and in coordination/organometallic chemistry as ligands.3 Since the substitution of hydrogen for fluorine can significantly change both physical and chemical properties of compounds,4 it is anticipated that polyfluorinated carboranes would have great potential as key components of n-type organic semiconductors and donor−acceptor type of materials owing to their super electron-deficient nature,5a−d of new BNCT agents due to enhanced biostability,4 and of weakly coordinating anions because of improved chemical inertness.5e In general, there are two conventional synthetic methods leading to the cage boron substituted carboranes: electrophilic substitution at cage BH vertices by electrophiles6 and capitation reaction of nido-C2B9H11 2 − with boron halides.7 Direct electrophilic halogenation is known to be an effective and convenient method for the preparation of cage boron chlorinated, brominated, and iodinated carboranes.8 Selective mono-, di-, or polyhalogenation of carboranes can be achieved under appropriate conditions. In contrast, the reaction of carborane with elemental fluorine is not selective, resulting in the formation of deca-B-fluorocarborane C2B10H2F10.