Discovery and Mechanistic Insights of Main-Group Catalyzed C–H Functionalization Reactions of Dicoordinate Carbocations
- Author(s): Shao, Brian
- Advisor(s): Nelson, Hosea M
- et al.
This dissertation focuses on the discovery and developments of novel carbon–carbon (C–C) bond forming reactions through dicoordinate carbocation intermediates. Chapter one provides a brief introduction into the known reactivity of relevant aryl and vinyl carbocations. In particular, C–C bond forming transformations will be the main focus of this discussion.
Our experimental work begins with the account of serendipitous discovery of intermolecular C–H insertion reactions of an aryl cation intermediate. We detail the reactivity-driven nature of our initial hypothesis and its development into a novel hydrocarbon arylation methodology. It was demonstrated that ortho-silylated aryl fluorides can be used as precursors for generating aryl cations capable of C–H insertion. The insertion chemistry exhibits inherent terminal selectivity in linear alkanes and is capable of activating one of the most challenges C–H bonds in methane.
Subsequently, we followed the reactivity of our aryl cations to other dicoordinate carbocations as in vinyl cations. Utilizing a similar strategy hinged upon the use of silylium–weakly coordinating anion catalysis, vinyl cations were found to also undergo intermolecular insertion into sp3 C–H bonds. Here, we establish a reductive coupling process between vinyl triflates and various hydrocarbons. Extensive deuterium labeling studies also helped to uncover key features of the dicoordinate carbocation insertion mechanism. A detailed account of our hypotheses in establishing our mechanistic probe experiments is reported.
In efforts to bring our new methodology to a more broadly applicable chemical space, we were determined to further develop our system for heteroatom compatibility. Our progress has culminated in a new mode of generation for vinyl cations under highly basic conditions, and results in the C–H insertion reactions in the presence of a wide variety of heteroatom-containing substrates. In these studies, 3-substitued cyclooctenyl vinyl triflates was a substrate class used to highlight the increased functional group tolerance of our new method. Utilization of lithium-weakly coordinating anions has allowed entry for our dicoordinate carbocation insertion chemistry into applications such as fine chemical syntheses.