Alkaline Salts of Sodium and Potassium: from C–X Reduction to C–H Functionalization and Beyond

Citation

Toutov, Anton Alexandrovich (2017) Alkaline Salts of Sodium and Potassium: from C–X Reduction to C–H Functionalization and Beyond. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9VM499F. https://resolver.caltech.edu/CaltechTHESIS:10172016-133616811

Abstract

The discovery and contemplations of Gilbert N. Lewis (1875–1946) regarding the concept of electron pair acceptors has led to an improved fundamental understanding of molecular interactions. Lewis’s recognition that acidic character can exist in substances not containing hydrogen (i.e., Brønsted acids) led to the classification of a new group of reagents and catalysts for organic synthesis: Lewis acids. Over the last half-century, the application of these reagents and catalysts has in turn led to the discovery of a plethora of new chemical reactions, enabling previously unknown transformations. It has also been appreciated that electron pair donors (i.e., Lewis bases) are characterized by analogous and opposite behavior. Perhaps most intriguing is that in certain cases Lewis bases are capable of modifying simultaneously the electrophilic and nucleophilic character of the substance to which they are coordinated. It is also known that neutral tetravalent silicon can act as a Lewis acid for a variety of nucleophiles (i.e., Lewis bases) generating pentavalent Si species; these adducts are observed to have enhanced electrophilicity at Si and enhanced electron density at the ligands bound to silicon. In the case of organosilanes wherein at least one of the groups on silicon is a hydrogen (i.e., a hydrosilane), the reaction with Lewis bases can lead to pentavalent adducts with weakened Si–H bonds wherein the H has enhanced hydridic character. This property has been exploited by researchers in a number of ways, perhaps most prevalently in the development of hydrosilanes as mild reducing agents for the reduction of carbonyl compounds or for the mechanistically-related carbonyl hydrosilylation reaction.

This thesis details the discovery and development of fundamentally new chemical reactivity of silanes enabled by their interaction with basic salts of certain alkali metals (and includes some, but certainly not all of the work that I have performed in this area). First, it was found that specific combinations of hydrosilanes with basic alkali metal salts – in particular KOt-Bu – under certain conditions form exceptionally powerful reductive couples capable of selectively cleaving strong aromatic C–O and C–S bonds with exceptional effectiveness and novel selectivity. Second, I found that certain modifications and elaborations of this chemical system lead to dramatic changes in the operative reaction manifold: from C–X bond cleavage to E–Si bond formation. I determined that this concept of activating hydrosilanes with alkaline salts of the alkali metals can be harnessed for the mild and efficient construction of a wide array of E–Si bond classes by catalytic crossdehydrogenative coupling. Surprisingly, these challenging chemistries all occur in the absence of transition metal species, providing new horizons and opportunities for investigating Earth-abundant elements as catalysts and reagents for a host of applications.

Thesis Files