Evidence of Phosphonium-Carbenium Dication Formation in a Superacid: Precursor to Fluorinated Phosphine Oxides

Accumulation, Characterization and Reactivity of Chiral Ammonium-Carboxonium Dications in Superacid

The accumulation of chiral ammonium-oxocarbenium dications in superacid is evidenced by low-temperature NMR spectroscopy, X-ray diffraction analysis and confirmed by DFT calculations. Its potential for the diastereoselective remote hydrofunctionalization of non-activated alkene is also explored.

Exploring F/CF3 substituted oxocarbenium ions for the diastereoselective assembly of highly substituted tetrahydrofurans

Understanding the influence of emerging fluorinated motifs is of a crucial importance in the context of the exponentially growing exploitation of fluorine in many fields. Herein, we report on the dramatic effect of a local partial charge inversion by replacing a CHCH₃ group by a CFCF₃. This strategy allows the diastereoselective reduction of 5-membered ring oxocarbenium ions to access highly substituted tetrahydrofurans.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.