Manganese is interesting as a cheaper and less toxic alternative to palladium and other established transition metals as catalysts in organic chemistry.
Transition metals are known to be excellent catalysts in a number of important organic chemistry reactions. While palladium is the most exploited transition metal for the purpose due to its high catalytic efficiency, efforts are directed towards finding other metals which are cheaper and less toxic. The project focuses on use of manganese catalysts in radical coupling reactions.
Catalytic cross-coupling involves a catalyst in the reaction mechanism leading to a repeating catalytic pathway – the catalytic cycle. Dating back to the beginning of the 20th century, the first types of coupling reactions formed the homo coupling product and used stoichiometric amounts of metal. From the early 1970’ies and later on, a number of much more efficient cross-coupling reactions with transition metals as catalysts were found. Especially, palladium was recognized as highly efficient – leading to the 2010 Nobel Prize in chemistry being award to Heck, Negishi, and Suzuki, three pioneers in palladium catalyzed cross-couplings.
Palladium remains the catalyst of choice in a number of industrial and academic research applications, but the high price of this metal along with toxicity issues has initiated attempts to identify cheaper and less toxic alternatives. The aim of this project was to expand the conditions of the known cross-couplings to include homogeneous manganese catalysts.
Firstly, a procedure for N-arylations through anon-cross-coupling mechanism was explored, but found too difficult to control.
Secondly, a Buchwald-Hartwig Catalyzed Cross-Couplings procedure was examined. It was not possible to reproduce findings from the literature. Instead, the reaction was shown to be catalyzed by 10-100 ppm of a copper catalyst. Likewise, it was impossible to reproduce literature findings for a Manganese Catalyzed Stille Cross-Couplings procedure. Instead, the reaction was shown to be catalyzed by 30 ppm of a palladium catalyst.
Further, a Manganese Catalyzed Kumada Cross-Couplings procedure was examined. The scope of this reaction was limited for the electrophile, which was attributed to an aryl radical anion intermediate that was indicated by a clock experiment.
Finally, Dimethyl Zinc Mediated Radical Alkylationof β–Bromostyrenes was investigated. The attempts at a manganese catalyzed Negishi cross-coupling resulted in the discovery of a radical coupling of β–bromostyrenes with ethers and tertiary amines.
Composite building blocks and framework view adapted from Database of Zeolite Structures.