N‐Ligand‐Enabled Aromatic Nucleophilic Amination of 1,2‐Diaryl‐o‐Carboranes with (R₂N)₂Mg for Selective Synthesis of 4‐R₂N‐o‐Carboranes and 2‐R₂N‐m‐Carboranes

Abstract

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)aminated o-carboranes. DFT calculations showcased a stepwise BÀ N formation/BÀ H cleavage process, in which MgÀ H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tertbutyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/ cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated mcarboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage BÀ H amination reaction offers an alternative strategy for selective amination of o-and m-carboranes.