UC Berkeley

The following dissertation discusses the development of silylations of C–H bonds that occur with high selectivity and investigations of the mechanisms by which they occur. The work includes intramolecular silylations of aliphatic C–H bonds, catalyzed by Rh complexes, and intermolecular silylations of aromatic C–H bonds, catalyzed by Ir complexes.

Chapter 1 provides a review of catalysts, reagents, and conditions for the undirected functionalization of C–H bonds with high selectivity. This review focuses on catalysts that alter or improve the selectivity of borylation and silylation reactions.

Chapter 2 discusses the development of a rhodium complex containing the Xantphos ligand which catalyzes the intramolecular silylation of alkyl C–H bonds with unusual selectivity for the C–H bonds located δ to the oxygen atom of a silyl ether over typically more reactive C–H bonds more proximal to the same oxygen atom. This chapter describes experimental studies on the mechanism of the silylation and DFT studies on the origin of the selectivity with which it occurs.

Chapter 3 describes the discovery of new catalysts for the silylation of aromatic C–H bonds. Analysis of initial rates uncovered the high reactivity of iridium catalysts containing a hindered phenanthroline ligand but accompanying rapid inhibition by hydrogen gas. With this catalyst, under a flow of nitrogen to remove hydrogen, electron-rich arenes, including those containing sensitive functional groups, undergo silylation in high yield for the first time.

Chapter 4 presents a detailed mechanistic analysis of the iridium-catalyzed silylation of aromatic C–H bonds. Until this study, no experimental data on the identity of complexes related to the mechanism of this process or the mechanisms by which they react to functionalize C–H bonds had been reported. In this chapter, we identify the resting state of the catalyst as an iridium disilyl hydride complex. Studies on the kinetics of the reaction revealed that the rate-limiting step varies with the electronic properties of the arene.

Chapter 5 describes the discovery of new catalysts for the silylation of heteroaromatic C–H bonds with high sterically derived selectivity. A novel pyridyl-imidazoline ligand is shown to improve the selectivity of the reaction for silylation of the least hindered C–H bond. The origin of selectivity was investigated and we determined that the rates of formation of the C–Si bond from isomeric heteroaryliridiuim complexes influences the selectivity of the silylation reaction more than the rates of the cleavage of C–H bonds to form these complexes.