Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12-Vertex Monocarborane Anion by B–H Activation

A series
of cage penta-arylated carboranes have been synthesized by palladium-catalyzed
intermolecular coupling of the C-carboxylic acid of the monocarba-closo-dodecaborate anion [CB11H12]− with iodoarenes by direct cage B–H bond functionalization.
These transformations set a record in terms of one-pot directing group-mediated
activation of inert bonds in a single molecule. The methodology is
characterized by high yields, good functional group tolerance, and
complete cage regioselectivity. The directing group COOH can be easily
removed during or after the intermolecular coupling reaction. The
mechanistic pathways were probed using density functional theory calculations.
A Pd­(II)–Pd­(IV)–Pd­(II) catalytic cycle is proposed,
in which initial coupling is followed by preferred B–H activation
of the adjacent boron vertex, and continuation of this selectivity
results in a continuous walking process of the palladium center. The
methodology opens a new avenue toward building blocks with 5-fold
symmetry.