Increasing the Stability and Turnover Number of a Highly Active Iron Catalyst for the Asymmetric Transfer Hydrogenation of Ketones

A highly active iron catalyst for the asymmetric transfer hydrogenation (ATH) of ketones was reported by the Morris group in 2013. This catalyst surpassed the activity of platinoid metals by reaching TOFs up to 200 per second, but the highest reported turnover number (TON) was only 6060. To compete with the platinoid metals industrially, the focus of this thesis was to increase the TON of the iron catalyst through mechanistic investigation of potential deactivation pathways, switching to potassium formate and water as the hydride and proton source to obtain quantitative conversion, as well as the heterogenization of the precatalyst for eventual use in a continuous flow system. The orthophenylene catalysts in Chapter 3 displayed lower activity due to the rigidity of the triphenylphosphine donor, which places a phenyl group into the active site of the catalyst. Despite this hindrance, one catalyst achieved a TON of 8821 while retaining a remarkably high ee of 90%. The maximum TON of this catalyst was 150 000 which was achieved by heating the reaction at 75 °C with sequential injections of acetophenone and isopropanol (iPrOH). Unfortunately, since the ATH with iPrOH is an equilibrium process, the product alcohol racemized over time. The ATH in water using potassium formate as the hydride source lead to decreased activity due to the mass-transfer limitations caused by the formation of a biphasic reaction mixture. The maximum TON was 199, which rivals platinoid metal catalysts in similar biphasic systems. Finally, a variant of the iron catalyst was successfully covalently bonded to polystyrene, silica gel, and polyethylene glycol for use in batch chemistry. While maintaining a pH of 12.6, the iron catalyst remained covalently bonded to the polymeric support and could be recycled one time to achieve a total turnover number (TTN) of 164. Further studies are required to optimize the batch chemistry reaction for later use in a continuous flow system.Ph.D