Transition‐Metal‐Catalyzed Selective Cage B−H Functionalization of o‐Carboranes

Abstract Carboranes are a class of carbon–boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o ‐carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C−H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B−H activation/functionalization among chemically very similar ten B−H vertices is very challenging. Considering the differences in electron density of ten cage B−H bonds in o ‐carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron‐rich transition metal catalysts for the functionalization of the most electron‐deficient B(3,6)−H vertices (bonded to both cage CH vertices); using electron‐deficient transition‐metal catalysts for the functionalization of relatively electron‐rich B(8,9,10,12)−H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)−H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B‐functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B‐functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts.