Base metal catalyzed aerobic benzylic oxygenations
13 June 2018
Campus Middelheim, A.143 - Middelheimlaan 1 - 2020 Antwerpen (route: UAntwerpen, Campus Middelheim
Organization / co-organization:
Department of Chemistry
PhD defence Hans Sterckx - Faculty of Science, Department of Chemistry
Since the emergence of green chemistry in the early nineties and an, albeit slowly, growing public awareness for the importance of the well-being of nature/planet Earth, several areas of organic chemistry have received increasing interest. Aerobic oxidation, the oxidation of (organic) molecules using dioxygen as the terminal oxidant, can be considered one of these areas. A fact that in no doubt can be attributed to the abundance of O2 in the atmosphere and the green character of the reagent, after all, water is the only byproduct formed. Considering the fact that our feedstock of organic compounds is still derived from petroleum, oxidation reactions are regarded as a key technology to transform hydrocarbons into chemicals of a higher oxidation state required for further transformations.
These bulk scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale O2 is the only economically viable oxidant. The produced commodity chemicals are however simple in structure and usually possess a high degree of symmetry thereby avoiding selectivity issues. When we look at the production of fine chemicals we see that preference is given to classical oxidants such as HNO3, Cl2, MnO2, CrO3, or H2O2. Remarkably, in fine chemicals industry oxidations are generally not very common reactions. This avoidance of O2 as oxidant can be explained by several factors, first there are the inherent safety issues accompanying the use of O2 as it forms flammable/explosive mixtures with most organic solvents. Furthermore all elements besides gold react highly exothermically with O2 giving rise to heat transfer issues.
Secondly, it is generally considered to be a less selective oxidant compared to the alternatives. While the first issue has recently seen some great solutions via the application of flow chemistry, the second issue mostly arises by a lack of understanding of the underlying mechanisms of aerobic oxidation. Given the large amount of new aerobic oxidation reactions being developed in academia the need for a deeper understanding increases even further as this will allow rational optimization and upscaling of these processes.
To this end, it was decided to further develop the previously discovered selective oxidation protocol of benzylpyridines in our research group and in addition study the reaction mechanism in detail. While this research was in progress it spawned the development of several related oxidation protocols in the group which again were studied in detail.