Homogeneous gold catalysts : development of applications for gold(I) catalysts bearing N-heterocyclic carbene ligands

Abstract

Recently established as an excellent activator for π-systems, efforts made in gold chemistry have increased enormously, resulting in a new ‘Gold Rush’ in chemistry. This thesis is a small contribution to it. There are two main aspects dominating the following chapters: gold catalysts bearing N-heterocyclic carbenes (NHCs) as supporting ligand, and H₂O assisted catalysis. The initial motivation for the presented work was to specifically demonstrate the potential of [(NHC)AuCl] as suitable catalysts for both known and new organic transformations and to establish these commercially available catalysts in gold chemistry, a field currently dominated by phosphine bearing gold complexes. Water mediated catalysis became the next repeatingly occurring aspect of this thesis by pursuing this initial aim and finding water as a useful solvent or agent, respectively. Various useful applications for gold-NHC complexes are presented, starting with the Meyer-Schuster rearrangement of propargylic alcohols as a continuation of the work realized with propargylic acetates by the Nolan group in early investigations on gold catalysts. Next, a study on alkyne hydration is presented with focus on low catalysts loadings to establish gold catalysts as a powerful choice for such a highly relevant reaction. The catalytic system is then advantageously adapted to a silver-free variation, still active at low catalyst loadings and with further mechanistic insight. Inspired by gold activation of alkynes, a gap of reactivity in gold catalysis is closed by a successful demonstration of nitrile hydration, a functionality previously thought to be inert towards gold activation. In this context, formation and role of dinuclear hydroxy-bridged gold complexes is investigated highlighting these complexes as a possible resting state of gold complexes in the presence of water. Next, the formation of furanones via alkoxylation/lactonization of propargylic propiolates is presented, an observation initially made when exploring the scope of the Meyer-Schuster rearrangement. The dissertation finally closes with the gold-catalyzed formation of amides, this time however achieved from aldoximes reacting via dehydration/hydration mechanism.