Silver-free Homogeneous Gold Catalysis Using Isolated Cationic Complexes, Functionalized Ligands and Acid Additives

Activation of π-bonds towards a nucleophilic attack is an atom economic way
of making new bonds, thus creating new molecules. The application of
transition metal catalysts enables us to perform these reactions faster and in
milder conditions. Amongst the metals, gold has the ability to act as a strong
Lewis acid and activate alkenes, allenes and alkynes to react with a variety of
nucleophiles. Hence, gold catalysis has become a widespread tool in the
laboratories for crafting molecular complexity in mild conditions.
The Lewis acidity of gold can be increased by using cationic complexes. The
majority of homogeneous gold catalysis involves silver, which abstracts the
halide counteranion and creates the cationic complex. In many cases this leads
to successful catalysis, but there are numerous reports where silver interferes
or completely inhibits the desired reaction. To overcome this obstacle chemists
have developed various silver-free activation methods for homogeneous
cationic gold catalysis, which are explored in the literature review of this
thesis. The experimental results have been published in three peer-reviewed
journals and consist of alkyne activation in synthesis of essential organic
molecules and catalyst development using silver-free conditions.
The first publication of this thesis is a study of a gold catalyzed
hydroamination of alkynes. A comparison of different catalysts was performed
in the synthesis of 4-quinolones, where a preformed commercial cationic gold
complex was the most efficient. The catalyst screening was done partly in the
presence of silver, but no specific effects stemming from the other metal could
be observed. Non-cationic gold salts were screened for their reactivity, but
inferior results were obtained compared to the cationic complexes.
The second publication describes a self-activating Au(I) complex which
contains a functionalized NHC-ligand. The self-activation is based on the
interaction of H-bond donors in the ligand and substrate with the chloride
counteranion. The effect of moisture on catalysis was investigated and optimal
level of moisture was recognized. The scope of reactivity could be expanded by
using acid additives, which led to effective cyclization of 1,6-enynes.
Finally, the third publication presents the synthesis of cyclometallated
NHC-Au(III) compounds, which act as stable precatalysts. These complexes
can be activated by an equimolar amount of a Brønsted acid, which enables
the utilization of the desired counteranions without expensive additives. The
activated catalyst is effective in alkyne activation towards hydrophenoxylation.
The nature of the catalyst was examined by 1H-NMR spectroscopic
measurements, which suggest that the NHC-Au(III) is the principal catalytic
species in the studied reaction.