Synthesis of B(4)-Alkynylated o-Carboranes
Icosahedral carboranes are a class of polyhedral boron hydride clusters in which one or more of the BH vertices are replaced by CH units, which can be viewed as three-dimensional relatives of benzene. Their exceptional thermal and chemical stabilities as well as 3D structures make them useful building blocks for boron neutron capture therapy agents in medicine, versatile ligands in coordination/organometallic chemistry, and functional units in supramolecular design/optoelectronic materials. Classic routes to functionalized carboranes rely on the polarized cage C–H/B–H bonds: the weakly acidic C–H proton and basic B–H hydride.
To tackle regioselectivity problems during B–H activation, the group of Professor Zuowei Xie from the Chinese University of Hong Kong (P. R. of China) introduced a carboxyl group at the cage carbon to control the regioselectivity and facilitate cage B–H activation. Subsequently, transition-metal-catalyzed cage B(4)-alkenylation, B(4,5)-dialkenylation, and B(4,5)-diarylation have been achieved.
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