Lithium Aryltrifluoroborate as a Catalyst for Halogen Transfer

The catalysis of two successive bromine–lithium
exchange
reactions, referred to as a “halogen dance”, by lithium
aryltrifluoroborate is reported. At a loading of 10 mol %, this catalyst
exhibited efficient activity for bromo group transfer and was applicable
to various brominated pyridines. Density functional theory calculations
were consistent with the proposed mechanism, where the catalyst provided
a bromine atom to the mono- or dibrominated substrate and the resulting
debrominated catalyst then abstracted another bromine atom from the
di- or tribrominated substrate, leading to the transposition of the
bromo group on the substrate with catalyst regeneration.