Boratriptycenes

: Development and Reactivity of Non-Planar Boranes

Abstract

Mentioned for the first time more than 50 years ago as a theoretical curiosity, the parent 9-boratriptycene was in-situ generated and trapped under the form of bench stable Lewis adducts, providing glimpse of its reactivity. Extensive investigations of its electronic properties revealed that the Lewis acidity of this non-planar borane is governed by two major factors: i) the pre-pyramidalization of the boron atom which reduces the reorganization energy of the Lewis acid upon coordination with a Lewis base and ii) the absence of pz–π-conjugation between the boron atom and the aromatic rings of the triptycene scaffold preventing electron donation from the aromatic π-system into the boron pz-orbital. The introduction of a sulfonium bridgehead allowed to increase the stability towards protodeborylation as well as the Lewis acidity, approaching the one of silylium ions. Synthesis of bench stable “ate”-complexes with weakly coordinating anions and hydride- and fluoride-bridged dimers shed light on the steric protection of the boron σ*-orbital provided by the triptycene scaffold. This revealed the impossibility to perform SN2 type reactions at the boron center, conferring a remarkable stability to even the weakest “ate”-complexes or Lewis adducts and allowing the synthesis of borylated equivalents of protonated sulfuric, triflic and triflimidic acids. The propensity of this 9-sulfonium-10-boratriptycene to perform Csp2–H borylation reactions was highlighted. The reaction conditions were optimized and a wide array of electron rich to electron depleted arenes could be borylated. A mechanistic investigation was performed revealing that the donor-free Lewis acid is only a transient species which is found as several complexes formed with the solvent or a counter-anion. The impact of the pre-pyramidalization of the boron atom was then demonstrated by synthesizing the selenenium derivatives and comparing it with the sulfonium derivative.

Date of Award	26 Jun 2023
Original language	English
Awarding Institution	University of Namur
Sponsors	Fund for Research Training in Industry and Agriculture (FRIA)
Supervisor	Guillaume Berionni (Supervisor), Johan Wouters (Jury), Steve Lanners (Jury), Didier Bourissou (Jury) & Holger Braunschweig (Jury)