Aromatic boracycles are continuing to attract tremendous interest in a wide range of different research areas as heteroatom analogs of prototypical carbon-based aromatics. Due to the inherent electron deficiency of boron, such molecules often feature unique chemical and photophysical properties. Over the last decades, extensive research has led to the isolation of a myriad of diverse boron containing π aromatics, which provide metallomimetic platforms for small molecule activation and catalysis. While neutral Lewis base-stabilized borabenzenes are investigated thoroughly, the chemistry of neutral diboraacenes is much more recent and unexplored. In 2016, Braunschweig reported the first neutral 1,2-diborabenzene 1 stabilized by cyclic alkyl(amino)carbene (CAAC) ligands, and in addition to the known N-heterocyclic carbene adduct, Braunschweig recently described the synthesis of the bis(CAAC)-stabilized 9,10-diboraanthracene 2 showing an open-shell singlet biradical ground state with an exceedingly small singlet-triplet gap. In this work, we present the reactivity of the diborabenzene 1 and the diboraanthracene 2 towards various substrates yielding rigid bicyclic structures. For instance, reactions with benzyne and oxygen led to the formation of the highly strained diborabarrelen 3 and the endoperoxide 4, respectively. Reactivity and computational studies highlight the differences and similarities between these unprecedented diboraacenes.

Neutral diboraacenes: Harnessing the power of cooperating, spatially-seperated subvalent boron atoms

ACS Spring 2022

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