Expanding the Scope of the Paternò-Büchi Reaction Methodology Development and Mechanistic Studies

Michalska, Weronika and Coote, Susannah (2023) Expanding the Scope of the Paternò-Büchi Reaction Methodology Development and Mechanistic Studies. PhD thesis, Lancaster University.

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Abstract

This thesis describes the development of the Paternò-Büchi reaction between aliphatic ketones and maleic acid derivatives. Chapter 1 discusses the current literature around oxetanes, and methods for their synthesis, particularly via the Paternò-Büchi reaction. The potential and importance of oxetane rings is highlighted, alongside current scope limitations of the Paternò-Büchi reaction. Chapter 2 investigates the Paternò-Büchi reaction between acetone and maleic anhydride to yield an oxetane product that requires further transformations to allow complete purification. A series of transformation reactions was developed to easily and diversely functionalize the formed oxetane products. A side product formed through dimerization of maleic anhydride was successfully removed after the reaction was complete. Chapter 3 expands on the scope of the Paternò-Büchi reaction by using cyclic ketones in the reaction with maleic anhydride to form a series of spirocyclic oxetanes. The initial reaction conditions were based on work presented in Chapter 2 and these conditions were optimized; the dimerization of maleic anhydride was successfully supressed with p-xylene, further increasing the isolated yield of the oxetane product. Different cyclic ketones were used in the Paternò-Büchi reaction, creating a library of oxetane-containing spirocycles. Chapter 4 includes mechanistic studies performed alongside the formation of oxetanes, such as the use of molecular oxygen in order to quench potential triplet excited states in the reaction mixture. UV-vis spectra of starting materials and reagents were collected, and the different absorption range of maleimide compared to maleic anhydride allowed for distinguishing between different possible reaction pathways of the Paternò-Büchi reaction. Chapter 5 shows a series of different electron-poor alkenes tested in the Paternò-Büchi reaction, giving mixed results. Chapter 6 provides overall conclusions, highlighting the expansion of the scope of the Paternò-Büchi reaction, as well as highlighting difficulties in establishing a full mechanistic picture for photochemical reactions. Additionally, ideas for further work are discussed. Chapter 7 provides the full experimental details and characterization of the novel compounds that have been reported in this thesis (including X-ray crystal structures).