Défense publique de thèse de doctorat en Sciences chimiques - Arnaud OSI

Boratriptycenes : Development and Reactivity of Non-Planar Boranes

Jury

• Prof. WOUTERS Johan (département de chimie, UNamur), président
• Prof. BERIONNI Guillaume (département de chimie, UNamur), promoteur et secrétaire
• Prof. LANNERS Steve (département de chimie, UNamur)
• Prof. BOURISSOU Didier (laboratoire HFA, Université Paul Sabatier-Toulouse III)
  
• Prof. BRAUNSCHWEIG Holger (Institut für Anorganische Chemie, Universität Würzburg)

Abstract

Mentioned for the first time more than 50 years ago as a theoretical curiosity for its predicted exceptional Lewis acidity, 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 p_z–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 p_z-orbital. The 9-boratriptycene showed a high propensity to undergo protodeborylation leading to the decomposition of the compound.
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 S_N2 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 the highlighted. The reaction conditions were extensively optimized and a wide array of electron rich to electron depleted arenes could be successfully borylated. A mechanistic investigation was performed revealing that the donor-free Lewis acid is only a transient species which is found in solution under several complexed 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.

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