Examining the Role of Nitrate and Nitrite Anions in Oxidative C-H Bond Functionalisation at Palladium

The synthesis of a range of palladacyclic complexes with C^N ligand backbones is described in this thesis. Novel palladium complexes containing anionic nitrite and nitrate ligands have been prepared and characterised, and the geometry and linkage isomerism at Pd have been examined. Density Functional Theory calculations have been used to rationalise the experimentally observed characteristics of the complexes.

Several novel palladium complexes prepared were found to be highly active precatalysts for oxidative C–H bond functionalisation co-catalysed by nitrate, achieving yields superior to Pd(OAc)2 at low loadings of co-catalyst. In the model reaction of the acetoxylation of 8-methylquinoline, a novel C^N complex containing NO3– and CH3CN ligands was found to catalyse the reaction in the absence of a sodium nitrate additive, showing that the complex can act as a source of catalytically relevant nitrite/nitrate. Preliminary substrate scope investigations have shown that this complex is able to catalyse the functionalisation of sp2 C–H bonds, representing a significant improvement on the literature conditions.

Detailed mechanistic investigations have been conducted into the role played by nitrate in the catalytic cycle and the intermediates involved. Isotopic labelling experiments (18O) have been used to elucidate the source of the oxygen in the acetoxylated product, which is proposed to derive from acetic acid (solvent). Stoichiometric experiments have shown that reductive elimination of functionalised products from PdII is not favorable, and thus high oxidation state intermediates are likely to be implicated in catalysis. Through a series of NMR experiments, several transient species were observed, and structures have been assigned to three proposed catalytic intermediates. Based on these results, a bimetallic PdIII/III mechanism is proposed. Work is ongoing to isolate and characterise the high oxidation state intermediates that result from the presence of NaNO3 and identify their specific role in the catalytic cycle.