Forging highly congested stereocenters in an asymmetric manner is of high interest to the fine chemical and pharmaceutical industry as these sterically stressed motifs (typically being tertiary or quaternary stereocenters) are found within the structures of many bio-active molecules and synthetic drugs. In the last couple of decades, new converging strategies have been developed using transition metal (TM) based catalytic methods providing a convenient way to form compounds featuring such appealing stereogenic centers. While transformations leading to these targets have long been dominated by the use of 2^nd 3^rd row transition metals such as Pd and Ir, recent work has uncovered the significant potential of first-row transition metals such as Cu, Ni and Co, which represent more sustainable and less expensive alternatives.

Our group has developed various asymmetric allylic substitution processes mediated by TMs during the last 8 years, spanning the use of 2^nd and 3^rd row to 3d (transition) metals. In order to design more and novel sustainable methodologies for the synthesis of scaffolds that feature stereogenic tertiary/quaternary carbon centers, an accurate analysis of the selectivity and activity parameters controlling these transformations is desired.

This thesis presents three different case studies that focus on a comprehensive mechanistic rational of allylic and propargylic substitution reactions developed in our group over the years. In each of these cases, a different metal catalyst was used and our efforts allowed us to investigate both more classical 2-electron but also less explored 1-electron pathways in transition metal catalysis. A range of techniques were applied to gather a complete mechanistic picture of the selected transformations including NMR, EPR, kinetics, UV-Vis spectroscopy, X-ray crystallography, MS, React-IR and computational methods. This thesis illustrates the value of mechanistic studies to reveal unexpected reaction intermediates and manifolds controlling the overall efficiency of protocols that furnish sterically frustrated stereocenters.