B–N, B–O, and B–CN Bond Formation via Palladium-Catalyzed Cross-Coupling of B-Bromo-Carboranes

Abstract

Carboranes are boron-rich molecules that can be functionalized through metal-catalyzed cross-coupling. Here, for the first time, we report the use of bromo-carboranes in palladium-catalyzed cross-coupling for efficient B-N, B-O, and unprecedented B-CN bond formation. In many cases bromo-carboranes outperform the traditionally utilized iodo-carborane species. This marked difference in reactivity is leveraged to circumvent multistep functionalization by directly coupling small nucleophiles (-OH, -NH2, and -CN) and multiple functional groups onto the boron-rich clusters.

Iridium-catalysed regioselective borylation of carboranes via direct B–H activation

“…On the other hand, carboranes have their own unique characteristics such as spherical geometry and hydrophobic molecular surface12, which make them attractive building blocks for boron neutron capture therapy agents in medicine3456, functional units in supramolecular design/materials7891011121314151617 and versatile ligands in coordination/organometallic chemistry181920212223. These research activities have drawn growing interests in the selective functionalization of carboranes12242526.…”

Transition‐Metal‐Catalyzed Regioselective Functionalization of Monophosphino‐o‐Carboranes

“…Halogenated carboranes are useful starting materials in transition‐metal‐catalyzed cross‐coupling reactions to construct B−E (E=C, O, N) bonds . In general, B ‐halogenated o‐ carboranes can be readily synthesized by electrophilic substitution of o‐ carboranes .…”

“…In general, B ‐halogenated o‐ carboranes can be readily synthesized by electrophilic substitution of o‐ carboranes . The transition‐metal‐catalyzed regioselective halogenation of B−H bonds is much less investigated . To our surprise, upon switching from t BuOLi to Li 2 CO 3 , the reaction of compound 1 with 1‐bromo‐3‐methoxybenzene afforded only the B 6 ‐brominated product 11 in a yield of 69 % in 24 h at 110 °C using 5 mol % loading of PdCl 2 as pre‐catalyst (Scheme and Table S2 in the Supporting Information).…”

Palladium‐Catalyzed Selective Mono‐Chlorination of o‐Carboranes: Changing the Concept of FeCl3 from Lewis Acid to Chlorine Source in Carboranes

“…In order to derivate the formed B‐Cl bond, some typical cross‐coupling reactions such as Suzuki type reaction, Heck type reaction etc. based on 2a were conducted with many palladium sources including the synthesized palladium catalyst reported by Spokoyny for functionalization of B‐Br bond, unfortunately, these reactions could not proceeded at all. This result indicate the B‐Cl bond is so inert that highly efficient catalyst is needed to be developed to addressing this derivatization …”