Structure–Reactivity Relationships of Monophosphine Ligands in Nickel-Catalyzed Cross-Coupling

Abstract

The selection of an appropriate ancillary ligand to control reaction outcome is among the most important considerations when developing a transition metal-catalyzed synthetic method. With thousands of unique ligand structures designed to complement equally diverse metal/substrate chemical space, identification of an effective ligand can be challenging, as the ability to rationalize and predict how ligands will impact catalyst structure often requires extensive study. Thorough mechanistic investigations have elucidated many of these ligand structure¬–reactivity relationships (SRRs) with Pd, leading to substantial advances in ligand design and catalytic methodology development. However, a similar mechanistic understanding of ligand effects is lacking for Ni, limiting its widespread adoption in synthesis as a practical alternative to precious metals.The works in this thesis describe the elucidation of monophosphine ligand structure–reactivity relationships in Ni catalysis. Chapter 1 provides historical context of Pd-catalyzed cross-coupling and ligand/catalyst design principles that have led to its success. Recent developments in Ni catalysts are then discussed with an overview of mechanism, ligand design, and outstanding challenges in the field. Chapter 2 describes the development of a computationally assisted, quantitative classification analysis for monodentate phosphines in Ni and Pd catalysis using a unique ligand steric parameter, %Vbur (min). Through organometallic studies, we demonstrated that %Vbur (min) best rationalizes the ligation state of phosphines when compared with previously developed steric descriptors. This analysis provided new insights into the ligand structural features that govern ligation state and the significance of speciation in successful Ni-catalyzed cross-couplings. Overall, the workflow will be of broad value in uncovering, modeling, and predicting structure–reactivity relationships of novel ligand scaffolds and reactions. Chapter 3 describes the elucidation of dialkylbiaryl (Buchwald-type) phosphine ligand SRRs in Ni catalysis. This study was guided by data-driven classification analysis, which together with mechanistic organometallic studies of structurally characterized Ni(0), Ni(I), and Ni(II) complexes allowed us to rationalize ligand reactivity in four representative Ni-catalyzed cross-coupling reactions. This has led to key insights into the use of these important ligands in Ni catalysis, along with the development of new air-stable, highly reactive precatalysts for room temperature cross-coupling reactions.