Manganese catalysed hydrogenation of carbonyl compounds

Abstract

The use of manganese catalysts represents a new area of research in the field of catalytic homogenous hydrogenation. This thesis covers the work on the synthesis and evaluation of manganese complexes in the hydrogenation of carbonyl groups. A N^P^N type ligand was synthesised and coordinated to ruthenium (II) and manganese (I) sources and evaluated in carbonyl reduction using hydrogen gas. The catalysts were found to be less active when compared to state-of-the-art catalysts derived from ruthenium and manganese and tridentate ligands. Both catalysts were found to work well using wet ethanol. Chapters 3-5 describe the development of new ligands derived from the PPFAPy (N-(2-methylpyridine)diphenylphosphinoferroceneamine) precursor and coordination of those ligands with manganese (I) and the evaluation of these catalysts in the hydrogenation of carbonyl compounds. The first catalyst was able to asymmetrically reduce ketones being the first reported manganese catalyst to do so using hydrogen as the reductant. The discovery of using mild bases such as potassium carbonate in hydrogenation using these catalysts was used to successfully reduce enantiomerically pure esters with minimal loss of purity. Kinetic experiments revealed the substrate to be 0th order and the catalyst 1st order in ketone hydrogenation indicating the hydrogen activation being the rate-determining step. In ester hydrogenation both the substrate and catalyst were found to be 1st order indicating that the rate-determining step was the ester reduction. By variation of the substituents on the ligand phosphine the activity of the catalyst can be altered with electron-rich phosphines increasing and electron-poor phosphines decreasing the catalytic activity. An electron-donating group in the 4-position of the pyridine gave a further increase in the activity, whilst an electron-withdrawing group in the same position led to a lowering in activity. The best combination was found to be an electron-rich phosphine paired with an electron-rich pyridine. The active manganese catalyst was able to reduce acetophenone at a catalyst loading of 0.01 mol%. Minimal impact on enantioselectivity was found with the substitution on either the phosphine or pyridine. The solubility of the catalysts was significantly improved by the increased substitution of the ligands.