To enable the rational development of CO2 utilisation processes, it is vital to understand the mechanisms of involved reactions. This thesis reports studies to improve the mechanistic understanding of two catalytic CO2 utilisation reactions and work towards the development of a novel CO2 utilisation process.
In Chapter 2, the role of guanidines in the reaction of propargylamines and CO2 to give cyclic carbonates is discussed. Guanidines have multiple potential mechanisms of catalytic activity. Through correlating the catalytic activity and solvent effects with observed guanidine-CO2 complexation, the origin of catalytic activity was determined.
Chapters 3-5 report results of a mechanistic investigation into the guanidine catalysed reductive functionalisation of CO2 with amines and hydrosilanes to yield formylamides, methylamines and aminal products. Through monitoring the reaction using GC, 1H & 13C NMR, in situ FT-IR and mass spectrometry, a number of competitive intertwined reaction pathways and intermediates were identified. At low temperatures formylamides are produced via transformylation of a formoxysilane intermediate with an amine. However, at higher temperatures a competing pathway enables further reduction of the formoxysilane to a bis(silyl)acetal, its subsequent reaction with amines yields aminal intermediates which can be further reduced under reaction conditions to yield methylamines and formylamides. A catalyst deactivation pathway was identified whereby formylation of the catalyst itself occurs. By using alkylated guanidines, the deactivation pathway was prevented and the alkylated catalysts could be used at 0.1 mol% loadings and achieved TON of 805 and TOF of 33.5 h-1.
In Chapter 6, efforts towards the development of a novel CO2 utilisation reaction are discussed. α-Ketocarboxylic acids were targeted from the insertion of CO2 into an aldehyde C-H bond, inspired by analogous hydroacylation chemistry. A range of reaction conditions, rhodium catalysts and substrates were investigated; however, although a number of products were identified, no incorporation of CO2 was observed.
