Synthesis of Fluorinated Dialkyl Halonium Ions via Strong Oxidants Formed under Lewis Superacid Conditions

Abstract

This thesis presents a systematic approach to the synthesis of highly electrophilic fluorinated dialkyl halonium ions, enabled by strong oxidizing noble gas compounds derived from Lewis superacidic systems. The study begins with the synthesis and characterization of Ga(OTeF5)3, a rare gallium-based Lewis superacid, obtained in quantitative yield via a solvent-free reaction between GaCl3 and ClOTeF5. In SO2ClF solution, Ga(OTeF5)3 forms a monomeric solvent adduct, while it adopts a dimeric structure in the solid state. Quantum-chemical calculations classify the free acid, its dimer, and the SO2ClF solvent adduct as Lewis superacids. This superacidity was further exploited to enhance the oxidative strength of Xe(OTeF5)2, enabling the oxidation of chloromethane and the subsequent formation of the dimethyl chloronium salt [Cl(CH3)2][Ga(OTeF5)4], highlighting the potential of Ga(OTeF5)3 in the synthesis of reactive cationic species. Building on this foundation, the thesis explores the synthesis and reactivity of the fluorinated dialkyl chloronium salts [Cl(CH2CF3)2][E(OTeF5)n] (E = Al, n = 4; E = Sb, n = 6), prepared via oxidation of CH2ClCF3 with [XeOTeF5][E(OTeF5)n]. The chloronium compounds were fully characterized by NMR and IR spectroscopy, as well as by single-crystal X-ray diffraction. Reactivity studies revealed that the chloronium ion can act as a CH2CF3 transfer reagent to weak nucleophiles or as a hydride abstraction reagent capable of activating linear aliphatic alkanes. The resulting branched carbocations, stabilized by the weakly coordinating [Sb(OTeF5)6]− anion, were characterized by NMR spectroscopy and the molecular structures of the tert-butyl and isopentyl cations were determined by single-crystal X-ray diffraction. The reactivity of the chloronium system was further used in the synthesis of other fluorinated dialkyl halonium ions, including [Br(CH2CF3)2]+, [I(CH2CF3)2]+, and [I(CH2CHF2)2]+. These compounds were characterized by NMR and IR spectroscopy as well as single-crystal X-ray diffraction. Additionally, the synthesis of fluorinated dipropyl halonium salts [Br(CH2CH2CF3)2]+ and [I(CH2CH2CF3)2]+ was achieved via oxidation or fluoroalkylation of the corresponding haloalkanes. A similar reaction, the oxidation of 2-chloro-1,1,1-trifluoropropane CHCl(CH3)(CF3), led to the formation of a highly reactive species capable of activating isobutane to generate the tert-butyl cation. Although the reaction product could not be directly observed due to its thermal instability and low solubility, quantum-chemical calculations support the formation of an asymmetric chloronium ion. Overall, this work introduces new synthetic routes to structurally diverse and highly electrophilic halonium ions and shows their utility in C‒H bond activation and carbocation formation, offering fundamental insights into the stabilization of reactive main-group species.