Probing Reaction Intermediates and Photochemical Mechanisms in the Synthesis of Heterocyclic Compounds Using In-Situ Spectroelectrochemistry: Original Contributions to Green Synthetic Chemistry

Heterocyclic compounds constitute the largest class of bioactive molecules, and understanding the mechanistic pathways of their formation under photochemical and electrochemical activation is essential for developing cleaner, atom-efficient synthetic routes. This dissertation uses in-situ spectroelectrochemical methods to probe reaction intermediates and elucidate photochemical mechanisms in the synthesis of three classes of nitrogen-containing heterocycles, specifically benzimidazoles, quinoxalines, and triazoles, under green chemistry conditions. Spectroelectrochemical cells coupling UV-vis, Raman, and FTIR spectroscopy with controlled potential electrochemical experiments were designed and fabricated in-house. Electro-oxidative and photocatalytic heterocyclisation reactions were conducted in ethanol-water solvent systems and ionic liquid media, monitored in real time by the coupled spectroscopic techniques. Transient intermediate characterisation was achieved by electron paramagnetic resonance spin-trapping and time-resolved fluorescence spectroscopy. Density functional theory calculations at the M06-2X/6-311+G(d,p) level modelled potential energy surfaces for proposed mechanistic pathways. Novel structure-reactivity insights were obtained by systematic variation of substrate electronic properties. Catalyst screening identified a copper-doped carbon nitride photocatalyst as optimal for benzimidazole synthesis, achieving 96 percent yield under visible light with atom economy exceeding 85 percent. For quinoxaline synthesis, electrochemical activation in ionic liquid media afforded complete selectivity control absent under thermal conditions. The dissertation makes original contributions to mechanistic organic electrochemistry and green heterocyclic synthesis methodology, with implications for pharmaceutical intermediate manufacturing in Nigeria. Keywords: spectroelectrochemistry, heterocyclic synthesis, reaction intermediates, photocatalysis, green chemistry