UBC Theses and Dissertations
Exploiting boron-fluorine bonds for fluorination and synthesis of potential bi-modal imaging agents Sadek, Omar
The B-F bond has an expansive and rich history in chemical transformations and the versatility of the B-F bond has also shown immense utility in fields as far reaching as PET and NIRF imaging. The application of B-F bonds for nucleophilic C-F bond formation and the development of novel fluorophores with potential applications in PET/NIRF bi-modal imaging are investigated in this work. Organotrifluoroborates are well known as indispensable synthetic tools for the elaboration of organic molecules. However, the trifluoroborate functionality is generally regarded as an auxiliary group, and its potential as a nucleophilic F- source has remained untested. In this context, the ability of organotrifluoroborates to serve as competent sources of F- was investigated. In an update to the historic Balz-Schiemann reaction, that traditionally uses BF₄- as a source of F-, organotrifluoroborates were shown to mediate the fluorodediazoniation of in situ generated aryl diazonium salts under mild reaction conditions (chapter 2). In reactions with an analogous substrate class, unsymmetrical diaryliodonium salts could also be fluorinated using phenyltrifluoroborate via thermal decomposition of the ion pair (chapter 3). Finally, the ability of the trifluoroborate moiety to mediate intramolecular fluorination reactions was investigated through the synthesis of various trifluoroborate-containing molecular scaffolds (chapter 4). I also investigated novel fluoroborate (B-Fn) complexes of the 2-aryl-benzothiazole scaffold, known for its interaction with Aβ aggregates, as fluorophores. Three complexes were isolated, characterised and their photophysical properties are reported. A 2,4-diaryl-benzothiazole monofluoroborate complex displaying the most promising photophysical properties, was shown to be stable to aqueous conditions. Fluoride abstraction from this complex was also demonstrated, providing promising preliminary results towards the viability of radiolabelling via ¹⁹F-¹⁸F isotope exchange. The monofluoroborate complex also showed interesting structural properties given the asymmetric boron centre. Enantiomers of the complex were successfully chirally resolved from the racemic mixture, characterised by Circular Dichroism and their stability to racemization via inversion investigated (chapter 5).
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