Site‐Selective Functionalization of Carboranes at the Electron‐Rich Boron Vertex: Photocatalytic B−C Coupling via a Carboranyl Cage Radical

Abstract

Functionalization of carboranes in a vertex‐specific manner is a perennial challenge. Here, we report a photocatalytic B−C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo‐induced decarboxylation of carborane carboxylic acids to generate boron vertex‐centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single‐electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid‐state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

“Cage Walking” Synthetic Strategy for Unusual Unsymmetrical Supramolecular Cages

“…Icosahedral carboranes (C 2 B 10 H 12 ) are often considered three-dimensional analogues of benzene and have unique properties of spherical geometry, three-dimensional aromaticity conjugated by σ bonds, and inherent robustness . They have been utilized as unique building blocks for supramolecular coordination complexes (SCCs), − forming assemblies that exhibit unique spatial configurations and chemical properties distinct from those using only organic ligands alone. − With development in recent years, transition-metal-catalyzed cage B–H functionalization can promote a series of vertex-specific functionalizations of carboranes. − However, the construction of supramolecular structures by this process has not been reported because functionalized carboranes no longer have potential sites to coordinate with metal ions. This problem could be solved if metal ions could migrate from one B-vertex to another in the process of cage B–H functionalization, a process known as a “walking metal” on the cage (or “cage walking”). − Through this strategy, coordination and covalent interaction can be combined effectively to explore new topological structures.…”

Lewis Base–Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon–Heteroatom Bonds

“…Recently, the homolytic cleavage of the B–B bond of diboron compounds in the presence of a Lewis base has been found to be an effective pathway to generate boron-centered radicals, which has also shown a variety of important transformations . Moreover, boron-centered carboranyl radicals have recently emerged as another class of powerful species that can enable a range of valuable B–C coupling reactions. − Alternatively, in the 1980s, Roberts and co-workers discovered that Lewis base (amines or phosphines) ligated BH 3 complexes could generate relatively stable 4-center–7-electron type Lewis base–boryl radicals (LBRs) through hydrogen atom abstraction (HAA) with the aid of a radical initiator . Later, these LBRs were used as dehalogenation reagents and polarity-reversal catalysts (Scheme b).…”