Planar N-heterocyclic carbene diarylborenium ions: synthesis by cationic borylation and reactivity with Lewis bases.

The NHC-borane adduct (IBn)BH3 (1) (NHC= N-heterocyclic carbene; IBn=1,3-dibenzylimidazol-2ylidene) reacts with [Ph3 C][B(C6 F5 )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 [C3 H2 (NCH2 C6 H4 )2 B][B(C6 F5 )4 ] is resistant to quaternization at boron by Et2 O coordination, but forms classical Lewis acid-base adducts with the stronger donors Ph3 P, Et3 PO, or 1,4-diazabicyclo[2.2.2]octane (DABCO). Treatment of 3 with tBu3 P selectively yields the unusual oligomeric borenium salt trans-[(C3 H2 (NCH2 C6 H4 )2 B)2 (C3 H2 (NCHC6 H4 )2 B)][B(C6 F5 )4 ] (7).