Old Key Opens the Lock in Carborane: The in Situ NHC-Palladium Catalytic System for Selective Arylation of B(3,6)–H Bonds of o-Carboranes via B–H Activation

Abstract

An efficient in situ Pd-NHC catalytic system for regioselective arylation of B­(3,6)–H bonds of o-carborane has been developed for the first time. A series of symmetric and unsymmetric 3,6-diaryl-o-carboranes anchored with active groups have been synthesized with moderate to good yields under mild conditions. This work offers an efficient protocol for selective activation of B­(3,6)–H bonds and has important value in design coupling reactions for selective functionalization of o-carboranes.

Highly selective electrophilic B(9)-amination of o-carborane driven by HOTf and HFIP

“…9 In the past decade, the main work for the selective functionalization of o -carborane focused on the transition-metal catalyzed B–H bond activation. 10–12 However, in most cases, it required introducing a directing group to realize reactivity and selectivity. 12 From the aspects of economic and environmental considerations, metal-free B–H functionalization without a directing group is more attractive.…”

Migration Mechanism of the B−H Activation of Carboranes

“…On the contrary, boron vertices with different electron environments [15][16] hinder the synthesis of regioselective products, which result in multiple isomers. Xie, [17][18] Cao, [19] Herberhold [20] and Jin [21] et al performed regioselective BÀ H functionalisation on carboranes via nucleophilic substitution and transition metal-mediated catalysis for controllable BÀ H activation. Cao's group [22] synthesised high-yield aryl-substituted o-carboranes through regioselective arylation catalysed by Pdcomplexes.…”

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

“…The attempt to use the Suzuki cross-coupling reaction of 3-iodo- ortho -carborane with aryl boronic acids only gave good results for aryls containing electron-donating substituents, while reactions with aryl boronic acids containing electron-withdrawing substituents (-CN, -NO 2 ) only led to the desired carboranes in low yields [ 56 ]. Recently, the direct arylation of ortho -carborane via Pd-catalyzed B-H activation has been reported, but this approach has been optimized for the synthesis of the 3,6-diaryl derivatives rather than the 3-aryl derivatives [ 57 ]. A series of 3-aryl derivatives of ortho -carborane were prepared by Pd-catalyzed B-H activation reactions with aryl iodides under functional group assistance; however, these reactions are currently only of academic interest rather than real synthetic methods [ 58 , 59 ].…”