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.

一系列笼状五芳基化碳硼烷可通过钯催化的分子间偶联反应合成得到：以一碳闭合式十二硼烷阴离子（monocarba-closo-dodecaborate anion）[CB11H12]−的C-羧酸与碘代芳烃为反应底物，借助直接笼状B-H键官能化策略实现转化。该类转化在单分子内由导向基团介导的惰性键活化领域创下了一锅法反应的新纪录。此合成方法具有收率优异、官能团耐受性良好以及完全的笼状区域选择性等特点。其中的导向基团COOH可在分子间偶联反应过程中或反应结束后便捷脱除。研究团队通过密度泛函理论（density functional theory, DFT）计算对反应机理路径进行了系统探究。研究提出了Pd(II)–Pd(IV)–Pd(II)的催化循环路径：初始偶联反应完成后，钯催化剂优先活化相邻硼顶点的B-H键，这种选择性的持续推进将使钯中心发生连续的游走过程。该合成方法为构建具有五重对称性的合成砌块提供了全新的研究路径。