Development of Pd-Catalysed C–H Bond Functionalisation Methodologies for the Accession of Molecular Complexity

This thesis describes the development of novel Pd-catalysed C–H bond functionalisation methodologies, with a view towards their application in sustainable chemical synthesis. The basis of this project focuses on the need for more efficient utilisation of precious metal catalysts, such as Pd, achieved by mechanistic understanding of the role of heterogeneous Pd nanoparticles (PdNPs) in such chemistry. An overview of observations from Pd-catalysed cross-coupling and C–H bond functionalisation chemistry is given initially, focusing on the mechanistic dichotomy between observed homogeneous and heterogeneous catalytic manifolds in these fields. The generation of potentially harmful stoichiometric byproducts in direct arylation methodologies is also examined for two classes of commonly-used electrophilic arylating agents, aryliodonium and aryldiazonium salts (Chapter 1).
The synthetic utility of C–H bond functionalisation chemistry has been exemplified through the development of complementary conditions for the direct arylation of the amino acid tryptophan (I) to form highly fluorescent 2-aryltryptophans (II), all of which have been evaluated using several key mass-based green metrics (Chapter 2). These conditions have also been shown to be effective for the functionalisation of tryptophan-containing peptides.
Initial rates kinetic analysis of the activity of several homogeneous and heterogeneous Pd catalysts in other direct arylation chemistry has highlighted remarkable similarities between apparently distinct catalysts, which suggests the formation of a comparable active catalyst phase. Heterogeneous Pd sources have also been successfully applied to the selective functionalisation of several biomolecules (Chapter 3).
The final part of this thesis describes fundamental studies on the nature of the ubiquitous Pd0 catalyst Pd2(dba)3. The major and minor isomers of this catalyst were characterised both in solution and in the solid state, which revealed that dynamic exchange between these species and free ligand varies significantly as a function of temperature. Crucially, this catalyst has also been shown by NMR and MS studies to be a source of catalytically competent PdNPs under commonly-found experimental conditions (Chapter 4).