| Description |
Linear free energy relationships have been a staple of reaction mechanistic studies for nearly 100 years, enabling the quantification of subtle steric and electronic interactions between ligand, catalyst, and substrate. Recent work has offered an integrated approach to both interrogate reaction selectivity origins and to predict more optimal conditions. Classic and modern approaches to analyze ligand effects are presented in Chapter 1. Chapter 2 focuses on the development of novel descriptors for monodentate phosphine ligands. The application of these parameters to a Suzuki reaction was complicated by multiple ligation states of the catalyst. Experimental outcomes indicated that two catalyst regimes are present in the reaction; thus, separation of the results into subclasses was necessary. Doing so simplified the selectivity models, revealing nuanced ligand effects that were quantified with the new parameters. Further applications of these phosphine descriptors are detailed in Chapter 3. First, two gold-phosphine catalyzed cycloisomerization reactions are investigated using physical organic techniques along with reaction selectivity correlations. Overall, these data are used to identify the origin of ligand induced chemoselectivity, and to predict a novel ligand to increase the desired product ratio. Second, studies of an alkyl-aryl Suzuki reaction are described. In this instance, the phosphine ligand is shown to affect the enantiospecificity and chemoselectivity in two different fundamental steps. Evidence of the role ligand size and electronics play in directing the reaction pathways are presented. Chapter 4 details our team's efforts to identify a catalyst system that favors the atypical oxidative addition pathway within a Buchwald-Hartwig coupling reaction of differentially halogenated hetero-aromatics. Bidentate phosphine ligands were found to induce moderate selectivity; thus, ligand parameterization was utilized. Guided by univariate correlations, an exceedingly selective diaminophosphine ligand was successfully predicted, the origins of which were additionally analyzed with density functional theory (DFT) calculations. Using similar multivariate techniques, Chapter 5 presents the parameterization of acyclic diaminocarbene ligands developed in the context of a gold-catalyzed rearrangement-cyclization reaction. Enantioselectivity in this case was found to be highly sensitive to two substituents on the ligand, and quantification of these effects enabled the identification of a reaction system that produces highly enantioenriched products. |
