The influence of heteroatom substitution on heterocyclic carbenes and their complexes

This thesis describes the influence of heteroatom substitution on heterocyclic carbenes and their corresponding complexes, using both density functional theory (DFT) and synthetic methods. The substitution of oxygen, sulfur or phosphorus for nitrogen in imidazol-2- ylidenes gives rise to a set of ten heterocyclic carbenes that were investigated using DFT. These replacements were found to have implications for both the structure and reactivity of the free carbene, which extended to complexes bearing ligands of this type. The balance between the ˜ìvâ-donation and ˜ìvÑ-withdrawal conferred by the heteroatoms onto the carbene centre was found to be a key factor influencing the reactivity at the carbenic carbon. Additional thermodynamic stability was observed for those carbenes that exhibited extensive ˜ìvÑ-delocalisation (aromaticity). The self-dimerisation of heteroatom-substituted carbenes is accelerated in the presence of protons. The activation barrier for dimerisation shows little correlation with the enthalpy of reaction, indicating that the estimation of carbene stability based purely on the enthalpy of reaction is not always appropriate. A number of novel palladium complexes bearing thiazol-2-ylidene ligands were prepared in the laboratory and their catalytic activity for Heck and Suzuki coupling assessed. While the catalysts were moderately active for both reactions, their performance was impeded by their insoluble nature and the lack of a second exocyclic ring substituent. DFT studies on the oxidative addition of imidazolium, thiazolium and oxazolium salts to zerovalent group-10 metals indicated that activation of C-C bonds resulting in the generation of hydrocarbyl-carbene complexes is likely to be achieved experimentally. Thiazolium salts and azoliums bearing 2-phenyl substituents show increased barriers to activation due to additional stabilizing interactions with the metal centre. The engineering of hydrocarbyl palladium carbene complexes that show enhanced stability to reductive elimination was undertaken using DFT. Complexes bearing imidazole-based ligands showed enhanced stability over imidazoline, thiazole and oxazole as a consequence of both their superior ˜ìvâ-donating ability and significant occupation of the carbene px-orbital. Widening of the auxiliary ligand bite-angle conferred a dramatic reduction in the barrier to reductive elimination, while the dihedral twist of the carbene relative to the PdL2 plane had little effect. Incorporation of increased steric bulk on nitrogen had little influence on the barrier to reductive elimination given its location distant from the active-site. More important were the ˜ìvâ-donating and ˜ìvÑ-withdrawing properties of the substituents, which conferred changes on the Pd-C bonding interaction resulting in an influence on the reductive elimination behaviour.