Catalytic Silylation of C-H Bonds: Reaction Development, Mechanism, and Applications and Development of Degradable Polymers from Biorenewable Sources
- Author(s): Cheng, Chen
- Advisor(s): Hartwig, John F
- et al.
The following dissertation discusses the development of catalytic silylation reactions of alkenyl, aryl, and heteroaryl C-H bonds, some mechanistic studies on the Ir-catalyzed silylation of terminal alkenes and Rh-catalyzed silylation of arenes, and applications of the Ir-catalyzed C-H silylation to the functionalization of complex active pharmaceutical ingredients (APIs).
Chapter 1 provides a review of the utility and importance of C-H silylation, challenges and limitations of traditional approaches to constructing C-Si bonds, and limitations of catalytic C-H silylation, and some mechanistic insight on C-H silylation.
Chapter 2 describes a method for the catalytic silylation of terminal alkenyl C-H bonds to construct either Z- or E-vinylsilanes with high diastereoselectivity under mild conditions. The switch in the diastereoselectivity is resulted from a switch in the ligand. Mechanistic studies suggest that the reaction proceed through insertion and beta-elimination, not through direct C-H activation.
Chapter 3 describes a method for the Rh-catalyzed silylation of unactivated aryl C-H bonds with high sterically derived regioselectivity. This method represents the first one to construct C-Si bonds from C-H and Si-H bonds under mild conditions with arene as the limiting reagent. Examples in which the regioselectivities are superior to or different from those of the C-H borylation are demonstrated. The resulting arylsilanes are stable to many typical organic functional groups interconversions yet amenable to further functionalization under conditions orthogonal to those of arylboranes, rendering this method useful for the construction of synthetic building blocks.
Chapter 4 discusses the mechanism of the Rh-catalyzed silylation, including isolation of the catalyst resting state, rate measurements, rate law derivation, and kinetic isotope effect (KIE) experiments. A plausible catalytic cycle is proposed. The influence of the electronic properties of the arene substituents on the reversibility and relative rates for individual steps of the mechanism, and on the regioselectivity of the C-H bond cleavage and functionalization, is discussed.
Chapter 5 describes a method for the Ir-catalyzed silylation of aryl and heteroaryl C-H bonds. This method requires slightly higher reaction temperature than the Rh-catalyzed silylation described in Chapter 3, but is compatible with a much broader range of functional groups, including many heteroaromatic moieties. Silylation and functionalization of APIs is demonstrated.
Chapter 6 provides a brief discussion on the current state of the art on C-H silylation and the challenges to be overcome.
Chapter 7 describes the synthesis of polysilylethers (PSEs) using a monomer derived from a biorenewable feedstock. The monomer contains an alcohol and a silyl hyride moiety, which allows for polymerization through catalytic dehydrogenative coupling of an alcohol and a silyl hydride to form polymers with silyl ether linkages. High molar mass products were achieved, and the degree of polymerization was controlled by varying the amount of an AA-type monomer in the reaction. The PSEs possess good thermal stability and a low glass transition temperature (Tg ≈ –67 °C). The PSEs was degraded in acidic aqueous solutions to a low-molecular weight diol, which could be further biodegraded or used as building blocks for other polymers. To demonstrate the utility of the PSEs, polyurethanes were synthesized with low molar mass hydroxy-telechelic PSEs.
Chapter 8 describes making new siloxane-containing, degradable polymers from biorenewable feedstock, as well as attempts to improve the synthetic route to access the monomer described in Chapter 7. Specifically, instead of using silyl ether linkages as handles for polymerization and degradation, Si-O-Si linkages were incorporated into monomers, which lead to polymers containing Si-O-Si linkages. Polyurethane, polycarbonate, polyesters, and polyamides were synthesized. Polyurethanes containing siloxane linkages were hydrolyzed under mildly acidic conditions, achieving controlled polymer degradation.
Chapter 9 describes the synthesis of a macrolactone containing a Si-O-Si linkage from undecenonic acid, and the ring-opening polymerization (ROP) of this macrolactone by a well-defined Zn-complex. ABA triblock copolymers were also synthesized with polyesters made from this macrolactone as the mid-block.