Reversible Assembly and Disassembly of σ‐Aromatic B 3 Rings

Electron‐deficient, planar oligoboron ring compounds fascinate due to their peculiar bonding properties (σ‐aromaticity) and reactivity; however, the directed synthesis of such compounds is challenging. In this work we report on the reaction of cationic diboranes with boranes to give cationic, σ‐aromatic B 3 ring compounds, with three‐center‐two‐electron bond between the three boron atoms, being analogues of B 3 H 8 − , but with opposite charge. Base‐induced, reversible disassembly of the cationic B 3 ring leads back to cationic diboranes and borane adducts. Quantum‐chemical calculations show that the thermodynamics of these reactions can be tuned by choice of the base. The reactions interconvert electron‐deficient boron three‐center bonds and electron‐precise two‐center bonds and disclose new synthetic access routes to cationic diboranes and σ‐aromatic B 3 rings.