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 ion and dehydrogenative cationic borylation(s) to give singly or doubly ring closed NHC–borenium salts 2 and 3. The planar doubly ring closed pr...

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Bibliographic Details
Published in:Angewandte Chemie International Edition 2015-04, Vol.54 (17), p.5214-5217
Main Authors: Farrell, Jeffrey M., Stephan, Douglas W.
Format: Article
Language:English
Subjects:
Adducts
boranes
Boron
borylation
Carbenes
Cationic
Dehydrogenation
Formations
frustrated Lewis pairs
Hydrides
Lewis base
steric hindrance
Synthesis
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Summary: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 ion 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.
ISSN:1433-7851
1521-3773
DOI:10.1002/anie.201500198