Planar N‐Heterocyclic Carbene Diarylborenium Ions: Synthesis by Cationic Borylation and Reactivity with Lewis Bases

The NHC–borane adduct (IBn)BH 3 ( 1 ) (NHC= N‐heterocyclic carbene; IBn=1,3‐dibenzylimidazol‐2ylidene) reacts with [Ph 3 C][B(C 6 F 5 ) 4 ] through sequential hydride abstraction and dehydrogenative cationic borylation(s) to give singly or doubly ring closed NHC–borenium salts 2 and 3 . The planar doubly ring closed product [C 3 H 2 (NCH 2 C 6 H 4 ) 2 B][B(C 6 F 5 ) 4 ] is resistant to quaternization at boron by Et 2 O coordination, but forms classical Lewis acid–base adducts with the stronger donors Ph 3 P, Et 3 PO, or 1,4‐diazabicyclo[2.2.2]octane (DABCO). Treatment of 3 with t Bu 3 P selectively yields the unusual oligomeric borenium salt trans ‐[(C 3 H 2 (NCH 2 C 6 H 4 ) 2 B) 2 (C 3 H 2 (NCHC 6 H 4 ) 2 B)][B(C 6 F 5 ) 4 ] ( 7 ).