Abstract

A key aspect of organic chemistry is the development of methods to gain an easier and more economical access to a variety of useful molecules. Since the discovery palladium-catalyzed cross-coupling reactions, transformations that employ an organometallic reagent as a nucleophilic coupling partner together with an aryl halide or pseudo-halide as the electrophile have set the standard for carbon-carbon bond formation. More recently, chemists have become increasingly aware of how their science affects the environment and that it has been strongly dependent on a finite amount of resources. Therefore principles of a greener chemistry have been applied to guide researchers in the development of novel reactions towards a more sustainable, less hazardous and less wasteful chemistry.

Decarboxylative cross-couplings employ aromatic carboxylic acids as replacement for the organometallic reagent and form only carbon dioxide as by-product, but decarboxylations of benzoic acids require a metal co-catalyst. Therefore, desulfinative cross-couplings, which rely on aryl sulfinate salts as the nucleophilic coupling-partner, have also gained attention. Bench-stable sulfinates can undergo metal-assisted desulfination under extrusion of sulfur dioxide in analogy to the decarboxylation of benzoates. This thesis started with the adaptation of conditions from a heteroaromatic decarboxylative cross-coupling towards a desulfinative reaction of aryl sulfinates with aryl bromides. The method gave good results with electron-poor aryl bromides and further studies of the reaction demonstrated that it is indeed a palladium(0)-catalyzed cross-coupling and neither a nucleophilic aromatic substitution nor a radical transformation.

During these studies, a tendency of the aryl sulfinate to undergo C-C homocoupling reactions was noted. We were interested in developing a catalytic reaction to improve access to symmetrical biphenyls. Conditions in aqueous media employing copper(II) dichloride for the reoxidation of the palladium catalyst as well as a reaction catalytic in palladium and TEMPO with molecular oxygen as terminal oxidant were successfully established. Further studies led to the development of a ligand-free desulfinative cross-coupling reaction that demonstrated an excellent reactivity of aryl sulfinates with bromobenzonitriles. Additional work to discover more sustainable reaction conditions resulted in the development of a method in isopropanol for bromobenzonitriles and attempts to adapt the reaction for aryl chlorides yielded a desulfinative cross-coupling with chlorobenzonitrile.

In summary, the research presented herein describes novel methods for the preparation of carbon-carbon bonds via palladium-catalyzed coupling reactions of aryl sulfinates. It increases the scope of synthetically applicable reactions of aryl sulfinates and enhances the knowledge on their reactivity.