Aromatic Substitution with Hypercloso C(BCH_3)_(11): A New Mechanism

摘要

We report aromatic substitution with the reactive intermediate C(BCH_3)_(11) (1, Chart 1), formally a hypercloso cation HC(BCH_3)_(11)~+ with a deprotonated carbon vertex but, according to calculations, better viewed in the ground state as a closo cluster with a 5-fold symmetric carbenoid/carbonium ylide structure C(BMe)_(11)reversible ~+C~-(BMe)_(11) (1a) carrying a vacant exocyclic orbital on carbon. The reaction yields highly hindered C-arylated and B-permethylated carborate anions (Me = CH_3): ArH+1 → Ar-C(BMe)_(11)~- + H~+(1) In the process, 1 exhibits a novel selectivity, unrelated to its known electrophilic affinity for lone pairs. To account for this result, we propose that this aromatic substitution proceeds by a new mechanism with a norcaradiene-like transition state. The properties of 1 may be relevant for the reactions and structures of the isomeric "boronium ylides" with a naked boron vertex, which occur as intermediates in electrophile-induced nucleophilic substitution (EINS) on polyhedral boranes.