Sustainable Methods in C–H Functionalizations

Abstract

Cross-coupling chemistry has matured in an attractive tool in synthetic organic chemistry to realized efficient formation of C‒C and C‒Het bonds. However, these transformations are also associated with certain drawbacks, especially with respect to the sustainability of the reactions, which is significantly reduced by the requirement of the prefunctionalization of the substrates. Contrary to this, the developed methods in the transition metal-catalyzed direct functionalization of omnipresent C‒H bonds presents a sustainable alternative for the construction of complex molecular scaffolds. Although the use of the direct C‒H functionalization approach is an important contribution towards Green Chemistry and environmentally-benign synthesis due to an improved atom- and step-economy, it is associated with certain limitations. These transformations typically require harsh reaction conditions with high temperatures, thus restricting an intendent broad applicability. Within our program on sustainable catalysis, the research is focused on the development of mild and sustainable strategies, which is also the central aspect of this thesis. Important strategies allowing for improved environmental aspects consist in the use of cost-efficient earth-abundant transition metal catalysts or the selection of green solvents to promote transformations. Especially, the employment of abundant manganese as catalyst is of great interest due to its low toxicity, for this reason, it was used for the functionalization of tryptophan derivatives. Here, the use of water and micellar catalysis was investigated to achieve mild and environmentally responsible conditions. Other projects were focused on new technologies and a sustainable reaction design. Besides the important application of electrocatalysis instead of the use of toxic metal salts as chemical oxidants, the irradiation with visible light was recognized as an elegant strategy to achieve mild reaction conditions. The beneficial effect of the blue LED light was further substantiated for ruthenium-catalyzed C‒H activation chemistry, as the synthesis of small molecules with a potential applicability as pharmaceutically active compounds was achieved at room temperature with an improved chemoselectivity and the avoidance of byproducts. Moreover, the new field of electrophotocatalysis is of great importance, where interesting findings were made with respect to different employed (photo)redox mediators.