Investigations into Carbon-Based Lewis Acids for Frustrated Lewis Pair Catalysis
Over the past decade, significant advancements in main group chemistry have led to an exciting renaissance in the field. The pioneering discoveries made in the mid 2000s by Dr. Stephan and Dr. Erker have paved the way for the flourishing field of frustrated Lewis pair (FLP) catalysis that we are familiar with today. Continued efforts have been made in the field to find alternative Lewis acids that are more robust, synthetically simple and air and moisture stable than the highly fluorinated aryl boranes commonly employed as FLP catalysts. A variety of group 14-16 element Lewis acids have been examined, however, to date none offer a competitive alternative. Recently, there has been interest in using carbon-based Lewis acids, specifically pyridinium based salts as FLPs. This thesis describes our attempts to synthesize carbon-based pyridinium Lewis acids and their application as FLP reduction catalysts. We report the synthesis of two 1,4-dihydropyridines and their corresponding pyridinium salts: N-phenyl Hantzsch ester (HE) and N-(4-trifluoromethyl)phenyl HE. We examined the activity of both pyridinium salts for the hydrosilylation of many carbon-heteroatom double bonds and determined that N-(4-trifluoromethyl)phenyl HE pyridinium was the most active catalyst. However, a troubling induction period was observed in all catalytic reactions. Upon changing the reaction solvent to 1,4-dioxane, the reaction rate was dramatically increased and no induction period was observed. Subsequently, while employing a control reaction, it was determined that N-(4-trifluoromethyl) phenyl HE pyridinium may not be the catalytically active species. Instead, in situ generated cationic silylium species, may be the active catalyst. More research must be completed to identify the catalytically active species and determine how these species are generated. A brief study of the potential of N-(4-trifluoromethyl)phenyl HE pyridinium as a hydrogenation catalyst for the reduction of aldimines has been presented. It was observed that the desired amine was generated in low quantities. Furthermore, N-(4-trifluoromethyl)phenyl HE pyridinium has the potential to be an active catalyst however, further optimization of catalytic conditions must be accomplished. The research described herein will detail the work completed to identify the potential of carbon-based N-aryl HE pyridinium salts as Lewis acids for FLP-type reduction catalysis.
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