| Description |
The palladium-catalyzed functionalization of sp3-hybridized carbons has been an active area of research in recent years. In order to accomplish such transformations, control of β-hydride elimination is one of the central issues to be addressed. In our group, palladium-catalyzed hydro- and difunctionalization reactions of alkenes have been a long-standing area of interest, and we have developed several catalytic systems that achieve precise control of β-hydride elimination from various Pd complexes. Three of these systems are presented in this thesis. In the first chapter, a hydroalkoxylation of styrenes is discussed. In this method, it is proposed that a Pd hydride is generated via Pd-catalyzed aerobic oxidation of an alcohol solvent. The substrate would then insert into the Pd-H bond to form a Pd alkyl intermediate, which undergoes nucleophilic substitution to yield the overall hydroalkoxylation product. Mechanistic experiments performed in parallel with the development of this transformation highlighted the precise control over relative rates of the different steps that is necessary for the reaction to proceed effectively. Importantly, the stabilization imparted by π-benzyl interactions on the Pd alkyl intermediates was found to be crucial to allow for their functionalization. In the second chapter, an asymmetric hydroarylation of styrenes and dienes is described. This was developed on the basis of several racemic styrene and diene hydroarylation reactions previously reported by our laboratory. Analogously to the hydroalkoxylation, oxidation of the alcohol solvent was proposed to provide the hydrogen incorporated into product; and formation of a Pd π-benzyl or π-allyl intermediate was found to be essential for further functionalization. Toward the development of an asymmetric variant of this reaction, several classes of ligands were explored, with bisoxazolines giving the highest enantioselectivities. Bisoxazolines were then systematically modified and evaluated. As the effort toward an asymmetric hydroarylation was only moderately successful, we chose to investigate other routes to access hydroarylation-type products, as presented in Chapter 3. Specifically, we decided to access Pd π-allyl complexes from homoallyl electrophiles by "walking" the Pd along the carbon chain of the substrate. This would result in a novel approach to Pd π-allyl complexes and their functionalization, which has the potential to be further developed into an asymmetric reaction. The development of this reaction as well as the preliminary scope and mechanism are presented. |
