Small Molecule Activation Facilitated by Main Group Elements

Abstract

Main group containing systems are emerging as an alternative to transition metal catalysts, often showing complementary reactivity. Transition metal complexes have provided the basis for fascinating catalytic reactions such as the reduction of unsaturated bonds, as well as the formation of new carbon-carbon and carbon-pnictogen bonds. The cost of many second and third row late transition metals, precious metals commonly used in catalysis, is high, and fluctuates, so discovery of alternative methods is important. Two paths have been taken to create alternative, more affordable systems. The first is harnessing the power of less expensive first row, or early transition metals and the second is using organic and main group containing catalysts to facilitate similar transformations, which is a focus of this thesis. Within this thesis, main group complexes containing boron or phosphorus are explored in bond activation chemistry. While the groundwork for hydrogenation via boron cations and neutral boranes has been reported, this thesis explores the application of bis(amino)cyclopropenylidene (BAC) carbenes in borane complexation through synthesis of new BAC borane adducts and carbenes. These adducts are explored in hydrogenation and hydrosilylation reactions with imines and some enamines to form amines including pharmaceutically relevant ones. The latter portion of this thesis explores the application of diazaphospholenes in bond activations. Recently diazaphospholenes have emerged as potent reductants in a variety of transformations as polar hydride donors. Diazaphospholenes have been made catalytic in these polar (two electron) transformations, and are readily synthesized from abundant materials. This thesis shows development of distinct radical (single electron) chemistry using diazaphospholenes, including phosphorus-carbon bond formation reactions, and radical cyclization reactions. The single-electron chemistry of diazaphospholenes was applied to the challenging problem of sulfur hexafluoride reduction. Sulfur hexafluoride is a highly potent greenhouse gas, which is used in the power industry with a global warming potential of approximately 23,500 times that of carbon dioxide. This thesis explores three new methods of the decomposition of this highly potent greenhouse gas. Reactions using stoichiometric phosphides or catalytic phosphines are shown, and finally a new route to sulfur hexafluoride reduction with magnesium (0) discovered during this work is shown.