A New Mode of Chemical Reactivity for Metal‐Free Hydrogen Activation by Lewis Acidic Boranes. (13th May 2019) - Bennett, Elliot L.
Abstract: We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H2 outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3, 5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H2, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids. Abstract : Aktivierung ohne Frustration : Wenn Lewis‐saure Borane ähnlich zu denen, die in der Chemie frustrierter Lewis‐Paare (FLP) eingesetzt werden, in Gegenwart gewöhnlicher Reduktionsreagenzien anstatt Lewis‐Basen H2 ausgesetzt werden, erfolgt die homolytische Spaltung von H2 . Eine Reihe von Intermediaten, die in dem Prozess an einem Hauptgruppen‐Borankomplex gebildet werden, konnten experimentell beobachtet und strukturell charakterisiert werden.