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-2-ylidene) reacts with [Ph3C][B(C6F5)(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 [C3H2(NCH2C6H4)(2)B][B(C6F5)(4)] is resistant to quaternization at boron by Et2O coordination, but forms classical Lewis acidbase adducts with the stronger donors Ph3P, Et3PO, or 1,4-diazabicyclo[2.2.2] octane (DABCO). Treatment of 3 with tBu(3)P selectively yields the unusual oligomeric borenium salt trans-[(C3H2(NCH2C6H4)(2)B)(2)(C3H2(NCHC6H4)(2)B)]-[B(C6F5)(4)] (7).