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 [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)2B][B(C6F5)4] is resistant to quaternization at boron by Et2O coordination, but forms classical Lewis acid-base adducts with the stronger donors Ph3P, Et3PO, or 1,4-diazabicyclo[2.2.2]octane (DABCO). Treatment of 3 with tBu3P selectively yields the unusual oligomeric borenium salt trans-[(C3H2(NCH2C6H4)2B)2(C3H2(NCHC6H4)2B)][B(C6F5)4] (7). Boron in action: Singly or doubly ring closed NHC-borenium salts (NHC= N-heterocyclic carbene) are prepared by dehydrogenative cationic borylation. Treatment of the planar doubly ring closed product (see picture, left) with tBu3P leads to the formation of an unusual trans C2-symmetric oligomeric borenium salt (right). Atom colors: C, black; N, blue; B, yellow. ? 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.