An investigation into the dynamic covalent chemistry of hydroxamic acids, boronic acids and 1,3,4,2-dioxazaboroles
Shepherd, Cathryn O.
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This thesis focuses on the development and characterisation of novel 1,3,4,2- dioxazaborole heterocycles that are formed from the reversible condensation of hydroxamic acids and boronic acids. Chapter 1 comprises of a brief literature review on dynamic covalent chemistry and includes some of the chemistries surrounding both heterocycle’s starting materials – hydroxamic acids and boronic acid. Also covered are previously studied dynamic reactions between salicylhydroxamic acid and benzoboronic acid, which suggest the dynamic formation of both a 5-membered 1,3,4,2-dioxazaborole and a 6-membered heterocycle. Chapter 2 begins the exploration into the chemistry of 1,3,4,2-dioxazaboroles, including both their formation and hydrolysis into starting materials. In addition, the 1,3,4,2-dioxazaborole was fully characterised using various analytical techniques, such as NMR spectroscopy, single-crystal X-ray diffraction and mass spectrometry, and calculated to contain aromatic character. Through a range of studies, discussed in Chapter 3, the Lewis acidity of the 3-coordinate boron atom in these species was determined, with a relatively low Acceptor Number, when compared to similar boronic acid esters, therefore, demonstrating potential applications in roles such as sensing due to the reversibility of certain Lewis bases binding to the Lewis acidic boron centre. These findings were corroborated by computational calculations. In Chapter 4, the dynamic character of these compounds was determined including observations of exchange occurring between both the hydroxamic acid and boronic acid sides of the heterocycle, in addition to its Lewis acidic dynamic character. The mechanisms of exchange for both the hydroxamic acid and boronic acid switches, including associative mechanisms in both acidic and basic conditions, as well as the fully dissociative mechanism were examined. The possibility of utilising 1,3,4,2-dioxazaboroles in applicationssuch as dynamic covalent chemistry (formation of COFs, polymers and discrete molecular cages) was explored in Chapter 5. This chapter discusses the project undertaken during a 6-week academic placement at Heidelberg University in Germany, wherein synthetic work towards molecular cages was undertaken. From this project, it was determined that the dynamic nature of 1,3,4,2-dioxazaboroles would allow for the production of discrete molecular cages, however, the correct conditions to allow for the required self-correction process still need to be determined. Additional supramolecular potential was also recognised through preliminary results from the coordination of transition metals to pyridine-functionalised 1,3,4,2-dioxazaboroles, through the characterisation by mass spectrometry of a 2:1 ditopic ligand containing two 1,3,4,2-dioxazaboroles bound to a Cu(II) metal centre. These findings suggest future possibilitiesfor 1,3,4,2-dioxazaboroles in the synthesis of coordination cages, for potential applications in catalysis, and gas separation and storage.