Organocatalytic Transformations with N-Heterocyclic Carbenes - An Investigation of Ionic Liquids as NHC Precatalysts and Aerobic Oxidative Esterifications
The call for more sustainable processes in the chemical society has led to the development of the field of green chemistry. The Twelve Principles of Green Chemistry serve as guidelines for the design of processes. With the rise of organocatalysis and N-heterocyclic carbenes (NHCs), the NHCs have become powerful alternatives in lieu of traditional metal-catalysts; being inexpensive and non-toxic. In this thesis, two different applications of NHC catalysis are presented.
In the first part of the thesis an imidazole-based ionic liquid is used as an NHC-precursor, forming oxo triphenylhexanoates (OTHOs) in a highly stereo- and regioselective manner. The reaction negates the requirements for laborious work-up procedures usually required for the isolation of products.
The second part of the thesis have demonstrated the use of molecular oxygen as a terminal oxidant, combined with coupled electron transfer mediators for oxidative NHC catalysis enabling access to various α,β-unsaturated esters and lactones. The catalytic protocol permits the substitution of stoichiometric high molecular weight oxidants, thereby facilitating scale-up reactions as well as reducing the amount of chemical waste generated.
Both of these routes have been designed according to the principles of green chemistry in an attempt to meet the demands for more environmentally benign synthetic strategies.