Design of precatalysts and phosphine ligands for Pd-catalyzed transformations
Author(s)Ingoglia, Bryan Taylor.
Massachusetts Institute of Technology. Department of Chemistry.
Stephen L. Buchwald.
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The work described in this thesis pertains to the formation of carbon-heteroatom bonds facilitated by palladium catalysts supported by bulky phosphine ligands. The first chapter is a summary of how biaryl monophosphine ligands have been used for carbon-heteroatom bond formations, including a ligand selection guide. The second chapter demonstrates how phosphinesupported Pd(II) oxidative addition complexes can be used as precatalysts in a variety of cross-coupling reactions. The third chapter presents a systematic study of the ligand architecture in an effort to rationally design new ligands capable of facilitating the challenging C-F reductive elimination from Pd(II). The fourth chapter highlights a structurally interesting side-product that resulted during ligand synthesis.Chapter 1: Biaryl Monophosphine Ligands in Palladium-Catalyzed C-N Coupling: An Updated User's Guide Over the past three decades, Pd-catalyzed cross-coupling reactions have become a mainstay of organic synthesis. In particular, catalysts derived from biaryl monophosphines have shown wide utility in forming C-N bonds under mild reaction conditions. This work summarizes a variety of C-N cross-coupling reactions using biaryl monophosphines as supporting ligands, with the goal of directing synthetic chemists toward the ligands and conditions best suited for a particular coupling. Chapter 2. Oxidative Addition Complexes as Precatalysts for Cross-Coupling Reactions Requiring Extremely Bulky Biarylphosphine Ligands. Palladium-based oxidative addition complexes were found to be effective precatalysts for C-N, C-O, and C-F cross-coupling reactions with a variety of aromatic electrophiles.These Pd(II) complexes are easily prepared and offer a convenient alternative to previously developed classes of precatalysts as they can be formed even with extremely large phosphine ligands, for which palladacycle-based precatalysts do not readily form. The complexes were found to be stable to long-term storage under ambient conditions. Chapter 3. Structure-Activity Relationship of Phosphine Ligands for the Fluorination of Five-membered Heteroaromatic Compounds Palladium catalysts supported by bulky dialkyl triaryl monophosphine ligands have been shown to promote the coupling of metal fluorides with (hetero)aryl bromides and triflates in good yield. A limitation of this methodology is the use of five-membered heteroaryl bromides, as the reductive elimination is more challenging due to the smaller size and electron-rich nature of the aryl electrophiles.In order to understand which structural features of the ancillary ligand are critical to facilitating the desired transformation, the ligand backbone was systematically varied and the initial rate of fluorination was monitored. These studies revealed that substitution at the 2" and 6" positions of the ligand scaffold has a dramatic impact on the reaction rate. As a result of these studies, new ligands were proposed which may be better able to accelerate the fluorination reaction. Chapter 4: Discovery of a Sterically Encumbered Hexasubstituted Arene through the Pdmediated Dearomative Rearrangement of Biaryl Monophosphine Ligands A key feature of the Pd-catalyzed aromatic fluorination reaction is the presence of the aryl group at the 3' position of the ligand backbone.It has been shown that supporting ligands lacking substitution at this position can be modified through a dearomative rearrangement, which incorporates one catalytic equivalent of the aryl electrophile into the ligand backbone when very bulky biarylphosphines are used. In Chapter 3, it was demonstrated that this rearrangement reaction is useful for rapidly accessing a variety of dialkyl triaryl monophosphine derivatives. During these studies, it was noted that for electron-rich aryl groups, this arylation occurred twice to form an unusual sterically congested hexasubstituted arene. X-ray crystallographic data indicates that the fully substituted aromatic ring is not planar.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019Cataloged from PDF version of thesis.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemistry
Massachusetts Institute of Technology